Molecular genetics of type 2 diabetes
Abstract: Type 2 diabetes is a common and chronic disease caused by interactions between genetic and environmental factors. The Goto-Kakizaki (GK) rat is a well-established genetic model of type 2 diabetes. Since several aspects of the pathophysiology of diabetes are shared between human and GK rats, we used this model to perform the first genome-wide scan for quantitative trait locus (QTL) of type 2 diabetes. A genetic linkage map with 530 microsatellite markers was constructed in a (GK x F344)F2 intercross. The map covers the whole rat genome evenly, and has a total length of 1827 centimorgans (cM), an average interval of 3 cM between markers and the maximum interval less than 20 cM. Subsequently, this map was used as a framework for the integration of 6 linkage maps and 2 radiation hybrid maps of the rat. The unified map comprises the majority of available rat markers (a total of 8627 markers) in a single map, allowing us to assemble a comparative genetic map for type 2 diabetes between human and rat. LOD scores > 2.9 for diabetes were identified in the 23 rat orthologous fragments, previously reported to be linked to diabetes in human. A total of 51 human candidate orthologous genes for type 2 diabetes were also suggested from the glucose QTLs on rat chromosomes 1, 5, and 10. Seven gender-influenced QTLs significantly affecting diabetes were assigned to chromosomes 1, 2, 5, 7, 10, and 18 with LOD scores > 4.0. These findings indicate that hyperglycemia, body weight, and insulin resistance can be genetically controlled in a sex-specific manner. A genome-wide scan for gene-gene interactions between loci indicated eleven epistatic interactions (P < 0.0001) responsible for diabetic susceptibility in the F2 rats. We longitudinally examined fasting blood glucose and insulin values, glucose tolerance, insulin secretion, body weight, metabolic responses to high-fat-diet, and glucose response to insulin injections in 230 (GK x F344)F72 intercross progeny with a 3 1 -week follow-up. We found that the incidence of diabetes in the F2 progeny significantly increased from day 31 to day 50. Four QTLs were mapped for the early development of diabetes on chromosomes 1, 2, 10, and 14 with LOD scores of 12.4, 5.1, 4.9, and 5.8. Additionally, we found that the incidence of diabetes in the F2 progeny increased after a treatment with a high fat diet (HFD). Four loci responsible for the increased incidence of diabetes after the HFD were located on chromosomes 1, 2, 11, and 14 with LOD scores of 4.8, 5.1, 4.5, and 4.7. A locus on chromosome 10 was also found to contribute to lowering scrum cholesterol levels with LOD score of 5.4. The GK alleles at this locus also resulted in decreased body weight and better glucose response to insulin injections, as well as a decreased rate of disease progression. A locus on chromosome 5 with LOD score of 4.5 was responsible for increased fasting glucose in response to pentobarbital injections as well. These findings suggest that the susceptibility genes contributing to the early-onset diabetes may also accelerate the disease progression during HFD. To our knowledge, this is the first genome- wide scan for the QTLs associated with early-onset and HFD-induced diabetes. The mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase (mtGPD) plays an important role in the regulation of insulin secretion. In the present study, the rat mtGPD gene (Gpd2) was cloned and mapped to the region of rat chromosome 3 that contains a region linked to diabetes in the GK rat. The insulin receptor gene is expressed as two alternative splice forms with or without exon 11. The encoded protein has two isoforms with discrete functional differences. In HepG2 cells, we demonstrated that dexamethasone treatment dose-dependently increased the expression of the splice form with exon 11, while low concentrations of glucose decreased the expression of this splice form. In conclusion, these results strongly indicate that the genome-wide linkage analysis combined with the follow-up study would outline the most significant loci influencing the onset and progression of diabetes. The present study suggests that the genetics of diabetes/metabolic syndrome in the GK-rat is a moving target that changes according to biological factors like age, gender, parent-of-origin, gene-gene interactions, and environmental factors like diet. Moreover, the effects of drugs in this model appear to be genetically regulated.
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