Mitochondrial complex I deficiency is a frequent biochemical condition, causing about one third of respiratory chain disorders. Partly due to the large number of genes necessary for its assembly and function only a small proportion of complex I deficiencies are yet confirmed at the molecular genetic level. Now, next generation sequencing approaches are applied to close the gap between biochemical definition and molecular diagnosis. Nevertheless such approaches result in a long list of novel rare single nucleotide variants. Identifying the causative mutations still remains challenging. Here we describe the identification and functional confirmation of novel NDUFS1 mutations using a cellular rescue-assay. Patient-derived complex I-defective fibroblast cell lines were transduced with wild type and mutant NDUFS1-cDNA and subsequently analyzed on the functional and protein level. We established the pathogenic nature of identified rare variants in four out of five disease alleles. This approach is a valuable add-on in disease genetics and it allows the analysis of the functional consequences of genetic variants in metabolic disorders.
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