Helicase-inactivating BRIP1 mutation yields Fanconi anemia with microcephaly and other congenital abnormalities.

Fanconi anemia is a genetically and phenotypically heterogeneous disorder characterized by congenital anomalies, bone marrow failure, cancer, and sensitivity of chromosomes to DNA cross-linking agents. One of the 22 genes responsible for Fanconi anemia is BRIP1, in which biallelic truncating mutations lead to Fanconi anemia group J and monoallelic truncating mutations predispose to certain cancers. However, of the more than 1000 reported missense mutations in BRIP1, very few have been functionally characterized. We evaluated the functional consequence of BRIP1 p.R848H (c.2543G > A), which was homozygous in two cousins with low birth weight, microcephaly, upper limb abnormalities, and imperforate anus and for whom chromosome breakage analysis of patient cells revealed increased mitomycin C sensitivity. BRIP1 p.R848H alters a highly conserved residue in the catalytic DNA helicase domain. We show that BRIP1 p.R848H leads to a defect in helicase activity. Heterozygosity at this missense has been reported in multiple cancer patients but, in the absence of functional studies, classified as of unknown significance. Our results support that this mutation is pathogenic for Fanconi anemia in homozygotes and for increased cancer susceptibility in heterozygous carriers.

Mutation of KREMEN1, a modulator of Wnt signaling, is responsible for ectodermal dysplasia including oligodontia in Palestinian families.

Tooth development is controlled by the same processes that regulate formation of other ectodermal structures. Mutations in the genes underlying these processes may cause ectodermal dysplasia, including severe absence of primary or permanent teeth. Four consanguineous Palestinian families presented with oligodontia and hair and skin features of ectodermal dysplasia. Appearance of ectodermal dysplasia was consistent with autosomal recessive inheritance. Exome sequencing followed by genotyping of 56 informative relatives in the 4 families suggests that the phenotype is due to homozygosity for KREMEN1 p.F209S (c.626 T>C) on chromosome 22 at g.29,521,399 (hg19). The variant occurs in the highly conserved extracellular WSC domain of KREMEN1, which is known to be a high affinity receptor of Dickkopf-1, a component of the Dickkopf-Kremen-LRP6 complex, and a potent regulator of Wnt signaling. The Wnt signaling pathway is critical to development of ectodermal structures. Mutations in WNT10A, LRP6, EDA, and other genes in this pathway lead to tooth agenesis with or without other ectodermal anomalies. Our results implicate KREMEN1 for the first time in a human disorder and provide additional details on the role of the Wnt signaling in ectodermal and dental development.

Five novel loci for inherited hearing loss mapped by SNP-based homozygosity profiles in Palestinian families.

In communities with high rates of consanguinity and consequently high prevalence of recessive phenotypes, homozygosity mapping with SNP arrays is an effective approach for gene discovery. In 20 Palestinian kindreds with prelingual nonsyndromic hearing loss, we generated homozygosity profiles reflecting linkage to the phenotype. Family sizes ranged from small nuclear families with two affected children, one unaffected sibling, and parents to multigenerational kindreds with 12 affected relatives. By including unaffected parents and siblings and screening 250 K SNP arrays, even small nuclear families yielded informative profiles. In 14 families, we identified the allele responsible for hearing loss by screening a single candidate gene in the longest homozygous region. Novel alleles included missense, nonsense, and splice site mutations of CDH23, MYO7A, MYO15A, OTOF, PJVK, Pendrin/SLC26A4, TECTA, TMHS, and TMPRSS3, and a large genomic deletion of Otoancorin (OTOA). All point mutations were rare in the Palestinian population (zero carriers in 288 unrelated controls); the carrier frequency of the OTOA genomic deletion was 1%. In six families, we identified five genomic regions likely to harbor novel genes for human hearing loss on chromosomes 1p13.3 (DFNB82), 9p23-p21.2/p13.3-q21.13 (DFNB83), 12q14.3-q21.2 (DFNB84; two families), 14q23.1-q31.1, and 17p12-q11.2 (DFNB85).