Genetics of MSUD
Human branched chain alpha-ketoacid dehydrogenase (BCKDH) is a protein complex found in the mitochondria of all cells. The BCKDH consists of three catalytic subunits: a branched-chain alpha-keto acid decarboxylase (E1), a dihydrolipoyl transacylase (E2), and dihydrolipoamide dehydrogenase (E3). The activity of the complex is regulated by specific kinase/phosphatase. To date BCKDH has been purified from several tissues and species including humans. The complex is highly conserved in the size of the proteins and their amino acid composition.
E1 decarboxylase is composed of 2 subunits, the larger subunit binding thiamine pyrophosphate (TPP) in formation of the binding site for the keto acid substrate. The role of E2 subunits is to form the structural core of the enzyme to which other subunits are attached through noncovalent interactions. The E2 subunit has three domain structures: lipoyl-bearing, E3-binding, and "inner core" domain. The E3 protein is also found in pyruvate and alpha-ketoglutarate dehydrogenase complexes.
Each subunit of BCKDH is encoded by a different gene (Table 1). Transcripts from these genes must be correctly spliced to form BCKDH preproteins, which are then imported in mitochondria and processed before assembly into an active complex.
Table 1. Genes of BCKDH
| Gene Product | Locus | Size (kb) | Introns | Exons |
| E1a | 19q13.1 | 55 | 8 | 9 |
| E1b | 6p21-22 | 100 | 9 | 10 |
| E2 | 1p21-31 | 68 | 10 | 11 |
| E3 | 7q31-32 | - | - | - |
Impaired BCKDH activity leads to Maple Syrup Urine Disease (MSUD). Defects in the kinase or phosphatase proteins also could result in varied expression of BCKDH activity and present as a mild form of MSUD. In certain cases it has been possible to establish links between genetic defects and the biochemistry of the disesase. In a patient with a classical type of MSUD caused by a deficiency of BCKDH-E2 subunit was found to have a 78-bp deletion in the mRNA of the BCKDH-E2 subunit. The sequencing of the cloned DNA of the patient revealed abnormal splicing of the mRNA. The amino acid sequence from the mutated cDNA revealed that the mutant E2 peptide lacks 26 amino acids in the inner core domain. This region in the inner E2 core of BCKDH is possibly involved in maintaining the normal functions of the subunit protein. Subsequently a mutant subunit of E2 is synthesized and incorporated into mitochondria together with the normal E1 and E3 subunits of BCKDH.