Etiologies of Diabetes Mellitus

Symposium on Diabetes Mellitus

Etiologies of Diabetes Mellitus loan Albin, M.D., * and Harold Rijkin, M.D. t

Recent evidence from work on the etiology and pathogenesis of diabetes mellitus has increased our appreciation of the heterogeneity of the disease. The only common denominator used to define diabetes mellitus is an abnormal blood glucose, and there has been both national and international disagreement as to what constitutes abnormal glucose tolerance. This lack of consistency in the definition of diabetes and the increasing awareness of the genetic and clinical heterogeneity of diabetes led to the formation of the National Diabetes Data Group in 1978. The goal of this group was to analyze the existing body of knowledge and to propose a new scheme for the classification and diagnosis of diabetes mellitus. 58 The purpose of the classification is to provide a uniform framework for clinical and epidemiologic research so that more meaningful and comparative data can be obtained. The subclassification of idiopathic diabetes mellitus separates the insulindependent patients (Type 1) from those who are insulin independent (Type 2) regardless of age. The third subclassification of idiopathic diabetes mellitus is termed "other types" and refers to the association of hyperglycemia with a wide variety of genetic syndromes, pancreatic diseases, hormonal abnormalities, drug or chemical exposures, and insulin receptor abnormalities. Type 1 or insulin-dependent diabetes (IDDM) is characterized clinically by abrupt onset of symptoms, insulinopenia, proneness to ketosis, and dependence on injected insulin to sustain life. Prior to the abrupt onset of symptoms, there may be a variable period of time of decreased beta cell function. Since this may be difficult to ascertain on clinical criteria alone, the patient may be considered to have Type 2 diabetes until it becomes apparent that insulin is required to sustain life. In Type 2 or non-insulindependent diabetes (NIDDM), the onset of disease is usually insidious and there may be low, normal, or supranormallevels of insulin associated with insulin resistance. Patients with NIDDM are not prone to ketosis and not

*Assistant

Professor of Medicine, Albert Einstein College of Medicine; Attending Physician, Montefiore Medical Center, Bronx, New York tClinical Professor of Medicine, Alhert Einstein College of Medicine; Principal Consultant, Diabetes Research and Training Center; Attending Physician, Montefiore Medical Center, Bronx; Attending Physician, Lenox Hill Hospital, New York, New York Supported in part by NIH Grant No. AM 20541.

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dependent on insulin treatment to sustain life. However, they may require insulin for correction of hyperglycemia if this cannot be achieved with diet or oral agents. The purpose of this article is to review current concepts of the pathogenesis of both IDDM and NIDDM.

INHERITANCE OF TYPE 1 DIABETES Although it has long been observed that "diabetes runs in families," the precise risk of developing diabetes has been difficult to establish because of such variables as diet, obesity, ethnic background, age of onset, and the variety of diagnostic criteria. Family studies have not been consistent with any single mode of inheritance. This has led to the concept of genetic heterogeneity, that is, diabetes mellitus is a symptom complex which is the result of many different genetic mechanisms. 1 This concept is supported by twin studies, family studies, and analysis of the inheritance patterns in the category "other types." For example, the hyperglycemia associated with cystic fibrosis is probably transmitted by an autosomal recessive mode of inheritance, whereas the hyperglycemia associated with myotonic dystrophy is transmitted by an autosomal dominant mode. Since MacDonald found an equal incidence of adult-onset diabetes among ancestors of juvenile diabetics and nondiabetics, this suggests different modes of inheritance for Type 1 and Type 2 diabetes. 51 In long-term studies of monozygotic twins, Tattersall and Pyke found a 93 per cent concordance rate for adult-onset disease (greater than age 40 at diagnosis) and a 50 per cent concordance rate for juvenile-onset disease. 80 Therefore, genetic factors appear to be predominant in NIDDM, but presumably environmental factors are needed to trigger the onset of IDDM. Another approach to the study of the inheritance of IDDM is human leukocyte antigen (HLA) typing. These cell surface antigens are associated with the rejection of tissue transplants. The genes coding for HLA antigens A, B, C, D, and DR (D-related) occupy 4 loci along the short arm of chromosome number 6. The HLA system is polymorphic at each locus, and there is linkage disequilibrium between the various loci. For example, the antigens HLA-B8 and HLA-Dw3 are positively associated in Caucasians and when one of these antigens occurs with increased frequency, there is a secondary increase of the other antigen. The prevalence of the HLA antigens varies considerably in different populations. Each individual inherits one set of antigens or haplotype from one parent and the other set of alleles or haplotype from the other parent. Both alleles are expressed as cell surface proteins and can be identified on leukocytes by serologic techniques. A few years after tissue typing was in clinical use, it became apparent that certain HLA antigens were found with unusually high frequency in patients with specific diseases. The strength of this association was termed "relative risk" and refers to the increased risk of disease in the group with a particular marker antigen as compared with the general population. Initial population studies found a higher proportion of HLA antigens B8, B15, and Cw3 in insulin-dependent diabetics than in controIs. 8, 59 In fact, these and subsequent studies showed no change in the

