A prior genomic linkage scan in Pima Indians indicated an obesity susceptibility locus on chromosome 11q23-24 (LOD=3.6). There was also evidence that the same genomic region contained a susceptibility locus for type 2 diabetes mellitus (T2DM)(LOD=1.7). Bivariate linkage analysis for the combined phenotype ?diabesity? gave the strongest evidence for a disease locus (LOD= 5.2). The region of linkage spans 24 Mb. To narrow this region, single nucleotide polymorphisms (SNPs) are being genotyped at a 10 kb density, across the entire 24 Mb, for linkage disequilibrium (LD) mapping. To date, my lab has genotyped 3400 SNPs across chromosome 11q23-25 in the same subjects that were analyzed in our linkage study (N=1300). Several of our most interesting SNPs have been genotyped in the FUSION study of Finns, independent case/control samples for either T2DM or obesity from Botnia, Helsinki, and Malmo, a large collection of metabolically phenotyped Caucasians from Denmark, Caucasian children from the Leipziger School Children study, Caucasians from the UK (Oxford), Mexican Americans from the VAGES study, and Mexican Americans from Starr County. Results from these replication groups for genes in our region of linkage on chromosome 11q23-25 have shown that variants near ETS-1 on chromosome 11 have replicated in three out of five studies. A variant in TREH on chromosome 11 showed significant associations in 6 out of seven studies, but the risk allele was inconsistent among the various ethnic groups, so it remains debatable whether this variant ?replicated.? A variant in CD3E was tested in 1 Mexican American group where it showed a non-significant trend for risk of T2DM in the same direction as Pima Indians.? ? My lab has further followed up on significant associations identified by LD mapping by directly sequencing 74 positional candidate genes under the peak of linkage on chromosome 11q23-25. This work is still in progress, but our gene sequencing efforts on chromosome 11 suggest that variants within IGSF4, CD3E, TREH, ETS-1, SNX-19, and OPCML are most convincingly associated with BMI and/or T2DM. We have recently completed functional analysis of variants in one of these genes, TREH, which encodes trehalase. We measured serum trehalase activity in 570 non-diabetic subjects and found that trehalase activity was highly associated with a non-synonymous Arg486Trp variant and a promoter variant at -100 b.p. (p=10-12 and p=10-15, respectively, adjusted for age, sex, heritage and family membership). Haplotype analysis showed that subjects who were homozygous for 3 variants (Trp486, Ala389, and C at -100 b.p.) had the lowest trehalase activity and had the lowest prevalence of T2DM. However, in a prospective study, we were unable to demonstrate that a low plasma trehalase activity (independent of genotype) predicted the development of T2DM. Therefore, we believe we have uncovered the genetic basis for the bimodal appearance of trehalase activity (14), and although these variants are associated with T2DM, they may not be causative for T2DM.
