The genetics of non-insulin-dependent diabetes mellitus: tools and aims

Bases Genéticas do Diabetes Mellitus Tipo 2

A patogênese do diabetes mellitus tipo 2 (DM2) é complexa, associando fatores genéticos e fatores ambientais. A hiperglicemia é secundária à combinação de defeitos tanto na sensibilidade à insulina quanto na disfunção das células beta-pancreáticas. Vários estudos estabeleceram claramente a importância dos fatores genéticos na predisposição ao DM2. No momento, conhecemos alguns genes implicados em formas monogênicas de diabetes (MODY, diabetes mitocondrial). No entanto, nas formas mais comuns da doença de caráter poligênico, conhecemos apenas poucos genes que são associados à doença de uma forma reprodutível nos diferentes grupos populacionais estudados. Cada um destes poligenes apresenta um papel isolado muito pequeno, atuando na modulação de fenótipos associados ao diabetes. Nestas formas tardias poligênicas de DM2 é evidente a importância dos fatores ambientais que modulam a expressão clínica da doença. Nesta revisão abordamos os avanços mais relevantes das bases genéticas do DM2.

The Q allele variant (GLN121) of membrane glycoprotein PC-1 interacts with the insulin receptor and inhibits insulin signaling more effectively than the common K allele variant (LYS121)

When overexpressed, the membrane glycoprotein PC-1 may play a role in human insulin resistance through the inhibition of insulin receptor (IR) autophosphorylation. A PC-1 variant (K121Q, with lysine 121 replaced by glutamine) is also associated with whole-body insulin resistance when not overexpressed. To better understand the effects of the Q allele on IR function and downstream signaling, we transfected cultured cells with cDNAs for either the Q or the K alleles. In human MCF-7 cells, the Q allele was severalfold more effective (P < 0.05-0.01) than the K allele in reducing insulin stimulation of IR autophosphorylation, insulin receptor substrate-1 phosphorylation, phosphatidylinositol 3-kinase activity, glycogen synthesis, and cell proliferation. Similar data on IR autophosphorylation inhibition were also obtained in mouse R-/hIR and human HEK 293 cell lines. In transfected MCF-7 cells, 125I-labeled insulin binding and IR content were unchanged, and PC-1 overexpression did not influence IGF-1 stimulation of IGF-1 receptor autophosphorylation. Both the Q and K alleles directly interacted with the IR, as documented by coimmunoprecipitation assays. This interaction was greater for the Q allele than for the K allele (P < 0.01), suggesting that direct PC-1-IR interactions are important for the PC-1 inhibitory effect on insulin signaling. In conclusion, the Q allele has stronger inhibitory activity on IR function and insulin action than the more common K allele, and this is likely a consequence of the intrinsic characteristics of the molecule, which more strongly interacts with the IR.

Bivariate genetic analysis of fasting insulin and glucose levels

The main aim of this study was to estimate the relative influence of genes and environment on fasting insulin levels, which were considered a proxy of insulin resistance. Possible sex differences in genetic and environmental influences, and the origin of the covariance between fasting insulin and glucose were investigated. Subjects were 209 pairs of middle-aged twins, divided into 5 sex-by-zygosity groups. A general bivariate model and a reciprocal causation model including fasting insulin and glucose were used in the analyses. For both quantitative genetic models, a model specifying additive genetic and unique environmental factors, which were the same in males and females, showed the best fit to the data. Heritability estimates were modest and highly similar in both models: 20-25% of the variance in fasting insulin, and around 50% of the variance in fasting glucose levels could be attributed to genetic factors. The two models could not be discriminated on the basis of their fit to the data. A submodel of the general bivariate model suggested that the covariance between glucose and insulin has a unique environmental basis, whereas for the reciprocal causation model both causal paths were needed to explain the phenotypic correlation between insulin and glucose and estimates of the reciprocal paths were of opposite sign, an indication for the expected negative feedback loop.

No evidence for linkage at candidate type 2 diabetes susceptibility loci on chromosomes 12 and 20 in United Kingdom Caucasians

Several studies have identified evidence for linkage between type 2 diabetes and the regions on chromosomes 12 and 20 containing the maturity-onset diabetes of the young (MODY) genes, hepatocyte nuclear factor-1alpha (HNF-1alpha) and HNF-4alpha. Two studies examining the HNF-1alpha region have demonstrated evidence for linkage at genome-wide levels of significance, whereas four studies examining the HNF-4alpha locus have resulted in evidence for linkage at more suggestive levels of significance. The demonstration of linkage to these regions in additional patient series will strengthen the evidence that susceptibility alleles exist at these loci. We therefore assessed the evidence for linkage to these regions using a large cohort of United Kingdom Caucasian type 2 diabetes-affected sibling pairs. A maximum total of 315 affected full sibling pairs were typed for microsatellite markers across the MODY regions and, in a subset of families, for markers spanning the whole of chromosome 20. Evidence for linkage was assessed using a multipoint, mode of inheritance-free method. Linkage analysis did not reveal any significant evidence for excess allele sharing at any of the regions studied. Loci contributing sibling recurrence risks, relative to the general population risk, of 1.75 and 1.25 could be excluded for the HNF-1alpha and HNF-4alpha regions, respectively. We have not confirmed in United Kingdom Caucasians the evidence for linkage previously reported on 12q and 20q. Our results highlight further the problems of replicating previous positive linkage results across different ethnic groups.

