Mutations in a novel retina-specific gene cause autosomal dominant retinitis pigmentosa.

Inherited retinal diseases are a common cause of visual impairment in children and young adults, often resulting in severe loss of vision in later life. The most frequent form of inherited retinopathy is retinitis pigmentosa (RP), with an approximate incidence of 1 in 3,500 individuals worldwide. RP is characterized by night blindness and progressive degeneration of the midperipheral retina, accompanied by bone spicule-like pigmentary deposits and a reduced or absent electroretinogram (ERG). The disease process culminates in severe reduction of visual fields or blindness. RP is genetically heterogeneous, with autosomal dominant, autosomal recessive and X-linked forms. Here we have identified two mutations in a novel retina-specific gene from chromosome 8q that cause the RP1 form of autosomal dominant RP in three unrelated families. The protein encoded by this gene is 2,156 amino acids and its function is currently unknown, although the amino terminus has similarity to that of the doublecortin protein, whose gene (DCX) has been implicated in lissencephaly in humans. Two families have a nonsense mutation in codon 677 of this gene (Arg677stop), whereas the third family has a nonsense mutation in codon 679 (Gln679stop). In one family, two individuals homozygous for the mutant gene have more severe retinal disease compared with heterozygotes.     

Mutations in the human delta homologue, DLL3, cause axial skeletal defects in spondylocostal dysostosis

Spondylocostal dysostosis (SD, MIM 277300) is a group of vertebral malsegmentation syndromes with reduced stature resulting from axial skeletal defects. SD is characterized by multiple hemivertebrae, rib fusions and deletions with a non-progressive kyphoscoliosis. Cases may be sporadic or familial, with both autosomal dominant and autosomal recessive modes of inheritance reported. Autosomal recessive SD maps to a 7.8-cM interval on chromosome 19q13.1-q13.3 that is homologous with a mouse region containing a gene encoding the Notch ligand delta-like 3 (Dll3). Dll3 is mutated in the X-ray-induced mouse mutant pudgy (pu), causing a variety of vertebrocostal defects similar to SD phenotypes. Here we have cloned and sequenced human DLL3 to evaluate it as a candidate gene for SD and identified mutations in three autosomal recessive SD families. Two of the mutations predict truncations within conserved extracellular domains. The third is a missense mutation in a highly conserved glycine residue of the fifth epidermal growth factor (EGF) repeat, which has revealed an important functional role for this domain. These represent the first mutations in a human Delta homologue, thus highlighting the critical role of the Notch signalling pathway and its components in patterning the mammalian axial     

A null mutation in the rhodopsin gene causes rod photoreceptor dysfunction and autosomal recessive retinitis pigmentosa.

Mutations within the rhodopsin gene are known to give rise to autosomal dominant retinitis pigmentosa (RP), a common hereditary form of retinal degeneration. We now describe a patient with autosomal recessive RP who is homozygous for a nonsense mutation at codon 249 within exon 4 of the rhodopsin gene. This null mutation, the first gene defect identified in autosomal recessive retinitis pigmentosa, should result in a functionally inactive rhodopsin protein that is missing the sixth and seventh transmembrane domains including the 11-cis-retinal attachment site. We also found a different null mutation carried heterozygously by an unrelated unaffected individual. Heterozygous carriers of either mutation had normal ophthalmologic examinations but their electroretinograms revealed an abnormality in rod photoreceptor function.     

Mutations in a gene encoding an ABC transporter cause pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a heritable disorder characterized by calcification of elastic fibres in skin, arteries and retina that results in dermal lesions with associated laxity and loss of elasticity, arterial insufficiency and retinal haemorrhages leading to macular degeneration. PXE is usually found as a sporadic disorder, but examples of both autosomal recessive and autosomal dominant forms of PXE have been observed. Partial manifestations of the PXE phenotype have also been described in presumed carriers in PXE families. Linkage of both dominant and recessive forms of PXE to a 5-cM domain on chromosome 16p13.1 has been reported (refs 8,9). We have refined this locus to an 820-kb region containing 6 candidate genes. Here we report the exclusion of five of these genes and the identification of the first mutations responsible for the development of PXE in a gene encoding a protein associated with multidrug resistance (ABCC6).     

