Type 1 Diabetes: Cellular, Molecular & Clinical Immunology

Chapter 9 - Epidemiology of Type I Diabetes
Marian Rewers, MD, PhD¹, Jill Norris, PhD², Adam Kretowski, MD, PhD
¹Barbara Davis Center for Childhood Diabetes, UCD, Aurora, CO
²Department of Preventive Medicine & Biometrics, UCD, Aurora, CO

Type 1 Diabetes
Type 1 diabetes accounts for about 10% of all diabetes, affecting approximately 1.4 million people in the U.S., and 10-20 million globally (1,2). About 40% of persons with type 1 diabetes develop the disease before 20 years of age, thus making it one of the most common severe chronic diseases of childhood. In the U.S., where 30,000 new cases occur each year, type 1 diabetes affects 1:300 children and as many as 1:100 adults during the life span (3,4). Type 1 diabetes is the leading cause of end-stage renal disease, blindness, and amputation, and a major cause of cardiovascular disease and premature death in the general population (5). This disease results in over $5 billion in medical care expenditures per year, with costs for patients over 10 times those for persons without diabetes (6). Both the U.S. Public Health Service (7) and the American Diabetes Association (8) have identified needs in the area of prevention of type 1 diabetes. Population-based studies, as well as family studies and laboratory data, have provided new insights into the pathogenesis of type 1 diabetes, its associated risk factors and its natural history. They also allowed development of plans for primary and secondary prevention of diabetes in persons with ongoing ß-cell autoimmunity.
This chapter considers the whole spectrum of type 1 diabetes - from genetic susceptibility, through ß-cell autoimmunity without diabetes, to clinical type 1 diabetes. Special emphasis is given to epidemiological data relevant to the primary and secondary prevention.
Natural History of Type 1a Diabetes
The American Diabetes Association and the World Health Organization have revised the classification of diabetes mellitus (9,10). The new classification of diabetes is now primarily based on pathogenesis rather than on the requirement for insulin therapy. Based on the new system, type 1 diabetes (previously defined as insulin-dependent or juvenile diabetes) is caused by ß cell destruction, often immune mediated, that leads to loss of insulin secretion and absolute insulin deficiency. The etiologic agents that cause the autoimmune process and ß cell destruction are not well established. Type 1 diabetes also includes cases that are thought not to be immune mediated, but are characterized by absolute insulin deficiency (insulinopenia).
The current classification of diabetes mellitus distinguishes type 1a (autoimmune) (11) and type 1b (not immune-mediated) form that remains poorly defined (12,13). Type 1a is the most common form of diabetes among children and adolescents of European origin, usually characterized by acute onset and dependence on exogenous insulin for survival. In adults, the disease is nearly as frequent as in children, but often less dramatic onset may lead to misclassification as type 2 and a delayed insulin treatment. About 60% of persons with type 1 diabetes are diagnosed as adults.
The natural history of type 1a diabetes (Figure 1) includes four distinct stages:
1) pre-clinical ß-cell autoimmunity with progressive defect of insulin secretion;
2) onset of clinical diabetes;
3) transient remission; and
4) established diabetes associated with acute and chronic complications and premature death.

