Late onset isolated sulfite oxidase deficiency: a case report

Background: Isolated sulfite oxidase deficiency (ISOD) is an autosomal recessive disorder caused by a deficiency of sulfite oxidase, which is encoded by the sulfite oxidase gene (SUOX). Clinically, the disorder is classified as one of two forms: the late-onset mild form or the classic early-onset form. The latter is life-threatening and always leads to death during early childhood. Mild ISOD cases are rare and may benefit from dietary therapy. Herein, we present a mild ISOD case with a stable clinical course.

Case presentation: This report presents a 4 ½ years old female patient with late onset ISOD manifesting with developmental delay. She had two novel compound heterozygous mutations in the SUOX gene (NM_001032386.2; c. 892C > T [p.L298F] and c.1084G > A [p.G362S]). Her plasma homocysteine level was low. Urine S-sulphocysteine level was high 11.41 mmol/mol kreatinin (nv: 5 mmol/mol kreatinin). Her MRI revealed diffusion restriction in globus pallidus. She exhibited stable course with dietary therapy containing low cystine and methionine.

Conclusion: This is the first report of mild ISOD case with a stable clinical course from Turkey. Our case expand the clinical spectrum of ISOD. Furthermore, we highlight the importance of including ISOD in the differential diagnosis for patients presenting with late-onset symptoms, bilaterally symmetric regions of abnormal intensities in the basal ganglia, and decreased plasma homocysteine levels.