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frequency of various HLA types associated with NIDDM. In contrast to the A, B, and C loci, which are determined by specific antisera that induce lymphocytotoxic effects if the antigen is present, the HLA-D locus antigens are determined by reactions in mixed lymphocyte culture. The DR- or Drelated antigens are serologically determined on B lymphocytes. The reagents may have cross-reactivity between various antigens, and work is in progress to confirm and identifY other specificities. The letter "w" denotes provisional recognition by the International Histocompatibility Workshop, so that the methodology is uniform worldwide. Therefore, as work proceeded on HLA typing, even stronger associations with a 3- to 6fold relative risk of ID D M were found with Dw3 and Dw4. 60 In 3 population studies on 199 Caucasian patients with IDDM, the relative risk associated with DRw3 was 5.69, and in 76 Caucasian patients, the relative risk associated with DRw4 was 2.S4. 6 Furthermore, when more than one high risk allele is present in the same individual, the likelihood of developing IDDM increases markedly. The available data suggest that the significant increases and decreases in the HLA B locus alleles are probably secondary to corresponding HLA D or DR associations. In contrast, other antigens such as HLA All, Aw32, B5, B7, Dw2, and DRw2 were found less commonly in populations with IDDM than in control populations, and it has been postulated that these alleles may exert a "protective" effect for the development of Type 1 diabetes. 6, 9, 48 The majority of population studies have been done on Caucasians in North America and Europe. In Europe, HLA BI5 is found more frequently in northern European whites while HLA BIS is found more frequently in the southern European population. Relatively few investigations have been conducted thus far on non-white populations. In Japan, where HLA BS occurs rarely and HLA BI5 is common in the general population, B22, DRw3 and DRw4 antigens are found in a statistically significant proportion of patients with IDDM.77, 84 In the black American population, where the frequency of HLA BS is low, IDDM is strongly associated with DRw3 and DRw4 antigens. 71 Similarly, Mexican-American patients with IDDM have a ~ignificant increase in HLA DR4 compared with a control population. 87 The Eskimo population has a very low frequency of HLA BS, and the prevalence of IDDM is extremely low. 6,76 To date, S3 population studies have thus far demonstrated an increased relative risk for the development of IDDM associated with HLA BS, BI5, BIS, Cw3, Dw3, Dw4, DR3, and DR4 types in Caucasian populations. The presence of Dw3, DRw3, and DRw4 in the Japanese, black American and Mexican-American populations appears to have the highest correlation with IDDM. Further studies on non-white populations are needed to better define the genetic profile of the group at risk to develop IDDM. Family studies have the advantage of excluding artifacts of population structure and have provided further support for the association between specific HLA types and IDDM. Rubinstein et al. studied HLA types in 31 families having one or more children with juvenile diabetes. 75 The clinical criteria for the children were onset of disease at 16 years of age or younger, normal body weight, presence of urine ketones, blood glucose greater than 200 mg per dl, and an absolute need for insulin therapy. In 23 families, 50

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per cent of the HLA-identical siblings were diabetic, whereas only 6 per cent of the nonidentical siblings were diabetic. Further analysis of the HLA types suggested that the strength of this association was best correlated with the D locus. Studies on Danish Caucasian families by Nerup et al. included 1967 controls and 146 diabetics. 6o The prevalence of diabetes in siblings of juvenile diabetics was highest (20.6 per cent) in siblings with both HLA B8 and Bw15. Dw3 and/or Dw4 were found in 80 per cent of patients compared with 20 per cent of the control population, but no family correlation studies were reported. The available data on family studies show that siblings of insulin-dependent diabetics have a 5 to 10 per cent risk of developing diabetes compared with the approximate 0.15 per cent prevalence in the general pOPLllation. 74 The studies on identical twins suggest that the risk in siblings would not be greater than 50 per cent. 30, 80 Recent evidence from studies on 64 unrelated families demonstrated and confirmed that the probable empiric risk for developing IDDM is approximately 30 per cent for HLA-identical siblings. 27 Siblings who share neither haplotype or who are HLA haplo identical are at considerably less risk. However, different subsets of IDDM may have different inheritance patterns. In susceptible individuals, environmental factors, such as viral infections, may trigger the onset of disease. In these individuals the risk of IDDM would be in part dependent on the prevalence of environmental factors.