Mutations in the gene encoding lamin A/C cause autosomal dominant Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by early contractures of elbows and Achilles tendons, slowly progressive muscle wasting and weakness, and a cardiomyopathy with conduction blocks which is life-threatening. Two modes of inheritance exist, X-linked (OMIM 310300) and autosomal dominant (EDMD-AD; OMIM 181350). EDMD-AD is clinically identical to the X-linked forms of the disease. Mutations in EMD, the gene encoding emerin, are responsible for the X-linked form. We have mapped the locus for EDMD-AD to an 8-cM interval on chromosome 1q11-q23 in a large French pedigree, and found that the EMD phenotype in four other small families was potentially linked to this locus. This region contains the lamin A/C gene (LMNA), a candidate gene encoding two proteins of the nuclear lamina, lamins A and C, produced by alternative splicing. We identified four mutations in LMNA that co-segregate with the disease phenotype in the five families: one nonsense mutation and three missense mutations. These results are the first identification of mutations in a component of the nuclear lamina as a cause of inherited muscle disorder. Together with mutations in EMD (refs 5,6), they underscore the potential importance of the nuclear envelope components in the pathogenesis of neuromuscular disorders.     

Mutation of TBCE causes hypoparathyroidism-retardation-dysmorphism and autosomal recessive Kenny-Caffey syndrome

The syndrome of congenital hypoparathyroidism, mental retardation, facial dysmorphism and extreme growth failure (HRD or Sanjad-Sakati syndrome; OMIM 241410) is an autosomal recessive disorder reported almost exclusively in Middle Eastern populations. A similar syndrome with the additional features of osteosclerosis and recurrent bacterial infections has been classified as autosomal recessive Kenny-Caffey syndrome (AR-KCS; OMIM 244460). Both traits have previously been mapped to chromosome 1q43-44 (refs 5,6) and, despite the observed clinical variability, share an ancestral haplotype, suggesting a common founder mutation. We describe refinement of the critical region to an interval of roughly 230 kb and identification of deletion and truncation mutations of TBCE in affected individuals. The gene TBCE encodes one of several chaperone proteins required for the proper folding of alpha-tubulin subunits and the formation of alpha-beta-tubulin heterodimers. Analysis of diseased fibroblasts and lymphoblastoid cells showed lower microtubule density at the microtubule-organizing center (MTOC) and perturbed microtubule polarity in diseased cells. Immunofluorescence and ultrastructural studies showed disturbances in subcellular organelles that require microtubules for membrane trafficking, such as the Golgi and late endosomal compartments. These findings demonstrate that HRD and AR-KCS are chaperone diseases caused by a genetic defect in the tubulin assembly pathway, and establish a potential connection between tubulin physiology and the development of the parathyroid.     

Ganglioside-induced differentiation-associated protein-1 is mutant in Charcot-Marie-Tooth disease type 4A/8q21.

We previously localized and fine-mapped Charcot Marie Tooth 4A (CMT4A), the autosomal recessive, demyelinating peripheral neuropathy, to chromosome 8. Through additional positional cloning, we have identified a good candidate gene, encoding ganglioside-induced differentiation-associated protein-1 (GDAP1). We found three different mutations in four different Tunisian families-two nonsense and one missense mutation. How mutations in GDAP1 lead to CMT4A remains to be understood.     

