Molybdenum cofactor deficiencies (MoCD) are a group of inborn errors of metabolism that result in impaired synthesis of molybdenum cofactor, crucial for the function of three oxidases (sulfite oxidase, xanthine oxidase and aldehyde oxidase). Most patients present with severe neonatal-onset epileptic encephalopathy, hypotonia, poor feeding and apnoea, with death typically occurring within the first three years of life. Whilst there is now an emerging therapy for MoCD Type A (cPMP/fosdenopterin), this treatment is not effective for MoCD Type B and there is no treatment for isolated sulfite oxidase deficiency (ISOD). Liver directed gene delivery is a potential alternative therapy for sulfite intoxication disorders. We report an attempt to use hepatocyte transplantation as a treatment option for MoCD Type B, in an infant with a strong family history of neonatal-onset disease and early mortality. Six transfusions of hepatocytes were given between Day 1 and Day 18 of life, totalling around 1 × 109 cells with immunosuppressive cover. Concomitantly dietary protein restriction was maintained at 2 g/kg, including 0.7 g/kg of methionine- and cyst(e)ine-free amino acid mixture. The aim was to utilize hepatocyte transplantation as a bridge to liver transplantation. Whilst there was evidence of biochemical stabilization with reduction in concentrations of sulfite and S-sulfocysteine and a moderate increase in urate levels compared to the sibling, the treatment was not able to prevent acute brain injury from sulfite toxicity which was evident in neuroimaging at 35 h of age. This correlated clinically with ongoing seizures as well as minimal developmental progress.

Sulfite oxidase (SO) deficiency, an inherited disease that causes severe neonatal neurological problems and early death, arises from defects in the biosynthesis of the molybdenum cofactor (Moco) (general sulfite oxidase deficiency) or from inborn errors in the SUOX gene for SO (isolated sulfite oxidase deficiency, ISOD). The X-ray structure of the highly homologous homonuclear dimeric chicken sulfite oxidase (cSO) provides a template for locating ISOD mutation sites in human sulfite oxidase (hSO). Catalysis occurs within an individual subunit of hSO, but mutations that disrupt the hSO dimer are pathological. The catalytic cycle of SO involves five metal oxidation states (MoVI, MoV, MoIV, FeIII, FeII), two intramolecular electron transfer (IET) steps, and couples a two-electron oxygen atom transfer reaction at the Mo center with two one-electron transfers from the integral b-type heme to exogenous cytochrome c, the physiological oxidant. Several ISOD examples are analyzed using steady-state, stopped-flow, and laser flash photolysis kinetics and physical measurements of recombinant variants of hSO and native cSO. In the structure of cSO, Mo…Fe = 32 Å, much too long for efficient IET through the protein. Interdomain motion that brings the Mo and heme centers closer together to facilitate IET is supported indirectly by decreasing the length of the interdomain tether, by changes in the charges of surface residues of the Mo and heme domains, as well as by preliminary molecular dynamics calculations. However, direct dynamic measurements of interdomain motion are in their infancy.

Molybdenum cofactor deficiency (MoCD) includes three ultrarare autosomal recessive inborn errors of metabolism (MoCD type A [MoCD-A], MoCD-B, and MoCD-C) that cause sulfite intoxication disorders. This natural history study analyzed retrospective data for 58 living or deceased patients (MoCD-A, n = 41; MoCD-B, n = 17). MoCD genotype, survival, neuroimaging, and medical history were assessed retrospectively. Prospective biomarker data were collected for 21 living MoCD patients. The primary endpoint was survival to 1 year of age in MoCD-A patients. Of the 58 MoCD patients, 49 (MoCD-A, n = 36; MoCD-B, n = 13) had first presenting symptoms by Day 28 (neonatal onset; median: 2 and 4 days, respectively). One-year survival rates were 77.4% (overall), 71.8% (neonatal onset MoCD-A), and 76.9% (neonatal onset MoCD-B); median ages at death were 2.4, 2.4, and 2.2 years, respectively. The most common presenting symptoms in the overall population were seizures (60.3%) and feeding difficulties (53.4%). Sequelae included profound developmental delay, truncal hypotonia, limb hypertonia that evolved to spastic quadriplegia or diplegia, dysmorphic features, and acquired microcephaly. In MoCD-A and MoCD-B, plasma and urinary xanthine and S-sulfocysteine concentrations were high; urate remained below the normal reference range. MOCS1 mutation homozygosity was common. Six novel mutations were identified. MoCD is a severe neurodegenerative disorder that often manifests during the neonatal period with intractable seizures and feeding difficulties, with rapidly progressive significant neurologic disabilities and high 1-year mortality rates. Delineation of MoCD natural history supports evaluations of emerging replacement therapy with cPMP for MoCD-A, which may modify disease course for affected individuals.