MECHANISM OF ACTION OF HLA-LINKED GENES What are the biological implications of the association of diabetes with the major histocompatibility complex? The principal explanations for an association of a disease of unknown cause with a genetic marker such as HLA type are ethnic stratification, causation, or association. The family and population studies in Caucasians and non-Caucasians negate the simple association of IDDM with a particular ethnic group in which certain HLA types are more common. Causation implies that the HLA type is in a dire~t chain of events leading to the disease, and as yet there is no evidence to support this possibility. Association implies that specific HLA alleles are linked to one or more alleles responsible for the predisposition to develop IDDM. Analogy to work done on immune responses in a mouse animal model provides a conceptual framework to support this possibility. Specific genes in the mouse that control immune response (Ir) are linked or in the same region of the chromosome as the genes for the major histocompatibility antigens of the mouse. By analogy, it is conceivable that genetically controlled differences in the immune response influence the development of diabetes. High-risk alleles associated with the HLA complex might code for a deficient immune response to agents which preferentially attack beta cells, thereby allowing for damage to beta cells. Conversely, the protective alleles might enhance the host's immune response to such agents. Alternatively, the high risk of HLA alleles associated with IDDM may be related to the development of autoimmune destruction of the beta cell either prior to or following a noxious insult. The evidence for the possible etiolc -Sic roles for infection and autoimmune phenomena has increased over the past several years.

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THE ROLE OF VIRAL INFECTION OR TOXIC CHEMICALS IN TYPE 1 DIABETES There are several lines of evidence to support the role of viral infections in the etiology of IDDM. Indirect evidence for the association of viral infections with the acute onset ofIDDM includes data on seasonal variation and the temporal association of childhood viral infection with onset of disease. These types of studies are retrospective, depend on data from parental recall or clinical records, and are based on the assumption that there is a short interval between infection and onset of disease. Despite these obvious shortcomings, the increased prevalence of new cases of IDDM in the winter, late summer, and early autumn does correspond with the increased prevalence of common viral infections in the community and suggests a possible cause and effect relationship. The estimated time of onset of IDDM in 2511 children living in 4 different American states followed a similar seasonal pattern. 20, 53 The same analysis of the onset of diabetes in 851 Australian children showed peaks of onset in fall and winter months, albeit in different calendar months. Similar studies in Great Britain show the same seasonal pattern. 23 Since the onset of disease occurs throughout all months of the year, this suggests that the association of seasonal viral infections with IDDM is only one of many contributing genetic or environmental factors. Specific infections reported to be associated with the onset ofIDDM include rubella, mumps, Coxsackie virus, infectious hepatitis, infectious mononucleosis, and cytomegalovirus. 7 The development of diabetes days or months after mumps virus infection has been reported in case studies since 1899. 33, 35, 41, 55 Sultz et al. found an increased prevalence of new cases of diabetes in Erie County, New York several years after the community-wide outbreak of mumps.79 Although pancreatitis is common in mumps virus infection, there are no reported pathologic studies that demonstrate specific insular lesions associated with mumps virus infection. Menser reported on the clinical features of 17 patients with congenital rubella and diabetes mellitus and reviewed 24 other reported cases. 54 Nine of these patients were part of a long-term prospective study of congenital rubella in 45 affected persons. Thus, the incidence of diabetes in this group of patients at age 36 was 20 per cent (9 of 45). The onset of diabetes in this subgroup was in the third decade, whereas the detection of disease in the other reported patients ranged from age 1 to 28 years. This great variability in age of detection is again consistent with multiple theories of pathogenesis. Either the diabetes is caused by a direct destructive viral infection of the beta cells, or the virus induces an immunologic reaction to islets, which then leads to gradual destruction of beta cells. Chronic persistent rubella virus infection of the pancreatic tissue of infants with congenitally acquired disease has been demonstrated, but islet cell lesions have not been described in pathologic studies. 12, 57, 78 Epidemiologic studies of viral antibodies have reported an association between infection with Coxsackie virus (Group B Type IV) and IDDM.24 The most direct evidence for the role of viral infections in the pathogenesis of IDDM was reported by,Yoon et al. 86 They recovered Coxsackie virus (Group B Type IV) from the pancreatic tissue of a child

