Disrupted valine metabolism and SCPC-mediated neurological toxicity in ECHS1 deficiency

ECHS1 Deficiency (ECHS1D) is a rare neurometabolic disorder caused by loss of function in the mitochondrial enzyme short-chain enoyl-CoA hydratase (ECHS1), leading to developmental delay, dystonia, seizures, and early mortality. Impaired ECHS1 activity disrupts valine catabolism, resulting in accumulation of reactive valine-derived intermediates associated with disease severity, though direct experimental evidence of neuronal toxicity and therapeutic modifiability remains limited. This study aimed to define the consequences of impaired ECHS1 activity, focusing on valine-derived intermediates and their impact on neuronal mitochondrial function.

Wild-type and Echs1-deficient (Echs1D) mouse neuroblastoma (N2A) cells were utilized to investigate effects of patient-relevant valine intermediates S-(2-carboxypropyl)cysteine (SCPC), S-(2-carboxyethyl)cysteine (SCEC), and 2,3-dihydroxy-2-methylbutyric (DHMB) on mitochondrial function, oxidative stress, and cell viability. ECHS1D mice were used to quantify valine intermediates, evaluate pyruvate dehydrogenase complex (PDC) function, and assess seizure susceptibility under typical, excess, or restricted dietary valine intake.

In wild-type N2A cells, SCPC exposure induced dose-dependent cell toxicity and impaired mitochondrial respiration, while SCEC and DHMB exerted distinct mitochondrial effects. In contrast, Echs1D N2A cells displayed preserved mitochondrial fitness with premature neuronal differentiation, indicating cell-type-specific metabolic adaptations. ECHS1D mice accumulated SCPC, which was exacerbated by excess dietary valine and associated with reduced pyruvate dehydrogenase complex activity, and worsened seizure susceptibility. Valine restriction initiated during adolescence failed to reduce SCPC or improve seizure outcomes and instead increased SCEC levels.

These studies demonstrate that accumulation of ECHS1D-specific valine intermediates, particularly SCPC, contributes to mitochondrial dysfunction and disease severity in ECHS1D. Dietary valine modulation during adolescence showed limited therapeutic benefit and induced broader metabolic perturbations, underscoring the need for improved therapeutic strategies.