Leen M. 't Hart

Gene variants in the novel type 2 diabetes loci CDC123/CAMK1D, THADA, ADAMTS9, BCL11A and MTNR1B affect different aspects of pancreatic beta cell function.

OBJECTIVE Recently, results from a meta-analysis of genome-wide association studies have yielded a number of novel type 2 diabetes loci. However, conflicting results have been published regarding their effects on insulin secretion and insulin sensitivity. In this study we used hyperglycemic clamps with three different stimuli to test associations between these novel loci and various measures of β-cell function. RESEARCH DESIGN AND METHODS For this study, 336 participants, 180 normal glucose tolerant and 156 impaired glucose tolerant, underwent a 2-h hyperglycemic clamp. In a subset we also assessed the response to glucagon-like peptide (GLP)-1 and arginine during an extended clamp (n = 123). All subjects were genotyped for gene variants in JAZF1, CDC123/CAMK1D, TSPAN8/LGR5, THADA, ADAMTS9, NOTCH2/ADAMS30, DCD, VEGFA, BCL11A, HNF1B, WFS1, and MTNR1B. RESULTS Gene variants in CDC123/CAMK1D, ADAMTS9, BCL11A, and MTNR1B affected various aspects of the insulin response to glucose (all P < 6.9 × 10−3). The THADA gene variant was associated with lower β-cell response to GLP-1 and arginine (both P < 1.6 × 10−3), suggesting lower β-cell mass as a possible pathogenic mechanism. Remarkably, we also noted a trend toward an increased insulin response to GLP-1 in carriers of MTNR1B (P = 0.03), which may offer new therapeutic possibilities. The other seven loci were not detectably associated with β-cell function. CONCLUSIONS Diabetes risk alleles in CDC123/CAMK1D, THADA, ADAMTS9, BCL11A, and MTNR1B are associated with various specific aspects of β-cell function. These findings point to a clear diversity in the impact that these various gene variants may have on (dys)function of pancreatic β-cells.

Molecular mechanisms of mitochondrial diabetes (MIDD).

Mitochondria provide cells with most of the energy in the form of adenosine triphosphate (ATP). Mitochondria are complex organelles encoded both by nuclear and mtDNA. Only a few mitochondrial components are encoded by mtDNA, most of the mt‐proteins are nuclear DNA encoded. Remarkably, the majority of the known mutations leading to a mitochondrial disease have been identified in mtDNA rather than in nuclear DNA. In general, the idea is that these pathogenic mutations in mtDNA affect energy supply leading to a disease state. Remarkably, different mtDNA mutations can associate with distinct disease states, a situation that is difficult to reconcile with the idea that a reduced ATP production is the sole pathogenic factor. This review deals with emerging insight into the mechanism by which the A3243G mutation in the mitochondrial tRNA (Leu, UUR) gene associates with diabetes as major clinical expression. A decrease in glucose‐induced insulin secretion by pancreatic beta‐cells and a premature aging of these cells seem to be the main process by which this mutation causes diabetes. The underlying mechanisms and variability in clinical presentation are discussed.

Heteroplasmy levels of a mitochondrial gene mutation associated with diabetes mellitus decrease in leucocyte DNA upon aging

We showed previously that a mutation in the mitochondrial tRNALeu(UUR) gene at position 3243 associates with maternally inherited diabetes and deafness (MIDD). This mutation shows heteroplasmy in DNA from peripheral blood and other tissues. To examine whether heteroplasmy levels in peripheral blood DNA change upon aging, heteroplasmy levels were determined in DNA samples from peripheral blood, collected recently and 1.5–6 years ago, from 18 individuals carrying the 3243 mutation. It was found that 17 out of 18 carriers showed a decrease upon aging (P = 0.001), the average change being −0.69 ± 0.61% per year. These data indicate a continuous selection against haematopoietic (precursor) cells carrying high levels of the 3243 mutation. Moreover, they imply that heteroplasmy levels may decrease below the detection limit if DNA from peripheral blood is analyzed from elderly individuals. DNA from oral mucosa cells was found to be a good alternative as heteroplasmy levels for the 3243 mutation are on the average 1.7 fold higher than in DNA from peripheral blood.