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who died with meningoencephalitis and new-onset diabetes. Pathologic studies of the pancreas demonstrated insulinitis. The isolated virus was then injected into susceptible mice and resulted in beta cell destruction and diabetes. This striking case study provides conclusive evidence for the viral etiology of diabetes in this particular patient and further raises the level of suspicion for the role of multiple viral infections in the pathogenesis of IDDM. Data from studies on animal models infected with encephalomyocarditis virus, Venezuelan encephalitis virus, rubella virus, cytomegalic virus, Western equine encephalitis virus, and Coxsackie virus provide another indirect line of evidence. 7 The experimental work in these animal models provides further support for the importance of genetic and metabolic factors in the development of insular lesions and hyperglycemia in the infected animals. It is also possible that viruses may be only one of the many causes of Type 1 diabetes and that other environmental insults such as toxic chemicals may damage beta cells and cause hyperglycemia. In experimental animals, drugs such as alloxan and streptozotocin selectively destroy the beta cells of islets and induce insulin deficiency. Further work on these agents by Like and Rossini showed that whereas a single large dose of streptozotocin is .directly toxic to the beta cells, multiple small doses of the drug appear to act indirectly by altering the beta cells so that they become more vulnerable to attack by the immune system of the animal. 45 Since only certain strains of mice are susceptible to this type of experimental destruction of islets, it appears that genetic factors are important for mediation of the chemical insult and/or the immunologic response. 72 In the last decade, the rodent poison Vacor, which has a molecular structure similar to the nitrosamine, streptozotocin, was introduced into the United States. After accidental injection of this rodenticide, an insulin-deficient, ketosis-prone form of diabetes mellitus associated with severe toxic neuropathy has been reported. 56• 66 In two of the fatal cases examined at autopsy, there was eosinophilic degeneration, degranulation and vacuolization of islet cell cytoplasm, and coagulation necrosis of islets. The exocrine pancreas appeared normal, and no lymphocytic infiltration was noted. Therefore, in the susceptible host, there may be multiple environmental toxins or viruses involved in the destruction of beta cells.

DISORDERED IMMUNE MECHANISMS IN THE ETIOLOGY OF TYPE 1 DIABETES The role of immunologic factors in initiating or perpetuating destruction of islet cells is supported by the pathologic studies of pancreata of children with diabetic ketoacidosis. Mononuclear cell infiltrates in and around 'he islets of Langerhans have been demonstrated in these pancreata. 25 In 1940, von Meyenburg coined the term "insulitis" to describe lymphocytic infiltration of the islets. 83 There is now unanimous agreement that this is a specific lesion found in patients with Type 1 diabetes who die early in the course of the disease. 26 In patients with Type 2 diabetes, insulitis has never

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been observed. Insulitis is more common in very young diabetic patients but has also been documented in two subjects, age 62 and 72 years. The actual frequency of lymphocytic infiltration of the islets is controversial because of problems of tissue sampling, variation in the extent oflymphocytic infiltrates, and duration of disease. In pancreata of patients who died after a short duration of disease, the islets were variable in size. The large islets are composed mainly of beta cells that show signs of tremendous functional hyperactivity and a small proportion of well-granulated glucagon-producing A cells and somatostatin-producing D cells. After a long duration of disease, the A, B, and D endocrine cells identified with immunocytochemical methods may be found scattered within the exocrine pancreas. The residual islets are small and composed of thin cords of small cells in a fibrous stroma. Immunocytochemical techniques have demonstrated that these previously regarded atrophic islets contain glucagon-producing A cells and somatostatinproducing D cells. 63 Another type of islet containing mainly pancreatic polypeptide-producing pp cells is found near ductal epithelium. Although the cause of the destruction of beta cells in IDDM has not been elucidated, the morphologic evidence of insulitis suggests that an autoimmune reaction to beta cells, either primary or triggered by viral infection in a genetically susceptible host, may be operative. Another line of evidence to support the role of autoimmunity has been the clinical association between diabetes and autoimmune endocrine disorders such as Hashimoto's thyroiditis, Graves' disease, Addison's disease, autoimmune oophoritis and orchitis, idiopathic hypoparathyroidism, and hypophysitis. Other associations between diabetes and nonendocrine autoimmune disease include pernicious anemia, vitiligo, rheumatoid arthritis, idiopathic thrombocytopenic purpura, myasthenia gravis, Sjogren's syndrome, and chronic active hepatitis. 82 Ogle first reported on the coexistence of diabetes and adrenal insufficiency in 1866. Subsequent studies have shown that the prevalence of diabetes in patients with Addison's disease approximates a 6-fold excess over the estimated prevalence of diabetes in the general population of the United States or Creat Britain, and the excess prevalence of Addison's disease in diabetic patients appears to be in the order of a 5-fold increase over the general population. Similarly, multiple reports have suggested an association between diabetes and autoimmune thyroid disorders.49 In these disorders, the high prevalence of organ-specific antibodies to the thyroid and adrenal glands has been well documented, and in 1974 two independent studies confirmed the existence of islet cell antibodies in the sera of Type 1 diabetic patients with other autoimmune glandular disease. 5 , 50 These cytoplasmic islet cell antibodies (leA) were detected by an indirect immunofluorescence assay on human pancreatic sections from donors of blood group O. Studies on large series of patients with this methodology demonstrated that this antibody was not detected in healthy control subjects and rarely found in the patients with NIDDM.ll, 36 In patients treated with insulin, the prevalence of leA was 38 per cent with associated overt organ-specific autoimmune disease and 22 per cent without organ-specific autoimmune disease. At the time of diagnosis of IDDM, 65 to 85 per cent of cases were positive for leA, but after 3 years of disease only about 20 per cent of patients had detectable antibody.36 These initial

