Uniparental isodisomy as a cause of mitochondrial complex I respiratory chain disorder due to a novel splicing NDUFS4 mutation.

Uniparental disomy (UPD) is an underestimated cause of autosomal recessive disorders. In this study, we aim to raise awareness about the possibility of UPD in mitochondrial disorders - where it is a hardly described event -, by functionally characterizing a novel variant in a structural subunit of complex I (CI) of the mitochondrial oxidative phosphorylation system. Using next-generation sequencing, we identified a new intronic homozygous c.350 + 5G > A variant in the NDUFS4 gene in a one-year-old girl (being alive at the age of 7) belonging to a non-consanguineous family presenting with encephalopathy, psychomotor delay, lactic acidosis and a single CI deficiency, a less severe phenotype than those previously reported in most NDUFS4 patients. One parent lacked the variant, and microsatellite genotyping showed complete paternal uniparental isodisomy of the non-imprinted chromosome 5. We demonstrated in patient's skeletal muscle and fibroblasts splicing abnormalities, low expression of NDUFS4, undetectable NDUFS4 protein, defects in cellular respiration (decreased oxygen consumption and ATP production), and impaired assembly or stability of mitochondrial supercomplexes containing CI. Our findings support that c.350 + 5G > A variant is pathogenic, and reinforce that UPD, although rare, should be considered as a possible cause of mitochondrial diseases in order to provide accurate genetic counselling.