B-cell autoimmunity
In most patients, the etiology of the autoimmune process and ß-cell destruction is not known (14). The process is mediated by macrophages and T lymphocytes with detectable autoantibodies to various ß-cell antigens. Currently, autoimmunity is defined by the presence of autoantibodies (11), because their measurement is reliable and standardized across laboratories, in contrast to the cellular markers. The initial islet cell antibody (ICA) assay, using immunofluorescence and pancreatic tissue (15) has been notoriously difficult to standardize and has been replaced by a combination of specific ß-cell autoantibodies to insulin (IAA) (16,17,18), glutamic acid decarboxylase (GAD) (19,20), and tyrosine phosphatase ICA512 (IA-2) (21). These tests have been shown to be quite sensitive and predictive in relatives of type 1 diabetes patients (22,23) and in the general population (24). However, to avoid false or only transiently positive results duplicate testing and independent sample retesting should be considered in the prediction strategy (25,26,27,28).
Progression from ß-cell autoimmunity to clinical diabetes
The duration of pre-clinical ß-cell autoimmunity is variable and precedes the diagnosis of diabetes by up to 13 years (29,30). In persons with persistent autoantibodies, there is an early loss of spontaneous pulsatile insulin secretion, progressive reduction in the acute insulin response to intravenous glucose load, followed by decreased response to other ß-cell secretagogues, impaired oral glucose tolerance and fasting hyperglycemia (31). However, a non-progressive ß-cell defect has been shown to exist for many years in monozygotic twins and other relatives of type 1 diabetes persons. The recent studies show that detailed characterization of islet autoantibodies that include determination of titer, epitope specificity, IgG subclass and affinity would improve diabetes prediction in autoantibody-positive relatives and subjects at risk from general population (32,33,34). The highest risks for progession to type 1 diabetes is associated with high-titer IA-2A and IAA, IgG2, IgG3, and/or IgG4 subclass of IA-2A and IAA, and antibodies to the IA-2-related molecule IA-2beta (32,35,36). Using models based on these antibody characteristics, autoantibody-positive relatives can be classified into groups with risks of diabetes ranging from 7 to 89% within 5 years (36).
Moreover IAA affinity, measured by competitive radiobinding assay, can further stratify the risk of type 1 diabetes development (36,37,38). IAA affinity in multiple antibody-positive children is on average 100-fold higher than in children who remained single IAA positive or became autoantibody negative. All high-affinity IAAs require conservation of human insulin A chain residues 8-13 and are reactive with proinsulin, while lower-affinity IAAs are dependent on COOH-terminal B chain residues and do not bind proinsulin. 92% of children who developed multiple islet autoantibodies or diabetes are correctly identified by high-affinity IAA and 82% who did not develop multiple islet autoantibodies or diabetes are correctly identified by low-affinity IAA (36).
Studies in the first-degree relatives of type 1 diabetes patients (30,39,40) and in school children with no family history of type 1 diabetes (41,42,43,44,45) have reported ICA "remission" rates between 10-78%. Newer data suggest that while individual islet-specific autoantibodies may fluctuate in titers, it is very unusual to observe remission after two or more of such autoantibodies were present for even a few months (46). Some people may loose ß-cell autoantibodies or remain positive but do not progress to diabetes due to incomplete penetrance of susceptibility genes or insufficient exposure to the causative environmental agent(s). It is possible that ß-cell autoimmunity remits spontaneously in genetically resistant persons or when the offending factor is removed, similar to celiac disease. Age also plays a role, because children younger than 10 years have a 3-fold increased risk of progressing from autoimmunity to type 1 diabetes, compared to older relatives (47).
ß-cell autoimmunity may remit and reappear in the course of viral infections or variable exposure to dietary causal factors (26,27). The cumulative ß-cell damage and increases in insulin resistance with obesity and physical inactivity, may eventually cause diabetes at a later age. Those people in whom the disease process is slow may present with type 1 diabetes as adults, develop diabetes that does not require insulin treatment, or may even escape diabetes altogether. Markers of autoimmunity can be detected in 14-33% of diabetes patients classified on clinical grounds as “Type 2” (48,49) and are associated with early failure of oral hypoglycemic drug therapy and insulin dependence in these patients. A term “latent autoimmune diabetes of adults” (LADA) (50) has been coined for this slowly progressing form of type 1 diabetes.
Clinical Onset of Type 1 Diabetes
Most epidemiological data on type 1 diabetes are based on a clinical, standard definition that has been widely used for a long time: 1) physician diagnosis of diabetes , 2) daily insulin injections instituted at the time of diagnosis, and 3) residence by the patient in the area of registration at the time of first insulin administration. This practical definition is not taking into account the nuances of etiological classification into Type 1a, 1b, and other forms of diabetes.
In industrialized countries, 20-40% of type 1 diabetes patients younger than 20 years presents in diabetic ketoacidosis (DKA) (51,52,53,54). Younger age, female gender, HLA-DR4 allele (55), lower socioeconomical status and lack of family history of diabetes have been associated with more severe presentation. Younger children present with more severe symptoms at diagnosis, because children younger than 7 have lost on average 80% of the islets, compared to 60% in those 7-14 year old and 40% in those older than 14 (56). Case fatality in industrialized countries ranges between 0.4-0.9% (57). While brain edema is believed to be the major cause of onset death, the risk factors are poorly understood and heterogeneous.
Both DKA and onset deaths are largely preventable, because most of the patients have typical symptoms of polyuria, polydipsia, and weight loss for 2-4 weeks prior to diagnosis. The diagnosis is straightforward in almost all cases, based on the symptoms, random blood glucose over 200 mg/dl and/or HbA1c >7%. Oral glucose tolerance test (OGGT) is rarely needed for diagnosis. In questionable cases, negative OGGT allows ruling out current diabetes and, in combination with negative ß-cell autoantibodies, offers reassurance that diabetes is not around the corner. Traditionally, nearly all children with newly diagnosed type 1 diabetes were hospitalized. More recently, an increasing proportion of these children have been managed on an outpatient basis, especially in urban centers with specialized diabetes education and treatment facilities. In Colorado, USA, the proportion of children receiving only outpatient care at onset increased from 6% in 1978 to 35% in 1988 (58) and 75% in 2000. Hospitalization at onset does not improve short-term outcomes such as readmission for DKA or severe hypoglycemia (51,54), if adequate family education and follow-up is available on outpatient basis. Onset hospitalizations and subsequenty acute complications have similar predictors: biological (younger age, lower endogenous insulin secretion) and psychosocial (lower socioeconomic status, limited access to health care, dysfunctional family).
Prevalence of type 1 diabetes
Initial examination of a disease generally begins with cross-sectional data on its prevalence (i.e., the number of people in the population who have the disease at a given point in time). Selected estimates of type 1 diabetes prevalence in different populations with at least 90% ascertainment of cases are shown in Table 9.1 (see the end of the chapter).
The prevalence of type 1 diabetes in children aged less than 15 years ranges from 0.05 to 0.3% in most European and North American populations (59). Comparisons of prevalence data may be subject to considerable bias, since prevalence is determined not only by disease incidence, but also by case survival, which may vary markedly across populations. Prevalence data, however, are useful in determining the public health impact of type 1 diabetes. For example, in the early 1990's, the number of type 1 diabetic patients 0-19 years of age in United States was estimated at approximately 123,000 individuals (60). However, in 2000, with the growth of this segment of the U.S. population to over 80 million, there were approximately 160,000 children with type 1 diabetes.
Incidence of type 1 diabetes
Geographic location: One of the most striking characteristics of type 1 diabetes is the large geographic variability in the incidence (61,62,63) (Fig.9. 2). Scandinavia and the Mediterranean island of Sardinia have the highest incidence rates in the world while Oriental populations have the lowest rates. A child in Finland is 400 times more likely to develop diabetes than one in China. While there is a strong south-north gradient in the incidence, ‘hot-spots’ in warm climates have been reported (Sardinia, Puerto Rico, Kuwait). The geographic and ethnic variations in type 1 diabetes reflect the prevalence of susceptibility genes or that of causal environmental factors, or both.
In the general population, the prevalence of ß-cell autoimmunity appears to be roughly proportional to the incidence of type 1 diabetes in the populations (64,65). In contrast, the prevalence of ß-cell autoimmunity in first-degree relatives of type 1 diabetic persons does not differ dramatically between high and low risk countries. The incidence of ß-cell autoimmunity is higher in relatives younger than 5 years (3.7%/yr), compared to those 5-9 yr old (0.5%/yr) (66). During 5 years of follow-up, none of the relatives older than 10 has developed ß-cell autoimmunity (66), suggesting that ß-cell autoimmunity develops primarily before the age of 5 and that it may remit in many cases (44,45).
The clinical picture of the disease is similar in low- and high-risk areas (67,68), making it unlikely that the inter-population difference is due to misclassification of different types of diabetes. However, in many populations (69,70) type 2 diabetes is an increasing or already the predominant form of diabetes in children (71) making correct diagnosis and treatment increasingly difficult.

Age: Type 1 diabetes incidence peaks at the ages of 2, 4-6 and 10-14 years, perhaps due to alterations in the pattern of infections or increases in insulin resistance. Recently published data suggest that in many populations the highest rate of incidence is observed in the 10-14 age group, while the highest annual increase is in the 0-4 yrs age children (Figure 9.2.1). The age-distribution of type 1 diabetes onset is similar across geographic areas and ethnic groups (68) (see also Figure 3). There have been only a few studies that have examined the incidence of type 1 diabetes in adults, mainly because of the difficulty of distinguishing type 1 diabetes from insulin-requiring type 2 diabetes in older individuals. The incidence decreases in the third decade of life (72,73,74), only to increase again in the fifth to seventh decades of life (75,76). It has been speculated that the incidence of type 1 diabetes increases again in the fifth through seventh decades of life (39,40), but there is no hard evidence of such increase, and it is not known whether there are etiologic differences between childhood- and adult-onset type 1 diabetes.

A. incidence of Type 1 diabetes

B. annual increase in incidence

Figure 9.2.1. Current (1988-2002) trends in incidence of type 1 diabetes (A) and rates of anual incidence increase (B) in different age groups: 0-4, 5-9 and 10-14 yrs.

The prevalence of ICA decreased with age in first-degree relatives participating in DPT-1 screening. It is not known whether the etiology differs between childhood- and adult-onset type 1diabetes, but it was not apparent among adult participants in the UKPDS. Over 30% of those aged 25-34 were positive for ICA and/or GAD autoantibodies, but the prevalence decreased with age, to less than 10% in those aged 55-65 (77). The presence of the autoantibodies and age of presentation of diabetes were strongly associated with the presence of the HLA-DRB1*03/DRB1*04-DQB1*0302 genotype (78).
Race and ethnicity: Theincidence data from early 1990 suggested a significant racial difference in type 1 diabetes risk in multiracial populations, although not of the same magnitude as the geographic differences (Table 9.2). In the U.S., non-Hispanic whites were about one and a half times as likely to develop type 1 diabetes as African Americans (79) or Hispanics (80) (Figure 9.3). This was similar to the differences reported from Montreal, where children of British descent had about one and a half the risk of type 1 diabetes in children of French descent (84) (Table 9.2).

T1a=positive auto-antibodies
T1=insulinopenia without auto-antibodies
T2=high insulin secretion without auto-antibodies
Hybrid=features of both T1a and T2
Untyped=preserved insulin secretion without auto-antibodies

NHW=Non-Hispanic White
AA-African American
H-Hispanic
API-Asian/Pacific Islander
AI-America Indian

Figure 9.2.2. Epidemiology of diabetes in children and adolescents 0-9 yrs and 10-19 yrs old in USA (2002).

