Abstract
Abstract Defects in NADH:ubiquinone oxidoreductase (complex I), the largest complex of the mitochondrial respiratory chain, account for most cases of respiratory chain deficiency in human. Complex I contains at least 45 subunits, 7 of which are encoded by mitochondrial DNA (mtDNA). Here we report a novel 10197G>A mutation of the ND3 gene in three unrelated families with Leigh syndrome (LS) or dystonia. Variable degrees of heteroplasmy were found in all tissues tested and a high percentage of mutant mtDNA was observed in muscle. The 10197G>A mutation modifies a hydrophobic alanine residue into a hydrophilic threonine (A47T) in a highly conserved domain of ND3 subunit. Furthermore, this defect could be transferred along with the mutant mtDNAs to ρ° lymphoblastoid cells in cybrid experiments. However, nuclear modifier genes may also play a role in the phenotypic expression and severity of the 10197G>A mutation. The association of the 10197G>A ND3 mutation with an isolated biochemical defect involving complex I and the discovery of the 10197G>A mutation with a similar phenotype in three unrelated families establish its pathogenicity and demonstrate that the amino acid position A47 is important for the function of complex I. These results show that the 10197G>A mutation in the mitochondrial ND3 gene should be considered as a common mtDNA mutation responsible for LS and dystonia. © 2006 Wiley‐Liss, Inc.