Dominant and recessive deafness caused by mutations of a novel gene, TMC1, required for cochlear hair-cell function

Positional cloning of hereditary deafness genes is a direct approach to identify molecules and mechanisms underlying auditory function. Here we report a locus for dominant deafness, DFNA36, which maps to human chromosome 9q13-21 in a region overlapping the DFNB7/B11 locus for recessive deafness. We identified eight mutations in a new gene, transmembrane cochlear-expressed gene 1 (TMC1), in a DFNA36 family and eleven DFNB7/B11 families. We detected a 1.6-kb genomic deletion encompassing exon 14 of Tmc1 in the recessive deafness (dn) mouse mutant, which lacks auditory responses and has hair-cell degeneration. TMC1 and TMC2 on chromosome 20p13 are members of a gene family predicted to encode transmembrane proteins. Tmc1 mRNA is expressed in hair cells of the postnatal mouse cochlea and vestibular end organs and is required for normal function of cochlear hair cells.     

Mutations in the human homologue of mouse dl cause autosomal recessive and dominant hypohidrotic ectodermal dysplasia.

X-linked hypohidrotic ectodermal dysplasia results in abnormal morphogenesis of teeth, hair and eccrine sweat glands. The gene (ED1) responsible for the disorder has been identified, as well as the analogous X-linked gene (Ta) in the mouse. Autosomal recessive disorders, phenotypically indistinguishable from the X-linked forms, exist in humans and at two separate loci (crinkled, cr, and downless, dl) in mice. Dominant disorders, possibly allelic to the recessive loci, are seen in both species (ED3, Dlslk). A candidate gene has recently been identified at the dl locus that is mutated in both dl and Dlslk mutant alleles. We isolated and characterized its human DL homologue, and identified mutations in three families displaying recessive inheritance and two with dominant inheritance. The disorder does not map to the candidate gene locus in all autosomal recessive families, implying the existence of at least one additional human locus. The putative protein is predicted to have a single transmembrane domain, and shows similarity to two separate domains of the tumour necrosis factor receptor (TNFR) family.     

Recessive Robinow syndrome, allelic to dominant brachydactyly type B, is caused by mutation of ROR2

The autosomal recessive form of Robinow syndrome (RRS; MIM 268310) is a severe skeletal dysplasia with generalized limb bone shortening, segmental defects of the spine, brachydactyly and a dysmorphic facial appearance. We previously mapped the gene mutated in RRS to chromosome 9q22 (ref. 4), a region that overlaps the locus for autosomal dominant brachydactyly type B (refs 5,6). The recent identification of ROR2, encoding an orphan receptor tyrosine kinase, as the gene mutated in brachydactyly type B (BDB1; ref. 7) and the mesomelic dwarfing in mice homozygous for a lacZ and/or a neo insertion into Ror2 (refs 8,9) made this gene a candidate for RRS. Here we report homozygous missense mutations in both intracellular and extracellular domains of ROR2 in affected individuals from 3 unrelated consanguineous families, and a nonsense mutation that removes the tyrosine kinase domain and all subsequent 3' regions of the gene in 14 patients from 7 families from Oman. The nature of these mutations suggests that RRS is caused by loss of ROR2 activity. The identification of mutations in three distinct domains (containing Frizzled-like, kringle and tyrosine kinase motifs) indicates that these are all essential for ROR2 function.     

The mouse X chromosome is enriched for multicopy testis genes showing postmeiotic expression

According to the prevailing view, mammalian X chromosomes are enriched in spermatogenesis genes expressed before meiosis and deficient in spermatogenesis genes expressed after meiosis. The paucity of postmeiotic genes on the X chromosome has been interpreted as a consequence of meiotic sex chromosome inactivation (MSCI)--the complete silencing of genes on the XY bivalent at meiotic prophase. Recent studies have concluded that MSCI-initiated silencing persists beyond meiosis and that most genes on the X chromosome remain repressed in round spermatids. Here, we report that 33 multicopy gene families, representing approximately 273 mouse X-linked genes, are expressed in the testis and that this expression is predominantly in postmeiotic cells. RNA FISH and microarray analysis show that the maintenance of X chromosome postmeiotic repression is incomplete. Furthermore, X-linked multicopy genes exhibit a similar degree of expression as autosomal genes. Thus, not only is the mouse X chromosome enriched for spermatogenesis genes functioning before meiosis, but in addition, approximately 18% of mouse X-linked genes are expressed in postmeiotic cells.     