Interestingly, recent data from the SEARCH study have shown a rising trend in type 1 diabetes not only in non-Hispanic white population, but also among Hispanic and African American children (Figure 9.2.2) (81). In fact, however the number of new type 1 diabetic patients of African-American origin aged 10–14 years has risen significantly during the last 10-15 years and the incidence of type 1 diabetes in this age group is almost equal in the white and African-American populations, it is still almost twofold difference between non-Hispanic white and African-American children in the incidence of type 1 type in younger children (0-9 yr) (81,82). One could suspect that the marked increase in incidence in the African-American population may be in part due to misclassification of cases actually having type 2 diabetes, as there is an epidemic of type 2 diabetes in children in the U.S., which largely affects African-American children over the age of 10 years and many cases of type 2 diabetes require treatment with insulin at the time of diagnosis. It is unlikely, however, that the increased incidence of type 1 diabetes in African-American children can be explained only by misclassification. The diagnosis type 1 diabetes in the SEARCH study was confirmed by the presence of insulinopenia and diabetic auto-antibodies (81). Similarly to this observation, the high annul increase in the incidence of type 1 diabetes has been recently reported among the children of south Asian immigrants (Indian, Pakistani, Bangladeshi) in UK (83). Children living in south Asia have a low incidence of type 1 diabetes, so it is worth to emphasize that migrants to the UK have similar overall rates of type 1 diabetes to the indigenous British population (83).

Figure 9.3. Incidence of T1 DM in Colorado, 1978-88.

Little data concerning ß-cell autoimmunity is available for non-Caucasian populations. Among 86,000 relatives of type 1 diabetic patients screened for the DPT-1 trail, lower prevalence of ICA was observed in Asian Americans (2.6%) and in Hispanics (2.7%), compared to African Americans (3.3%) or non-Hispanic whites (3.9%). Lower prevalence of GAD antibodies has also been reported in Oriental compared to Caucasian type 1 diabetes patients (84,85).
Gender: In general, males and females have similar risk of type 1 diabetes (86), with the pubertal peak of incidence in females preceding that in males by 1-2 years. In lower-risk populations, such as Japan or U.S. blacks, there is a female preponderance, while in high-risk groups, there is a slight male excess (3,61,87). DPT-1 screening data have shown that male sex was associated with the appearance of autoimmunity (the presence of ICA and having two or more antibodies), but not with type 1 diabetes (88). It seems to suggest that male relatives with the known risk factor of ICA are less likely to progress to overt disease than comparable female relatives or that women develop different antibody responses (88). Interestingly, even within Europe, all populations with an incidence higher than 20/100,000 (Sardinia, UK, Italy, Finland, Norway, etc) had male excess, whereas those with a rate below 4.5/100,000 (the Baltic countries, Macedonia, Yugoslavia, Romania, etc) had female excess (59,89,90).

Figure 9.4. Gender differences in the incidence of type 1 diabetes.

Type 1 diabetes diagnosed in adulthood seems to be associated with male excess with a male:female ratio between 1.3 and 2.15 in most populations of European origin (91,92,93,94,95), but there is considerably less information concerning type 1 diabetes with onset in adult life. These findings contrast with data from animal models of type 1 diabetes (the NOD mouse), in which diabetes progression is almost twice as common in females (96). Several pathways for these differences have been explored, such as the effects of sex hormones, hormone substitution, and pregnancy on autoimmunity, the relationship between genetic risk factors for diabetes and sex (97), but the reasons for these differences are yet not known.
Time: The incidence of type 1 diabetes varies markedly over time, both seasonally and annually. In the Northern Hemisphere, the incidence declines during the warm summer months; similarly in the Southern Hemisphere, the seasonal pattern exhibits a decline during the warm months of December and January, implicating a climatic factor (98). This seasonal pattern appears to occur only in older children (99,100), suggesting that factors triggering diabetes may be related to school attendance. The observed seasonality does not appear to be an artifact of health care seeking or access, but the seasonal patterns differ by the HLA-DR genotype (101,102).
Most population-based registries have shown an increase in type 1 diabetes incidence over time (103,104,105,106,107,108). Periodic outbreaks, sometimes of pandemic proportion, e.g., during 1984-86 (104) appear to be superimposed on a steady secular increase in incidence.

Figure 9.5. Type 1 diabetes incidence is rising 3-5% /year in different geographic region with different incidence rate.

While the increase in type 1 diabetes incidence has affected all age groups, several studies reported particular increase among the youngest children (109,110,111,112,113). No reliable information is available concerning potential seasonal or annual variation in the incidence of ß-cell autoimmunity.
Genetic models are unable to explain the apparent temporal changes in the incidence (114). Few studies have analyzed time trends using modeling procedures, looking specifically at the effects of age, calendar period, or birth cohort on the incidence of type 1 diabetes (115,116,117,118). Figure 9.5 displays examples of different types of changes in the incidence of childhood onset type 1 diabetes that have been observed from late 1960s to mid 1980s: periodic outbreaks superimposed on a steady secular increase (Finland), and outbreaks with little secular increase (Poland). These non-linear calendar period effects were attributed to environmental factors causing epidemic peaks. The most recent study covering the period from 1984 to 1996 found only a strong linear effect of an increase of 2.3% per year, suggesting that the causal role of unknown environmental factors is diffusing over time (118). Importantly, the increase in incidence was seen in the age group 0 to 14 years and also in the age group on 15-29 years. The incidence is increasing both in low and high incidence populations. The rates have doubled over the past 30 years in countries with very high (Finland) and moderately high incidence (Colorado, USA) and even have tripled in countries known as a low risk area (Poland) (Figure 9.5).
A recent analysis of data on published incidence trends showed that the incidence of type 1 diabetes is globally increasing by 3.0% per year, and that the incidence of type 1 diabetes will be 40% higher in 2010 than in 1998 (119). The polio model, where autoimmune diabetes results from delayed exposure to infection that is benign when encounter in early childhood, could explain the recent increase in the incidence, but not the shift in diagnosis to earlier ages. Alternative explanation invokes the congenital rubella model where increased hygiene has led to a decline in herd immunity to common infections among women in child-bearing age (120). These women are more likely to develop viremia during pregnancy resulting in congenital persistent infection of ß-cells and early onset type 1 diabetes in the offspring. This model could explain both the increasing incidence of diabetes and the decreasing age of disease onset.
Genetic factors
Family history of type 1 diabetes. In moderate type 1 diabetes risk areas, such as the United States, the risk of type 1 diabetes by the age of 20 years is approximately 1:300 (Table 9.3). The risk is increased to about 1:50 in offspring of type 1 diabetes mothers and 1:15 in offspring of type 1 diabetes fathers (the reason for this parental gender difference is not known). The risk to siblings of type 1 diabetes probands ranges from 1:12 to 1:35 (121,122) and is further increased, in HLA-identical siblings (123). Recent analysis in Colorado has shown that in siblings, the overall risk of type 1 diabetes by age 20 years is 4.4%, and significantly higher in siblings of probands diagnosed under age 7 years than in those diagnosed later. In parents, the overall risk by age 40 years is 2.6% and twofold higher in fathers (3.6%) than in mothers (1.7%) of probands (124). It is estimated that by the age of 60 years approximately 10% of the first degree relatives develop type 1 diabetes (125). Family history of type 1 diabetes is a surrogate measure of the combination of type 1 diabetes genes and environmental exposures shared by family members.
The risk of ß-cell autoimmunity is higher, because not all autoantibody positive children develop diabetes by the age of 20. Prevalence estimates from cross-sectional studies shown in Table 9.1 are obviously less precise than cumulative incidence rates available for clinical diabetes.

