The insulin receptor in South African families with lipo-atrophic diabetes mellitus : DNA studies and variations in insulin binding, including the effect of pregnancy

Objectives. To determine insulin receptor (INSR) binding in 2 patients with lipo-atrophic diabetes mellitus (LDM) and their families, to assess relevant aspects of their carbohydrate and lipid metabolism, and to investigate whether mutations in the INSR gene occur in these families. Design. [ 125 I]-insulin binding to monocyte receptors was determined by radioreceptor assay. Fasting blood samples were analysed for glucose, insulin, C-peptide, cholesterol, triglycerides and non-esterified fatty acids. The hyperinsulinaemic euglycaemic clamp technique was used to assess insulin-mediated glucose disposal. Genomic DNA was amplified by the polymerase chain reaction (PCR) and screened for single nucleotide differences by the single-stranded conformation polymorphism (SSCP) technique in the insulin-binding domain (exons 2 - 5) and the tyrosine kinase domain (exons 17 - 21) of the INSR gene; this was followed by DNA sequencing. Setting. Department of Medicine, Johannesburg Hospital. Results. Different combinations of INSR-binding abnormalities were found in the two families. In family 1, the patient had decreased insulin binding as a result of reduced receptor concentration. She fell pregnant and was re-evaluated in the third trimester, when her insulin binding doubled because of an increase in receptor concentration. This was reflected clinically by a reciprocal reduction in her insulin requirements. In family 2, affinity was increased and insulin-binding levels were low in all members, most severely in the patient. SSCP analysis detected three mutational patterns in exon 17. DNA sequencing revealed that the SSCP variants corresponded to the silent polymorphisms TAC/TAT at codon 984, and CAC/CAT and CAT/CAT at codon 1058. Conclusions. Although these polymorphisms in the INSR gene are of interest, they do not explain the defective INSR function seen in these families. The variations in insulin binding observed in LDM, which are subject to physiological fluctuations, do not appear to be the primary cause of the insulin resistance found in this syndrome.