Colloquium, Faija Akter, UWM Chemistry

Molybdenum cofactor (Moco) deficiency (MoCD) is a rare autosomal recessive disorder characterized by neonatal-onset myoclonic epileptic encephalopathy and dystonia, with cerebral MRI changes resembling hypoxic–ischemic lesions. The molecular basis of the disease is the loss of sulfite oxidase (SOX) activity, one of four Moco-dependent enzymes in humans. Accumulating toxic sulfite causes a secondary increase of metabolites such as S-sulfocysteine (SSC) and thiosulfate, as well as a decrease in cysteine and its oxidized form, cystine. These biochemical derangements trigger neurodegeneration through NMDA receptor-mediated excitotoxicity and ferroptosis resulting from glutathione depletion.

Moco is synthesized by a three-step biosynthetic pathway involving the gene products of MOCS1, MOCS2, MOCS3, and GPHN. Depending on which synthetic step is impaired, MoCD is classified as type A, B, or C. This distinction is clinically relevant because the metabolic block in MoCD type A can be circumvented by administering cPMP, marketed as fosdenopterin (Nulibry), which received FDA approval in 2021. Substitution therapy with cPMP is highly effective in reducing sulfite toxicity and restoring biochemical homeostasis, while clinical outcome critically depends on the degree of brain injury prior to the start of treatment. Notably, recent case studies reveal that brain injury begins prenatally between 22-28 weeks gestation, underscoring the need for prenatal intervention strategies.

In the absence of specific treatments for MoCD type B/C and isolated SOX deficiency, this presentation summarizes recent progress in understanding the underlying metabolic changes in cysteine homeostasis and explores novel therapeutic interventions — including dietary restriction, NMDA receptor antagonists, sulfite scavenging, and ferroptosis inhibition to circumvent these pathological changes.