REGULATION OF HUMAN INSULIN-RECEPTOR RNA SPLICING IN-VIVO

Alternative splicing involving the inclusion or exclusion of exon 11 i n insulin receptor mRNA results in two isoforms of the alpha subunit. The two subunits display tissue-specific variation in relative abundan ce at both RNA and protein levels and discrete differences in biologic al properties. We have previously reported a small decrease in the rel ative level of RNA molecules lacking exon 11 (Ex 11-) in skeletal musc le of non-insulin-dependent diabetes mellitus (NIDDM) patients. In the present study, we describe a drastically altered ratio in favor of Ex 11- RNA in a NIDDM patient with markedly impaired insulin-mediated gl ucose utilization. The ratio between the splice variants changed from 74% to 48% Ex 11- RNA after initiation of insulin treatment, which con siderably improved his blood glucose concentrations and insulin-stimul ated glucose utilization rate. This shows that splicing can be regulat ed by metabolic and/or hormonal factors in response to changes in the in vivo milieu. No genomic deletion or base substitution in either the coding regions or exon-intron borders was found that explains the alt ered splicing. Heterozygous mutations were excluded in sequences of pu tative importance for splicing outside the analyzed regions as both al leles were expressed and spliced in an identical fashion. Furthermore, these results suggest that this patient fails to regulate alternative splicing of exon 11 in the manner observed in most NIDDM patients and that this defect is associated with the extreme impairment in insulin action.