Identification and characterization of a protein containing formin homology (FH1/FH2) domains

A novel member of the Formin/Diaphanous family of proteins was cloned and characterized. A 4kB mRNA is ubiquitously expressed but is found in abundance in the spleen. FHOS (Formin Homologue Overexpressed in Spleen) contains a 3414bp open reading frame and encodes for an approximately 128kDa protein. FHOS has sequence homology to Diaphanous and Formin proteins within the Formin Homology (FH)1 and FH2 domains. FHOS also contains a coiled-coil, a collagen-like domain, two nuclear localization signals, and several potential PKC and PKA phosphorylation sites. FHOS-specific antiserum was generated and used to determine that FHOS is a predominantly cytoplasmic protein and is expressed in a variety of human cell lines. FHOS was mapped to chromosome 16q22 between framework markers WI-5594 and WI-9392.

Clinical evidence for dystrophin dysfunction as a cause of hearing loss in locus DFN4

OBJECTIVE

Locus DFN4 is an X-linked nonsyndromic hearing loss locus originally mapped to Xp21.2. Recently, we have mapped deafness in a second family from Turkey to the same region, refining the location to within the Duchenne muscular dystrophy (DMD) locus. The objective of this study was to characterize the clinical phenotype of the Turkish family with comprehensive audiovestibular testing and high-resolution temporal bone computerized tomography.

METHODS

Fourteen members of a three-generation family were studied in detail including two deaf affected males. Members of the family underwent general physical and otologic examination, vestibular testing, pure-tone audiometry, otoacoustic emissions, and immitance testing. An affected male underwent high-resolution computerized tomography of the temporal bone, electroretinogram (ERG), electromyography, electroneurography, and determination of serum creatinine phosphokinase level.

RESULTS

Affected males were congenitally deaf with normal vestibular function. Carrier females showed a mild sensorineural hearing loss affecting all frequencies and absent otoacoustic emissions. Otoacoustic emissions in a younger, 3-year-old carrier girl were normal. In an affected male, ERG demonstrated subnormal scotopic b-wave typically seen in DMD. Computerized tomography of the temporal bone was normal. With the exception of the ERG finding, there was no clinical or laboratory evidence of DMD or Becker muscular dystrophy (BMD).

CONCLUSION

The abnormal ERG in the Turkish family in conjunction with mapping of the DFN4 locus to within DMD strongly suggests that a defect in dystrophin is responsible for the hearing loss in this family. Patients with DMD and BMD should be screened systematically for sensorineural hearing loss. This family provides additional evidence for the critical role of cytoskeletal proteins in normal hearing.

A gene upregulated in the acoustically damaged chick basilar papilla encodes a novel WD40 repeat protein

The chick WDR1 gene is expressed at higher levels in the chick basilar papilla after acoustic overstimulation. The 3.3-kb WDR1 cDNA encodes a novel 67-kDa protein containing nine WD40 repeats, motifs that mediate protein-protein interactions. The predicted WDR1 protein has high sequence identity to WD40-repeat proteins in budding yeast (Saccharomyces cerevisiae), two slime molds (Dictyostelium discoideum and Physarum polycephalum), and the roundworm (Caenorhabditis elegans). The yeast and P. polycephalum proteins bind actin, suggesting that the novel chick protein may be an actin-binding protein. Sequence database comparisons identified mouse and human cDNAs with high sequence identity to the chick WDR1 cDNA. The mouse Wdr1 and human WDR1 proteins showed 95% sequence identity to each other and 86% identity to the chick WDR1 protein. Northern blot analysis of total RNA from the chick basilar papilla after noise trauma revealed increased levels of a 3.1-kb transcript in the lesioned area. The WDR1 gene was mapped to human chromosome 4, between 22 and 24 cM from the telomere of 4p.

