Mutations in TNFRSF11A, affecting the signal peptide of RANK, cause familial expansile osteolysis

Familial expansile osteolysis (FEO, MIM 174810) is a rare, autosomal dominant bone disorder characterized by focal areas of increased bone remodelling. The osteolytic lesions, which develop usually in the long bones during early adulthood, show increased osteoblast and osteoclast activity. Our previous linkage studies mapped the gene responsible for FEO to an interval of less than 5 cM between D18S64 and D18S51 on chromosome 18q21.2-21.3 in a large Northern Irish family. The gene encoding receptor activator of nuclear factor-kappa B (RANK; ref. 5), TNFRSF11A, maps to this region. RANK is essential in osteoclast formation. We identified two heterozygous insertion mutations in exon 1 of TNFRSF11A in affected members of four families with FEO or familial Paget disease of bone (PDB). One was a duplication of 18 bases and the other a duplication of 27 bases, both of which affected the signal peptide region of the RANK molecule. Expression of recombinant forms of the mutant RANK proteins revealed perturbations in expression levels and lack of normal cleavage of the signal peptide. Both mutations caused an increase in RANK-mediated nuclear factor-kappaB (NF-kappaB) signalling in vitro, consistent with the presence of an activating mutation.     

Mutations in a new gene encoding a thiamine transporter cause thiamine-responsive megaloblastic anaemia syndrome.

Thiamine-responsive megaloblastic anaemia syndrome (TRMA; MIM 249270) is an autosomal recessive disorder with features that include megaloblastic anaemia, mild thrombocytopenia and leucopenia, sensorineural deafness and diabetes mellitus. Treatment with pharmacologic doses of thiamine ameliorates the megaloblastic anaemia and diabetes mellitus. A defect in the plasma membrane transport of thiamine has been demonstrated in erythrocytes and cultured skin fibroblasts from TRMA patients. The gene causing TRMA was assigned to 1q23.2-q23.3 by linkage analysis. Here we report the cloning of a new gene, SLC19A2, identified from high-through-put genomic sequences due to homology with SLC19A1, encoding reduced folate carrier 1 (refs 8-10). We cloned the entire coding region by screening a human fetal brain cDNA library. SLC19A2 encodes a protein (of 497 aa) predicted to have 12 transmembrane domains. We identified 2 frameshift mutations in exon 2. a 1-bp insertion and a 2-bp deletion, among four Iranian families with TRMA. The sequence homology and predicted structure of SLC19A2, as well as its role in TRMA, suggest that its gene product is a thiamine carrier, the first to be identified in complex eukaryotes.     

EIF2AK3, encoding translation initiation factor 2-alpha kinase 3, is mutated in patients with Wolcott-Rallison syndrome

Wolcott-Rallison syndrome (WRS) is a rare, autosomal recessive disorder characterized by permanent neonatal or early infancy insulin-dependent diabetes. Epiphyseal dysplasia, osteoporosis and growth retardation occur at a later age. Other frequent multisystemic manifestations include hepatic and renal dysfunction, mental retardation and cardiovascular abnormalities. On the basis of two consanguineous families, we mapped WRS to a region of less than 3 cM on chromosome 2p12, with maximal evidence of linkage and homozygosity at 4 microsatellite markers within an interval of approximately 1 cM. The gene encoding the eukaryotic translation initiation factor 2-alpha kinase 3 (EIF2AK3) resides in this interval; thus we explored it as a candidate. We identified distinct mutations of EIF2AK3 that segregated with the disorder in each of the families. The first mutation produces a truncated protein in which the entire catalytic domain is missing. The other changes an amino acid, located in the catalytic domain of the protein, that is highly conserved among kinases from the same subfamily. Our results provide evidence for the role of EIF2AK3 in WRS. The identification of this gene may provide insight into the understanding of the more common forms of diabetes and other pathologic manifestations of WRS.     

