Molecular phenotypes in cultured maple syrup urine disease cells. Complete E₁α cDNA sequence and mRNA and subunit contents of the human branched chain α-keto acid dehydrogenase complex

The activity of the branched-chain α-keto acid dehydrogenase complex is deficient in patients with the inherited maple syrup urine disease (MSUD). To elucidate the molecular basis of this metabolic disorder, we have isolated three overlapping cDNA clones encoding the E₁α subunit of the human enzyme complex. The composite human E₁α cDNA consists of 1783 base pairs encoding the entire human E₁α subunit of 400 amino acids with calculated M(r) = 45,552. The human E₁α and the previously isolated human E₂ cDNAs were used as probes in Northern blot analysis with cultured fibroblasts and lymphoblasts from seven unrelated MSUD patients. The results along with those of Western blotting have revealed five distinct molecular phenotypes according to mRNA and protein-subunit contents. These consist of type I, where the levels of E₁α mRNA and E₁α and E₁β subunits are normal in cells, but E₁ activity is deficient; Type II, where the E₁α mRNA is present in normal quantity, whereas the contents of E₁α and E₁β subunits are reduced; Type III, where the level of E₁α mRNA is markedly reduced with a concomitant loss of E₁α and E₁β subunits; Type IV, where the contents of both E₂ mRNA and E₂ subunits are markedly reduced; and Type V, where the E₂ mRNA is normally expressed, but the E₂ subunit is markedly reduced or completely absent. Type V includes thiamin-responsive (WG-34) and certain classical MSUD cells. These molecular phenotypes have demonstrated the complexity of MSUD and identified the affected gene in different patients for further characterization.