Hanan E. Shamseldin

Mutations in lectin complement pathway genes COLEC11 and MASP1 cause 3MC syndrome

3MC syndrome has been proposed as a unifying term encompassing the overlapping Carnevale, Mingarelli, Malpuech and Michels syndromes. These rare autosomal recessive disorders exhibit a spectrum of developmental features, including characteristic facial dysmorphism, cleft lip and/or palate, craniosynostosis, learning disability and genital, limb and vesicorenal anomalies. Here we studied 11 families with 3MC syndrome and identified two mutated genes, COLEC11 and MASP1, both of which encode proteins in the lectin complement pathway (collectin kidney 1 (CL-K1) and MASP-1 and MASP-3, respectively). CL-K1 is highly expressed in embryonic murine craniofacial cartilage, heart, bronchi, kidney and vertebral bodies. Zebrafish morphants for either gene develop pigmentary defects and severe craniofacial abnormalities. Finally, we show that CL-K1 serves as a guidance cue for neural crest cell migration. Together, these findings demonstrate a role for complement pathway factors in fundamental developmental processes and in the etiology of 3MC syndrome.

Mutations in the RNA Granule Component TDRD7 Cause Cataract and Glaucoma

A Tudor domain protein mediates posttranscriptional control of gene expression and is required for eye-lens development. The precise transcriptional regulation of gene expression is essential for vertebrate development, but the role of posttranscriptional regulatory mechanisms is less clear. Cytoplasmic RNA granules (RGs) function in the posttranscriptional control of gene expression, but the extent of RG involvement in organogenesis is unknown. We describe two human cases of pediatric cataract with loss-of-function mutations in TDRD7 and demonstrate that Tdrd7 nullizygosity in mouse causes cataracts, as well as glaucoma and an arrest in spermatogenesis. TDRD7 is a Tudor domain RNA binding protein that is expressed in lens fiber cells in distinct TDRD7-RGs that interact with STAU1-ribonucleoproteins (RNPs). TDRD7 coimmunoprecipitates with specific lens messenger RNAs (mRNAs) and is required for the posttranscriptional control of mRNAs that are critical to normal lens development and to RG function. These findings demonstrate a role for RGs in vertebrate organogenesis.

Recessive Mutations in DOCK6, Encoding the Guanidine Nucleotide Exchange Factor DOCK6, Lead to Abnormal Actin Cytoskeleton Organization and Adams-Oliver Syndrome

Adams-Oliver syndrome (AOS) is defined by the combination of aplasia cutis congenita (ACC) and terminal transverse limb defects (TTLD). It is usually inherited as an autosomal-dominant trait, but autosomal-recessive inheritance has also been documented. In an individual with autosomal-recessive AOS, we combined autozygome analysis with exome sequencing to identify a homozygous truncating mutation in dedicator of cytokinesis 6 gene (DOCK6) which encodes an atypical guanidine exchange factor (GEF) known to activate two members of the Rho GTPase family: Cdc42 and Rac1. Another homozygous truncating mutation was identified upon targeted sequencing of DOCK6 in an unrelated individual with AOS. Consistent with the established role of Cdc42 and Rac1 in the organization of the actin cytoskeleton, we demonstrate a cellular phenotype typical of a defective actin cytoskeleton in patient cells. These findings, combined with a Dock6 expression profile that is consistent with an AOS phenotype as well as the very recent demonstration of dominant mutations of ARHGAP31 in AOS, establish Cdc42 and Rac1 as key molecules in the pathogenesis of AOS and suggest that other regulators of these Rho GTPase proteins might be good candidates in the quest to define the genetic spectrum of this genetically heterogeneous condition.

Mutations in CCNO result in congenital mucociliary clearance disorder with reduced generation of multiple motile cilia

Using a whole-exome sequencing strategy, we identified recessive CCNO (encoding cyclin O) mutations in 16 individuals suffering from chronic destructive lung disease due to insufficient airway clearance. Respiratory epithelial cells showed a marked reduction in the number of multiple motile cilia (MMC) covering the cell surface. The few residual cilia that correctly expressed axonemal motor proteins were motile and did not exhibit obvious beating defects. Careful subcellular analyses as well as in vitro ciliogenesis experiments in CCNO-mutant cells showed defective mother centriole generation and placement. Morpholino-based knockdown of the Xenopus ortholog of CCNO also resulted in reduced MMC and centriole numbers in embryonic epidermal cells. CCNO is expressed in the apical cytoplasm of multiciliated cells and acts downstream of multicilin, which governs the generation of multiciliated cells. To our knowledge, CCNO is the first reported gene linking an inherited human disease to reduced MMC generation due to a defect in centriole amplification and migration.

Genomic analysis of mitochondrial diseases in a consanguineous population reveals novel candidate disease genes

Objective To investigate the utility of autozygome analysis and exome sequencing in a cohort of patients with suspected or confirmed mitochondrial encephalomyopathy. Methods Autozygome was used to highlight candidate genes for direct sequencing in 10 probands, all born to consanguineous parents. Autozygome was also used to filter the variants from exome sequencing of four probands. Results In addition to revealing mutations in known mitochondrial genes, the analysis revealed the identification of two novel candidate disease genes: MFF and FARS2, encoding the mitochondrial fission factor and phenylalanyl-tRNA synthetase, respectively. Interpretation These findings expand the repertoire of genes that are mutated in patients with mitochondrial disorders and highlight the value of integrating genomic approaches in the evaluation of these patients.