Amnionless,essential for mouse gastrulation,is mutated in recessive hereditary megaloblastic anemia

The amnionless gene, Amn, on mouse chromosome 12 encodes a type I transmembrane protein that is expressed in the extraembryonic visceral layer during gastrulation. Mice homozygous with respect to the amn mutation generated by a transgene insertion have no amnion. The embryos are severely compromised, surviving to the tenth day of gestation but seem to lack the mesodermal layers that normally produce the trunk. The Amn protein has one transmembrane domain separating a larger, N-terminal extracellular region and a smaller, C-terminal cytoplasmic region. The extracellular region harbors a cysteine-rich domain resembling those occurring in Chordin, found in Xenopus laevis embryos, and Sog, found in Drosophila melanogaster. As these cysteine-rich domains bind bone morphogenetic proteins (Bmps), it has been speculated that the cysteine-rich domain in Amn also binds Bmps. We show that homozygous mutations affecting exons 1-4 of human AMN lead to selective malabsorption of vitamin B12 (a phenotype associated with megaloblastic anemia 1, MGA1; OMIM 261100; refs. 5,6) in otherwise normal individuals, suggesting that the 5' end of AMN is dispensable for embryonic development but necessary for absorption of vitamin B12. When the 5' end of AMN is truncated by mutations, translation is initiated from alternative downstream start codons.     

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.     

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.     

Heterozygous missense mutations in BSCL2 are associated with distal hereditary motor neuropathy and Silver syndrome

Distal hereditary motor neuropathy (dHMN) or distal spinal muscular atrophy (OMIM #182960) is a heterogeneous group of disorders characterized by an almost exclusive degeneration of motor nerve fibers, predominantly in the distal part of the limbs. Silver syndrome (OMIM #270685) is a rare form of hereditary spastic paraparesis mapped to chromosome 11q12-q14 (SPG17) in which spasticity of the legs is accompanied by amyotrophy of the hands and occasionally also the lower limbs. Silver syndrome and most forms of dHMN are autosomal dominantly inherited with incomplete penetrance and a broad variability in clinical expression. A genome-wide scan in an Austrian family with dHMN-V (ref. 4) showed linkage to the locus SPG17, which was confirmed in 16 additional families with a phenotype characteristic of dHMN or Silver syndrome. After refining the critical region to 1 Mb, we sequenced the gene Berardinelli-Seip congenital lipodystrophy (BSCL2) and identified two heterozygous missense mutations resulting in the amino acid substitutions N88S and S90L. Null mutations in BSCL2, which encodes the protein seipin, were previously shown to be associated with autosomal recessive Berardinelli-Seip congenital lipodystrophy (OMIM #269700). We show that seipin is an integral membrane protein of the endoplasmic reticulum (ER). The amino acid substitutions N88S and S90L affect glycosylation of seipin and result in aggregate formation leading to neurodegeneration.     

HRPT2, encoding parafibromin,is mutated in hyperparathyroidism-jaw tumor syndrome

We report here the identification of a gene associated with the hyperparathyroidism-jaw tumor (HPT-JT) syndrome. A single locus associated with HPT-JT (HRPT2) was previously mapped to chromosomal region 1q25-q32. We refined this region to a critical interval of 12 cM by genotyping in 26 affected kindreds. Using a positional candidate approach, we identified thirteen different heterozygous, germline, inactivating mutations in a single gene in fourteen families with HPT-JT. The proposed role of HRPT2 as a tumor suppressor was supported by mutation screening in 48 parathyroid adenomas with cystic features, which identified three somatic inactivating mutations, all located in exon 1. None of these mutations were detected in normal controls, and all were predicted to cause deficient or impaired protein function. HRPT2 is a ubiquitously expressed, evolutionarily conserved gene encoding a predicted protein of 531 amino acids, for which we propose the name parafibromin. Our findings suggest that HRPT2 is a tumor-suppressor gene, the inactivation of which is directly involved in predisposition to HPT-JT and in development of some sporadic parathyroid tumors.     

Cornelia de Lange syndrome is caused by mutations in NIPBL, the human homolog of Drosophila melanogaster Nipped-B

Cornelia de Lange syndrome (CdLS; OMIM 122470) is a dominantly inherited multisystem developmental disorder characterized by growth and cognitive retardation; abnormalities of the upper limbs; gastroesophageal dysfunction; cardiac, ophthalmologic and genitourinary anomalies; hirsutism; and characteristic facial features. Genital anomalies, pyloric stenosis, congenital diaphragmatic hernias, cardiac septal defects, hearing loss and autistic and self-injurious tendencies also frequently occur. Prevalence is estimated to be as high as 1 in 10,000 (ref. 4). We carried out genome-wide linkage exclusion analysis in 12 families with CdLS and identified four candidate regions, of which chromosome 5p13.1 gave the highest multipoint lod score of 2.7. This information, together with the previous identification of a child with CdLS with a de novo t(5;13)(p13.1;q12.1) translocation, allowed delineation of a 1.1-Mb critical region on chromosome 5 for the gene mutated in CdLS. We identified mutations in one gene in this region, which we named NIPBL, in four sporadic and two familial cases of CdLS. We characterized the genomic structure of NIPBL and found that it is widely expressed in fetal and adult tissues. The fly homolog of NIPBL, Nipped-B, facilitates enhancer-promoter communication and regulates Notch signaling and other developmental pathways in Drosophila melanogaster.     

