Insulin resistance and exaggerated insulin sensitivity triggered by single-gene mutations in the insulin signaling pathway.

Whereas the genetic basis of insulin sensitivity is determined by variation in multiple genes, mutations of single genes can give rise to profound changes in such sensitivity. Mutations of the insulin receptor gene ()-which trigger type A insulin resistance, Rabson-Mendenhall, or Donohue syndromes-and those of the gene for the p85α regulatory subunit of phosphoinositide 3-kinase (), which give rise to SHORT syndrome, are the most common and second most common causes, respectively, of single-gene insulin resistance. Loss-of-function mutations of the genes for the protein kinase Akt2 () or for TBC1 domain family member 4 () have been identified in families with severe insulin resistance. Gain-of-function mutations of the gene for protein tyrosine phosphatase nonreceptor type 11 (), which negatively regulates insulin receptor signaling, give rise to Noonan syndrome, and some individuals with this syndrome manifest insulin resistance. Gain-of-function mutations of the gene for the p110α catalytic subunit of phosphoinositide 3-kinase () have been identified in individuals with segmental overgrowth or megalencephaly, some of whom also manifest spontaneous hypoglycemia. A gain-of-function mutation of was also found in individuals with recurrent hypoglycemia. Loss-of-function mutations of the gene for phosphatase and tensin homolog (), another negative regulator of insulin signaling, give rise to Cowden syndrome in association with exaggerated metabolic actions of insulin. Clinical manifestations of individuals with such mutations of genes related to insulin signaling thus provide insight into the essential function of such genes in the human body.