Table 9.3. Risk, by the age of 20 years, of type 1 diabetes and beta-cell autoimmunity in the general population and family members of type 1 diabetic patients.

'Familial' cases represent about 10% of type 1 diabetes and do not appear to be etiologically different from 'sporadic' cases in terms of the HLA-DR, DQ gene frequencies, seasonality of onset, prevalence of islet autoantibodies (126). 'Familial' cases tend to have lower HbA1c and higher C-peptide levels than 'sporadic' cases, because relatives recognize diabetes symptoms earlier, however, these differences disappear soon after diagnosis.
Candidate genes: The primary loci of genetic susceptibility to type 1 diabetes have been mapped to the HLA-DR, DQ (127,128,129,130) and recently also to the DP region (131). While 50 percent of non-Hispanic whites in the United States have HLA-DR3 or DR4 allele, at least one of these alleles is present in 95 percent of patients with type 1 diabetes (132,133). The estimated risk for general population children who have the HLA-DR3/4,DQB1*0302 genotype is approximately 1:15 (134). Only 2.4% of the general population carries this genotype, compared to 30-40% of type 1 diabetes patients.
No particular HLA type seems to be associated with ß-cell autoimmunity, although associations between different patterns of insulin, IA-2A or GAD autoantibodies and HLA-DR, DQ phenotypes have been reported (135,136,137). In the DAISY study HLA DR3/4-DQ8 genotype predicted both persistence of autoantibodies and progression to diabetes among young first degree relatives of T1D patients and infants identified by newborn screening for HLA genotypes associated with diabetes (136). Significant difference in the prevalence of IAA, IA-2A, GADA and multiple autoantibodies has been observed between siblings of T1D children with the strongest susceptibility DR3/DR4 genotype and those with alleles other then DR3 and DR4 (136,137). 90% of the first-degree relatives (137) and general population children who stay persistently autoantibody positive (41,138) express the HLA-DRB1*04, DQB1*0302.
Data from the Type 1 Diabetes Prediction and Prevention Study have shown that titres of ICA, GADA and IA2A antibodies in unaffected children form general population are also dependent on the genetic risk (137).
The HLA-DR2, DQB1*0602 haplotype, which almost completely protects from type 1 diabetes (127), is found in about 15% of GAD and IAA positive young relatives of type 1 diabetes patients (135,136,137). None of the sibling, taking part in the DIPP study, who carried DQB1*0602 allele, had IAA, IA-2A or multiple antibodies. The frequency of children tested positive for ICA (1.3%) or GADA (0.3%) also tended to be lower in comparison to those without protective allele (137). In addition, it was recently observed that among ICA positive relatives with DQB1*0602, identified by the DPT-1 study, only 19% had multiple “biochemical type” autoantibodies and these markers were not associated with abnormal glucose tolerance (139).
Persistent islet autoimmunity was also found to be associated with non-HLA genes: insulin gene (INS-23Hph1 polymorphism in children with DR3/4 genotype) and genes coding cytokines involved in the Th1 regulatory pathway: IL-13, IL-4, IL4 receptor; but not with CTLA-4 SNPs (140).
Candidate genes outside the HLA region are being identified (141,142,143,144,145, 146,147). Genes encoding proteins involved in T-cell activation: CTLA-4, PTPN22, VNTR of insulin gene have been reported to have the functional variants associated with type 1 diabetes and contribute in 5-10% to genetic risk (148,149,150,151,152).
It is increasingly apparent that the identification of true genetic associations in common multifactorial disease will require studies comprising thousands rather than the hundreds of individuals employed to date (153).Perhaps even more subjects with ß-cell autoimmunity need to be genotyped to precisely determine the role of HLA and additional type 1 diabetes candidate genes in the initiation of autoimmunity and progression to diabetes.

Figure 9.7. Genes encoding proteins involved in T-cell activation (HLA II class genes, CTLA-4, PTPN22, VNTR insulin gene) play a key role in type 1 diabetes.