Mutations in KERA, encoding keratocan, cause cornea plana

Specialized collagens and small leucine-rich proteoglycans (SLRPs) interact to produce the transparent corneal structure. In cornea plana, the forward convex curvature is flattened, leading to a decrease in refraction. A more severe, recessively inherited form (CNA2; MIM 217300) and a milder, dominantly inherited form (CNA1; MIM 121400) exist. CNA2 is a rare disorder with a worldwide distribution, but a high prevalence in the Finnish population. The gene mutated in CNA2 was assigned by linkage analysis to 12q (refs 4, 5), where there is a cluster of several SLRP genes. We cloned two additional SLRP genes highly expressed in cornea: KERA (encoding keratocan) in 12q and OGN (encoding osteoglycin) in 9q. Here we report mutations in KERA in 47 CNA2 patients: 46 Finnish patients are homozygous for a founder missense mutation, leading to the substitution of a highly conserved amino acid; and one American patient is homozygous for a mutation leading to a premature stop codon that truncates the KERA protein. Our data establish that mutations in KERA cause CNA2. CNA1 patients had no mutations in these proteoglycan genes.     

Identification of the gene responsible for methylmalonic aciduria and homocystinuria, cblC type

Methylmalonic aciduria and homocystinuria, cblC type (OMIM 277400), is the most common inborn error of vitamin B(12) (cobalamin) metabolism, with about 250 known cases. Affected individuals have developmental, hematological, neurological, metabolic, ophthalmologic and dermatologic clinical findings. Although considered a disease of infancy or childhood, some individuals develop symptoms in adulthood. The cblC locus was mapped to chromosome region 1p by linkage analysis. We refined the chromosomal interval using homozygosity mapping and haplotype analyses and identified the MMACHC gene. In 204 individuals, 42 different mutations were identified, many consistent with a loss of function of the protein product. One mutation, 271dupA, accounted for 40% of all disease alleles. Transduction of wild-type MMACHC into immortalized cblC fibroblast cell lines corrected the cellular phenotype. Molecular modeling predicts that the C-terminal region of the gene product folds similarly to TonB, a bacterial protein involved in energy transduction for cobalamin uptake.     

Identification of the gene causing mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive, neurodegenerative, lysosomal storage disorder characterized by psychomotor retardation and ophthalmological abnormalities including corneal opacities, retinal degeneration and strabismus. Most patients reach a maximal developmental level of 12?15 months. The disease was classified as a mucolipidosis following observations by electron microscopy indicating the lysosomal storage of lipids together with water-soluble, granulated substances. Over 80% of the MLIV patients diagnosed are Ashkenazi Jews, including severely affected and mildly affected patients. The gene causing MLIV was previously mapped to human chromosome 19p13.2-13.3 in a region of approximately 1 cM (ref. 7). Haplotype analysis in the MLIV gene region of over 70 MLIV Ashkenazi chromosomes indicated the existence of two founder chromosomes among 95% of the Ashkenazi MLIV families: a major haplotype in 72% and a minor haplotype in 23% of the MLIV chromosomes (ref. 7, and G.B., unpublished data). The remaining 5% are distinct haplotypes found only in single patients. The basic metabolic defect causing the lysosomal storage in MLIV has not yet been identified. Thus, positional cloning was an alternative to identify the MLIV gene. We report here the identification of a new gene in this human chromosomal region in which MLIV-specific mutations were identified.     

Mutation of POLG is associated with progressive external ophthalmoplegia characterized by mtDNA deletions.

Progressive external ophthalmoplegias (PEO) characterized by accumulation of large-scale mitochondrial DNA (mtDNA) deletions are rare human diseases. We mapped a new locus for dominant PEO at 15q22-q26 in a Belgian pedigree and identified a heterozygous mutation (Y955C) in the polymerase motif B of the mtDNA polymerase gamma (POLG). We identified three additional POLG missense mutations compatible with recessive PEO In two nuclear families. POLG is the only DNA polymerase responsible for mtDNA replication.     

