Isolated sulfite oxidase deficiency

Isolated sulfite oxidase deficiency (ISOD) is an extremely rare autosomal recessive metabolic disorder caused by mutations in the SUOX gene, which encodes the enzyme sulfite oxidase. This enzyme is essential for the final step in the degradation of sulfur-containing amino acids (methionine and cysteine), converting toxic sulfite to sulfate. When this enzyme is deficient, sulfite and other toxic metabolites (S-sulfocysteine, thiosulfate, taurine) accumulate in the body, causing severe damage primarily to the central nervous system. The disease predominantly presents in the neonatal period with intractable seizures, feeding difficulties, and progressive encephalopathy. Affected infants typically develop severe intellectual disability, spasticity, microcephaly, and characteristic lens dislocation (ectopia lentis). Brain imaging often reveals extensive white matter changes, cystic encephalomalacia, and cerebral atrophy. The clinical presentation closely resembles molybdenum cofactor deficiency, which also impairs sulfite oxidase activity but additionally affects xanthine dehydrogenase and aldehyde oxidase. The two conditions can be distinguished biochemically, as isolated sulfite oxidase deficiency shows elevated sulfite and S-sulfocysteine but normal uric acid and xanthine levels. A milder, late-onset form has been described in rare cases, presenting with developmental delay and movement abnormalities rather than the devastating neonatal course. Unfortunately, there is currently no effective cure or disease-modifying treatment for isolated sulfite oxidase deficiency. Management is primarily supportive, focusing on seizure control (which is often refractory to standard anticonvulsants), nutritional support, and physical therapy. Dietary restriction of methionine and cysteine has been attempted with limited success. The prognosis for the severe neonatal form is generally poor, with significant morbidity and early mortality.

Common questions about Isolated sulfite oxidase deficiency