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studies provided support for the possible pathogenetic role of autoimmune mechanisms in diabetes mellitus, but the key question remains: are the islet cell antibodies merely an epiphenomenon as the result of islet cell damage by viruses or toxins, or do they participate in producing progressive islet cell damage? In a hypothetical model involving beta cell recognition by the cellular or humoral immune system, the target antigen(s) would most probably reside in the plasma membrane. Since the initially described ICA reacted with cytoplasmic elements, other assay systems, such as islet cell surface antibodies, complement fixing islet cell antibodies, complementdependent cytotoxic islet cell antibodies, and antibody-dependent cellular cytotoxic islet cell antibodies, have been developed. 42 Islet cell surface antibodies (ICSA) have been detected by indirect immunofluorescence on cultured insulinoma cells, suspensions of viable rat islets, and human fetal pancreatic cultures in sera of Type 1 diabetic patients. 43, 52, 67 Subsequent studies have shown that in the presence of complement, ICSA-positive sera caused a block in insulin release from perfused pancreatic beta cells and lysis of cultured rat islets. 13, 38 In the latter study, 25 per cent of the serum samples from nondiabetic first-degree relatives of diabetic probands were ICSA-positive and cytotoxic for beta cells. Thus, the presence of these antibodies in nondiabetic family members and the observation that not all diabetic patients have cytotoxic ICSA suggest that the presence of these antibodies alone is not sufficient to produce diabetes. However, the presence of islet cell antibodies may be a useful predictive marker for identifYing persons who are at a greater risk for the development of diabetes. In a prospective study of the prediabetic period, HLA genotypes and ICA (by two different techniques) were determined on 582 nondiabetic first-degree relatives of 160 children with insulin-dependent diabetes. 29 After 2 years of observation, all of the 6 persons in whom diabetes developed were known to have circulating ICA. Thus, the beginning of the pathogenetic process could well be several years before the abrupt onset of the classic diabetic symptoms. The time sequence may be analogous to other autoimmune endocrine disease in which the presence of antithyroid antibodies may precede the development of hypothyroidism by years. The HLA genotypes of the family members have not as yet been reported in this prospective study. GinsbergFellner et al. studied HLA types and ICA in 247 first-degree relatives of 74 children dependent on insulin and demonstrated no significant correlation between HLA genotypes and the presence of ICA. 27 However, two of the children who had hyperglycemia were HLA-identical to their siblings with diabetes (DR3 and DR4) and had detectable ICA before the onset of diabetes. This latter study was done with the methods that detect cytoplasmic ICA. Comparative studies of the prevalence of cytoplasmic ICA and ICSA in insulin-dependent diabetics do not always yield concordant results. 21 Therefore, future studies are needed to define the character of the islet cell antigen(s), to determine quantitatively the titer of antibody, and to standardize the assay procedures. Then, the value of islet cell antibodies as a marker for continuing destruction of beta cells can be assessed. Currently, the exact role of ICA in the pathogenesis of diabetes remains unclear. Another line of investigation being actively pursued is the possibility

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that the critical role in beta cell damage is played not by circulating antibodies but by lymphocytes and macrophages. Again, more sophisticated experimental work in this area is needed before the importance of immunopathologic mechanisms in the disease can be thoroughly assessed. A recently discovered model of spontaneous diabetes in the BB/W rat shares the following characteristics with Type 1 diabetes: genetic predisposition, absence of obesity, life sustaining requirement for insulin therapy, lymphocytic insulitis with destruction of pancreatic beta cells, and autoantibodies to thyroid colloid, smooth muscle, and islet cell surface antibodies. 15. 47 In this animal model, considerable experimental evidence exists for the cell-mediated autoimmune pathogenesis of diabetes. Future work may better define this process and the applicability to human Type 1 diabetes.

