Rabson-Mendenhall syndrome (RMS) is a genetic disorder characterized by severe insulin resistance and somatic characteristics. Recombinant insulin-like growth factor 1 (r-IGF-1) is used to treat RMS, as the IGF-1 and insulin receptors share homology. However, the effect of r-IGF-1 varies in patients and it is difficult to manage metabolic status appropriately in r-IGF-1 resistant cases. We report a Japanese boy with RMS who showed resistance to r-IGF-1 therapy and a novel mutation in the insulin receptor in the tyrosine kinase domain. Mutations in this region disturb tyrosine kinase catalytic activity in IGF-1 receptors as a result of dominant negative effects. We consider this mutation to be the cause of resistance to r-IGF-1. The patient also exhibited radiographical features of medullary sponge kidney and had severe nephrocalcinosis and hypokalemia, indicating Bartter syndrome. However, analysis revealed no mutations in the responsible genes and the etiology of the renal abnormalities therefore remains unknown.
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