Polymorphism in the 5'-flanking region of the insulin gene and its potential relation to cardiovascular disease risk: observations in a biracial community. The Bogalusa Heart Study

The insulin gene variable number tandem repeat and risk of type 2 diabetes in a population-based sample of families and unrelated men and women

Abnormalities in insulin regulation are central to the pathogenesis of type 2 diabetes. We assessed variation in the insulin gene variable number tandem repeat (INS VNTR) minisatellite (using the -23Hph1 A/T single nucleotide polymorphism) as a risk factor for 92 cases of incident type 2 diabetes in 883 unrelated Framingham Heart Study (FHS) subjects and in a separate sample of 698 members of 282 FHS nuclear families with 62 diabetes cases. In the unrelated sample, the -23Hph1 TT genotype frequency was 8.0% and was associated with a diabetes hazard ratio of 1.89 [95% confidence interval (CI), 1.01-3.52; P = 0.045] compared with the AA genotype using diabetes age of onset as the time failure variable in a proportional hazards model adjusted for age, offspring sex, body mass index, parental diabetes, and sex by parental diabetes interactions. In sex-stratified analyses, TT increased risk for diabetes in women (hazard ratio, 4.25; 95% CI, 1.76-10.3), but not men (hazard ratio, 1.01; 95% CI, 0.39-2.60). Using a family-based association test to assess transmission disequilibrium in the sample of related subjects, the age- and sex-adjusted z-score for diabetes associated with the T allele was 2.07 (P = 0.04), and a family-based association test using age of onset in a proportional hazards model was also statistically significant (P = 0.03), indicating that increased risk of diabetes was not attributable to population admixture. These data support the hypothesis that the INS VNTR is a genetic risk factor for type 2 diabetes, with the TT genotype accounting for about 6.6% of cases in the FHS population.

Human type 1 diabetes and the insulin gene: principles of mapping polygenes

We review the strategy used to identify a susceptibility locus (IDDM2) for type 1 (insulin dependent) diabetes mellitus. As type 1 diabetes is becoming the paradigm for dissecting multifactorial disease genetics, the approach described provides important general guidelines for positional cloning of human disease polygenes. Main topics include: (a) historical conspectus of the mapping and identification of IDDM2--a critical survey of the work leading up to the conclusion that IDDM2 most likely corresponds to allelic variation at the insulin gene minisatellite (VNTR) locus; (b) the nature of allelic (length and sequence) variation at the VNTR locus; (c) gene interactions and disease pathogenesis; (d) mechanism of action of the INS VNTR in type 1 diabetes--insulin gene expression, parent-of-origin effects (genomic imprinting); and (e) summary and future prospects--alleles of the insulin VNTR that are protective for type 1 diabetes appear to encode susceptibility to type 2 diabetes.

In Finland insulin gene region encoded susceptibility to IDDM exerts maximum effect when there is low HLA-DR associated risk. DiMe (Childhood Diabetes in Finland) Study Group

An association between insulin-dependent diabetes mellitus (IDDM) and polymorphisms of the insulin gene on chromosome 11p15 (INS) is a consistent finding in Europid populations. While one study suggested that the INS association is restricted to HLA-DR4-positive individuals, studies in other Europid populations have shown the disease-associated INS genotype to confer susceptibility independently of HLA-DR. We have investigated the role of INS in susceptibility to IDDM in Finland, which has the highest incidence of diabetes mellitus in the world, at two polymorphic restriction sites, 5' and 3' to the insulin gene. From the DiMe (Childhood Diabetes in Finland) Study we studied 154 diabetic children without regard to HLA-DR type; 108 DR4 positive/non-DR3 diabetic children; 39 DR3 positive/non-DR4 diabetic children; 30 DR4/DR3 positive diabetic children; 31 non-DR4/non-DR3 diabetic children; 96 matched DiMe control subjects and 86 other healthy, non-diabetic Finnish control subjects. We found an overall association between IDDM and INS in the high-risk Finnish population only with the 5' polymorphism and identified an INS haplotype negatively associated with IDDM in Finland. However, among diabetic subjects with a reduced HLA-associated susceptibility (non-DR4/non-DR3) both 3' and 5' INS loci showed an association with IDDM (p values 0.02 and 0.0002, respectively). Thus, in the Finnish population insulin gene-encoded susceptibility to IDDM exerts a maximum effect in those with reduced HLA-associated risk.

