Treating a Disorder Caused by an Overactive Enzyme: BCKD-Kinase Deficiency.

Branched-chain ketoacid dehydrogenase kinase (BCKDK) deficiency is a rare autosomal recessive disorder. Loss of this kinase leaves the branched-chain α-ketoacid dehydrogenase complex constitutively active, leading to depletion of branched-chain amino acids (BCAAs) rather than their accumulation. To date, 31 patients from 20 families have been reported. In the largest systematic series, global developmental delay and intellectual disability are universal, autism spectrum disorder occurs in 71%, epilepsy in 43%, and progressive postnatal microcephaly. Mean age at diagnosis is close to 6 years, partly because the clinical picture resembles many forms of idiopathic neurodevelopmental disability. Treatment with a high-protein diet (≥ 2 g/kg/day) and BCAA supplementation (100-250 mg/kg/day) can normalize plasma BCAA levels and appears to stabilize motor function and head circumference. In the limited data available, three patients who started treatment before age 2 did not develop autism, and the earliest-treated patient (8 months) was developing normally at 3 years of follow-up. However, current therapy has clear limitations. Tracer studies in Bckdk knockout mice show reduced incorporation of BCAA-derived nitrogen into brain glutamate even with dietary intervention, pointing to a gap between peripheral biochemical correction and what the brain actually receives. Emerging strategies include partial pharmacological suppression of BCKDH activity, stabilization of residual mutant BCKDK protein, and gene therapy. Newborn screening, feasible with existing technology, deserves consideration given the contrast in outcomes between early and late treatment. This review examines what current treatment achieves, where it falls short, and what additional strategies may be needed.