Browsing by Subject "Diabetes Mellitus, Type 2"
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Item The bHLH/PAS Transcription Factor SIM1 is a Novel Obesity Gene(2005-05-03) Holder, Jimmy Lloyd, Jr.; Zinn, Andrew R.Obesity is epidemic in the United States and other developed countries. Obesity is a major risk factor for type II diabetes, hypertension, hyperlipidemia and osteoarthritis. I report a unique girl with early-onset obesity (47.5 kg, +9.3 s.d. above mean at age 67 months) and a de novo balanced translocation between chromosomes 1 and 6. She has normal energy expenditure and a voracious appetite. I show that her translocation disrupts a transcription factor gene, SIM1, on chromosome 6q16.2. I also present data that Sim1 haploinsufficiency causes obesity in mice. Animals heterozygous for a Sim1 null allele fed a standard chow diet (4% fat) developed obesity around the time of sexual maturity, were 33-45% heavier than wild-type littermates by 5 months of age, and had increased adiposity by DEXA scans. In contrast, the human subject developed obesity by two years of age, well before puberty. To investigate whether differences in dietary fat consumption might explain this discrepancy in human and mouse phenotypes, I fed mutant mice and wild type littermates a "Westernized" diet (35% fat). Heterozygous Sim1 mice fed this diet became obese prior to 6 weeks of age. The obesity was also more severe, especially in females, who by 8 weeks of age weighed 72% more than controls compared with 13% on a low fat diet. Heterozygous Sim1 mice maintained on a 4% fat diet ate more than controls over a 5 day period (delta kcals 12-14%), and became even more hyperphagic when acutely challenged with increased dietary fat (delta kcals 46-68% over 5 days). This altered behavior was evident within the first day of exposure to the high fat diet: during this time, heterozygous Sim1 mice failed to significantly change the mass of food consumed, whereas wild-type littermates decreased their food consumption by >15%. These data suggest that Sim1 is critical for the acute and chronic homeostatic response to elevated dietary fat. This data demonstrates that normal Sim1 gene dosage is critical for proper regulation of feeding behavior and body weight regulation.Item Role of the Vitamin D Receptor in Insulin Secretion and Beta Cell Function(2012-07-20) Kjalarsdottir, Lilja; Repa, Joyce1,25-dihydroxyvitamin D3 (VitD) is a ligand for the Vitamin D Receptor (VDR, NR1I1), which is a member of the family of Nuclear Hormone Receptors (NHR). Previously, the Repa lab identified VDR as the fourth most abundant NHR in mouse islets based on mRNA levels, also, VDR is clearly present in human islets [1]. In the past years multiple epidemiological studies have implicated Vitamin D deficiency in the development of Type 2 Diabetes, however no reports have described any mechanism(s) linking VitD status with pancreatic islet function. Therefore, my studies have focused on the role of Vitamin D and VDR in islet biology. Preincubation of isolated mouse and human islets with Vitamin D results in enhanced glucose-stimulated insulin secretion (GSIS). This response is VDR-dependent, as no VitD-mediated change in GSIS is observed in islets obtained from Vdr-null mice. However, VitD causes no changes in gene expression of any of the major islet hormones, nor does it change glucose uptake into primary beta cells. VitD does however increase glucose-stimulated calcium uptake, suggesting that VitD affects transcription of genes involved in calcium flux into the beta cell. To identify molecular mechanisms linking VDR activity to increased insulin secretion and increased glucose-stimulated calcium uptake, we performed global gene expression profiling by microarray in mouse and human islets. These studies identified multiple genes associated with islet function, calcium transport and insulin secretion. One of these genes is the R-type voltage-gated calcium channel, CaV2.3, which is highly upregulated by VitD in human and mouse islets. We identified a strong VDR binding element within intron 7 of the Cav2.3 gene that is conserved in mouse and man. With previous reports linking Cav2.3 activity with Type 2 Diabetes, our findings support a role for vitamin D signaling in the regulation of CaV2.3 and calcium uptake to enhance glucose-stimulated insulin secretion by beta cells of the endocrine pancreas. A second VDR target gene we identified in the islet is klotho, a key regulator of phosphate homeostasis. We clearly establish that klotho mRNA and protein are detected in beta cells of mouse islets, at levels sufficient to mediate signal transduction pathways via klotho’s role as a co-receptor for FGF23. By analysis of islets from Klotho-/- mice, we also show that the sialidase activity of klotho may modulate the membrane localization of GLUT-2 to affect glucose-stimulated insulin secretion. In summary, my studies suggest that vitamin D status may impact the beta cell’s capacity to sense glucose levels and respond appropriately to secrete the anabolic hormone, insulin. Future studies involving beta cell-selective deletion of VDR, klotho, and Cav2.3 are now warranted, to elucidate the contribution of islet vitamin D signaling pathways in glucose homeostasis in vivo. The results of studies for my dissertation research provide a needed mechanistic approach, which complements the current clinical and observational reports that exist, regarding potential roles for Vitamin D in the progression of Diabetes. In addition, our identification of numerous Vitamin-D regulated genes of the human and mouse islet can form the basis for future hypothesis-driven research efforts to identify novel therapeutic targets to affect insulin secretion and beta cell function. [Keywords: vitamin D, vitamin D receptor, VDR, islets, diabetes, insulin secretion, microarray, voltage-gated calcium channel, Klotho, bile acids]Item Using Balanced Translocation to Discover Novel Disease Causing Genes: Potential Involvement of Basonuclin 2 in Congenital Heart Disease and Palmitoylated Membrane Protein 7 in Maturity-Onset Diabetes of the Young(2010-05-14) Bhoj, Elizabeth Joyce; Zinn, Andrew R.Patients with balanced chromosomal translocations and disease have been used to identify novel disease-causing genes by examining the genes affected by the chromosomal breakpoints. We have identified a man with unexplained maturity-onset diabetes of the young (MODY) and a balanced translocation (7q22;10p12), and a man carrying a balanced translocation associated with partial anomalous pulmonary venous return (PAPVR) and hypospadias. / In our first proband, the translocation breakpoints were mapped to high resolution using FISH and somatic cell hybrids and the junctions amplified by PCR and sequenced. No annotated genes were disrupted by the translocation. Three of the four genes bordering the translocation breakpoints showed biallelic transcription in lymphoblastoid cells; the other gene did not have any informative SNPs. The 10p12 breakpoint was 220 kb 5’ to the Membrane Protein, Palmitoylated (MPP7) gene, which encodes a protein required for proper cell polarity. As this biological function is shared by HNF4A, a known MODY gene, the translocation may cause islet cell dysfunction by altering MPP7 expression in a tissue- or a cell-specific fashion. / In our second proband, tiling array comparative genomic hybridization (aCGH) for chromosomes 9 and 13 was performed on DNA from cultured cells from the productions of conception of his offspring with karyotype (46,XX,der(9)t(9;13)(p22;q22). The results mapped both breakpoints to kilobase resolution. The chromosome 13 breakpoint lies in an intergenic region. The nearest gene, Lim-domain Only 7 (LMO7), is ~400 kb away. The chromosome 9 breakpoint lies within an intron of basonuclin 2 (BNC2), a gene of unknown function with a complex pattern of alternative transcripts. Expression of LMO7 and BNC2 is enriched in fetal heart and penis. Transcripts from both alleles of LMO7 and BNC2 were present in immortalized lymphocytes with the balanced translocation. One BNC2 missense mutation of unclear significance was identified among 10 patients with sporadic total or PAPVR.