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      The Use of High-Density SNP Array to Map Homozygosity in Consanguineous Families to Efficiently Identify Candidate Genes: Application to Woodhouse-Sakati Syndrome


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          Two consanguineous Qatari siblings presented for evaluation: a 17-4/12-year-old male with hypogonadotropic hypogonadism, alopecia, intellectual disability, and microcephaly and his 19-year-old sister with primary amenorrhea, alopecia, and normal cognition. Both required hormone treatment to produce secondary sex characteristics and pubertal development beyond Tanner 1. SNP array analysis of both probands was performed to detect shared regions of homozygosity which may harbor homozygous mutations in a gene causing their common features of abnormal pubertal development, alopecia, and variable cognitive delay. Our patients shared multiple homozygous genomic regions; ten shared regions were >1 Mb in length and constituted 0.99% of the genome. DCAF17, encoding a transmembrane nuclear protein of uncertain function, was the only gene identified in a homozygous region known to cause hypogonadotropic hypogonadism. DCAF17 mutations are associated with Woodhouse-Sakati syndrome, a rare disorder characterized by alopecia, hypogonadotropic hypogonadism, sensorineural hearing loss, diabetes mellitus, and extrapyramidal movements. Sequencing of the coding exons and flanking intronic regions of DCAF17 in the proband revealed homozygosity for a previously described founder mutation (c.436delC). Targeted DCAF17 sequencing of his affected sibling revealed the same homozygous mutation. This family illustrates the utility of SNP array testing in consanguineous families to efficiently and inexpensively identify regions of genomic homozygosity in which genetic candidates for recessive conditions can be identified.

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          The application of next-generation sequencing in the autozygosity mapping of human recessive diseases.

          Autozygosity, or the inheritance of two copies of an ancestral allele, has the potential to not only reveal phenotypes caused by biallelic mutations in autosomal recessive genes, but to also facilitate the mapping of such mutations by flagging the surrounding haplotypes as tractable runs of homozygosity (ROH), a process known as autozygosity mapping. Since SNPs replaced microsatellites as markers for the purpose of genomewide identification of ROH, autozygosity mapping of Mendelian genes has witnessed a significant acceleration. Historically, successful mapping traditionally required favorable family structure that permits the identification of an autozygous interval that is amenable to candidate gene selection and confirmation by Sanger sequencing. This requirement presented a major bottleneck that hindered the utilization of simplex cases and many multiplex families with autosomal recessive phenotypes. However, the advent of next-generation sequencing that enables massively parallel sequencing of DNA has largely bypassed this bottleneck and thus ushered in an era of unprecedented pace of Mendelian disease gene discovery. The ability to identify a single causal mutation among a massive number of variants that are uncovered by next-generation sequencing can be challenging, but applying autozygosity as a filter can greatly enhance the enrichment process and its throughput. This review will discuss the power of combining the best of both techniques in the mapping of recessive disease genes and offer some tips to troubleshoot potential limitations.
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            Mutations in C2orf37, encoding a nucleolar protein, cause hypogonadism, alopecia, diabetes mellitus, mental retardation, and extrapyramidal syndrome.

            Hypogonadism, alopecia, diabetes mellitus, mental retardation, and extrapyramidal syndrome (also referenced as Woodhouse-Sakati syndrome) is a rare autosomal recessive multisystemic disorder. We have identified a founder mutation consisting of a single base-pair deletion in C2orf37 in eight families of Saudi origin. Three other loss-of-function mutations were subsequently discovered in patients of different ethnicities. The gene encodes a nucleolar protein of unknown function, and the cellular phenotype observed in patient lymphoblasts implicates a role for the nucleolus in the pathogenesis of this disease. Our findings expand the list of human disorders linked to the nucleolus and further highlight the developmental and/or maintenance functions of this organelle.
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              A syndrome of hypogonadism, alopecia, diabetes mellitus, mental retardation, deafness, and ECG abnormalities.

              A distinct and previously undescribed syndrome has been observed in six Saudi Arabian patients from two highly inbred families. The parents were normal, indicating an autosomal recessive pattern of inheritance. All the patients have a distinctive facial appearance, hypogonadism, sparse or absent hair, diabetes mellitus, mental retardation, mild deafness, and variable S-T and T wave abnormalities on the electrocardiograph.

                Author and article information

                Case Rep Genet
                Case Rep Genet
                Case Reports in Genetics
                Hindawi Publishing Corporation
                17 November 2015
                : 2015
                1McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA
                2Cytogenomics Laboratory, Johns Hopkins Hospital, Baltimore, MD 21287, USA
                3Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA
                Author notes
                *Molly B. Sheridan: msherid3@ 123456jhmi.edu

                Academic Editor: Mohnish Suri

                Copyright © 2015 Molly B. Sheridan et al.

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

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