Charlotte Granhall

Epigenetic regulation of PPARGC1A in human type 2 diabetic islets and effect on insulin secretion.

Aims/hypothesisInsulin secretion in pancreatic islets is dependent upon mitochondrial function and production of ATP. The transcriptional coactivator peroxisome proliferator activated receptor gamma coactivator-1 alpha (protein PGC-1α; gene PPARGC1A) is a master regulator of mitochondrial genes and its expression is decreased and related to impaired oxidative phosphorylation in muscle from patients with type 2 diabetes. Whether it plays a similar role in human pancreatic islets is not known. We therefore investigated if PPARGC1A expression is altered in islets from patients with type 2 diabetes and whether this expression is influenced by genetic (PPARGC1A Gly482Ser polymorphism) and epigenetic (DNA methylation) factors. We also tested if experimental downregulation of PPARGC1A expression in human islets influenced insulin secretion.MethodsThe PPARGC1A Gly482Ser polymorphism was genotyped in human pancreatic islets from 48 non-diabetic and 12 type 2 diabetic multi-organ donors and related to PPARGC1A mRNA expression. DNA methylation of the PPARGC1A promoter was analysed in pancreatic islets from ten type 2 diabetic and nine control donors. Isolated human islets were transfected with PPARGC1A silencing RNA (siRNA).ResultsPPARGC1A mRNA expression was reduced by 90% (p < 0.005) and correlated with the reduction in insulin secretion in islets from patients with type 2 diabetes. After downregulation of PPARGC1A expression in human islets by siRNA, insulin secretion was reduced by 41% (p ≤ 0. 01). We were able to ascribe reduced PPARGC1A expression in islets to both genetic and epigenetic factors, i.e. a common PPARGC1A Gly482Ser polymorphism was associated with reduced PPARGC1A mRNA expression (p < 0.00005) and reduced insulin secretion (p < 0.05). In support of an epigenetic influence, the PPARGC1A gene promoter showed a twofold increase in DNA methylation in diabetic islets compared with non-diabetic islets (p < 0.04).Conclusions/interpretationWe have shown for the first time that PPARGC1A might be important in human islet insulin secretion and that expression of PPARGC1A in human islets can be regulated by both genetic and epigenetic factors.

Overexpression of Alpha2A-Adrenergic Receptors Contributes to Type 2 Diabetes

Ratting Out a Diabetes Gene Inbred animals with inherited susceptibility to disease can be especially informative regarding pathogenetic mechanisms because they carry naturally occurring genetic variants of the same type that cause disease in humans. This principle is illustrated by Rosengren et al. (p. 217; published online 19 November), whose analysis of an inbred strain of rats prone to develop type 2 diabetes led to the discovery of a gene whose aberrant overexpression suppresses pancreatic insulin secretion in both rats and humans. The culprit gene, ADRA2A, encodes the alpha2A adrenergic receptor and is potentially a valuable lead for diabetes therapy because it can be targeted pharmacologically. Sequence variations in an adrenergic receptor gene cause reduced insulin secretion and contribute to type 2 diabetes. Several common genetic variations have been associated with type 2 diabetes, but the exact disease mechanisms are still poorly elucidated. Using congenic strains from the diabetic Goto-Kakizaki rat, we identified a 1.4-megabase genomic locus that was linked to impaired insulin granule docking at the plasma membrane and reduced β cell exocytosis. In this locus, Adra2a, encoding the alpha2A-adrenergic receptor [alpha(2A)AR], was significantly overexpressed. Alpha(2A)AR mediates adrenergic suppression of insulin secretion. Pharmacological receptor antagonism, silencing of receptor expression, or blockade of downstream effectors rescued insulin secretion in congenic islets. Furthermore, we identified a single-nucleotide polymorphism in the human ADRA2A gene for which risk allele carriers exhibited overexpression of alpha(2A)AR, reduced insulin secretion, and increased type 2 diabetes risk. Human pancreatic islets from risk allele carriers exhibited reduced granule docking and secreted less insulin in response to glucose; both effects were counteracted by pharmacological alpha(2A)AR antagonists.

High-Resolution Quantitative Trait Locus Analysis Reveals Multiple Diabetes Susceptibility Loci Mapped to Intervals <800 kb in the Species-Conserved Niddm1i of the GK Rat

Niddm1i, a 16-Mb locus within the major diabetes QTL in the diabetic GK rat, causes impaired glucose tolerance in the congenic NIDDM1I strain. Niddm1i is homologous to both human and mouse regions linked with type 2 diabetes susceptibility. We employed multiple QTL analyses of congenic F2 progeny selected for one recombination event within Niddm1i combined with characterization of subcongenic strains. Fine mapping located one hyperglycemia locus within 700 kb (Niddm1i4, P = 5 × 10−6). Two adjacent loci were also detected, and the GK allele at Niddm1i2 (500 kb) showed a glucose-raising effect, whereas it had a glucose-lowering effect at Niddm1i3 (400 kb). Most proximally, Niddm1i1 (800 kb) affecting body weight was identified. Experimental data from subcongenics supported the four loci. Sorcs1, one of the two known diabetes susceptibility genes in the region, resides within Niddm1i3, while Tcf7l2 maps outside all four loci. Multiple-marker QTL analysis incorporating the effect of cosegregating QTL as cofactors together with genetically selected progeny can remarkably enhance resolution of QTL. The data demonstrate that the species-conserved Niddm1i is a composite of at least four QTL affecting type 2 diabetes susceptibility and that two adjacent QTL (Niddm1i2GK and Niddm1i3GK) act in opposite directions.

