Tumour necrosis factor-alpha gene promoter polymorphism and decreased insulin resistance

Insulin resistance is a feature of non-diabetic relatives of non-insulin-dependent diabetic (NIDDM) families. Tumour necrosis factor-alpha (TNF alpha) expression is linked with insulin resistance, and is under strong genetic control. We examined the relationship between insulin resistance and two polymorphisms of the TNF alpha promoter region (positions -238 and -308). Non-diabetic relatives (n = 123) of NIDDM families and control subjects (n = 126) with no family history of diabetes were studied. Insulin resistance was determined by homeostasis model assessment (HOMA) and short insulin tolerance test (ITT), and genotyping was by restriction digest. The -238 polymorphism (TNFA-A allele) was carried by 14 relatives and 11 control subjects, and all were heterozygotes. To examine the relationship between the -238 polymorphism and insulin resistance independent of potentially confounding factors, the relatives with the TNFA-A allele were individually pair-matched for age, sex, waist-hip ratio, body mass index, and glucose tolerance with relatives homozygous for the wild-type allele. Relatives with the TNFA-A allele had decreased insulin resistance (HOMA index: 2.0, 3.6 +/- 2.1 [means +/- SD of differences], p = 0.03), and this was true for comparable pair-matched control subjects (HOMA index: 1.1, 1.9 +/- 0.8, p = 0.01). Combining relative (n = 7) and control (n = 4) pairs that had undergone an ITT, subjects with the TNFA-A allele had an increased K(ITT) (3.8, 3.0 +/- 1.0%/min, p = 0.04) similarly indicating decreased insulin resistance. There was no significant relationship between the -308 polymorphism and insulin resistance. We conclude that the TNFA-A allele is associated with decreased insulin resistance as assessed by two independent methods, and may protect against the future development of NIDDM in susceptible individuals.