A Gain-of-Function Mutation on BCKDK Gene and Its Possible Pathogenic Role in Branched-Chain Amino Acid Metabolism

Abstract

BCKDK is an important key regulator of branched-chain ketoacid dehydrogenase complex activity by phosphorylating and so inactivating branched-chain ketoacid dehydrogenases, the rate-limiting enzyme of the branched-chain amino acid metabolism. We identified, by whole exome-sequencing analysis, the p.His162Gln variant of the BCKDK gene in a neonate, picked up by newborn screening, with a biochemical phenotype of a mild form of maple syrup urine disease (MSUD). The same biochemical and genetic picture was present in the father. Computational analysis of the mutation was performed to better understand its role. Extensive atomistic molecular dynamics simulations showed that the described mutation leads to a conformational change of the BCKDK protein, which reduces the effect of inhibitory binding bound to the protein itself, resulting in its increased activity with subsequent inactivation of BCKDC and increased plasmatic branched-chain amino acid levels. Our study describes the first evidence of the involvement of the BCKDK gene in a mild form of MSUD. Although further data are needed to elucidate the clinical relevance of the phenotype caused by this variant, awareness of this regulatory activation of BCKDK is very important, especially in newborn screening data interpretation.

Keywords: branched-chain amino acid metabolism; branched-chain ketoacid dehydrogenase kinase; genetic analysis; leucinosis; maple syrup urine disease; molecular dynamics simulations; newborn screening; whole-exome sequencing.

Figure 1

BCKDK three-dimensional structure cocrystallized with ADP and the synthetic inhibitor S-α-chloroisocaproate (PDB accession code: 3TZ4). The red ball represents the location of the mutation p.His192Gln.

Figure 2

Schematic representation of the regulation of BCKDC. BCAAs undergo a transamination reaction by BCAAT. The resulting α-ketoacids are then oxidatively decarboxylated by BCKDH. The overall activity of BCKDH is regulated by phosphorylation/dephosphorylation by BCKDK and DCKDP, respectively. On its turn, BDKDK is negatively regulated by the α-ketoacids, leading to an increase in the activation of BCKDH (dephosphorylated state).

Figure 3

(A) Interaction (distance in angstrom) between Gln162 and Asp164 of BCKDK-p.His162Gln after 500 ns of MD simulation. (B) Superimposition of BCKDK-wt (light brown) and BCKDK-p.His162Gln (light blue) after MD simulations. The red arrows represent a deviation higher than 3 Å.

Figure 4

Graphical representation of the trend of BCAAs and alloisoleucine during follow-up and after protein intake liberalization.