Background: Many patients with extreme insulin resistance in combination with acanthosis nigricans are reported to have defective insulin receptor genes. Because acanthosis nigricans is commonly accompanied by severe hyperinsulinemia and obesity, and obesity is a major factor in insulin resistance, this study was initiated to assess the prevalence of mutations in the insulin receptor gene in Chinese patients with extreme insulin resistance defined by hyperinsulinemia, obesity and acanthosis nigricans.
Methods: Exons 1-22 of the insulin receptor gene were amplified by polymerase chain reaction (PCR) by from genomic DNA from 13 young subjects with clinical and metabolic features of extreme insulin resistance. They were also screened for nucleotide variation using single-strand conformation polymorphism (SSCP) combined with nucleotide sequence analysis.
Results: Variant SSCP patterns were detected in exons 1, 3, 6, 11, 12 and 17 of the insulin receptor gene. However, sequencing of amplified DNA fragments revealed that none of the variations were mutations. The SSCP variant in exon 1 was caused by a novel intron polymorphism (G-->T at position 13, 5' intron), while the SSCP variant in exon 12 was caused by a novel silent polymorphism Thr789 (ACG-->ACA). Variants in exons 3 and 17 corresponded to known silent polymorphisms: Gln276 (CAA-->CAG) and His1068 (CAC-->CAT), respectively. Another three variants in exons 3, 6 and 11 were also identified as known polymorphisms in the flanking introns. In addition, two alterations, a silent polymorphism Gly8 (GGA-->GGG) and a polymorphism in the 3' flanking intron (T-->G at position 74), were observed in exon 1 of the insulin receptor gene.
Conclusions: Although the prevalence of insulin receptor gene mutations in this Chinese study population might be underestimated because of the sensitivity of SSCP, these results suggest that mutations at the insulin receptor locus are uncommon in subjects with features of hyperinsulinemia, obesity and acanthosis nigricans.