Effectors for the Rho GTPases

The Rho GTPases are simple enzymes with complex roles in regulating cell morphology, gene transcription, cell cycle progression, apoptosis and tumour progression. The picture has been further complicated by the steady rise in the number of known Rho GTPases as well as in the number of known regulators and target proteins of these GTPases. Recent implications of Rho effectors in human disease, however, might give important clues to how specificity is achieved in cell signalling pathways employing Rho GTPases.

Vestibular and hearing loss in genetic and metabolic disorders

Hearing loss affects about 4% of people under 45 years of age and comprises a broad spectrum of clinical presentations (congenital or late-onset, conductive or sensorineural, and syndromic or nonsyndromic). Approximately 30% of genetically determined deafness is reported to occur in syndromic form and 70% in nonsyndromic form. This review highlights recent advances in the molecular and genetic basis of hearing loss, which will help in understanding the biology of normal and abnormal hearing.

Genomics and Hearing Impairment

Hearing impairment is clinically and genetically heterogeneous. There are >400 disorders in which hearing impairment is a characteristic of the syndrome, and family studies demonstrate that there are at least 30 autosomal loci for nonsyndromic hearing impairment. The genes that have been identified encode diaphanous (HDIA1), α-tectorin (TECTA), the transcription factorPOU4F3, connexin 26 (GJB2), and two unconventional myosins (MYO7A and MYO15), and four novel proteins (PDS,COCH, DFNA5, DFNB9). The same clinical phenotype in hearing-impaired individuals, even those within the same family, can result from mutations in different genes. Conversely, mutations in the same gene can result in a variety of clinical phenotypes with different modes of inheritance. For example, mutations in the gene encoding MYO7A cause Usher syndrome type IB, autosomal-recessive nonsyndromic hearing impairment (DFNB2), and autosomal-dominant nonsyndromic hearing impairment (DFNA11). Additionally, the mouse ortholog of theMYO7A gene is the shaker-1 gene. Mouse models such asshaker-1 have facilitated the identification of genes that cause hearing impairment in humans. The availability of high-resolution maps of the human and mouse genomes and new technologies for gene identification are advancing molecular understanding of hearing impairment and the complex mechanisms of the auditory system.

A second family with nonsyndromic sensorineural hearing loss linked to Xp21.2: refinement of the DFN4 locus within DMD

X-linked inherited hearing impairment is a group of heterogeneous disorders accounting for less than 2% of hereditary hearing loss. DFN4, a sex-linked hearing impairment associated with profound sensorineural hearing loss, has been previously mapped to Xp21.2, a region containing the DMD locus. We have identified a family from Turkey with deafness in which the disease maps to and refines the DFN4 locus. In contrast to the previous family, the crossover points are entirely within the DMD locus. Two-point lod score analysis for the markers DXS 997, DXS 1214, and DXS 1219 showed a lod score of 2. 59. 5' and 3' crossovers were between DMD 44 and DXS 1219 and between DXS 1214 and DXS 985, respectively, suggesting that DFN4 is either an allele of DMD or a mutation in a DMD nested gene. The restriction of the DFN4 locus to DMD suggests that dystrophin may play an important role in hearing.

Mutations in a novel cochlear gene cause DFNA9, a human nonsyndromic deafness with vestibular dysfunction

DFNA9 is an autosomal dominant, nonsyndromic, progressive sensorineural hearing loss with vestibular pathology. Here we report three missense mutations in human COCH (previously described as Coch5b2), a novel cochlear gene, in three unrelated kindreds with DFNA9. All three residues mutated in DFNA9 are conserved in mouse and chicken Coch, and are found in a region containing four conserved cysteines with homology to a domain in factor C, a lipopolysaccharide-binding coagulation factor in Limulus polyphemus. COCH message, found at high levels in human cochlear and vestibular organs, occurs in the chicken inner ear in the regions of the auditory and vestibular nerve fibres, the neural and abneural limbs adjacent to the cochlear sensory epithelium and the stroma of the crista ampullaris of the vestibular labyrinth. These areas correspond to human inner ear structures which show histopathological findings of acidophilic ground substance in DFNA9 patients.

