PPM1K defects cause mild maple syrup urine disease: The second case in the literature.

Maple syrup urine disease (MSUD) is an inborn error of metabolism caused by the insufficient catabolism of branched-chain amino acids. BCKDHA, BCKDHB, DBT, and DLD encode the subunits of the branched-chain α-ketoacid dehydrogenase complex, which is responsible for the catabolism of these amino acids. Biallelic pathogenic variants in BCKDHA, BCKDHB, or DBT are characteristic of MSUD. In addition, a patient with a PPM1K defect was previously reported. PPM1K dephosphorylates and activates the enzyme complex. We report a patient with MSUD with mild findings and elevated BCAA levels carrying a novel homozygous start-loss variant in PPM1K. Our study offers further evidence that PPM1K variants cause mild MSUD.