EYS, encoding an ortholog of Drosophila spacemaker, is mutated in autosomal recessive retinitis pigmentosa

Using a positional cloning approach supported by comparative genomics, we have identified a previously unreported gene, EYS, at the RP25 locus on chromosome 6q12 commonly mutated in autosomal recessive retinitis pigmentosa. Spanning over 2 Mb, this is the largest eye-specific gene identified so far. EYS is independently disrupted in four other mammalian lineages, including that of rodents, but is well conserved from Drosophila to man and is likely to have a role in the modeling of retinal architecture.     

An abundance of X-linked genes expressed in spermatogonia

Spermatogonia are the self-renewing, mitotic germ cells of the testis from which sperm arise by means of the differentiation pathway known as spermatogenesis. By contrast with hematopoietic and other mammalian stem-cell populations, which have been subjects of intense molecular genetic investigation, spermatogonia have remained largely unexplored at the molecular level. Here we describe a systematic search for genes expressed in mouse spermatogonia, but not in somatic tissues. We identified 25 genes (19 of which are novel) that are expressed in only male germ cells. Of the 25 genes, 3 are Y-linked and 10 are X-linked. If these genes had been distributed randomly in the genome, one would have expected zero to two of the genes to be X-linked. Our findings indicate that the X chromosome has a predominant role in pre-meiotic stages of mammalian spermatogenesis. We hypothesize that the X chromosome acquired this prominent role in male germ-cell development as it evolved from an ordinary, unspecialized autosome.     

A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases

Sialic acid storage diseases (SASD, MIM 269920) are autosomal recessive neurodegenerative disorders that may present as a severe infantile form (ISSD) or a slowly progressive adult form, which is prevalent in Finland (Salla disease). The main symptoms are hypotonia, cerebellar ataxia and mental retardation; visceromegaly and coarse features are also present in infantile cases. Progressive cerebellar atrophy and dysmyelination have been documented by magnetic resonance imaging (ref. 4). Enlarged lysosomes are seen on electron microscopic studies and patients excrete large amounts of free sialic acid in urine. A H+/anionic sugar symporter mechanism for sialic acid and glucuronic acid is impaired in lysosomal membranes from Salla and ISSD patients. The locus for Salla disease was assigned to a region of approximately 200 kb on chromosome 6q14-q15 in a linkage study using Finnish families. Salla disease and ISSD were further shown to be allelic disorders. A physical map with P1 and PAC clones was constructed to cover the 200-kb area flanked by the loci D6S280 and D6S1622, providing the basis for precise physical positioning of the gene. Here we describe a new gene, SLC17A5 (also known as AST), encoding a protein (sialin) with a predicted transport function that belongs to a family of anion/cation symporters (ACS). We found a homozygous SLC17A5 mutation (R39C) in five Finnish patients with Salla disease and six different SLC17A5 mutations in six ISSD patients of different ethnic origins. Our observations suggest that mutations in SLC17A5 are the primary cause of lysosomal sialic acid storage diseases.     

SPG20 is mutated in Troyer syndrome,an hereditary spastic paraplegia

Troyer syndrome (TRS) is an autosomal recessive complicated hereditary spastic paraplegia (HSP) that occurs with high frequency in the Old Order Amish. We report mapping of the TRS locus to chromosome 13q12.3 and identify a frameshift mutation in SPG20, encoding spartin. Comparative sequence analysis indicates that spartin shares similarity with molecules involved in endosomal trafficking and with spastin, a molecule implicated in microtubule interaction that is commonly mutated in HSP.