DOI: 10.2337/db06-0414 © 2006 by the American Diabetes Association
The HADHSC Gene Encoding Short-Chain L-3-Hydroxyacyl-CoA Dehydrogenase (SCHAD) and Type 2 Diabetes SusceptibilityThe DAMAGE Study
1 Department of Molecular Cell Biology, Leiden University Medical Center, Leiden, the Netherlands Address correspondence and reprint requests to L.M. t Hart, PhD, Leiden University Medical Center, Department of Molecular Cell Biology, Building 2, Room R2-005, Postal Zone S1-P, P.O. Box 9600, 2300 RC Leiden, Netherlands. E-mail: l.m.t_hart{at}lumc.nl
Abbreviations:
NGT, normal glucose tolerance; SCHAD, short-chain L-3-hydroxyacyl-CoA dehydrogenase; SNP, single nucleotide polymorphism
The short-chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD) protein is involved in the penultimate step of mitochondrial fatty acid oxidation. Previously, it has been shown that mutations in the corresponding gene (HADHSC) are associated with hyperinsulinism in infancy. The presumed function of the SCHAD enzyme in glucose-stimulated insulin secretion led us to the hypothesis that common variants in HADHSC on chromosome 4q22-26 might be associated with development of type 2 diabetes. In this study, we have performed a large-scale association study in four different cohorts from the Netherlands and Denmark (n = 7,365). Direct sequencing of HADHSC cDNA and databank analysis identified four tagging single nucleotide polymorphisms (SNPs) including one missense variant (P86L). Neither the SNPs nor haplotypes investigated were associated with the disease, enzyme function, or any relevant quantitative measure (all P > 0.1). The present study provides no evidence that the specific HADHSC variants or haplotypes examined do influence susceptibility to develop type 2 diabetes. We conclude that it is unlikely that variation in HADHSC plays a major role in the pathogenesis of type 2 diabetes in the examined cohorts. Previously, it was shown that variation in HADHSC is associated with the development of hyperinsulinism of infancy (OMIM 601820) (1–3). HADHSC encodes the short-chain L-3-hydroxyacyl-CoA dehydrogenase enzyme (SCHAD; EC 1.1.1.35), an important enzyme in mitochondrial fatty acid oxidation that is highly expressed in pancreatic ß-cells (National Center for Biotechnology Information gene no. 3033). Disease-associated mutations in the gene result in severely reduced enzyme levels and activity in different tissues. Patients suffer from unregulated insulin hypersecretion resulting in hypoglycemia. The exact mechanisms behind this association remain, however, unclear. The fact that patients are responsive to the effects of the drug diazoxide on the closure of the pancreatic ATP-sensitive K+ channels suggests involvement of the ATP-sensitive K+ channel–dependent pathway of glucose-stimulated insulin secretion. Whether this is via increased ATP levels in the cytosol or other (mitochondrial) factors affecting channel function and membrane depolarization of the ß-cell remains unclear. Previously, it has been shown that (common) variants in other hyperinsulinism in infancy-associated genes are associated with enhanced susceptibility to develop type 2 diabetes (4–7). Glucose intolerance in these cases is most likely caused by defects in glucose-stimulated insulin secretion (8). Most prominent examples are variants in the genes encoding the two subunits of the ATP-sensitive potassium channel, ABCC8 and KCNJ11 and glucokinase (GCK) (4–9). Several other studies (10–13) have furthermore shown the importance of correct mitochondrial function in relation to insulin secretion, insulin resistance, glucose homeostasis, and type 2 diabetes. Together, this has led us to the hypothesis that polymorphisms in HADHSC might be associated with enhanced type 2 diabetes susceptibility. Therefore, we have performed an association study with HADHSC in four population cohorts from the Netherlands and Denmark involving in total 7,365 subjects.
Subjects participating in the association studies were selected from four study populations in the Netherlands and Denmark. The first cohort, HS1, consisted of 470 participants of the Hoorn Study, a population-based study in the Netherlands (14). Subjects were classified as having type 2 diabetes (n = 210, aged 65 ± 8 years, 48% men) or as glucose-tolerant control subjects (n = 260, aged 60 ± 7 years, 42% men) based on the baseline fasting oral glucose tolerance test according World Health Organization criterion. Furthermore, we used a second sample from the same study, the HS2 cohort (15). Subjects with either normal glucose tolerance (NGT) or type 2 diabetes at the year 2000 survey of the Hoorn Study were included in this cohort. To enhance power, the type 2 diabetes group was supplemented by subjects with screen-detected type 2 diabetes from the same geographic area (n = 217) (16). There was no overlap between the subjects in either cohort. In the type 2 diabetes group (n = 297), the mean age was 68 ± 8 years (48% men) and 69 ± 6 years for participants with NGT (n = 283, 50% men). Both Dutch cohorts are part of the DAMAGE (Diabetes and Mitochondrial Aging Genes) study. Furthermore, we genotyped two case-control study samples from Denmark, DK1 and DK2. DK1 was a group of unrelated type 2 diabetic patients recruited from the Steno Diabetes Center, and a group of unrelated subjects with NGT sampled at random through public registers at the Steno Diabetes Center and the Research Centre for Prevention and Health (17). In the group of type 2 diabetic patients (n = 603, 59% men), the mean age was 60 ± 10 years and 58 ± 10 years for participants with NGT (n = 527, 46% men) (17). DK2 consisted of unrelated type 2 diabetic patients recruited from the Steno Diabetes Center and the Research Centre for Prevention and Health (n = 756, aged 54 ± 10 years, 61% men), and a group of unrelated subjects with NGT sampled from the prospective Inter99 Study at the Research Centre for Prevention and Health (n = 4,429, aged 45 ± 8 years, 47% men) (18,19). All control subjects participating in the Danish studies underwent an oral glucose tolerance test according to World Health Organization criteria. Questionnaires were used to obtain other relevant information. All participants were Caucasian by self-report. Informed written consent was obtained from all subjects before participation. The study was approved by the appropriate medical ethical committees and was in accordance with the principles of the Declaration of Helsinki.