Environmental factors
Twin (154) and family studies indicate that genetic factors alone cannot explain the etiology of type 1 diabetes. Seasonality, increasing incidence and epidemics of type 1 diabetes as well as numerous ecological, cross-sectional and retrospective studies suggest that certain viruses and components of early childhood diet may cause type 1 diabetes (155).
Viruses: Herpesviruses (156,157), mumps (158,159), rubella (160,161), retroviruses (162,163), and rotavirus (164) have been implicated. Viral infections appear to initiate autoimmunity rather than precipitate diabetes in subjects with autoimmunity. Two or more infections with similar viruses may be needed - mice persistently expressing a viral protein in the ß-cells do not develop ß-cell autoimmunity unless exposed to the same virus later in life (165,166). ICA or IAA has been detected after mumps (158), rubella, measles, chickenpox (167), Coxsackie (168), ECHO4 (169), and rotavirus (164) infections. Newborns and infants are particularly likely to develop a persistent infection and among patients with congenital rubella syndrome, 70% have ICA (160).
An increased incidence of type 1 diabetes in patients with congenital rubella syndrome (CRS) is particularly interesting. While CRS is responsible for a minute proportion of type 1 diabetes and there is little evidence that postnatal rubella exposure to the wild strain (167) or to the MMR vaccine (170) causes type 1 diabetes, CRS provides an example of viral persistence leading to type 1 diabetes. The incubation period of type 1 diabetes in CRS patients is 5-20 years (161) and persistent rubella virus infection of the pancreas has been demonstrated in some cases. While CRS is not associated with particular HLA-DR alleles, the distribution of the HLA-DR3 and 4 alleles among patients with CRS and diabetes resembles that in non-CRS type 1 diabetes patients (160). Finally, a molecular mimicry has been reported between a rubella virus protein and a 52 kD ß-cell autoantigen (171).
The evidence is strongest for picornaviruses, which include human (enteroviruses and rhinoviruses) and animal pathogens (e.g., mouse EMC virus and Theiler's virus). Enteroviruses have been most strongly linked to human type 1 diabetes, but convincing proof of causality remains elusive (for review see 172,173). Case and autopsy reports (174,175), epidemics of type 1 diabetes associated with concurrent epidemics of enteroviruses (176,177) and multiple cross-sectional seroepidemiological studies (172) have been suggestive, but not entirely convincing. At least 90% of type 1 diabetes patients demonstrate prolonged period of ß-cell autoimmunity that is hardly compatible with an acute cytolytic enteroviral infection being a major cause. Enteroviral infection could, however, initiate ß-cell autoimmunity through molecular mimicry between CBV P2-C protein and GAD (178)) or a persistent ß-cell infection with impairment of insulin secretion and expression of self-antigens.
Cross-sectional studies of anti-Coxsackie antibodies in ß-cell autoimmunity have been week and inconclusive (179) and have been recently replaced by studies based on detection of picornaviral RNA in bodily fluids using PCR. Prospective studies of non-diabetic relatives and general population children found a strong relation between enteroviral infections, defined by PCR, and development of islet autoantibodies in Finland (180,181,182) but not in the U.S (616). Studies from Finland (180) and Sweden (183,184) have suggested in utero enteroviral infections can lead to type 1 diabetes in a significant proportion of the cases.
According to polio hypothesis low rate of enterovirus infections in background population is the reason that young infants may have increased susceptibility to the diabetogenic effect of enteroviruses, as they are not protected by maternal or their own antibodies (185,186,187,188). In line with this hypotesis there are recent observations from Europe, which shows that the frequency of enterovirus antibodies is higher in serum samples taken from pregnant women in countries with a low or intermediate incidence of type 1 diabetes compared with high-incidence countries (185). An inverse correlation between the incidence of type 1 diabetes and enterovirus infections is observed in the background population in different European countries (188).
There is accumulating evidence that mechanism of viral infection leading to b-cell destruction may be related to the induction of interferon a (189,190). Significant overexpression of INF-a in the pancreatic islets and higher serum levels INF-a have been documented in newly diagnosed patients with type 1 diabetes (189). It is also known that INF-a can decrease insulin synthesis/secretion, induce B-cell apoptosis and has a strong influence on innate immune system (190,191).
Viral infections through the generation of double-stranded RNA may induce INF-a production and by a direct cytolytic effect on pancreatic B-cells or indirect activation of innate immune system could trigger of autoimmunity in genetically susceptible individuals (G*). It was recently demonstrated that poly I:C (polyinosinic: polycytidylic acid) – a synthetic double-stranded RNA – stimulates INF-a and induces autoimmune diabetes mainly by activation of toll-like receptors, and its effect on an immune response is related the genetic background in different animal models (191).
Additional factors (186,192,193,194, 195) and season of birth (196) have been associated with type 1 diabetes.
Possible protective effect of infections: In animal models, viral infection may protect the host from developing type 1 diabetes (187,188). Evidence for such a protective effect in humans still needs confirmation (200,201,202).
According to so called “hygiene hypothesis” infectious agents could have a protective effect on autoimmunity development (203). The beneficial mechanisms of an early exposure (in utero or during the first weeks/months of life) of viral, bacterial or parasitic antigens are not known, however the role of the regulatory immune CD4 T cells stimulation, toll-like receptors and superantigens (by activation of T cell clones expressing specific receptor V gene) is discussed (203).
This hypothesis is supported by the evidence that multiple viral infections prevent autoimmune diabetes in NOD mouse and by the numerous studies showing that improvement of socio-economic status, higher level of sanitation and better medical care (for example: use of antibiotics) is associated with increased incidence of type 1 diabetes in humans (200,201,202).
Routine childhood immunization: None of the routine childhood immunization have been shown to increase the risk of diabetes (170,202,204,205) or pre-diabetic autoimmunity (206,207, 208).
Dietary factors: Cow's milk or wheat introduced at weaning trigger insulitis and diabetes in animal models (209) perhaps through a molecular mimicry (210). Human data are conflicting, but predominantly negative (211,212,213,214,215,216,217). An ecological study suggested an association between decrease in breast-feeding and increase in type 1 diabetes incidence between 1940 and 1980 (218). Subsequent case-control studies have shown a negative (219,220), positive (221) or no association (222,223,224). Certain studies (219,225,226) but not others (221,223,224) suggested a dose-response relationship between the duration of breast-feeding and protection from type 1 diabetes.