Mutations in SPTLC1, encoding serine palmitoyltransferase, long chain base subunit-1, cause hereditary sensory neuropathy type I

Hereditary sensory neuropathy type I (HSN1) is the most common hereditary disorder of peripheral sensory neurons. HSN1 is an autosomal dominant progressive degeneration of dorsal root ganglia and motor neurons with onset in the second or third decades. Initial symptoms are sensory loss in the feet followed by distal muscle wasting and weakness. Loss of pain sensation leads to chronic skin ulcers and distal amputations. The HSN1 locus has been mapped to chromosome 9q22.1-22.3 (refs. 3,4). Here we map the gene SPTLC1, encoding serine palmitoyltransferase, long chain base subunit-1, to this locus. Mutation screening revealed 3 different missense mutations resulting in changes to 2 amino acids in all affected members of 11 HSN1 families. We found two mutations to be located in exon 5 (C133Y and C133W) and one mutation to be located in exon 6 of SPTLC1 (V144D). All families showing definite or probable linkage to chromosome 9 had mutations in these two exons. These mutations are associated with increased de novo glucosyl ceramide synthesis in lymphoblast cell lines in affected individuals. Increased de novo ceramide synthesis triggers apoptosis and is associated with massive cell death during neural tube closure, raising the possibility that neural degeneration in HSN1 is due to ceramide-induced apoptotic cell death.     

Dominant mutations in ROR2, encoding an orphan receptor tyrosine kinase, cause brachydactyly type B

Inherited limb malformations provide a valuable resource for the identification of genes involved in limb development. Brachydactyly type B (BDB), an autosomal dominant disorder, is the most severe of the brachydactylies and characterized by terminal deficiency of the fingers and toes. In the typical form of BDB, the thumbs and big toes are spared, sometimes with broadening or partial duplication. The BDB1 locus was previously mapped to chromosome 9q22 within an interval of 7.5 cM (refs 9,10). Here we describe mutations in ROR2, which encodes the orphan receptor tyrosine kinase ROR2 (ref. 11), in three unrelated families with BDB1. We identified distinct heterozygous mutations (2 nonsense, 1 frameshift) within a 7-amino-acid segment of the 943-amino-acid protein, all of which predict truncation of the intracellular portion of the protein immediately after the tyrosine kinase domain. The localized nature of these mutations suggests that they confer a specific gain of function. We obtained further evidence for this by demonstrating that two patients heterozygous for 9q22 deletions including ROR2 do not exhibit BDB. Expression of the mouse mouse orthologue, Ror2, early in limb development indicates that BDB arises as a primary defect of skeletal patterning.     

Mutations in EFHC1 cause juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME) is the most frequent cause of hereditary grand mal seizures. We previously mapped and narrowed a region associated with JME on chromosome 6p12-p11 (EJM1). Here, we describe a new gene in this region, EFHC1, which encodes a protein with an EF-hand motif. Mutation analyses identified five missense mutations in EFHC1 that cosegregated with epilepsy or EEG polyspike wave in affected members of six unrelated families with JME and did not occur in 382 control individuals. Overexpression of EFHC1 in mouse hippocampal primary culture neurons induced apoptosis that was significantly lowered by the mutations. Apoptosis was specifically suppressed by SNX-482, an antagonist of R-type voltage-dependent Ca(2+) channel (Ca(v)2.3). EFHC1 and Ca(v)2.3 immunomaterials overlapped in mouse brain, and EFHC1 coimmunoprecipitated with the Ca(v)2.3 C terminus. In patch-clamp analysis, EFHC1 specifically increased R-type Ca(2+) currents that were reversed by the mutations associated with JME.     

Mutations in ATP2A2, encoding a Ca2+ pump, cause Darier disease

Darier disease (DD) is an autosomal-dominant skin disorder characterized by loss of adhesion between epidermal cells (acantholysis) and abnormal keratinization. Recently we constructed a 2.4-Mb, P1-derived artificial chromosome contig spanning the DD candidate region on chromosome 12q23-24.1. After screening several genes that mapped to this region, we identified mutations in the ATP2A2 gene, which encodes the sarco/endoplasmic reticulum Ca2(+)-ATPase type 2 isoform (SERCA2) and is highly expressed in keratinocytes. Thirteen mutations were identified, including frameshift deletions, in-frame deletions or insertions, splice-site mutations and non-conservative missense mutations in functional domains. Our results demonstrate that mutations in ATP2A2 cause DD and disclose a role for this pump in a Ca(2+)-signalling pathway regulating cell-to-cell adhesion and differentiation of the epidermis.