A physical map of 30,000 human genes

A map of 30,181 human gene-based markers was assembled and integrated with the current genetic map by radiation hybrid mapping. The new gene map contains nearly twice as many genes as the previous release, includes most genes that encode proteins of known function, and is twofold to threefold more accurate than the previous version. A redesigned, more informative and functional World Wide Web site (www.ncbi.nlm.nih.gov/genemap) provides the mapping information and associated data and annotations. This resource constitutes an important infrastructure and tool for the study of complex genetic traits, the positional cloning of disease genes, the cross-referencing of mammalian genomes, and validated human transcribed sequences for large-scale studies of gene expression.

Evidence for digenic inheritance of nonsyndromic hereditary hearing loss in a Swedish family

We investigated a Swedish family with nonsyndromic progressive bilateral sensorineural hearing loss. Thirteen candidate loci for autosomal dominant nonsyndromic hearing loss were tested for linkage in this family. We found significant LOD scores (>3) for markers at candidate locus DFNA12 (11q22-q24) and suggestive LOD scores (>2) for markers at locus DFNA2 (1p32). Our results for markers on chromosome 11 narrowed down the candidate region for the DFNA12 locus. A detailed analysis of the phenotypes and haplotypes shared by the affected individuals supported the notion that two genes segregated together with hearing impairment in the family. Severely affected family members had haplotypes linked to the disease allele on both chromosomes 1 and 11, whereas individuals with milder hearing loss had haplotypes linked to the disease allele on either chromosome 1 or chromosome 11. These observations suggest an additive effect of two genes, each gene resulting in a mild and sometimes undiagnosed phenotype, but both together resulting in a more severe phenotype.

cyk-1: a C. elegans FH gene required for a late step in embryonic cytokinesis

A maternally expressed Caenorhabditis elegans gene called cyk-1 is required for polar body extrusion during meiosis and for a late step in cytokinesis during embryonic mitosis. Other microfilament- and microtubule-dependent processes appear normal in cyk-1 mutant embryos, indicating that cyk-1 regulates a specific subset of cytoskeletal functions. Because cytokinesis initiates normally and cleavage furrows ingress extensively in cyk-1 mutant embryos, we propose that the wild-type cyk-1 gene is required for a late step in cytokinesis. Cleavage furrows regress after completion of mitosis in cyk-1 mutants, leaving multiple nuclei in a single cell. Positional cloning and sequence analysis of the cyk-1 gene reveal that it encodes an FH protein, a newly defined family of proteins that appear to interact with the cytoskeleton during cytokinesis and in the regulation of cell polarity. Consistent with cyk-1 function being required for a late step in embryonic cytokinesis, we show that the CYK-1 protein co-localizes with actin microfilaments as a ring at the leading edge of the cleavage furrow, but only after extensive furrow ingression. We discuss our findings in the context of other studies suggesting that FH genes in yeast and insects function early in cytokinesis to assemble a cleavage furrow.

A novel type of myosin encoded by the mouse deafness gene shaker-2

The mouse recessive deafness mutation, shaker-2(sh-2), represents a plausible model for an autosomal recessive form of human non-syndromic genetic deafness, DFNB3. Here we report the use of a positional cloning approach to show that the gene mutated in sh-2 mice encodes a novel type of unconventional myosin. A G-to-A transition changing cysteine to tyrosine in the conserved actin binding domain is detected in sh-2 but absent in laboratory strains and wild mice belonging to different mouse subspecies and species. This suggests that the novel myosin gene is a strong candidate for DFNB3.

Mechanosensation: swimming round in circles

Studies of inherited deafness disorders in mice and humans are providing new insights into the basis of hair-cell mechanosensitivity; this enterprise has been joined by large-scale genetic screening in the zebrafish, where a number of intriguing mutants defective in mechanosensation have recently been described.