Figure 9.8. Potential mechanisms of the association between infant diet and beta-cell autoimmunity.

A meta-analysis found a 50% increase in type 1 diabetes risk associated with a breast-feeding duration of less than 3 months, and exposure to breast-milk substitutes prior to 3 months of age (192), but a subsequent meta-analysis reported much lower risk estimates (227). Breast-feeding may be viewed as a surrogate for the delay in the introduction of diabetogenic substances present in formula or early childhood diet. The reports that newly diagnosed diabetic children, compared with age-matched controls, have higher levels of serum antibodies against cow's milk and beta-lactoglobulin (228) as well as against bovine serum albumin (229) have been difficult to reproduce (209). More recent cohort studies failed to find an association between infant diet exposures and beta-cell autoimmunity (207,215,217). Interestingly, a study from Finland suggested that current cow’s milk consumption was more closely linked to pre-diabetic autoimmunity and diabetes than infant exposure (230).
Preliminary data from the Finish TRIGR study suggests that incidence of ICA antibodies is significantly lower in children fed until the age of 6-8 months with the casein hydrolysate in comparison to the group with conventional cow's milk-based formula (186).
It has been also recently discovered by two prospective large studies (DAISY and German BABYDIAB) that early ingestion of cereals between ages 0 and 3 months contribute to increased development of anti-islet auto-antibodies in offsprings of type 1 diabetic mothers and children (relatives of T1DM subjects and from general population) with high risk HLA-DR3/4, DQB1*0302 genotypes (231,232).

BABY Diab offspring of T1D mothers Ziegler, JAMA 2003;290:1721

DAISY HLA-DR3/4,DQB1*0302 children relatives & general population Norris, JAMA 2003;290:1713

Figure 9.9. Timing of the solid cereals introduction into infant diet and the risk of islet autoantibodies development.

Vitamin D3 supplementation
There is increasing evidence that vitamin D3 might contribute to pathogenesis and prevention of type 1 diabetes. Active vitamin D3 prevents type 1 diabetes in animal models, modifies T-cell differentiation, modulates dendritic cell action and modulates cytokine secretion, shifting the balance to regulatory T cells. Maternal intake of vitamin D in food during pregnancy was significantly associated with a decreased risk of islet autoimmunity appearance in offspring (233). Recent evidence from the large European studies suggests that vitamin D deficiency in pregnancy increases the incidence of type 1 diabetes in offsprings and that high dose witamin D3 supplementation early in life have a protective effect (234). Use of cod liver oil, an important source of dietary vitamin D and the long-chain n-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid, during the first year of life was found to significantly reduce risk for disease (235).
It seems possible that birth seasonality in children and/or the presence of seasonal pattern at diagnosis of type 1 diabetes could be explained by variation in endogenous vitamin D production during different year season (236,237). The monthly averages of maximal daily temperature and daily hours of sunshine were inversely related to the number of new patients per month in Belgium (238).
Chemical compounds: (streptozotocin (239,240) or dietary nitrates and nitrozamines (241) induce ß-cell autoimmunity in animal models. Circumstantial evidence suggests a connection between type 1 diabetes and consumption of foods and water containing nitrates, nitrites or nitrosamines (242,243,244). Multiple hits of dietary ß-cell toxins may render genetically resistant individuals susceptible to diabetogenic viruses leading to type 1 diabetes (245).
Weight gain, insulin resistance - “ the accelerator hypothesis”: It has recently been hypothesized that excess weight gain and increase in insulin resistance in early childhood is a trigger event, which initiates the autoimmunity leading to b-cell destruction and type 1 diabetes development (246,247). The rising blood glucose (glucotoxicity) accelerates beta-cell apoptosis directly or by inducing beta-cell immunogens in genetically predisposed subjects. This so called 'Accelerator Hypothesis' seems to be supported by several epidemiological case-control and population based cohort studies (246,248,249,250). A study from Norway found almost linear correlation between incidence rate of type 1 diabetes and birth weight (249). The risk of type 1 diabetes was higher by more than twofold in children with birth weight > 4500g in comparison to newborns with the lowest birth weight (<2000 g) (249). In the Childhood Diabetes in Finland Study children <15 years of age who developed type 1 diabetes were heavier and taller throughout childhood than birth date- and sex-matched controls (250). In this nationwide case-control study ten percent increment in relative weight was associated with a 50-60% increase in the risk of type 1 diabetes before 3 years of age and a 20-40% increase from 3 to 10 years of age (250). The most recent epidemiological observations suggest that high birth weight could possibly result from a moderating effect on intrauterine growth of HLA genotypes conferring a high risk of diabetes (251).
The wide variation in childhood type 1 diabetes incidence rates within the different populations could also be partially explained by indicators of national and individual prosperity. These indicators could reflect differences in environmental risk factors such as nutrition or lifestyle that are important in determining the risk of type 1 diabetes. The EURODIAB study has shown a positive association of the incidence rates with the value of gross domestic product (252).
The Environmental Determinants of Diabetes in the Youth (TEDDY): To resolve the controversy about the role of environmental factors in the pathogenesis of type 1 diabetes -The Environmental Determinants of Diabetes in the Youth (TEDDY), a large international project, with the aim to evaluate the putative environmental triggers during the 15-year follow-up of several thousand newborn babies identified with HLA-DR, DQ genotypes associated with type 1 diabetes, has recently been initiated (155).
Gene-environment interactions: Type 1 diabetes is likely caused by an interactive effect of genetic and environmental factors within a limited age-window. While both the susceptibility genes and the candidate environmental exposures appear to be quite common, the disease is still uncommon, raising a possibility of low penetrance (253).
In mice, the host's genes restrict the diabetogenic effect of picornaviruses in a manner compatible with a recessive trait not related to the MHC. In humans, on the other hand, susceptibility to diabetogenic enteroviruses appears to be genetically restricted by HLA-DR and DQ alleles (172). However, the allelic specificity is controversial (55,254,255,256) and may depend on the viral type and epidemicity. In general, the HLA-DR3 allele, present in most patients with type 1 diabetes, is associated with viral persistence.

