Mutations of the mitochondrial holocytochrome c type synthase in X-linked dominant microphthalmia with linear skin defects syndrome

Abstract

The microphthalmia with linear skin defects syndrome (MLS, or MIDAS) is an X-linked dominant male-lethal disorder almost invariably associated with segmental monosomy of the Xp22 region. In two female patients, from two families, with MLS and a normal karyotype, we identified heterozygous de novo point mutations\u97a missense mutation (p.R217C) and a nonsense mutation (p.R197X)\u97in the HCCS gene. HCCS encodes the mitochondrial holocytochrome c\u96type synthase that functions as heme lyase by covalently adding the prosthetic heme group to both apocytochrome c and c1. We investigated a third family, displaying phenotypic variability, in which the mother and two of her daughters carry an 8.6-kb submicroscopic deletion encompassing part of the HCCS gene. Functional analysis demonstrates that both mutant proteins (R217C and 197\u96268) were unable to complement a Saccharomyces cerevisiae mutant deficient for the HCCS orthologue Cyc3p, in contrast to wild-type HCCS. Moreover, ectopically expressed HCCS wild-type and the R217C mutant protein are targeted to mitochondria in CHO-K1 cells, whereas the C-terminal\u96truncated 197\u96268 mutant failed to be sorted to mitochondria. Cytochrome c, the final product of holocytochrome c\u96type synthase activity, is implicated in both oxidative phosphorylation (OXPHOS) and apoptosis. We hypothesize that the inability of HCCS-deficient cells to undergo cytochrome c\u96mediated apoptosis may push cell death toward necrosis that gives rise to severe deterioration of the affected tissues. In summary, we suggest that disturbance of both OXPHOS and the balance between apoptosis and necrosis, as well as the X-inactivation pattern, may contribute to the variable phenotype observed in patients with MLS