HADHSC sequencing.
Genotyping.
Enzyme activity.
SCHAD protein expression.
mRNA expression levels.
Statistical analyses.
Databank analysis and direct sequencing of HADHSC cDNA revealed 17 common intronic SNPs (minor allele frequency >0.05) and three cSNPs in this locus. The three cSNPs were all nonsynonymous variants: P86L (rs4956145), H152Q (rs1051519), and D279E (rs17417368). The latter two seemed to be rare and were not detected in our (sequenced) samples. Therefore, we focused our research on the P86L variant and three tagging SNPs that capture all other alleles in the HapMap Phase I CEPH dataset (online appendix Fig. 1).
The minor allele frequency of the P86L variant varied between 6.6 and 9.2% in the different case-control groups from the Netherlands and Denmark (Table 1). Only in the DK1 cohort did the difference reach statistical significance (P = 0.04; Table 1). Meta-analysis was used to calculate a common OR after homogeneity testing revealed no significant differences between the studies (P > 0.05). The common OR was 1.08 (95% CI 0.93–1.26, P = 0.29) (Table 1 and online appendix Fig. 2). Adjustment for age and sex did not significantly alter the results. Analysis of the genotype distributions in the different cohorts assuming different genetic models and adjusted for age and sex yielded similar nonsignificant results (online appendix Table 1). Based on the observed allele frequency of the P86L variant (minor allele frequency 0.08), we had >80% power to detect effects with an OR
We determined SCHAD enzyme activity in mitochondrial fractions isolated from fibroblasts of 5 carriers and 10 noncarriers of the P86L variant. Enzyme activities were, however, not significantly different (0.64 ± 0.14 vs. 0.60 ± 0.07 units/UCS, P > 0.5). SCHAD enzyme activities in total fibroblast lysates showed a similar nonsignificant difference. Furthermore, there were no detectable differences in SCHAD protein and mRNA levels between carriers and noncarriers (data not shown). Together, this suggests that the P86L variant has no effect on enzyme levels or function, although small differences cannot be fully excluded. According to the Hapmap Phase I CEPH data, HADHSC is present on one single haploblock spanning the whole locus (online appendix Fig. 1). The four common haplotypes (>1%) in HADHSC can be tagged by four tagging SNPs, which were typed in subsets of the HS1 (n = 630) and DK1 (n = 520) studies. We also examined the performance of our tagging SNPs in the recently available HapMap Phase II CEPH (January 2006) dataset. Our tagging SNPs captured almost all of the SNPs identified in the HADHSC region from the Phase II dataset, capturing 48 of 55 common SNPs with a mean r2 of 0.991 and 100% of alleles with r2 > 0.8.
Haplotype frequency distributions are very similar in both populations. In a joint analysis of all haplotypes in relation to disease status, there was no significant difference between case and control subjects in both cohorts. There were also no significant differences in the frequency of the four most common haplotypes (all P > 0.1), although we observed similar trends in both populations, especially with the 0-0-0-1 haplotype (Table 3). Our combined case-control sample provides at least 80% power to detect an effect with an OR
To examine the possibility that the 0-0-0-1 haplotype is associated with altered enzyme function and a minor effect on type 2 diabetes susceptibility, we determined SCHAD activity in mitochondrial fractions from fibroblasts of four homozygous carriers of this haplotype and four homozygous carriers of the common 0-0-0-0 haplotype. Enzyme activity was not detectably different between the two haplotypes (0.57 ± 0.13 vs. 0.62 ± 0.11 for 0-0-0-0 haplotypes, P > 0.5). There was also no evidence for differences in expression levels between carriers and noncarriers (data not shown).
In this study, which is the first ever to examine the relationship between variations in HADHSC and type 2 diabetes, we have found no evidence for an association between variants at this gene locus and susceptibility to develop type 2 diabetes. Based on the above-described results, the fact that we found no evidence for alterations in circulating glucose, insulin, or lipid levels in NGT carriers of these gene variants, and given that the data show that the variants investigated in this study have no detectable effect on expression levels or enzyme function, we conclude that variation in HADHSC is unlikely to have a major effect on susceptibility to develop type 2 diabetes in these cohorts.
This study was supported by the Dutch Diabetes Research Foundation; the Netherlands Organisation for Health Research and Development; the Research Institute for Diseases in the Elderly (RIDE program); the Center for Medical Systems Biology; the Danish Medical Research Council; the Danish Center for Evaluation and Health Technology; the Danish Heart Foundation; Novo Nordic; Copenhagen County; the Danish Pharmaceutical Association; the Danish National Board of Health; the Velux, Augustinus, Ib Henriksen, and Becket Foundations; the Danish Diabetes Association; and European Economic Committee grants (BMH4-CT98-3084 and QLRT-CT-1999-00546). We thank all participants for their kind cooperation. A.W. Strauss (Nashville, TN) is kindly acknowledged for the gift of the SCHAD antibody. Annemette Forman, Inge Lise Wantzin, Marianne Stendal, and the staff from the Research Center for Prevention and Health (Glostrup, Denmark) are thanked for dedicated and careful assistance and Grete Lademann for secretarial support.
Additional information for this article can be found in an online appendix at http://diabetes.diabetesjournals.org. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Received for publication March 29, 2006 and accepted in revised form July 27, 2006
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