Wimplinger Isabella, Morleo Manuela, Rosenberger Georg, Iaconis Daniela, Orth Ulrike, Meinecke Peter, Lerer Israela, Ballabio Andrea, Gal Andreas, Franco Brunella, Kutsche Kerstin
Institut fur Humangenetik, Universitatsklinikum Hamburg-Eppendorf, Butenfeld 42, Hamburg, Germany.
Am J Hum Genet. 2006 Nov;79(5):878-89. doi: 10.1086/508474. Epub 2006 Sep 6.
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.
小眼畸形伴线性皮肤缺损综合征（MLS，或MIDAS）是一种X连锁显性男性致死性疾病，几乎总是与Xp22区域的节段性单体相关。在来自两个家庭的两名患有MLS且核型正常的女性患者中，我们在HCCS基因中鉴定出杂合的新生点突变——一个错义突变（p.R217C）和一个无义突变（p.R197X）。HCCS编码线粒体全细胞色素c型合酶，该酶通过将辅基血红素基团共价添加到脱辅基细胞色素c和c(1)上来发挥血红素裂解酶的作用。我们研究了第三个表现出表型变异性的家庭，其中母亲及其两个女儿携带一个8.6 kb的亚显微缺失，该缺失包含部分HCCS基因。功能分析表明，与野生型HCCS相比，两种突变蛋白（R217C和Delta 197 - 268）均无法补充缺乏HCCS同源物Cyc3p的酿酒酵母突变体。此外，异位表达的HCCS野生型和R217C突变蛋白在CHO - K1细胞中靶向线粒体，而C末端截短的Delta 197 - 268突变体未能分选到线粒体。全细胞色素c型合酶活性的最终产物细胞色素c与氧化磷酸化（OXPHOS）和细胞凋亡均有关。我们推测，缺乏HCCS的细胞无法进行细胞色素c介导的细胞凋亡，可能会使细胞死亡倾向于坏死，从而导致受影响组织的严重恶化。总之，我们认为OXPHOS以及细胞凋亡与坏死之间的平衡受到干扰，以及X染色体失活模式，可能导致了MLS患者中观察到的可变表型。