CLINICAL ASPECTS Is there a relationship between HLA type and progression of microvascular disease? Knowles has shown a progressive increase of clinically significant microangiopathy in IDDM, which reaches 80 per cent by 25 to 30 years' duration of disease. 39 Two other reports on the clinical course of patients dependent on insulin after 20 to 40 years of disease show that clinically significant microvascular disease or neuropathy was absent in approximately 20 per cent of patients. 61 . 65 In retrospective chart reviews, there was no discernible clinical difference between those patients with or without significant complications. Then, the question has remained, "What determines the good prognosis in this select group of patients?" Pyke and Tattersall noted that retinopathy was frequently more severe in concordant twins than it was in discordant twins. 68 Data of this type suggest that a genetic component may be associated with the development of complications. Since HLA type appears to confer susceptibility to diabetes, different HLA-linked alleles may confer susceptibility to microvascular complications such as retinopathy. However, inv€stigations of HLA type in patients with diabetic retinopathy have produced conflicting results. These discrepancies may have been caused by selection bias in the control and study groups, variations in duration of disease, age, ethnic background, degree of hyperglycemia, and the type of genetic marker. For example, in two studies of patients with diabetic retinopathy, there was no evidence for an increased frequency of a particular HLA genotype. 3, 4 Other studies were reviewed by Dornan and co-workers who addressed the issue of the effect of glycemic control on susceptibility to retinopathy. 14 They analyzed the HLA types and control of blood glucose in 127 insulindependent diabetics and found that either or both the presence of HLADR4 and poor control significantly increased the risk of retinopathy. Further studies are needed to elucidate the role of HLA type or other genetic factors besides hyperglycemia in the development of microvascular complications. The three interacting causes in IDDM appear to be genetic susceptibility, environmental factors, and autoimmune processes. Specific therapy to decrease the incidence and severity of ID D M depends on further delineation of these variables. The HLA genotype may affect the infective

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potential of various viruses or the efficacy of repair processes after injury induced by viruses or toxins. Thus, vaccination against viruses such as Coxsackie B4, rubella, mumps, and infectious mononucleosis may reduce the incidence of IDDM. The cost-effectiveness of immunization to prevent IDDM has been explored. I7 The authors concluded that vaccinating all children at age 3 would be preferable to HLA-screening and vaccinating only persons genetically predisposed to diabetes. Even if only 15 per cent of diabetes were virally induced, and if a vaccine were developed that was only 13 per cent effective, vaccinating everyone would still be cost-effective. This type of analysis was based on numerous assumptions that must be validated, but it suggests the feasibility of this type of intervention in the future. If the hypothesis that Type 1 diabetes is an autoimmune disease is correct, then immunotherapy before the destruction of beta cells might also be effective prevention. In animal models of Type 1 diabetes, the experimental success of immunotherapy with antilymphocytic serum in preventing the destruction of beta cells is impressive. 46, 73 The available data in human studies demonstrate an association between islet cell antibodies and some insulindependent persons but do not as yet support a cause-and-effect relationship. In one recently reported study, 17 children with newly diagnosed diabetes and dependent on insulin were treated with prednisone for the first 12 months after diagnosis to preserve function of beta cells. 16 Comparative studies of the urinary C peptide excretion, an index of the residual function of beta cells, were performed in the treated and 23 control subjects over a 24-month period, Both the children in the control group and the children receiving corticosteroids with initially low urinary C peptide showed no improvement in the second year of disease. However, as a group, the children with diabetes receiving corticosteroids excreted significantly more C peptide than the control group at 18 and 24 months. Islet cell antibodies were not detected in the majority of children and if positive showed nQ change with treatment. In another similar pilot study, the effect of prednisone therapy on T cell subsets and islet cell antibodies was examined, but the duration of observation was too short to assess the effect on residual beta cell function. 37 If one considers the undesirable side effects of corticosteroid therapy, the obscure mechanism of action of prednisone, and the variable natural history of endogenous insulin secretion in these children, this form of therapy requires further study. Nevertheless, there are profound clinical implications of the recent work on the cause of IDDM.

PHYSIOLOGIC HETEROGENEITY IN TYPE 2 DIABETES In contrast to Type 1 diabetes, NIDDM or Type 2 diabetes is characterized by the absence of inflammatory cells in the islets, no circulating islet cell antibodies, no particular HLA association, and no seasonal trend

