2 ABERRANT SPLICINGS CAUSED BY MUTATIONS IN THE INSULIN-RECEPTOR GENEIN CULTURED LYMPHOCYTES FROM A PATIENT WITH RABSON-MENDENHALLS-SYNDROME

Rabson-Mendenhall's syndrome is one of the most severe forms of insuli n resistance syndrome. We analyzed an English patient described elsewh ere and found novel mutations in both alleles of the insulin receptor gene. One is a substitution of G for A at the 3' splice acceptor site of intron 4, and the other is an eight-base pair deletion in exon 12. Both decrease mRNA expression is a cis-dominant manner, and are predic ted to produce severely truncated proteins. Surprisingly, nearly norma l insulin receptor levels were expressed in the patient's lymphocytes, although the level of expression assessed by immunoblot was similar t o 10% of the control cells. Insulin binding affinity was markedly redu ced, but insulin-dependent tyrosine kinase activity was present. Analy zing the insulin receptor mRNA of the patient's lymphocytes by reverse transcription PCR, we discovered aberrant slicing caused by activatio n of a cryptic splice site in exon 5, another aberrant splicing, was f ound in both the patient and the mother who had the heterozygotic muta tion, whereas activation of th cryptic splice site occurred almost exc lusively in the patient. Transfectional analysis in COS cells revealed that the mutant receptor produced by cryptic site activation has the same characteristics as those expressed in patient's lymphocytes. We s peculate that this mutant receptor may be involved in the relatively l ong survival of the patient by rescuing otherwise more severe phenotyp es resulting from the complete lack of functional insulin receptors.