Linkage of type 2 diabetes mellitus and of age at onset to a genetic location on chromosome 10q in Mexican Americans

Since little is known about chromosomal locations harboring type 2 diabetes-susceptibility genes, we conducted a genomewide scan for such genes in a Mexican American population. We used data from 27 low-income extended Mexican American pedigrees consisting of 440 individuals for whom genotypic data are available for 379 markers. We used a variance-components technique to conduct multipoint linkage analyses for two phenotypes: type 2 diabetes (a discrete trait) and age at onset of diabetes (a truncated quantitative trait). For the multipoint analyses, a subset of 295 markers was selected on the basis of optimal spacing and informativeness. We found significant evidence that a susceptibility locus near the marker D10S587 on chromosome 10q influences age at onset of diabetes (LOD score 3.75) and is also linked with type 2 diabetes itself (LOD score 2.88). This susceptibility locus explains 63.8%+/-9.9% (P=. 000016) of the total phenotypic variation in age at onset of diabetes and 65.7%+/-10.9% (P=.000135) of the total variation in liability to type 2 diabetes. Weaker evidence was found for linkage of diabetes and of age at onset to regions on chromosomes 3p, 4q, and 9p. In conclusion, our strongest evidence for linkage to both age at onset of diabetes and type 2 diabetes itself in the Mexican American population was for a region on chromosome 10q.

The global epidemiology of non-insulin-dependent diabetes mellitus and the metabolic syndrome

Non-insulin-dependent diabetes mellitus (NIDDM) constitutes about 85% of all cases of diabetes in developed countries and it has now reached epidemic proportions in many developing nations, as well as disadvantaged groups in developed countries, e.g., Mexican- and African-Americans and Australian Aborigines and Torres Strait Islanders. The diagnosis of NIDDM is usually made after the age of 50 years in Europids, but it is seen at much younger age in these high prevalence populations, which also include Pacific Islanders, Native Americans, and migrant Asian Indians and Chinese. There is enormous variation in NIDDM prevalence between populations, and exceptionally high rates have been documented in populations who have changed from a traditional to a modern lifestyle, e.g., American Pima Indians, Micronesians, and other Pacific Islanders, Australian Aborigines, migrant Asian Indians, and Mexican-Americans. Over the next decade, following the initial phase of the NIDDM epidemic, macro- and microvascular complications will emerge as a major threat to future public health throughout the world with huge economic and social costs. The major cause of death in NIDDM is macrovascular disease (coronary artery, peripheral vascular, and cerebrovascular), which accounts for at least two-thirds of NIDDM mortality. A key strategy in reducing macrovascular disease lies in the better understanding of the Deadly Quartet or Metabolic Syndrome. New data suggest that hyperleptinemia rather than hyperinsulinemia may play an important and central role in the genesis of the cardiovascular disease risk factor cluster that constitutes the Metabolic Syndrome.

NIDDM is associated with loss of pancreatic beta-cell L-type Ca2+ channel activity

Development of non-insulin-dependent diabetes mellitus (NIDDM) is associated with defects in glucose-stimulated insulin secretion. We have investigated Zucker diabetic fatty rats (ZDF), an animal model of NIDDM, and found that, compared with control islets, the expression of mRNA encoding C- and D-isoforms of alpha 1-subunits of beta-cell L-type voltage-dependent Ca2+ channels (VDCC) was significantly reduced in islets isolated from ZDF rats. This correlated with a substantial reduction of L-type Ca2+ currents (ICa) in ZDF beta-cells. Intracellular Ca2+ concentration responses in ZDF islets after glucose, KCI, or BAY K 8644 stimulation were markedly attenuated, whereas responses evoked by carbachol were unimpaired, consistent with a specific decrease in ICa in the diabetic islets. This reduction was accompanied by loss of pulsatile insulin secretion from ZDF islets treated with oscillatory increases of external glucose concentration. Our findings suggest that the attenuation of ICa in diabetic islets may contribute to the abnormal glucose-dependent insulin secretory responses associated with NIDDM and indicate that this defect is caused by decreased expression of genes encoding beta-cell VDCC alpha 1-subunits.

Human diabetes and obesity: tracking down the genes

Diabetes mellitus and obesity are heterogeneous disorders that affect up to 5-8% of the population of Western Europe. The importance of genetic factors in these disorders is well established; however, the genes involved and the defects leading to the manifestation of disease are largely unknown. Family studies are a powerful tool with which to localize chromosomal regions linked to a genetic disorder. A genome-wide search for the genes associated with susceptibility to diabetes mellitus and obesity, combined with a more-specific, candidate-gene approach, should enable the identification of the loci involved in these diseases. Once regions linked to disease are identified, positional-cloning techniques can be used to track down the gene(s) responsible.