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or suggestion of association with viral illnesses. Clinically, the age of onset of Type 2 diabetes is usually over 40 years. The appearance of symptoms is insidious, metabolic ketoacidosis is rare, and obesity is common. In Type 1 diabetes, the circulating insulin levels are characteristically diminished, whereas in Type 2 diabetes the insulin levels may be decreased, normal, or increased. These variable patterns of insulin secretion strongly suggest heterogeneous causes. The sites of abnormality may be in either the beta cell secretory response to glucose or the target tissue response to insulin, or both. In Type 2 subjects with normal or increased secretion of basal insulin, glucose-stimulated release of insulin is diminished. However, in these subjects, the beta cell responsiveness to other insulin secretagogues such as Isuprel, arginine, glucagon, and tolbutamine is normal. 31,64 These studies suggest a specific abnormality involving glucose recognition, or metabolism by the beta cell, or both. A new experimental model for NIDDM has been developed in which neonatal rats are injected with streptozotocin at 2 days of age. After transient hyperglycemia and return to near normoglycemia, nonketotic diabetes develops in rats at age 6 weeks. 85 Isolated perfused pancreata from these rats demonstrated approximately 25 per cent reduction of beta cells and a selective defect in glucose-stimulated release of insulin with preservation of response to other agents. Further studies in this model may shed light on the development of abnormal function of beta cells in human Type 2 diabetes. Studies of pancreatic pathology in Type 2 diabetes demonstrate extremely variable and nonspecific findings. 26 The general appearance of the islets are nondiagnostic, and more sophisticated quantitative studies on islet composition with immunocytochemical methods are needed to better assess the role for disturbed intra-islet intercellular relationships in Type 2 diabetes. The number and size of these islets may be decreased. Islet fibrosis and deposits of hyaline substance, now considered to be a form of amyloid, may also be present. These same findings occur in many nondiabetic elderly patients. In contrast to pathologic findings early in the course of Type 1 diabetes, there is no evidence of islet hyperplasia, and this suggests a decreased responsiveness to hyperglycemia. Possible functional abnormalities of the beta cell could include limitations of islet cell replication, abnormalities of insulin biosynthesis and storage, deficient glucoreceptors in the beta cell membrane, defective adenylate cyclase system, impairment of calcium flux into the beta cells, and derangement of the microtubular-microfilamentous system necessary for secretion. In fact, patients have recently been described in whom the biosynthesis of a structurally modified insulin with greatly impaired biologic activity accounted for hyperglycemia. 28 • 32 The patients had increased levels of immunoreactive insulin without hypoglycemia and responded appropriately to exogenous administration of insulin. Familial hyperproinsulinemia has been described in 18 family members of a child who was evaluated for a seizure disorder. 22 This genetic defect in producing or processing proinsulin had no apparent relationship to hypo glycemia or the development of diabetes

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in the index case or the affected progeny. Other abnormalities of glucosemediated release of insulin need to be defined to understand better the mechanism of beta cell deficiency. The high incidence of obesity in Type 2 diabetes suggests the pathogenetic importance of insulin resistance. Obesity is associated with resistance of muscle, liver, and adipose tissue to the action of insulin. Numerous studies have documented a reduction of insulin receptors on these cells as well as on circulating monocytes. This reduction may contribute to the insulin resistance of obesity. 2,62 The reduced insulin binding to circulating monocytes and in vivo resistance to the action of exogenously infused insulin has been observed in nonobese as well as obese patients with Type 2 diabetes. 10 If the basic defect in NIDDM is loss of normal tissue sensitivity to insulin, then to maintain normal glucose homeostasis, the pancreas secretes increased amounts of insulin. As the need to augment secretion of insulin continues, the beta cell may lose the ability to compensate, and frank hyperglycemia may develop. However, the available data suggest heterogeneity of sensitivity to insulin as well as secretion of insulin in NIDDM. Then, the degree of glucose intolerance in an individual will vary as a function of the relative severity of these two metabolic defects. The confounding variables in studies designed to understand these defects in the pathogenesis of NI DD M are the effects of insulin deficiency on resistance to insulin and the effects of obesity on secretion of insulin and sensitivity to it.69

GENETIC HETEROGENEITY OF TYPE 2 DIABETES The most impressive evidence for a genetic component in the pathogenesis of Type 2 diabetes is the observation of the high concordance rate of NID D M in monozygotic twins, even when they are geographically' separated. 80 In 31 pairs of twins, the concordance rate for diabetes was 93 per cent when the index twin became diabetic after age 40. The index twin in this group had primarily Type 2 diabetes. When diabetes was found before age 40 in the index twin (Type 1), the concordance rate for diabetes in 64 monozygotic pairs was only 50 per cent. These findings suggest that environmental modifications may not appreciably modifY the incidence of NIDDM but may affect the clinical course and onset of disease. Aside from the rare clinical situation of monozygotic twins, the pattern of genetic inheritance for the majority of people with Type 2 diabetes remains unknown. However, there is again evidence for genetic heterogeneity from the observations of Fajans and Tattersall on a group of patients with maturity onset diabetes of the young (referred to as MODY).18, 81 MODY was defined as a patient in whom diabetes was discovered before the age of 25 years and in whom fasting hyperglycemia could be normalized without insulin for more than 2 years. Studies of 26 families

