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--a missense mutation (p.R217C) and a nonsense mutation (p.R197X)--in the HCCS gene. HCCS encodes the mitochondrial holocytochrome c-type synthase that functions as heme lyase by covalently adding the prosthetic heme group to both apocytochrome c and c(1). 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 Delta 197-268) 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-truncated Delta 197-268 mutant failed to be sorted to mitochondria. Cytochrome c, the final product of holocytochrome c-type synthase activity, is implicated in both oxidative phosphorylation (OXPHOS) and apoptosis. We hypothesize that the inability of HCCS-deficient cells to undergo cytochrome c-mediated 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.

CASK mutations are frequent in males and cause X-linked nystagmus and variable XLMR phenotypes

“…13,14 cDNA preparation and RT-PCR Reverse transcriptase PCR (RT-PCR) was carried out using standard techniques as previously described. 15,16 …”

Goltz–Gorlin (focal dermal hypoplasia) and the microphthalmia with linear skin defects (MLS) syndrome: no evidence of genetic overlap

“…Analysis of the X chromosome inactivation (XCI) status using the androgen receptor assay revealed severely skewed XCI in all four patients with a pattern ranging from 96:4 to 100:0 (K. Kutsche, unpublished data) that is in line with extensive XCI skewing in patients with MLS and HCCS mutation. 10,19 Ethical approval for this study was obtained and informed consent for the genetic analyses was received from parents or their legal guardians.…”

“…Heterozygous mutations in the HCCS gene (Xp22.2), encoding the mitochondrial holocytochrome c-type synthase are responsible for MLS. 10 In patients with FDH, heterozygous mutations in Here, we present results of PORCN mutation screening in 13 female patients with FDH and four female patients displaying clinical manifestations compatible with the diagnosis of MLS syndrome, who had been previously tested negative for a mutation in HCCS.…”

Genetics of Congenital Diaphragmatic Hernia

“…Additional findings include sclerocornea, occasional cardiovascular malformations and genital anomalies (Wimplinger et al, 2006).…”