Variants in the sulphonylurea receptor gene: association of the exon 16-3t variant with type II diabetes mellitus in Dutch caucasians

Aims/hypothesis. We have analysed to what extent two previously reported single nucleotide polymorphisms in the sulphonylurea receptor gene (SUR1) are associated with Type II (non-insulin-dependent) diabetes mellitus in The Netherlands. Furthermore, we estimated haplotype frequencies in control and diabetic populations, including data extracted from three other studies. Methods. Subjects with Type II diabetes (n = 388) and normoglycaemic subjects (n = 336) were randomly selected from two population-based studies, the Hoorn and Rotterdam studies. DNA was typed for variants in exon 16 (-3c→t variant in the splice acceptor site) and exon 18 (Thr759Thr, ACC→ACT). Results. The genotype frequencies in both populations were similar. We observed an association of the exon 16–3t variant with Type II diabetes (allele frequencies 0.41 % vs 0.48 % in NGT and Type II diabetes, respectively, p = 0.01). There was no association between Type II diabetes and the variant in exon 18 or the combination of both variants (p > 0.5). A strong linkage disequilibrium between the exon 16 and exon 18 variants was observed in the diabetic groups but not, or less pronounced, in the control groups from the different studies. Haplotype estimation shows that several different risk haplotypes exist in different Caucasian populations. Conclusion/interpretation. The exon 16–3t allele of the SUR1 gene is associated with Type II diabetes in the Netherlands. Based on estimated haplotype frequencies in different Caucasian populations we conclude that multiple haplotypes on the SUR1 gene seem to confer a risk for developing Type II diabetes in Caucasians. [Diabetologia (1999) 42: 617–620]

Association of Polymorphism in the Receptor for Advanced Glycation End Products (RAGE) Gene with Circulating RAGE Levels

OBJECTIVE The receptor for advanced glycation end products (RAGE)-ligand interaction has been linked to vascular complications. The family of soluble forms of RAGE (sRAGE) consists of splice variants and proteolytically cleaved and shed forms of RAGE. sRAGE may be a reflection of cell-bound RAGE. Because genetic variation in the RAGE gene may be associated with individual differences in sRAGE concentration and outcome, we investigated whether RAGE single-nucleotide polymorphisms (SNPs) were associated with circulating levels of sRAGE. METHODS Nine SNPs, covering the common RAGE gene variation, were genotyped in a Dutch cohort of subjects with normal glucose metabolism (n = 301), impaired glucose metabolism (n = 127), and type 2 diabetes mellitus (n = 146). We used linear regression analyses adjusted for age, sex, and glucose metabolism status to compare sRAGE levels across genotypes. RESULTS SNP rs2060700 (Gly82Ser) showed an association with sRAGE levels. Specifically, after adjustments for age, sex, and glucose metabolism, subjects with CT genotype had -527 pg/ml (95% confidence interval -724 to -330, P < 0.001) lower sRAGE levels compared with the CC genotype (age, sex, and glucose metabolism adjusted mean +/- SE values of 836 +/- 99 and 1369 +/- 26 pg/ml, respectively, P < 0.001). These results were confirmed in a subsample of a second cohort study of subjects with CT (n = 37) and CC genotype (n = 37). Immunoblotting using antibodies against amino acids 39-55 and 100-116 of RAGE also showed a similar decrease of sRAGE levels in the CT genotypes. No other SNPs showed an association with sRAGE levels. In addition, no associations between SNPs and the advanced glycation end products N(epsilon)-(carboxymethyl)lysine and N(epsilon)-(carboxyethyl)lysine were found. CONCLUSION The CC genotype of SNP rs2070600 (Gly82Ser) was strongly associated with higher sRAGE levels in a Dutch population. The mechanism by which Gly82Ser polymorphism alters the sRAGE levels remains to be elucidated.