Separately inherited defects in insulin exocytosis and beta-cell glucose metabolism contribute to type 2 diabetes.

The effects of genetic variation on molecular functions predisposing to type 2 diabetes are still largely unknown. Here, in a specifically designed diabetes model, we couple separate gene loci to mechanisms of β-cell pathology. Niddm1i is a major glucose-controlling 16-Mb region in the diabetic GK rat that causes defective insulin secretion and corresponds to loci in humans and mice associated with type 2 diabetes. Generation of a series of congenic rat strains harboring different parts of GK-derived Niddm1i enabled fine mapping of this locus. Congenic strains carrying the GK genotype distally in Niddm1i displayed reduced insulin secretion in response to both glucose and high potassium, as well as decreased single-cell exocytosis. By contrast, a strain carrying the GK genotype proximally in Niddm1i exhibited both intact insulin release in response to high potassium and intact single-cell exocytosis, but insulin secretion was suppressed when stimulated by glucose. Islets from this strain also failed to respond to glucose by increasing the cellular ATP-to-ADP ratio. Changes in β-cell mass did not contribute to the secretory defects. We conclude that the failure of insulin secretion in type 2 diabetes includes distinct functional defects in glucose metabolism and insulin exocytosis of the β-cell and that their genetic fundaments are encoded by different loci within Niddm1i.

High-resolution QTL Analysis Reveals Multiple Diabetes Susceptibility Loci Mapped to Intervals less than 800-kb in the Species Conserved Niddm1i of the GK Rat.

Niddm1i, a 16-Mb locus within the major diabetes QTL in the diabetic GK rat, causes impaired glucose tolerance in the congenic NIDDM1I strain. Niddm1i is homologous to both human and mouse regions linked with type 2 diabetes susceptibility. We employed multiple QTL analyses of congenic F2 progeny selected for one recombination event within Niddm1i combined with characterization of subcongenic strains. Fine mapping located one hyperglycemia locus within 700 kb (Niddm1i4, P = 5 × 10−6). Two adjacent loci were also detected, and the GK allele at Niddm1i2 (500 kb) showed a glucose-raising effect, whereas it had a glucose-lowering effect at Niddm1i3 (400 kb). Most proximally, Niddm1i1 (800 kb) affecting body weight was identified. Experimental data from subcongenics supported the four loci. Sorcs1, one of the two known diabetes susceptibility genes in the region, resides within Niddm1i3, while Tcf7l2 maps outside all four loci. Multiple-marker QTL analysis incorporating the effect of cosegregating QTL as cofactors together with genetically selected progeny can remarkably enhance resolution of QTL. The data demonstrate that the species-conserved Niddm1i is a composite of at least four QTL affecting type 2 diabetes susceptibility and that two adjacent QTL (Niddm1i2GK and Niddm1i3GK) act in opposite directions.

Hyperactivity in adrenergic signalling via alpha2A receptors contributes to human type 2 diabetes

Prevalence of lipid abnormalities before and after the introduction of lipid modifying therapy among Swedish patients with type 2 diabetes and/or coronary heart disease (PRIMULA Sweden)In the ACTION (A Coronary disease Trial Investigating Outcome with Nifedipine GITS) trial, the benefits of adding nifedipine GITS to the treatment of patients with stable symptomatic coronary artery disease were particularly apparent in those with concomitant hypertension. This further analysis has assessed whether or not the addition of nifedipine GITS is particularly beneficial in the treatment of patients with the combination of diabetes mellitus and chronic stable angina.Different sets of risk factors for the development of albuminuria and renal impairment in type 2 diabetes : the Swedish National Diabetes register (NDR)

High-Resolution Quantitative Trait Locus Analysis Reveals Multiple Diabetes Susceptibility Loci Mapped to Intervals Niddm1i of the GK Rat

Niddm1i, a 16-Mb locus within the major diabetes QTL in the diabetic GK rat, causes impaired glucose tolerance in the congenic NIDDM1I strain. Niddm1i is homologous to both human and mouse regions linked with type 2 diabetes susceptibility. We employed multiple QTL analyses of congenic F2 progeny selected for one recombination event within Niddm1i combined with characterization of subcongenic strains. Fine mapping located one hyperglycemia locus within 700 kb (Niddm1i4, P = 5 × 10−6). Two adjacent loci were also detected, and the GK allele at Niddm1i2 (500 kb) showed a glucose-raising effect, whereas it had a glucose-lowering effect at Niddm1i3 (400 kb). Most proximally, Niddm1i1 (800 kb) affecting body weight was identified. Experimental data from subcongenics supported the four loci. Sorcs1, one of the two known diabetes susceptibility genes in the region, resides within Niddm1i3, while Tcf7l2 maps outside all four loci. Multiple-marker QTL analysis incorporating the effect of cosegregating QTL as cofactors together with genetically selected progeny can remarkably enhance resolution of QTL. The data demonstrate that the species-conserved Niddm1i is a composite of at least four QTL affecting type 2 diabetes susceptibility and that two adjacent QTL (Niddm1i2GK and Niddm1i3GK) act in opposite directions.