Figure 9.6. Odds ratios for type 1 diabetes by exposure to whole cow’s milk prior to 3 months of age in low- and high- risk individuals, as determined by a genetic marker on the HLA-DQB1 chain (255).

Very few studies have examined a possibility of an interaction between the HLA genes and dietary exposured (255,230) (see Fig. 9.6). The epidemiological data are limited, but suggest that an early exposure to cow's milk in relatives with HLA-DR3/4,DQB1*0302, DR3/3 or DRx/4,DQB1*0302 is not associated with development of ß-cell autoantibodies (257,258). It is unclear whether other genes are involved.
Remission ("Honeymoon Period"): Shortly after clinical onset, most of the patients experience a transient fall in insulin requirements due to improving ß-cell function. Total and partial remissions have been reported in, respectively, 2-12% and 18-62% of young type 1 diabetes patients (54,259,260). Older age and less severe initial presentation of diabetes (259,260,261,262) and low or absent ICA (262,263,264) or IA-2 (265) have been consistently associated with deeper and longer remission (5). Evidence relating GAD autoantibodies (262,265,266), non-Caucasian origin, HLA-DR3 allele, female gender and family history of type 1 diabetes to a less severe presentation, greater frequency of remission and slower deterioration of insulin secretion is inconclusive. Most studies (259,263), but not all (260,261), agree that preserved ß-cell function is associated with better glycemic control (lower HbA1c) and preserved ß-cell glucagon response to hypoglycemia (267). The prevalence of ICA (but not GAA) decreases from 87% at the time of type 1 diabetes diagnosis to 38-62% 2-3 years later (263,268), faster in young boys, subjects lacking HLA-DR3 and 4, and those diagnosed between July and December (268).
The natural remission is always temporary, ending with a gradual or abrupt increase in exogenous insulin requirements. Destruction of ß-cells is complete within 3 years of diagnosis in most young children, especially those with the HLA DR3/4 phenotype (218). It is much slower and often only partial in older patients (270), 15% of who still have some ß-cell function preserved 10 years after diagnosis (271).
Established Diabetes
Acute complications: Acute complications of type 1 diabetes: DKA, hypoglycemia and infections are described in detail in other chapters. The risk of hospital admission for acute complication is 30/100 patient-years (p-yrs) in the first year of the disease and 20/100 p-yrs in the subsequent 3 years (54). Age and sex-specific incidence pattern suggest that the risk of ketoacidosis is increased in adolescent girls (272). The preventable or potentially modifiable risk factors comprise lack of health insurance, high HbA1C levels and mental problems (272,273). An estimated 26% of the patients have at least one episode of severe hypoglycemia within the initial 4 years of diagnosis, with little relation to the demographic or socioeconomical factors. The incidence of severe hypoglycemic episodes varies between 6 and 20 per 100 person-years, depending on age, geographic location, and intensity of insulin treatment (54,272).
Mortality: Insulin treatment dramatically prolongs survival but it does not cure diabetes. Although the absolute mortality at onset and within the first 20 years of type 1 diabetes is low (3-8%), it is 5 times higher for diabetic males and 12 times higher for diabetic females, compared to the general population (274,275). In the Allegheny County population-based registry (Pennsylvania, USA) as of January 1999 the cumulative survival rates were 98.0% at 10 years, 92.1% at 20 years and 79.6% at 30 years duration of type 1 diabetes (276). A significant improvement in the survival rate between the group of patients diagnosed during 1965-1969 and the cohort diagnosed ten years later (1975-1979) has been observed (276). Higher mortality compared with the background population, but lower compared to previous studies and other countries has been also recently reported from Norway (277). The overall mortality was higher in males than females and the excess mortality was similar for both genders (277).
Cardiovascular disease and renal disease are the most common causes of death of type 1 diabetic subjects accounting for 44% and 21% respectively (278). Analyses of mortality from the cohort of patients with type 1 diabetes have also shown that cerebrovascular mortality (stroke of nonhemorrhagic origin) is raised at all ages in these patients (279). The risk of mortality from ischaemic heart disease is exceptionally high in young adult women, with rates similar to those in men under the age of 40 with type 1 diabetes (280,281).
The excess mortality is lowest in Scandinavia, intermediate in the U.S. and highest in countries where type 1 diabetes is rare, e.g. Japan (282,283), probably due to a combination of the quality of care and access. Even in Finland, at least a half of the death is due to currently preventable causes such as acute complications, infections and suicide (284). On the other hand, 40% of the patients survive over 40 years and a half of these have no major complications.
Moreover WHO Multinational Study of Vascular Disease in Diabetes performed in 10 centers throughout the word from 1975 to 1987 showed that age-adjusted all cause mortality rates were significantly higher in type 1 diabetes compared with type 2 diabetes.
Survival and avoidance of complications have been related to better metabolic control (285), but genetic factors also appear to be involved. In the EURODIAB study the predictors for cardiovascular mortality were blood pressure, serum cholesterol, proteinuria and retinopathy, but not fasting plasma glucose (286). In line with this observation, insulin resistance-related factors, but not glycemia, predicted coronary artery disease in type 1 diabetes during 10-year follow-up in the Pittsburgh Epidemiology of Diabetes Complications study (287).
The strongest predictors for five-year mortality in patients with type 1 are amputations (hazard ratio, HR=5.08) and poor visual acuity (HR=1.74) (275).
Inconsistent associations have been reported between diabetic nephropathy and HLA-DR4 (288) and several genes involved in blood pressure regulation (285,289,290,291,292,293,294,295). Polymorphisms of paraoxonase (296,297) and A-IV (298) appear to play an important role in development of coronary artery disease in type 2 diabetes patients, but have not been extensively studied in persons with type 1 diabetes.
Microvascular complications: The high incidence, associated severe morbidity (299), mortality (300), and enormous health care expenditures(301,302,303) make T1DM a prime target for interventions.
The Diabetes Control and Complications Trial demonstrated the efficacy of tight glycemic control in reducing the risk of late complications of type 1 diabetes (304,305,306). Unfortunately, increased risk of hypoglycemia associated with tight control (307,308) and compliance problems have hampered full implementation of the trial recommendations, especially in children. Importantly, studies, including ours, have shown that factors other than hyperglycemia contribute significantly to development of microvascular complications.
Diabetic retinopathy: Diabetes is the most common cause of new cases of blindness in the U.S. adults. After 15+ years of T1DM, 80% of the patients have diabetic retinopathy and 25% have vision-threatening proliferative diabetic retinopathy (PDR) (309). Fundus photography is an effective method for detecting; however, this technique is used to detect disease associated with anatomic changes in the retina. Fluorescein angiography may detect earlier changes, such as breakdown of the blood-retinal barrier manifested by leakage of fluorescein, however, it is costly and associated with complications and is generally used to evaluate more severe diabetic retinopathy, e.g., macular edema, prior to laser photocoagulation. Novel sensitive and specific markers are needed to detect meaningful early changes predictive of incidence and progression of diabetic retinopathy and subsequent visual loss. Such markers could also potentially shorten the duration of clinical trials or allow smaller sample sizes by allowing more precise detection of regression or progression of diabetic retinopathy.
Alterations in retinal blood flow and loss of retinal pericytes (310) precede the earliest clinical stages of diabetic retinopathy. In contrast to the loss of retinal pericytes, changes in retinal arterial blood flow can be detected non-invasively using laser Doppler velocimetry (311), video fluorescein angiography (312) or pulsatile ocular blood flow (313). T1 DM patients with no retinopathy or mild NPDR show dilated major retinal arteries with reduced blood flow velocity, compared to that in non-diabetic controls (312,314). With increasing duration of diabetes and progression of retinopathy, retinal blood flow shows a bimodal pattern of a transition from reduced to increased (311,315).
Diabetic nephropathy: Historically, diabetic nephropathy would affect 30-50% of T1DM patients throughout the initial 20 years of the disease (316,317,318). While there has been an evidence for a secular decline in the incidence of nephropathy in T1DM (317,319,320), this decline may have leveled off (321) and the cumulative risk remains at least 20%-30%.