Combined Risk Allele Score of Eight Type 2 Diabetes Genes Is Associated With Reduced First-Phase Glucose-Stimulated Insulin Secretion During Hyperglycemic Clamps

OBJECTIVE At least 20 type 2 diabetes loci have now been identified, and several of these are associated with altered β-cell function. In this study, we have investigated the combined effects of eight known β-cell loci on insulin secretion stimulated by three different secretagogues during hyperglycemic clamps. RESEARCH DESIGN AND METHODS A total of 447 subjects originating from four independent studies in the Netherlands and Germany (256 with normal glucose tolerance [NGT]/191 with impaired glucose tolerance [IGT]) underwent a hyperglycemic clamp. A subset had an extended clamp with additional glucagon-like peptide (GLP)-1 and arginine (n = 224). We next genotyped single nucleotide polymorphisms in TCF7L2, KCNJ11, CDKAL1, IGF2BP2, HHEX/IDE, CDKN2A/B, SLC30A8, and MTNR1B and calculated a risk allele score by risk allele counting. RESULTS The risk allele score was associated with lower first-phase glucose-stimulated insulin secretion (GSIS) (P = 7.1 × 10−6). The effect size was equal in subjects with NGT and IGT. We also noted an inverse correlation with the disposition index (P = 1.6 × 10−3). When we stratified the study population according to the number of risk alleles into three groups, those with a medium- or high-risk allele score had 9 and 23% lower first-phase GSIS. Second-phase GSIS, insulin sensitivity index and GLP-1, or arginine-stimulated insulin release were not significantly different. CONCLUSIONS A combined risk allele score for eight known β-cell genes is associated with the rapid first-phase GSIS and the disposition index. The slower second-phase GSIS, GLP-1, and arginine-stimulated insulin secretion are not associated, suggesting that especially processes involved in rapid granule recruitment and exocytosis are affected in the majority of risk loci.

Reduced second phase insulin secretion in carriers of a sulphonylurea receptor gene variant associating with Type II diabetes mellitus.

Aims/hypothesis. The sulphonylurea receptor is a subunit of the ATP-sensitive potassium channel in the pancreatic beta cell. Mutations at nt –3 of the splice acceptor site of exon 16 and a silent mutation in exon 18 of the gene for the sulphonylurea receptor (SUR1) associate with Type II (non-insulin-dependent) diabetes mellitus in several independent populations. We investigated whether these gene variants associate with changes in the pattern of glucose-stimulated insulin secretion.¶Methods. Subjects who had normal glucose tolerance (n = 67) and subjects with an impaired glucose tolerance (n = 94), originating from two independent studies, were included in the study. Beta-cell function and insulin sensitivity were assessed by the hyperglycaemic clamp.¶Results. Frequencies of the exon 16 –3t allele in the normal and impaired glucose tolerant groups were 46 % and 44 % respectively (p = NS). The more rare exon 18 T allele showed frequencies of 5 and 7 % respectively (p = NS). We observed an approximately 25 % reduced second-phase insulin secretion in carriers of the exon 16 –3t allele in both groups (p < 0.05). Estimates of insulin sensitivity did not show differences between carriers and non-carriers. The variant in exon 18 and the combined presence of variants in exon 16 and exon 18 were not associated with differences in insulin secretion or insulin sensitivity in our study groups.¶Conclusion/interpretation. The diabetes associated exon 16 –3t variant of the SUR1 gene associates with a functional change of the beta cell as reflected by reduced second-phase insulin secretion in response to a standardized hyperglycaemia in normal and impaired glucose tolerant subjects. [Diabetologia (2000) 43: 515–519]

Evaluation of the Importance of Maternal History of Diabetes and of Mitochondrial Variation in the Development of NIDDM

