Genealogical Rabson-Mendenhall syndrome caused by INSR gene mutation.

Rabson-Mendenhall syndrome (RMS) is a rare autosomal recessive disorder caused by mutations in the insulin receptor gene (INSR), leading to severe insulin resistance. Clinical manifestations of RMS include hypertrichosis and acanthosis nigricans. A 3-yr-old male patient presented with darkened skin on the neck, without any apparent precipitating factors, and did not exhibit symptoms of polyuria or polydipsia. Both the patient and his older sister displayed signs of hypertrichosis and acanthosis nigricans. Laboratory investigations revealed significantly elevated levels of insulin and C-peptide. Genetic testing identified two mutations in the INSR gene: c.3614C>T in exon 20 and c.3670G>A in exon 21, with the latter being a novel mutation previously unreported in RMS. His sister also exhibited similar clinical features and harbored the same mutations. Consequently, both siblings were diagnosed with RMS. The novel mutation c.3670G>A in exon 21, inherited from the father, is likely to impair insulin receptor function by disrupting tyrosine kinase activity, thereby contributing to the pathogenesis of genealogical RMS.