Considerable data support adiponectin as an important adipose-derived insulin sensitizer that enhances fatty acid oxidation and alters hepatic gluconeogenesis. Adiponectin acts by way of two receptors, ADIPOR1 and ADIPOR2. ADIPOR1 is widely expressed in tissues, including muscle, liver, and pancreas, and binds the globular form of adiponectin with high affinity. To test the hypothesis that sequence variations in or near the ADIPOR1 gene contribute to the risk of developing type 2 diabetes and the metabolic syndrome, we screened the eight exons (including the untranslated exon 1) of the ADIPOR1 gene with flanking intronic sequences and the 5 and 3 flanking sequences. We identified 22 single nucleotide polymorphisms (SNPs) in Caucasian and African-American subjects, of which a single nonsynonymous SNP (N44K) in exon 2 was present only in AfricanAmerican subjects. We typed 14 sequence variants that had minor allele frequencies >5%. No SNP was associated with type 2 diabetes in Caucasians or African Americans, and no SNP was a determinant of insulin sensitivity or insulin secretion among nondiabetic members of high-risk Caucasian families. However, the two alleles of a SNP in the 3 untranslated region were expressed unequally, and ADIPOR1 mRNA levels were significantly lower among transformed lymphocytes from diabetic African-American individuals than among control cell lines. This altered gene expression might suggest a role for ADIPOR1 in the metabolic syndrome.

IL-6 levels and polymorphisms have been implicated in type 2 diabetes mellitus (T2DM) and insulin resistance. The IL-6 receptor (IL-6R) comprises two subunits, IL-6R and gp130, of which IL-6R confers specificity to IL-6 action and is located in a region of replicated linkage to T2DM on chromosome 1q21. We screened this gene for variation in Northern European Caucasian and African-American ethnic groups. We identified 11 variants with a minor allele frequency over 5%, including two amino acid changes (D358A and V385I) and four variants in the 3' untranslated region. No variant was associated with obesity or measures of insulin sensitivity, but two single nucleotide polymorphisms in the 3' untranslated region showed a trend to an association with T2DM in all Caucasians, and three single nucleotide polymorphisms, including D358A, showed a trend (P < 0.06) to an association with T2DM among the subset of Northern European Caucasians. Variant V385I was unique to African-Americans and was significantly associated with diabetes and diabetic nephropathy (P < 0.05). Among individuals heterozygous for the four variants in the transcribed sequence, one allele was significantly overrepresented, thus suggesting the existence of a regulatory variant controlling mRNA stability or expression. IL-6R is not likely to explain the linkage to diabetes in this region, but our work supports a minor role of variants in T2DM risk and suggests that sequence variants may alter IL-6R mRNA levels and possibly levels of soluble IL-6R.

Increased flux of glucose through the hexosamine biosynthetic pathway has been implicated in insulin resistance, altered insulin secretion, and diabetic nephropathy. Glutamine:fructose-6-phosphate amidotransferase (GFPT), the rate limiting enzyme in hexosamine biosynthesis, is encoded by the unlinked but highly homologous genes GFPT1 and GFPT2. We tested the hypothesis that GFPT2 sequence variation contributed to the susceptibility to type 2 diabetes mellitus (T2DM) and diabetic nephropathy in Caucasian and African-American individuals. We identified 11 single nucleotide polymorphisms (SNPs), of which seven were common. A single variant in exon 14, I471V, altered the amino acid sequence, is conserved between human and mouse genes, and was associated with T2DM among Caucasians (P = 0.05). A trend to an association was noted with diabetic nephropathy among African-American individuals (P = 0.15). Several variants in the 3' untranslated region (UTR) and exon 18 were also associated with T2DM in Caucasian individuals (P < 0.05), and the SNP in the 3' UTR was associated with diabetic nephropathy in African-American subjects (P = 0.047). GFPT2 mRNA levels in transformed lymphocytes from study subjects were significantly increased among African-American subjects compared with Caucasian individuals, regardless of diagnosis. Furthermore, the associated allele of the 3' UTR SNP was approximately 2-fold overexpressed. We propose that the 3' UTR variant results in increased GFPT2 mRNA levels with resultant increased hexosamine flux. The I471V variant may contribute to altered protein function or may simply be in linkage disequilibrium with the 3' UTR.

We have identified a region on chromosome 1q21-q24 that was significantly linked to type 2 diabetes in multiplex families of Northern European ancestry and also in Pima Indians, Amish families, and families from France and England. We sought to narrow and map this locus using a combination of linkage and association approaches by typing microsatellite markers at 1.2 and 0.5 cM densities, respectively, over a region of 37 cM (23.5 Mb). We tested linkage by parametric and nonparametric approaches and association using both casecontrol and family-based methods. In the 40 multiplex families that provided the previous evidence for linkage, the highest parametric, recessive logarithm of odds (LOD) score was 5.29 at marker D1S484 (168.5 cM, 157.5 Mb) without heterogeneity. Nonparametric linkage (NPL) statistics (P ‫؍‬ 0.00009), SimWalk2 Statistic A (P ‫؍‬ 0.0002), and sib-pair analyses (maximum likelihood score ‫؍‬ 6.07) all mapped to the same location. The one LOD CI was narrowed to 156.8 -158.9 Mb. Under recessive, two-point linkage analysis, adjacent markers D1S2675 (171.5 cM, 158.9 Mb) and D1S1679 (172 cM, 159.1 Mb) showed LOD scores >3.0. Nonparametric analyses revealed a second linkage peak at 180 cM near marker D1S1158 (163.3 Mb, NPL score 3.88, P ‫؍‬ 0.0001), which was also supported by case-control (marker D1S194, 178 cM, 162.1 Mb; P ‫؍‬ 0.003) and family-based (marker ATA38A05, 179 cM, 162.5 Mb; P ‫؍‬ 0.002) association studies. We propose that the replicated linkage findings actually encompass at least two closely spaced regions, with a second susceptibility region located telomeric at 162.5-164.7 Mb. Diabetes 53:492-499, 2004 T ype 2 diabetes (MIM125853) likely encompasses a diverse set of diseases marked by elevated levels of plasma glucose. Among Caucasian populations, individuals with type 2 diabetes, individuals with the intermediate phenotype of impaired glucose tolerance, and likely individuals at risk of diabetes are all characterized by variable degrees of both decreased insulin action, particularly resistance to insulin-mediated muscle glucose uptake, and impaired insulin secretion in response to that decreased insulin action (1). Defects of both insulin action and insulin secretion among individuals with normal glucose tolerance predict later onset of diabetes (2). Despite the diverse phenotypic nature of type 2 diabetes, monozygotic and dizygotic twin studies, family studies, and marked differences in disease prevalence across populations all provide convincing evidence for an important role of genetic susceptibility loci in type 2 diabetes pathogenesis (1). Based on epidemiological data, the total sibling relative risk ( s ) has been estimated at 3-4 (3), although the number of loci that contribute to this risk is unclear.Based on these data supporting type 2 diabetes susceptibility genes, genome scans for both type 2 diabetes and type 2 diabetes-related traits have been undertaken by multiple laboratories in Caucasian, Pima Indian, AfricanAmerican, and Asian populations (1,4,5), among others. T...