Complex I, the largest respiratory chain complex is composed of 45 proteins and is located at the mitochndrial inner membrane. NDUFS6 encodes a 13 kDa subunit of complex I, which is part of the peripheral arm and is localized in the iron-sulfur fraction of the complex. NDUFS6 is important for complex I assembly. Mutations in the NDUFS6 gene were previously identified in only six patients.

We report on three siblings from a consanguineous family with neonatal encephalopathy, and severe lactic acidosis leading to early death. Brain MRI showed mesencephalic, cerebellar, cerebral peduncles, basal ganglia and corpus callosum areas of abnormal signal intensity with associated supratentorial subcortical and deep white matter edema together with involvement of the subcortical U fibers and lactate elevation. Whole exome sequencing showed homozygous novel pathogenic mutation in the NDUFS6 gene (c.80GA; p.Cys27Tyr).

Our study further emphasized that NDUFS6 gene should be analyzed in patients presenting with lethal neonatal lactic acidemia due to isolated complex I deficiency.