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with this disorder showed that 22 parents (or 85 per cent) had diabetes, and 12 grandparents had diabetes. The three-generation inheritance in 46 per cent of the families provided evidence for autosomal dominant inheritance in MODY. Furthermore, of the 47 siblings tested, 53 per cent were diabetic. Most of the affected individuals in these families did not need insulin. Theoretically, the clinical and metabolic characteristics and natural history of MODY have the potential to increase our understanding of the mechanism of the genetic determinants of this syndrome. Fajans et al. reported the long-term clinical course of families with MODY and demonstrated differences between families in the insulin responses to administration of glucose and in the occurrence of vascular complications. 19 There was a spectrum between the nonprogressive and slowly progressive clinical course of MODY for hyperglycemia and vascular complications. For example, the 23-year follow-up in one patient with a family history of diabetes in 4 consecutive generations was described. The patient was first noted to have impaired glucose tolerance at age 10 that improved with reduction of weight and deteriorated with weight gain and with pregnancy. Eighteen years after the initial diagnosis, fasting hyperglycemia developed that again responded to reduction of weight. The patient then required oral hypoglycemic agents and at age 33 required insulin to correct fasting hyperglycemia. Analysis of insulin response to glucose showed a progressive decline over the last 8 years of the clinical course. Further efforts to define distinguishing features of this autosomally dominant inherited form of Type 2 diabetes resulted in the theory of Leslie and Pyke that chlorpropamide-induced alcohol flushing may be a marker of this disorder. 44 The authors observed this phenomenon in a mother and her two daughters with considerable hyperglycemia and no discernible vascular complications. This prompted evaluation of other groups of diabetic patients. Facial flushing induced by chlorpropamide and alcohol was reported to be common in patients with Type 2 diabetes and rare in patients with Type 1 diabetes. , In twin and family studies, facial flushing induced by chlorpropamide and alcohol appeared to be a dominantly inherited trait. Other investigators did not confirm these striking findings. 40 An International Workshop formed in 1980 because of the potentially important implications of the chlorpropamide-alcohol reaction for research into the cause of diabetes. 34 The participants concluded that flushing of the face caused by chlorpropamide and alcohol consumption is an inherited trait, but its relationship to diabetes is still unclear. However, the observation that patients with Type 2 diabetes and flushing of the face may be relatively free from vascular complications requires further investigation. Thus, to date, the heterogeneity of the clinical course and vascular complications in families with MODY appears to be similar to the typical patient with Type 2 diabetes. Genetic factors appear to be extremely important in the cause of NIDDM, and current investigations have delineated abnormalities in the response of the beta cell to glucose, variable levels of circulating insulin, and insensitivity to insulin at the cellular site of insulin action in this group of patients.

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CONCLUSIONS

Studies in the last several years have provided compelling evidence for the heterogeneity of diabetes. Since an abnormal level of blood glucose has been the only marker for the definition of diabetes, the search for genetic, immunologic, biochemical, physiologic, or morphologic markers has resulted in increased awareness of the heterogeneity within diabetes. Type 1 and Type 2 diabetes clearly represent markedly different entities with fundamental genetic, etiologic, pathologic, and clinical differences. In Type 1 diabetes, the increased frequency of specific HLA types may relate to the genetic susceptibility of the beta cell to damage by viruses or other environmental toxins. Definition of appropriate genetic markers may allow one to identify and categorize vulnerable populations. Then, if these populations are exposed to identifiable "risks," immunization or other measures may one day prevent the onset of diabetes. The demonstration of insulitis, circulating islet cell antibodies, and the clinical association between Type 1 diabetes and other autoimmune diseases suggests that altered immune functions may be involved in the pathogenesis of diabetes. In the genetically susceptible individual, autoimmune destruction of the beta cell might occur as a primary event or in response to prior injury of beta cells. If this theory is correct, it follows that in the future specific immunotherapy may prevent or decrease damage to beta cells in subjects at risk. Alternatively, unique problems may develop with therapies, such as islet cell transplantation in subjects with genetically determined primary autoimmune destruction of beta cells. In contrast to Type 1 diabetes, the studies of monozygotic twins suggest that genetic factors are dominant in the cause of Type 2 diabetes. Although no genetic marker has as yet been identified, the existence of the MODY form of NIDDM with an autosomal dominant pattern of inheritance suggests genetic heterogeneity. Features of clinical heterogeneity include obesity, age of onset, and variable patterns of insulin sensitivity and secretion. The key question in the care of patients with diabetes has been "What is the role of tight control in preventing microvascular complications?" Although impressive epidemiologic, biochemical, and animal model studies support the importance of aggressive medical management in preventing long-term complications,70 the question may be definitely answered when the heterogeneity of human diabetes is fully appreciated. Theoretically, there may be subgroups of persons with diabetes and inexorable complications, others free of complications, forms of diabetes in which control of blood glucose is vital and other forms in which control is less important. In the future, genetic or other markers may identify these possible subgroups and enhance our understanding of the pathogenesis of diabetes and vascular complications. When each of the many disorders that comprise the diabetic syndrome are delineated, specific genetic counseling, prognostications, preventive therapy, and individualized medical therapy may be possible.

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