A nonsense mutation in the NDUFS4 gene encoding the 18 kDa (AQDQ) subunit of complex I abolishes assembly and activity of the complex in a patient with Leigh-like syndrome

Sequence analysis of mitochondrial and nuclear candidate genes of complex I in children with deficiency of this complex and exhibiting Leigh-like synd rome has revealed, in one of them, a novel mutation in the NDUFS4 gene enco ding the 18 kDa subunit. Phosphorylation of this subunit by cAMP-dependent protein kinase has previously been found to activate the complex. The prese nt mutation consists of a homozygous G-->A transition at nucleotide positio n +44 of the coding sequence of the gene, resulting in the change of a tryp tophan codon to a stop codon. Such mutation causes premature termination of the protein after only 14 amino acids of the putative mitochondrial target ing peptide. Fibroblast cultures from the patient exhibited severe reductio n of the rotenone-sensitive NADH-->UQ oxidoreductase activity of complex I, which was insensitive to cAMP stimulation. Two-dimensional electrophoresis showed the absence of detectable normally assembled complex I in the inner mitochondrial membrane. These findings show that the expression of the NDU FS4 gene is essential for the assembly of a functional complex I.