The incidence peaks 10-15 years after diagnosis and then declines, suggesting that only a subset of the patients is susceptible to diabetic nephropathy. Increased urinary albumin excretion or microalbuminuria (MA) has been shown to be an early manifestation of diabetic nephropathy, rather than merely a prognostic factor (322,323,324). Since MA strongly predicts progression to later stages of diabetic nephropathy (overt proteinuria and end-stage renal disease) (325,326), current standards of care include screening for MA and treatment of those positive for MA with ACE inhibitors (327,328,329,330). However, prediction of the progression to overt nephropathy based on the presence of MA and the other risk factors is imprecise. In addition, some patients in whom MA has not increased may nonetheless develop advanced renal lesions (331). Previously, increased GFR (332,333), increased ambulatory blood pressure (334,335), and autonomic neuropathy (335,336) have shown some promise but have not gained wide acceptance.
Recent reports from patients with type 1 and type 2 diabetes suggest that podocyte loss is associated with progression of diabetic glomerulosclerosis (337,338,339,340,341).
Diabetic neuropathy: The incidence of neuropathy in EURODIAB study group was independently associated with duration of diabetes, glycosylated hemoglobin values, BMI and smoking (342).
Risk factors for microvascular complications in type 1 diabetes: Traditional epidemiological distinction between micro- and macrovascular complictations of diabetes is becoming more fluid with the recognition that similar processes (e.g., glycation, hypertension, inflammation, and endothelial dysfunction) affect both vascular beds. Accurate markers of early retinal microvascular alterations may offer a powerful predictive tool concerning the risk of cumulative vascular burden and future micro- and macrovascular clinical events elsewhere in the body.
Hyperglycemia: Chronic hyperglycemia is probably the most accepted risk factor for development of microvascular complications in type 1 diabetes mellitus (304,343,344,345,346).
Blood pressure: Numerous previous studies have demonstrated that increased systolic or diastolic blood pressure is a powerful predicator of microvascular complications (347,348,349,350).
Cigarette smoking is an established risk factor for diabetic nephropathy (348,351,352). Interestingly, smoking does not appear to be an independent risk factor for diabetic retinopathy (353).
Lipids and lipoproteins: Elevated TG and low HDL-cholesterol (349,354,355), and increased total and LDL-cholesterol (348,356,357) have been reported to predict diabetic retinopathy and diabetic nephropathy.
Insulin resistance is an emergent risk factors for both macro- and microvascular complications in type 1 DM (354,355). However, it is unclear if this phenomenon is mediated by associated risk factors (350) or a common genetic determinant (358,359).
Visceral obesity, PAI-1 and markers of inflammation and endothelial dysfunction tend to cluster with microalbuminuria and insulin resistance in diabetic and nondiabetic persons and are clearly associated with macrovascular complications. The relationship of these markers to microvascular diabetic complications has been less studied (360,361). Visceral obesity (354,355), vWF and fibrynogen (362,349), C-reactive protein, IL-6, TNF-a (361) as well as PAI-1 levels (363) have been found to predict diabetic nephropathy and/or diabetic retinopathy. Most of the previous studies were cross-sectional and none has measured visceral obesity directly or analyzed all of these factors jointly. This prospective study will be in a unique position to shed new light on this cluster of novel microvascular risk factors.
Age at onset of diabetes, C-peptide: Greater severity of initial presentation, related largely to the HLA-DR/DQ genotype and age at diagnosis, predicts the faster loss of endogenous insulin production and poorer glycemic control. Lower endogenous insulin production (marked by undetectable C-peptide levels and associated with higher HbA1c) has been suggested to predict progression of diabetic retinopathy (364) and diabetic nephropathy (365). These observations are consistent with the association of microvascular complications with HLA-DR3/4 DR3/4 genotype (366,367) and faster loss of GAD autoantibodies (368).
Importantly, it has been recently found by DAISY study that early diagnosis of type 1 diabetes, in the group of children followed from prediabetic stage to diabetes onset, is associated with better preservation of insulin secretion, which resulted in lower mean insulin dose 12 months after diagnosis (369). The significance of residual insulin secretion with regard to metabolic control and to long-term complications was confirmed by data from DCCT study (370). Patients with preserved C-peptide and lower insulin requirements, fasting blood glocose and HbA1c in the intensive therapy group had 50% reduced risk for progression of retinopathy, development of microalbuminuria and 65% lower risk of severe hypoglycemia (370).
Genetics of microvascular complications: Despite the seemingly inevitable development of complications, at least half of the patients survive over 40 years and a quarter of these have no major complications (371). Retinopathy develops in virtually all patients, given enough time, but the more severe proliferative retinopathy and visual impairment appear in only up to half of the patients (371). Survival and avoidance of complications is related to better metabolic control, but patients developing microvascular complications, especially nephropathy, frequently have no identifiable classical risk factors, suggesting that the genetic component of renal disease is different from that of diabetes.
Diabetic nephropathy clearly clusters in certain families, apparently independent of glycemic control and both in type 1 (372,373,374) and type 2 diabetes (375,376,377,378,379,380). Familial clustering of diabetic nephropathy in T1DM families has been confirmed in a study incorporating the results of kidney biopsies in addition to more common measures of renal function such as urinary AER (381). The clustering is more pronounced in African Americans than Caucasians (376,378,382).
In segregation analysis of the adenine/creatinine ratio as a continuous trait in type 1 diabetes, adjusting for age, sex, and duration of diabetes, adenine/creatinine inheritance was most consistent with a Mendelian model with multifactorial inheritance (383), i.e.determined by a mixture of genes, variable effects, and environment factors such as diabetes and hypertension. Adenine/creatinine ratio heritability (h2) was estimated to be 0.27.
Inconsistent associations have been reported between diabetic nephropathy and HLA-DR4 (384) and several genes involved in blood pressure regulation (371,385,386,387,388,389). ApoE genotype may mediate well-known association between dyslipidemia and development of diabetic nephropathy (390,391,392). Matrix metalloproteinase-9 polymorphism may play a role (393). Additional genes (IL1RN, NHE5, NOS1, KLKB1, RAGE,) may be minor contributors to genetic risk. Extensive analysis of the chromosome 3 region suggested strong evidence of a nephropathy gene, but a detailed evaluation of the AGTR1 gene suggested that alleles of AGTR1 were not the source of the linkage.
The genetic components of retinopathy and other microvascular complications have been investigated to a lesser degree than nephropathy. Diabetic retinopathy clusters in families (307). Diabetic retinopathy has been associated with the presence of genetic variants of the aldose reductase promoter region, eNOS4 polymorphism of the endothelial nitric oxide synthase and DNA sequence variants of VEGF and vitamin D receptor genes (394,395,396,397,398,399,400, 401). Data concerning the role of HLA genotypes are inconsistent  (366,367,402,403,404,405) . The associations between diabetic nephropathy, diabetic retinopathy, and markers of insulin resistance may be mediated by a polymorphism in the PC-1 gene coding region (358,359).
Macrovascular complications
Coronary artery disease (CAD): CAD is the main cause of death in persons with type 1 diabetes and accounts for a large proportion of premature morbidity and mortality in the general population. Heart disease in type 1 diabetic patients occurs earlier in life, affects women as often as men, and associated mortality is dramatically higher than that in the general population (406,407,408,409). Women with type 1 diabetes are 9 to 29 times more likely to die of CAD than nondiabetic women; the risk for men is increased 4 to 9-fold. Patients with proteinuria are at a 15-37 times increased risk of fatal CAD while the risk of those without proteinuria is 3-4-fold, compared to the general population (406,410). However, it is far from clear whether the association of CAD with nephropathy is mediated by hypertension and dyslipidemia - features of renal failure - or rather by risk factors that predispose to both nephropathy and CAD. While conventional CAD risk factors (hypertension, smoking, low HDL cholesterol and high triglycerides) increase the risk, the role of hyperglycemia, autonomic neuropathy, endothelial dysfunction, insulin resistance and diabetes duration is less established (411,412,413,414). The Pittsburgh Epidemiology of Diabetes Complications Study has recently suggested that, among endothelial dysfunction markers, E-selectin concentration is an independent predictor of CAD (415,416). In type 1 diabetic patients, atherosclerosis is more diffuse (417,418), leading to higher case fatality (419,420), higher cardiac failure (421) and restenosis rates (422), and shorter survival (422,423), compared to the general population. These poor outcomes emphasize the need for primary prevention of CAD in type 1 diabetic patients. Silent ischemia is common - 24% of asymptomatic patients older than 35 yrs had ischemia on exercise test, Holter monitoring or dynamic thallium scintigraphy and 10% had coronary stenosis greater than 50% by angiography (424).
In asymptomatic young subjects with type 1 diabetes the incidence of CAD is increasing 1-2% per year and by their mid 40s 70% of men and 50% of women develop CAC (Coronary Artery Calcification) - a marker of atherosclerotic plaque (425,426).
Small clinical studies using B-mode imaging of carotid arteries have suggested that type 1 diabetic patients have significant atherosclerosis as early as at the age of 10-19 yrs and strongly associated with diabetes duration (427,428). This observation was recently confirmed by EDIC study (429). After six years of follow-up the progression of the intima-media thickness of the common carotid artery was significantly greater in patients with type 1 diabetes in comparison to the controls and was associated with age, mean HbA1C, LDL/HDL ratio, smoking, systolic blood pressure and urinary albumin excretion rate (429).
However, it is still little known aboutthe risk factors for progression of asymptomatic coronary artery disease to clinical endpoints. To address this issue, 6 years ago, the study Coronary Artery Calcification (CAC) in Type 1 Diabetes (CACTI), was initiated (430,431,432,433,434,435,436,437). The measurement of CAC by electron beam tomography is one of the new techniques (besides the ultrasound imaging of carotid arteries and potentially coronary magnetic resonance imaging) for noninvasive monitoring of cardiovascular disease progression (438,439). CAC has been shown to correlate well with the invasive method of coronary lumen estimation (angiography) and arterial wall intimal architecture (IVUS) (435,438). The initial data from the CACTI study suggest that gender-related differences in insulin sensitivity may explain the increase in CAC in women compared with men with type 1 diabetes (431). Moreover a common promoter polymorphism in the hepatic lipase gene (LIPC-480C>T) was found to be associated with the extent of coronary calcification in a dose dependent manner (431). Among type 1 diabetic patients greater progression of CAC was observed in subjects with HbA1C values >7.5%, higher levels of soluble IL-2 receptor and low plasma adiponectin (416,430,433) One third of the patients enrolled into the CACTI had hypertension, but only half of those were well treated and controlled (434). The other half was either treated, but uncontrolled or not treated at all. Even worse, nearly half of the patients had dyslipidemia, according to the ATP III criteria and very few of these were successfully treated (432). This CACTI data suggest that appropriate blood pressure and lipid disturbances treatment issues should be addressed before one move to interventions targeting novel risk factors, such as adiponectin or markers of inflammation (432,434).
Summary
Type 1 diabetes can be diagnosed at any age, but clinical course, genetic, and environmental determinants appear to be heterogenous by age. The common pathway begins with pre-clinical ß-cell autoimmunity with progressive defect of insulin secretion, followed by onset of hyperglycemia, transient usually partial remission, and finally complete insulinopenia associated with acute and chronic complications and premature death. Current research effort is focused on identification of the genetic and environmental determinants of this process and the ways they interact.

 * all types of diabetes
Table 9.1. Prevalence of childhood type 1 diabetes, from selected studies.

America

Europe

Australia, Oceania, Asia, Middle East

N.S.=not significant increase or no increase NA= no available data, I-increase

Table 9.2. Current incidence of type 1 diabetes (data published 2000-2005).

Reference List - links to PubMed available in Reference List.

Chapter 9 Powerpoint slide set - slides updated 12/09, new slides #1

Additional Slideset: Chapter9_Sardinia.ppt Type 1 Diabetes and Related ADs (M. Songini) - Added 1/06

Instructions for obtaining article and slideset for: Early Infant Feeding and Risk of Developing Type 1 Diabetes-Associated Autoantibodies. Ziegler AG, Schmid S, Huber D, Hummel M, Bonifacio E. JAMA 2003 Oct 1;290(13):1721-8

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