Clinical and molecular characterisation of Bardet–Biedl syndrome in consanguineous populations: the power of homozygosity mapping

Bardet–Biedl syndrome (BBS) is a ciliopathy with pleiotropic effect that manifests primarily as renal insufficiency, polydactyly, retinal dystrophy and obesity. The current phenotype–genotype correlation is insufficient to predict the likely causative mutation that makes sequencing of all 14 BBS genes an often necessary but highly complicated way to identify the underlying genetic defect in affected patients. In this study, homozygosity mapping is shown as a robust approach that is highly suited for genetically heterogeneous autosomal recessive disorders in populations in which consanguinity is prevalent. This approach allowed us to quickly identify seven novel mutations in seven families with BBS. Some of these mutations would have been missed by unguided routine sequencing, which suggests that missed mutations in known BBS genes could be more common than previously thought. This study, the largest to date on Saudi BBS families, also revealed interesting phenotypic aspects of BBS, including the first report of non-syndromic retinitis pigmentosa as a novel BBS phenotype.

Identification of a novel DLX5 mutation in a family with autosomal recessive split hand and foot malformation

Background Split hand and foot malformation (SHFM) refers to a genetically heterogeneous developmental disorder of the hands and feet that presents as median ray deficiency of varying severity. 7q21.3 (SHFM1) is one of six loci described to date, and although DLX5 and DLX6 are compelling candidates in that locus, no intragenic mutations have been described in either of these genes. Methods The authors combined autozygome analysis and exome sequencing to study a consanguineous family with a highly unusual SHFM phenotype, where there is associated dorsalisation of the hands. Results A novel missense mutation in a highly conserved residue of the homeobox domain of DLX5 was identified. Unlike previously reported position effect mutations in SHFM1, this first documented intragenic DLX5 mutation is also accompanied by abnormal dorsal-ventral patterning. Conclusion This study identified the first intragenic DLX5 mutation in SHFM and raises interesting possibilities about a dual role for DLX5 in limb development.

Exome sequencing reveals a novel Fanconi group defined by XRCC2 mutation

Background Fanconi anaemia (FA) is a group of disorders characterised by progressive bone marrow failure and a characteristic but variable craniofacial and skeletal involvement. Recessive mutations in any of 15 genes linked to FA lead to the pathognomonic increased susceptibility to double-strand DNA breaks. Methods Autozygome and exome analysis of a patient with classic FA phenotype Results The authors identified a novel truncating mutation in XRCC2. Consistent with the proposed causal link to FA, this gene is an essential non-redundant component of the RAD51 family of homologous repair proteins and its deficiency in a murine model has been shown to lead to a highly similar phenotype to that of this patient both at the cellular and organismal level. Conclusion This study implicates XRCC2 in the pathogenesis of FA and calls for further investigation of the potential contribution of XRCC2 mutations to the overall mutational load of FA.

POC1A Truncation Mutation Causes a Ciliopathy in Humans Characterized by Primordial Dwarfism

Primordial dwarfism (PD) is a phenotype characterized by profound growth retardation that is prenatal in onset. Significant strides have been made in the last few years toward improved understanding of the molecular underpinning of the limited growth that characterizes the embryonic and postnatal development of PD individuals. These include impaired mitotic mechanics, abnormal IGF2 expression, perturbed DNA-damage response, defective spliceosomal machinery, and abnormal replication licensing. In three families affected by a distinct form of PD, we identified a founder truncating mutation in POC1A. This gene is one of two vertebrate paralogs of POC1, which encodes one of the most abundant proteins in the Chlamydomonas centriole proteome. Cells derived from the index individual have abnormal mitotic mechanics with multipolar spindles, in addition to clearly impaired ciliogenesis. siRNA knockdown of POC1A in fibroblast cells recapitulates this ciliogenesis defect. Our findings highlight a human ciliopathy syndrome caused by deficiency of a major centriolar protein.

Mutations in CSPP1, Encoding a Core Centrosomal Protein, Cause a Range of Ciliopathy Phenotypes in Humans

Ciliopathies are characterized by a pattern of multisystem involvement that is consistent with the developmental role of the primary cilium. Within this biological module, mutations in genes that encode components of the cilium and its anchoring structure, the basal body, are the major contributors to both disease causality and modification. However, despite rapid advances in this field, the majority of the genes that drive ciliopathies and the mechanisms that govern the pronounced phenotypic variability of this group of disorders remain poorly understood. Here, we show that mutations in CSPP1, which encodes a core centrosomal protein, are disease causing on the basis of the independent identification of two homozygous truncating mutations in three consanguineous families (one Arab and two Hutterite) affected by variable ciliopathy phenotypes ranging from Joubert syndrome to the more severe Meckel-Gruber syndrome with perinatal lethality and occipital encephalocele. Consistent with the recently described role of CSPP1 in ciliogenesis, we show that mutant fibroblasts from one affected individual have severely impaired ciliogenesis with concomitant defects in sonic hedgehog (SHH) signaling. Our results expand the list of centrosomal proteins implicated in human ciliopathies.