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      Familial cases of a submicroscopic Xp22.2 deletion: genotype-phenotype correlation in microphthalmia with linear skin defects syndrome

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      1 , 1 , 2 , 2 , 1 ,
      Molecular Vision
      Molecular Vision

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          Abstract

          Purpose

          Microphthalmia with linear skin defects syndrome (MLS or MIDAS, OMIM #309801) is a rare X-linked male-lethal disorder characterized by microphthalmia or other ocular anomalies and skin lesions limited to the face and neck. However, inter- and intrafamilial variability is high. Here we report a familial case of MLS.

          Methods

          A mother and daughter with MLS underwent a complete ophthalmological examination, and extensive imaging, including anterior segment pictures, corneal topography and keratometry, autofluorescence, infrared reflectance and red free images, as well as spectral-domain optical coherence tomography. The mother also underwent full-field flash electroretinography. In addition, high-resolution array comparative genomic hybridization analysis was performed in both as well as in the maternal grandparents of the proband.

          Results

          Microphthalmia and retinal abnormalities were noted in the proband and the mother, whereas only the mother presented with scars of the typical neonatal linear skin defects. Array comparative genomic hybridization analysis revealed a 185–220 kb deletion on chromosome band Xp22.2 including the entire HCCS gene.

          Conclusions

          The identification of a deletion including HCCS led to the diagnosis of MLS in these patients. Retinal abnormalities can be part of the ocular manifestations of MLS.

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          Most cited references50

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          Challenges for CNV interpretation in clinical molecular karyotyping: lessons learned from a 1001 sample experience.

          Molecular karyotyping has moved from bench to bedside for the genetic screening of patients with mental retardation and/or congenital anomalies. The commercial availability of high-resolution microarray platforms has significantly facilitated this process. However, the notion that copy number variants are also abundantly present in the general population challenges the interpretation of the clinical significance of detected copy number variants (CNVs) in these patients. Moreover, the awareness of incomplete penetrance and variable expression, exemplified by the inheritance of causal CNVs from apparently unaffected parents, has further blurred the boundary between benign and pathogenic variation. We analyzed 1001 patients using a large insert clone array (298 patients) and an oligonucleotide-based (703 patients) platform. In this cohort we encountered several examples of causal imbalances that could have been easily interpreted as benign variants when relying on established paradigms. Based on our experience and the pitfalls we encountered, we suggest a decision tree that can be used as a guideline in clinical diagnostics. Using this workflow, we detected 106 clinically significant CNVs in 100 patients, giving a diagnostic yield of at least 10%. Of these imbalances, 58 occurred de novo, 22 were inherited and 26 of unknown inheritance. This underscores that inherited CNVs should not be automatically disregarded as benign variants. Among the clinically relevant CNVs were 11 single-gene aberrations, highlighting the power of high-resolution molecular karyotyping to identify causal genes.
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            Mutations of the mitochondrial holocytochrome c-type synthase in X-linked dominant microphthalmia with linear skin defects syndrome.

            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.
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              Dosage compensation of the mammalian X chromosome influences the phenotypic variability of X-linked dominant male-lethal disorders.

              In mammals females inactivate one of the two X chromosomes during early development to achieve an equal gene dosage between sexes. This process, named X chromosome inactivation (XCI), usually occurs randomly. However, in a few instances, non-random XCI may take place, thus modulating the phenotype observed in female patients carrying mutations in X-linked genes. Different aspects related to dosage compensation contribute to explain the influences of XCI on the phenotypic variability observed in female patients. The study of two X-linked dominant male-lethal disorders, such as the microphthalmia with linear skin lesions (MLS) syndrome and the oral-facial-digital type I (OFDI) syndrome, offers the opportunity to discuss this intriguing topic. In addition, recent data on the characterisation of a murine model for OFDI provide the opportunity to discuss how differences in the XCI between Homo sapiens and Mus musculus can justify the discrepancies between the phenotypes observed in OFDI patients and the corresponding murine model.
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                Author and article information

                Journal
                Mol Vis
                Mol. Vis
                MV
                Molecular Vision
                Molecular Vision
                1090-0535
                2013
                06 February 2013
                : 19
                : 311-318
                Affiliations
                [1 ]Center for Medical Genetics, Ghent University Hospital & Ghent University, Ghent, Belgium
                [2 ]Department of Ophthalmology, Ghent University Hospital & Ghent University, Ghent, Belgium
                Author notes

                The first two authors contributed equally to this paper.

                Correspondence to: Björn Menten, Center for Medical Genetics Ghent, Ghent University & Ghent University Hospital, De Pintelaan 185, 9000 Ghent, Belgium; Phone: +32 9 332 52 84; FAX: +32 9 332 65 49; email: bjorn.menten@ 123456ugent.be
                Article
                32 2012MOLVIS0211
                3566894
                23401659
                fdef4a3a-f4dc-4a25-8249-1f14018e72c4
                Copyright © 2013 Molecular Vision.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 02 May 2012
                : 04 February 2013
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                Vision sciences

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