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Urea cycle disorders (UCDs), including OTC deficiency (OTCD), are life-threatening diseases with a broad clinical spectrum. Early diagnosis and initiation of treatment based on a newborn screening (NBS) test for OTCD with high specificity and sensitivity may contribute to reduction of the significant complications and high mortality. The efficacy of incorporating orotic acid determination into routine NBS was evaluated. Combined measurement of orotic acid and citrulline in archived dried blood spots from newborns with urea cycle disorders and normal controls was used to develop an algorithm for routine NBS for OTCD in Israel. Clinical information and genetic confirmation results were obtained from the follow-up care providers. About 1147986 newborns underwent routine NBS including orotic acid determination, 25 of whom were ultimately diagnosed with a UCD. Of 11 newborns with OTCD, orotate was elevated in seven but normal in two males with early-onset and two males with late-onset disease. Orotate was also elevated in archived dried blood spots of all seven retrospectively tested historical OTCD patients, only three of whom had originally been identified by NBS with low citrulline and elevated glutamine. Among the other UCDs emerge, three CPS1D cases and additional three retrospective CPS1D cases otherwise reported as a very rare condition. Combined levels of orotic acid and citrulline in routine NBS can enhance the detection of UCD, especially increasing the screening sensitivity for OTCD and differentiate it from CPS1D. Our data and the negligible extra cost for orotic acid determination might contribute to the discussion on screening for proximal UCDs in routine NBS.

Defects of the mitochondrial oxidative phosphorylation (OXPHOS) system are frequent causes of neurological disorders in children. Linkage analysis and DNA sequencing identified a new founder p.G250V substitution in the C20ORF7 complex I chaperone in five Ashkenazi Jewish patients from two families with a combined OXPHOS complex I and IV defect presenting with Leigh's syndrome in infancy. Complementation with the wild type gene restored complex I, but only partially complex IV activity. Although the pathogenic mechanism remains elusive, a C20ORF7 defect should be considered not only in isolated complex I deficiency, but also in combination with decreased complex IV. Given the significant 1:290 carrier rate for the p.G250V mutation among Ashkenazi Jews, this mutation should be screened in all Ashkenazi patients with Leigh's syndrome prior to muscle biopsy.