MSUD is a deadly disorder that affects the breaks down of three amino acids called Leucine, Isoleucine, and Valine. When they’re not being used to build a protein, these three amino acids can either be recycled or broken down for energy. They are normally broken down by six proteins working together as a complex called BCKD (Branched-Chain Alpha-Ketoacid Dehydrogenase). Sufferers of MSUD have a mutation that renders one of the 6 proteins in the complex deficient. Therefore, they can’t break down aforementioned three amino acids, which end up with dangerously high levels of these amino acids in blood cause the rapid degeneration of brain cells leading to death of the patient.
Defects in any of the six subunits of the BCKD protein complex can cause MSUD. The most common defect is caused by a mutation in a gene on the q arm of chromosome 19 that encodes the alpha subunit of the BCKD complex (BCKDHA).