In 79 South Indian nuclear pedigrees ascertained via probands with NIDDM and both parents living, parental diabetic status was established through previously diagnosed NIDDM (n = 97) or oral glucose tolerance testing (n = 61). There was no significant difference between diabetes prevalence in mothers and fathers (60 vs 53 (76 % vs 67 %), respectively, p = 0.22). ‘Age at diabetes diagnosis’ survival curves did differ according to parental gender (p = 0.02) but this may reflect gender differences in health provision rather than pathophysiology. No maternal excess effects of the magnitude evident in previous studies were detected, suggesting either ethnic differences or overestimation of the maternal effect when reported histories of parental diabetes have been used. The tRNALeu(UUR) gene region was studied for diabetes‐associated variation given the role of mutations in this gene in some pedigrees displaying maternal transmission of NIDDM. None of 142 unrelated South Indian NIDDM subjects displayed the MELAS mutation at nt3243. However, sequencing identified two variants of potential importance: (a) at nt3290 in the tRNALeu(UUR) gene, seen in 7/142 diabetic and 1/85 control subjects (p = 0.11), (b) at nt3316 in the ND1 gene (4/142 vs 1/85 subjects, respectively (p = 0.51)). Further studies are needed to determine the relevance of these variants to the development of NIDDM.

Mitochondrial diabetes and its lessons for common Type 2 diabetes

Multiple pathogenic pathways are able to deregulate glucose homoeostasis leading to diabetes. The 3243A>G mutation in the mtDNA (mitochondrial DNA)-encoded tRNALeu,UUR gene was found by us to be associated with a particular diabetic subtype, designated MIDD (maternally inherited diabetes and deafness). This mutation causes an imbalance in the mitochondrion between proteins encoded by the nuclear and mitochondrial genomes, resulting in a gradual deterioration of glucose homoeostasis during life. Remarkably, carriers of the 3243A>G mutation are generally not obese. The mutation also results in enhanced radical production by mitochondria. We propose that this mutation leads to the development of diabetes due to an inappropriate storage of triacylglycerols within adipocytes. The result is a fatty acid-induced deterioration of pancreatic beta-cell function. In combination with an enhanced radical production in the beta-cell due to the mutation, this leads to an age-dependent, accelerated decline in insulin production. In common Type 2 (non-insulin-dependent) diabetes, which is generally associated with obesity, a decline in mitochondrial function in adipose cells seems to result in an inappropriate scavenging of fatty acids by beta-oxidation. As a consequence, a systemic overload with fatty acids occurs, leading to an enhanced decline in beta-cell function due to lipotoxicity.

DETECTION OF MITOCHONDRIAL DNA DELETIONS IN HUMAN SKIN FIBROBLASTS OF PATIENTS WITH PEARSON'S SYNDROME BY TWO-COLOR FLUORESCENCE IN SITU HYBRIDIZATION

Pearsons marrow/pancreas syndrome is a disease associated with a large mitochondrial DNA (mtDNA) deletion. The various tissues of a patient contain heteroplasmic populations of wild-type (WT) and deleted mtDNA molecules. The clinical phenotype of Pearsons syndrome is variable and is not correlated with the size and position of the deletion. The histo- and cytological distribution of WT and deleted mtDNA molecules may be factors that correlate with the phenotypical expression of the disease. Here we introduce a new application of two-color FISH to visualize WT and deleted mtDNA simultaneously in a cell population of in vitro cultured skin fibroblasts of two patients with Pearsons syndrome. At the third passage of culturing, fibroblasts showed a remarkable heterogeneity of WT and deleted mtDNA: about 90% of the cells contained almost 100% WT mtDNA, and 10% of the cells contained predominantly deleted mtDNA. At the tenth passage of culturing, fibroblasts showed a reduction of intercellular heteroplasmy from 10% to 1%, while intracellular heteroplasmy was maintained. This new approach enables detailed analysis of distribution patterns of WT and deleted mtDNA molecules at the inter- and intracellular levels in clinical samples, and may contribute to a better understanding of genotype-phenotype relationships in patients with mitochondrial diseases.