Maple syrup urine disease: It has come a long way

Maple syrup urine disease (MSUD) was first described in 1954 by Menkes et al. as a progressive neurologic degenerative disorder. In 1960, Dancis et al. established that the metabolic block in MSUD is at the decarboxylation of branched-chain α-ketoacids derived from leucine, isoleucine, and valine. The multienzyme complex affected in MSUD, the mitrochondrial branched-chain α- ketoacid (BCKD) dehydrogenase complex was purified in 1978 to homogeneity in Reed's laboratory. This led to the later cloning of cDNAs and genes for subunits of the human BCKD complex. Genetic heterogeneity in MSUD is now explained by the various mutations that occur in the E1α, E1β, E2, and E3 loci of the BCKD complex. Recently, we found that bacterial chaperonins GroEL and GroES promote folding and assembly of E1 decarboxylase component of the BCKD complex in Escherichia coli. Pulse-chase labeling in this system showed that a subset of E1α mutations notably the homozygous Y393N-α in Mennonite MSUD patients, impedes the assembly of the mutant E1α subunit with normal E1β. The assembly defect is associated with a rapid degradation of the normal E1β subunit in MSUD cells. Retrovirus-mediated transduction of lymphoblasts from a Mennonite MSUD patient with a normal E1α cDNA resulted in a complete restoration of BCKD activity. This was accompanied by a stabilization of the normal E1β subunit through assembly with recombinant E1α. The results demonstrated the feasibility of stable correction of E1α- deficient (type 1A) MSUD and provided a basis for the development of gene therapy.