The genetics of non-insulin-dependent diabetes mellitus in south India: an overview

Non-insulin-dependent diabetes mellitus affects approximately 10% of urban Indian and Indian migrant populations and as such carries major health implications for these groups. Whilst a strong genetic component to the aetiology of non-insulin-dependent diabetes mellitus is incontestable, progress in identifying the specific genetic determinants involved in its pathogenesis has been slow. In studies of South Indian pedigrees, preliminary segregation analysis indicates that non-insulin-dependent diabetes mellitus is likely to be a polygenic disease. A number of candidate genes have been studied with the aim of demonstrating either association or linkage with the disease; in South Indians the only positive results thus far have been associations between non-insulin-dependent diabetes mellitus and the genes for insulin, apolipoprotein D and complement component C4B. However, it seems likely that these genes contribute only a small proportion of the genetic susceptibility to non-insulin-dependent diabetes mellitus in this ethnic group and that the major genes underlying glucose intolerance remain to be determined.

Central obesity and hyperinsulinaemia in women are associated with polymorphism in the 5' flanking region of the human insulin gene

Obesity is a multifactorial disease with a marked genetic component. The situation is further complicated by the heterogeneity of obesity demonstrated by the topographical distribution of body fat, e.g. upper body (central) and lower body (gluteal) obesity. Furthermore, the distribution of fat shows a stronger heritable tendency compared with total body fat. Central obesity is characterized by hyperinsulinaemia and insulin resistance, a feature in common with non-insulin dependent diabetes mellitus, hypertension and atherosclerosis. In order to study the molecular genetics of central obesity we have examined 56 severely obese (mean body mass index 40), unrelated British Caucasoid young non-diabetic women for associations of restriction fragment length polymorphism of candidate genes with anthropometric measurements and indices of insulin secretion and resistance. The candidate genes examined were insulin receptor, insulin sensitive glucose transporter and insulin. An association of the class 3 allele of the hypervariable region in the 5' flanking region of the insulin gene was found with upper segment obesity (P = 0.005). Furthermore, the class 3 allele was also associated with fasting hyperinsulinaemia (P = 0.01), stimulated insulin secretion (P = 0.01) and insulin resistance as calculated from the homeostatic model of assessment (HOMA; P = 0.008). No such associations were found with the other candidate genes studied. This data suggests that polymorphisms in the 5' flanking region of the insulin gene may affect expression of the gene and thereby modulate insulin production in severely obese female subjects.

Use of DNA polymorphisms for genetic analysis of non-insulin dependent diabetes mellitus

Polymorphisms occur on the average of one out of every 500 base pairs of DNA, and these polymorphisms provide useful markers for genetic analysis. Hundreds of RFLP markers have been mapped at regular intervals throughout the human genome. Diabetes genes have not been mapped with these markers, however, only one MODY family has been partially evaluated. This type of analysis is further complicated if NIDDM is multigenic and/or polygenic. RFLPs have been used to evaluate specific candidate loci for NIDDM, e.g. the insulin, insulin receptor and glucose transporter genes. For these analyses, population and family studies (limited in number) have suggested that none of these loci are major contributors to the genetic susceptibility to NIDDM. In no case, however, could a contribution of 10% or less of these loci be confidently excluded, because of variable penetrance, different degrees of linkage disequilibrium between RFLPs and putative mutations, the frequencies of the RFLPs in non-diabetic populations, and inadequate sample size. The conclusions are clear: either (1) the correct candidate gene(s) has not been found, or (2) sample sizes need to be increased by at least an order of magnitude, or (3) newer methods of analysis must be adopted (e.g. use of extended haplotypes and associations with subphenotypes, or screening with allele specific oligonucleotide probes, denaturing gradient gel electrophoresis or direct genomic sequencing of polymerase chain reaction amplified DNA).

Genetics of non-insulin dependent diabetes mellitus

It is probable that NIDDM has a multifactorial origin in which environmental factors hasten the progression of the disease in genetically predisposed individuals. The importance of the genetic contribution to NIDDM has been established by the study of certain inbred populations, the almost 100% concordance of disease in monozygotic twins and by familial clustering. However, progress in identifying specific genetic factors involved in NIDDM has been slow and no consistent evidence has emerged supporting a major aetiological role for any of the genes so far studied. This may be due in part to methodological problems encountered in the identification of such disease susceptibility genes.