Brief Genetics Report Genetic Variants in Human Sterol Regulatory Element Binding Protein-1c in Syndromes of Severe Insulin Resistance and Type 2 Diabetes

The transcription factor sterol regulatory element binding protein (SREBP)-1c is intimately involved in the regulation of lipid and glucose metabolism. To investigate whether mutations in this gene might contribute to insulin resistance, we screened the exons encoding the aminoterminal transcriptional activation domain in a cohort of 85 unrelated human subjects with severe insulin resistance. Two missense mutations (P87L and P416A) were found in single affected patients but not in 47 control subjects. However, these variants were indistinguishable from the wild-type in their ability to bind DNA or to transactivate an SREBP-1 responsive promoter construct. We also identified a common intronic single nucleotide polymorphism (C/T) located between exon 18c and 19c. In a case-control study of 517 U.K. Caucasian case subjects and 517 ageand sex-matched control subjects, the T-allele at this locus was significantly associated with type 2 diabetes in men (odds ratio 1.42 [1.11–1.82], P 0.005) but not women. In a separate population-based study of 1,100 Caucasians, carriers of the T-allele showed significantly higher levels of total and LDL cholesterol (P < 0.05) compared with wild-type individuals. In summary, we have conducted the first study of the SREBP-1c gene as a candidate for human insulin resistance. Although the rare mutations identified were functionally silent in the assays used, we obtained some evidence, which requires conformation in other populations, that a common variant in the SREBP-1c gene might influence diabetes risk and plasma cholesterol level. Diabetes 53:842–846, 2004 Type 2 diabetes is characterized by peripheral insulin resistance, increased hepatic gluconeogenesis, loss of glucose-induced insulin secretion and, in most patients, obesity (1). Recent data suggest that dysregulated fatty acid metabolism might be a key unifying mediator of these disparate phenomena (2–4). Sterol regulatory element binding proteins (SREBPs) are transcription factors crucial in the regulation of fatty acid and cholesterol metabolism. To date, three SREBP isoforms have been identified: SREBP-1a and SREBP-1c, derived from a single gene (SREBF1) through alternative promoter usage, and SREBP-2, encoded by a separate gene (SREBF2). The aminoterminal segment of the SREBPs contains an acidic transactivation domain and a basic helix-loop-helix leucine zipper (bHLH-Zip) region that mediates protein dimerization and DNA binding. SREBPs are embedded in the membrane of the endoplasmic reticulum as 120-kDa precursor proteins. Following fatty acid or cholesterol depletion, 68-kDa aminoterminal fragments (mature SREBP) are cleaved proteolytically from these precursor proteins and migrate into the nucleus (5). There they activate different target genes, encoding key enzymes of fatty acid and cholesterol metabolism. Of the two isoforms, SREBP-1c is the predominant transcript in most organs, including liver and adipose tissue, of adult animals (6). The mouse isoform of SREBP-1c, also known as adipocyte differentiation and determination factor–1, was identified as a factor promoting differentiation of cultured preadipocytes (7). Dysregulation of SREBP-1 expression or function has been demonstrated in humans (8) and a number of animal models of insulin resistance and diabetes. Thus, overexpression of a constitutively active form of SREBP-1c in adipose tissue leads to lipodystrophic diabetes in mice (9), genetic deletion of SREBP-1 rescues the fatty liver phenotype of ob/ob mice (10), and manipulation of SREBP-1 levels in pancreatic -cells can modulate the effects of fatty acids on insulin secretion (11,12). There have been no reports of genetic studies of the SREBF1 gene in relation to human diabetes phenotypes; however, it has been suggested that the mutation in lamin A that leads to the Dunnigan Köbberling type of autosomal dominant face From the Department of Medicine and Clinical Biochemistry, University of Cambridge/Addenbrooke’s Hospital, Cambridge, U.K.; Incyte Genomics, Palo Alto, California; and the Department of Public Health and Primary Care, University of Cambridge, Cambridge, U.K. Address correspondence and reprint requests to Professor Stephen O’Rahilly, FRS, MD, Department of Clinical Biochemistry, University of Cambridge, Addenbrooke’s Hospital, Hills Road, Box 232, Cambridge, CB2 2QR U.K. E-mail: [email protected]. Received for publication 16 September 2003 and accepted in revised form 24 November 2003. M.L. and I.B. contributed equally to this work. bHLH-Zip, basic helix-loop-helix leucine zipper; PEP, primer extension preamplification; SNP, single nucleotide polymorphism; sre, sterol regulatory element; SREBP, sre binding protein; SREBP-1a-NT, aminoterminal domain of human SREBP-1a. © 2004 by the American Diabetes Association.