VARIANT SEQUENCES OF THE HEXOKINASE-II GENE IN FAMILIAL NIDDM

Hexokinase II (HKII) plays a central role in the intracellular metabol ism of glucose in skeletal muscle, catalysing the phosphorylation of g lucose to glucose 6-phosphate. It is therefore considered to be a pote ntially important candidate gene in the development of insulin resista nce and non-insulin-dependent diabetes mellitus (NIDDM). The aim of th is study was to screen the HKII gene for mutations in NIDDM subjects f rom insulin-resistant families. Insulin sensitivity was assessed in un affected first degree relatives from families with two or more living NIDDM subjects, and 15 families were identified as being insulin resis tant. In 15 NIDDM subjects (one from each family) and 4 normoglycaemic control subjects, all 18 exons of the HKII gene were amplified by the polymerase chain reaction, and the products screened for mutations us ing a combination of single-stranded conformational polymorphism analy sis and direct sequencing. Six sequence variations were detected in th e NIDDM subjects; four silent polymorphisms [CAT vs GAC at codon 251 i n exon 7, AAT vs AAC at codon 692 in exon 15, CCG vs CCC at codon 736 in exon 15, and CTG vs CTA at codon 766 in exon 16]; a single base cha nge [T --> C], 22 base pairs distal to the exon-intron junction of exo n 17 in the 5'-splice donor; and a single amino acid substitution [Gln (142) --> His] in exon 4, which was identified in 6 of the 15 NIDDM su bjects. The frequency of the mutated codon 142 allele however, was com parable between NIDDM subjects with familial NIDDM (n = 56) and normog lycaemic control subjects (n = 48) (18.8% and 14.6% for NIDDM subjects and control subjects respectively; chi(2) = 0.6, p > 0.25). In additi on, measures of insulin sensitivity were comparable in normal glucose tolerant subjects with (n = 20) and without (n = 40) the codon 142 pol ymorphism. In conclusion: (1) mutations in the coding regions of the H KII gene are unlikely to be major determinants in the development of i nsulin resistance and familial NIDDM; although (2) the influence of th e codon 142 mutation in combination with other abnormalities of the in sulin-signalling pathway on insulin action remain to be addressed.