Whole exome sequencing identified a homozygous novel mutation in SUOX gene causes extremely rare autosomal recessive isolated sulfite oxidase deficiency

Background: Isolated sulfite oxidase deficiency (ISOD) is a rare type of life-threatening neurometabolic disorders characterized by neonatal intractable seizures and severe developmental delay with an autosomal recessive mode of inheritance. Germline mutation in SUOX gene causes ISOD. Till date, only 32 mutations of SUOX gene have been identified and reported to be associated with ISOD. Methods: Here, we investigated a 5-days old Chinese female child, presented with intermittent tremor or seizures of limbs, neonatal encephalopathy, subarachnoid cyst and haemorrhage, dysplasia of corpus callosum, neonatal convulsion, hyperlactatemia, severe metabolic acidosis, hyperglycemia, and hyperkalemia. Results: Whole exome sequencing identified a novel homozygous transition (c.1227G > A) in exon 6 of the SUOX gene in the proband. This novel homozygous variant leads to the formation of a truncated sulfite oxidase (p. Trp409*) of 408 amino acids. This variant causes partial loss of the dimerization domain of sulfite oxidase. Hence, it is a loss-of-function variant. Proband's father and mother is carrying this novel variant in a heterozygous state. This variant was not found in 200 ethnically matched normal healthy control individuals. Conclusions: Our study not only expanded the mutational spectrum of SUOX gene associated with ISOD, but also strongly suggested the significance of whole exome sequencing for identifying candidate genes and novel diseasecausing variants.