No abstract

Autosomal recessive cutis laxa (ARCL) describes a group of syndromal disorders that are often associated with a progeroid appearance, lax and wrinkled skin, osteopenia and mental retardation. Homozygosity mapping in several kindreds with ARCL identified a candidate region on chromosome 17q25. By high-throughput sequencing of the entire candidate region, we detected disease-causing mutations in the gene PYCR1. We found that the gene product, an enzyme involved in proline metabolism, localizes to mitochondria. Altered mitochondrial morphology, membrane potential and increased apoptosis rate upon oxidative stress were evident in fibroblasts from affected individuals. Knockdown of the orthologous genes in Xenopus and zebrafish led to epidermal hypoplasia and blistering that was accompanied by a massive increase of apoptosis. Our findings link mutations in PYCR1 to altered mitochondrial function and progeroid changes in connective tissues.

The filamins are cytoplasmic proteins that regulate the structure and activity of the cytoskeleton by cross-linking actin into three-dimensional networks, linking the cell membrane to the cytoskeleton and serving as scaffolds on which intracellular signaling and protein trafficking pathways are organized (reviewed in refs. 1,2). We identified mutations in the gene encoding filamin B in four human skeletal disorders. We found homozygosity or compound heterozygosity with respect to stop-codon mutations in autosomal recessive spondylocarpotarsal syndrome (SCT, OMIM 272460) and missense mutations in individuals with autosomal dominant Larsen syndrome (OMIM 150250) and the perinatal lethal atelosteogenesis I and III phenotypes (AOI, OMIM 108720; AOIII, OMIM 108721). We found that filamin B is expressed in human growth plate chondrocytes and in the developing vertebral bodies in the mouse. These data indicate an unexpected role in vertebral segmentation, joint formation and endochondral ossification for this ubiquitously expressed cytoskeletal protein.Morphogenesis in vertebrate organisms requires the integration of extracellular signals with alterations in the cellular cytoskeleton. Filamins regulate the organization of cytoskeletal F-actin into either parallel bundles or orthogonal gel networks 3 and also mediate interactions between subcortical actin networks and transmembrane receptors to modulate cell-cell, cell-matrix and intracytoplasmic signal transduction 1,2,4 . Mammals have three filamin genes, FLNA, FLNB and FLNC. FLNA and FLNB seem to be ubiquitously expressed 5,6 ; FLNC is predominantly expressed in muscle. Human filamin genes are highly similar with conserved exon-intron structure, and there is ∼70% homology at the protein level 2,7 . The filamin monomer comprises an N-terminal actin binding domain (ABD) followed by a series of 24 β-sheet repeats that collectively bind many cytoplasmic and transmembrane proteins 1,2 . Filamins exist in vivo as dimers. Dimerization, leading to homo-and possibly heterodimer formation, is mediated by interactions between C-terminal sequences 5,8,9 . Mutations in FLNA produce a spectrum of X-linked malformation and osteochondrodysplasia syndromes. FLNA loss-of-function mutations are usually embryonically lethal in males and underlie a neuronal migration disorder in females 10 . Mutations producing structural changes in the protein lead to numerous developmental anomalies in the brain, skeleton and viscera 11 .Recently the gene associated with SCT, an autosomal recessive disorder characterized by short stature and vertebral, carpal and tarsal fusions 12,13 , was localized on chromosome 3p14 (ref. 14). These studies and further recombination mapping (data not shown) identified a 4.7-cM candidate region, which included a 1.4-Mb region of homozygosity containing 14 genes. Mutations were not found in the candidate genes WNT5A 14 , ASB14 and IL17RD (also known as SEF) in affected individuals from the linked families. The gene FLNB localizes to this interval and, considering the r...

Mandibulofacial dysostosis with microcephaly (MFDM) is a rare sporadic syndrome comprising craniofacial malformations, microcephaly, developmental delay, and a recognizable dysmorphic appearance. Major sequelae, including choanal atresia, sensorineural hearing loss, and cleft palate, each occur in a significant proportion of affected individuals. We present detailed clinical findings in 12 unrelated individuals with MFDM; these 12 individuals compose the largest reported cohort to date. To define the etiology of MFDM, we employed whole-exome sequencing of four unrelated affected individuals and identified heterozygous mutations or deletions of EFTUD2 in all four. Validation studies of eight additional individuals with MFDM demonstrated causative EFTUD2 mutations in all affected individuals tested. A range of EFTUD2-mutation types, including null alleles and frameshifts, is seen in MFDM, consistent with haploinsufficiency; segregation is de novo in all cases assessed to date. U5-116kD, the protein encoded by EFTUD2, is a highly conserved spliceosomal GTPase with a central regulatory role in catalytic splicing and post-splicing-complex disassembly. MFDM is the first multiple-malformation syndrome attributed to a defect of the major spliceosome. Our findings significantly extend the range of reported spliceosomal phenotypes in humans and pave the way for further investigation in related conditions such as Treacher Collins syndrome.

Genetic mutations responsible for oblique facial clefts (ObFC), a unique class of facial malformations, are largely unknown. We show that loss-of-function mutations in SPECC1L are pathogenic for this human developmental disorder and that SPECC1L is a critical organizer of vertebrate facial morphogenesis. During murine embryogenesis, Specc1l is expressed in cell populations of the developing facial primordial, which proliferate and fuse to form the face. In zebrafish, knockdown of a SPECC1L homolog produces a faceless phenotype with loss of jaw and facial structures, and knockdown in Drosophila phenocopies mutants in the integrin signaling pathway that exhibit cell-migration and -adhesion defects. Furthermore, in mammalian cells, SPECC1L colocalizes with both tubulin and actin, and its deficiency results in defective actin-cytoskeleton reorganization, as well as abnormal cell adhesion and migration. Collectively, these data demonstrate that SPECC1L functions in actin-cytoskeleton reorganization and is required for proper facial morphogenesis.