Novel MTND1 mutations cause isolated exercise intolerance, complex I deficiency and increased assembly factor expression

Complex I (CI) is the largest of the five multi-subunit complexes constituting the human oxidative phosphorylation (OXPHOS) system. Seven of its catalytic core subunits are encoded by mitochondrial DNA (ND1-ND6, ND4L), with mutations in all seven having been reported in association with isolated CI deficiency. We investigated two unrelated adult patients presenting with marked exercise intolerance, persistent lactic acidaemia and severe, muscle-restricted isolated CI deficiency associated with subsarcolemmal mitochondrial accumulation. Screening of the mitochondrial genome detected novel mutations in the MTND1 gene, encoding subunit of CI (Patient 1, m.3365T>C predicting p.(Leu20Pro); Patient 2, m.4175G>A predicting p.(Trp290*)) at high levels of mitochondrial DNA heteroplasmy in skeletal muscle. We evaluated the effect of these novel MTND1 mutations on complex assembly showing that CI assembly although markedly reduced, was viable, in the absence of detectable ND1 signal. Real-time PCR and western blotting showed overexpression of different CI assembly factor transcripts and proteins in patient tissue. Together our data indicate that the mechanism underlying the expression of the biochemical defect may involve a compensatory response to the novel MTND1 gene mutations, promoting assembly factor upregulation and stabilisation of respiratory chain supercomplexes, resulting in partial rescue of the clinical phenotype.
Source: Clinical Science - Category: Biomedical Science Authors: Source Type: research