To identify chromosomal regions containing susceptibility genes for NIDDM, we have analyzed evenly-spaced microsatellite markers on DNA samples from 1338 Pima Indians. These samples represent 264 nuclear families and 1862 sib-pairs (563 concordant affected; 396 concordant unaffected; 903 discordant). Sib-pair linkage analyses identified a genetic marker on chromosome 4q.12 (D4S2638) that is potentially linked (p<0.001) to NIDDM (age of onset <45). Alleles at this locus are also associated with NIDDM (p<0.04). D4S2638 was further found to be possibly linked with fasting plasma insulin concentrations (p<0.04) and insulin action at physiologic insulin concentrations during a hyperinsulinemic, euglycemic clamp (p<0.07) in 250 non-diabetic Pima sibpairs after adjusting for obesity, age and sex. The locus for the vitamin D binding protein (VDBP), which also maps to 4q.12, has been previously associated with NIDDM as well as subphenotypes of NIDDM. The locus for VDBP (also referred to as group-specific component, Gc) has previously been associated with NIDDM in 7 southwest Pacific Island populations and one European population. In addition, alleles of the VDBP locus are associated with fasting plasma insulin levels in the Dogrib Indians of the Canadian Nothwest Territories and fasting plasma glucose levels in a group of subarctic Amerindians. We have utilized the technique of Radiation Hybrid Mapping to show that the VDBP locus and D4S2638 are separated by less than 500 kb. Therefore, since it is entirely plausible that variation at the VDBP locus could account for our linkage observation at D4S2638, we are currently analyzing the coding sequences of VDBP for variability. As an initial screen, we have sequenced all of the coding exons of VDBP in 40 individuals. We have identified 4 single base polymorphisms that would result in amino acid substitutions at the protein level. Surprisingly, these 4 substitutions all reside in exon 11, and are within 7 codons of each other. We are currently sequencing our entire data set of 1338 Pimas for these 4 substitutions to determine whether any substitution is associated with NIDDM or a subphenotype of NIDDM.