Mutation in transcription factor POU4F3 associated with inherited progressive hearing loss in humans

The molecular basis for autosomal dominant progressive nonsyndromic hearing loss in an Israeli Jewish family, Family H, has been determined. Linkage analysis placed this deafness locus, DFNA15, on chromosome 5q31. The human homolog of mouse Pou4f3, a member of the POU-domain family of transcription factors whose targeted inactivation causes profound deafness in mice, was physically mapped to the 25-centimorgan DFNA15-linked region. An 8-base pair deletion in the POU homeodomain of human POU4F3 was identified in Family H. A truncated protein presumably impairs high-affinity binding of this transcription factor in a dominant negative fashion, leading to progressive hearing loss.

A human homologue of the Drosophila melanogaster diaphanous gene is disrupted in a patient with premature ovarian failure: evidence for conserved function in oogenesis and implications for human sterility

Premature ovarian failure (POF) is a defect of ovarian development and is characterized by primary or secondary amenorrhea, with elevated levels of serum gonadotropins, or by early menopause. The disorder has been attributed to various causes, including rearrangements of a large "critical region" in the long arm of the X chromosome. Here we report identification, in a family with POF, of a gene that is disrupted by a breakpoint. The gene is the human homologue of the Drosophila melanogaster diaphanous gene; mutated alleles of this gene affect spermatogenesis or oogenesis and lead to sterility. The protein (DIA) encoded by the human gene (DIA) is the first human member of the growing FH1/FH2 protein family. Members of this protein family affect cytokinesis and other actin-mediated morphogenetic processes that are required in early steps of development. We propose that the human DIA gene is one of the genes responsible for POF and that it affects the cell divisions that lead to ovarian follicle formation.

FH proteins as cytoskeletal organizers

Regulation of cell shape is a poorly understood yet central issue in cell biology. Recent experiments indicate that FH proteins link cellular signalling pathways to changes in cell shape. Members of the FH protein family play essential roles in cytokinesis and in driving alterations in cell polarity. This review discusses the structure and function of these proteins and examines the evidence that they interact specifically with Rho GTPases and profilin to organize the actin-based cytoskeleton.

SeqHelp: a program to analyze molecular sequences utilizing common computational resources

Here we describe a tool to analyze molecular sequences utilizing the internet and existing computational resources for molecular biology. The computer program SeqHelp organizes information from database searches, gene structure prediction, and other information to generate multiply aligned, hypertext-linked reports to allow for fast analysis of molecular sequences. The efficient and economical strategy in this program can be employed to study molecular sequences for gene cloning, mutation analysis, and identical sequence search projects.

Connexin-26 mutations in sporadic and inherited sensorineural deafness

BACKGROUND

Hearing impairment affects one infant in 1000 and 4% of people aged younger than 45 years. Congenital deafness is inherited or apparently sporadic. We have shown previously that DFNB1 on chromosome 13 is a major locus for recessive deafness in about 80% of Mediterranean families and that the connexin-26 gene gap junction protein beta2 (GJB2) is mutated in DFNB1 families. We investigated mutations in the GJB2 gene in familial and sporadic cases of deafness.

METHODS

We obtained DNA samples from 82 families from Italy and Spain with recessive non-syndromic deafness and from 54 unrelated participants with apparently sporadic congenital deafness. We analysed the coding region of the GJB2 gene for mutations. We also tested 280 unrelated people from the general populations of Italy and Spain for the frameshift mutation 35delG.

FINDINGS

49% of participants with recessive deafness and 37% of sporadic cases had mutations in the GJB2 gene. The 35delG mutation accounted for 85% of GJB2 mutations, six other mutations accounted for 6% of alleles, and no changes in the coding region of GJB2 were detected in 9% of DFNB1 alleles. The carrier frequency of mutation 35delG among people from the general population was one in 31 (95% CI one in 19 to one in 87).

INTERPRETATION

Mutations in the GJB2 gene are a major cause of inherited and apparently sporadic congenital deafness. Mutation 35delG is the most common mutation for sensorineural deafness. Identification of 35delG and other mutations in the GJB2 gene should facilitate diagnosis and counselling for the most common genetic form of deafness.