The gene TFR2 is mutated in a new type of haemochromatosis mapping to 7q22

Haemochromatosis is a common recessive disorder characterized by progressive iron overload, which may lead to severe clinical complications. Most patients are homozygous for the C282Y mutation in HFE on 6p (refs 1-5). A locus for juvenile haemochromatosis (HFE2) maps to 1q (ref. 7). Here we report a new locus (HFE3) on 7q22 and show that a homozygous nonsense mutation in the gene encoding transferrin receptor-2 (TFR2) is found in people with haemochromatosis that maps to HFE3.     

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.     

SPTLC1 is mutated in hereditary sensory neuropathy, type 1

Hereditary sensory neuropathy type 1 (HSN1, MIM 162400; ref. 1) genetically maps to human chromosome 9q22 (refs. 2-4). We report here that the gene encoding a subunit of serine palmitoyltransferase is located within the HSN1 locus, expressed in dorsal root ganglia (DRG) and mutated in HSN1.     

New views on RPE65 deficiency: the rod system is the source of vision in a mouse model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most serious form of the autosomal recessive childhood-onset retinal dystrophies. Mutations in the gene encoding RPE65, a protein vital for regeneration of the visual pigment rhodopsin in the retinal pigment epithelium, account for 10-15% of LCA cases. Whereas previous studies of RPE65 deficiency in both animal models and patients attributed remaining visual function to cones, we show here that light-evoked retinal responses in fact originate from rods. For this purpose, we selectively impaired either rod or cone function in Rpe65-/- mice by generating double- mutant mice with models of pure cone function (rhodopsin-deficient mice; Rho-/-) and pure rod function (cyclic nucleotide-gated channel alpha3-deficient mice; Cnga3-/-). The electroretinograms (ERGs) of Rpe65-/- and Rpe65-/-Cnga3-/- mice were almost identical, whereas there was no assessable response in Rpe65-/-Rho-/- mice. Thus, we conclude that the rod system is the source of vision in RPE65 deficiency. Furthermore, we found that lack of RPE65 enables rods to mimic cone function by responding under normally cone-isolating lighting conditions. We propose as a mechanism decreased rod sensitivity due to a reduction in rhodopsin content to less than 1%. In general, the dissection of pathophysiological processes in animal models through the introduction of additional, selective mutations is a promising concept in functional genetics.     

The gene mutated in juvenile nephronophthisis type 4 encodes a novel protein that interacts with nephrocystin

Nephronophthisis, the most common genetic cause of chronic renal failure in children, is a progressive tubulo-interstitial kidney disorder that is inherited as an autosomal recessive trait. The disease is characterized by polyuria, growth retardation and deterioration of renal function during childhood or adolescence. The most prominent histological features are modifications of the tubules with thickening of the basement membrane, interstitial fibrosis and, in the advanced stages, medullary cysts. Nephronophthisis can also be associated with conditions affecting extrarenal organs, such as retinitis pigmentosa (Senior-L?ken syndrome) and ocular motor apraxia (Cogan syndrome). Three loci are associated with the juvenile, infantile and adolescent forms, on chromosomes 2q13 (NPHP1; refs 5,6), 9q22 (NPHP2; ref. 7) and 3q21 (NPHP3; ref. 8), respectively. NPHP1, the only gene identified so far, encodes nephrocystin, which contains a Src homology 3 (SH3) domain and interacts with intracytoplasmic proteins involved in cell adhesion. Recently, a second locus associated with the juvenile form of the disease, NPHP4, was mapped to chromosome 1p36 (ref. 14). We carried out haplotype analysis of families affected with nephronophthisis that were not linked to the NPHP1, NPHP2 or NPHP3 loci, using markers covering this region. This allowed us to reduce the NPHP4 interval to a one centimorgan interval between D1S2795 and D1S2870, which contains six genes. We identified five different mutations in one of these genes, designated NPHP4, in unrelated individuals with nephronophthisis. The NPHP4 gene encodes a 1,250-amino acid protein of unknown function that we named nephrocystin-4. We demonstrated the interaction of nephrocystin-4 with nephrocystin suggesting that these two proteins participate in a common signaling pathway.     

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.     

The transmembrane protein meckelin (MKS3) is mutated in Meckel-Gruber syndrome and the wpk rat

Meckel-Gruber syndrome is a severe autosomal, recessively inherited disorder characterized by bilateral renal cystic dysplasia, developmental defects of the central nervous system (most commonly occipital encephalocele), hepatic ductal dysplasia and cysts and polydactyly. MKS is genetically heterogeneous, with three loci mapped: MKS1, 17q21-24 (ref. 4); MKS2, 11q13 (ref. 5) and MKS3 (ref. 6). We have refined MKS3 mapping to a 12.67-Mb interval (8q21.13-q22.1) that is syntenic to the Wpk locus in rat, which is a model with polycystic kidney disease, agenesis of the corpus callosum and hydrocephalus. Positional cloning of the Wpk gene suggested a MKS3 candidate gene, TMEM67, for which we identified pathogenic mutations for five MKS3-linked consanguineous families. MKS3 is a previously uncharacterized, evolutionarily conserved gene that is expressed at moderate levels in fetal brain, liver and kidney but has widespread, low levels of expression. It encodes a 995-amino acid seven-transmembrane receptor protein of unknown function that we have called meckelin.     

Mutations in a new photoreceptor-pineal gene on 17p cause Leber congenital amaurosis

Leber congenital amaurosis (LCA, MIM 204000) accounts for at least 5% of all inherited retinal disease and is the most severe inherited retinopathy with the earliest age of onset. Individuals affected with LCA are diagnosed at birth or in the first few months of life with severely impaired vision or blindness, nystagmus and an abnormal or flat electroretinogram (ERG). Mutations in GUCY2D (ref. 3), RPE65 (ref. 4) and CRX (ref. 5) are known to cause LCA, but one study identified disease-causing GUCY2D mutations in only 8 of 15 families whose LCA locus maps to 17p13.1 (ref. 3), suggesting another LCA locus might be located on 17p13.1. Confirming this prediction, the LCA in one Pakistani family mapped to 17p13.1, between D17S849 and D17S960-a region that excludes GUCY2D. The LCA in this family has been designated LCA4 (ref. 6). We describe here a new photoreceptor/pineal-expressed gene, AIPL1 (encoding aryl-hydrocarbon interacting protein-like 1), that maps within the LCA4 candidate region and whose protein contains three tetratricopeptide (TPR) motifs, consistent with nuclear transport or chaperone activity. A homozygous nonsense mutation at codon 278 is present in all affected members of the original LCA4 family. AIPL1 mutations may cause approximately 20% of recessive LCA, as disease-causing mutations were identified in 3 of 14 LCA families not tested previously for linkage.     

Gain-of-function mutations in TRPV4 cause autosomal dominant brachyolmia

The brachyolmias constitute a clinically and genetically heterogeneous group of skeletal dysplasias characterized by a short trunk, scoliosis and mild short stature. Here, we identify a locus for an autosomal dominant form of brachyolmia on chromosome 12q24.1-12q24.2. Among the genes in the genetic interval, we selected TRPV4, which encodes a calcium permeable cation channel of the transient receptor potential (TRP) vanilloid family, as a candidate gene because of its cartilage-selective gene expression pattern. In two families with the phenotype, we identified point mutations in TRPV4 that encoded R616Q and V620I substitutions, respectively. Patch clamp studies of transfected HEK cells showed that both mutations resulted in a dramatic gain of function characterized by increased constitutive activity and elevated channel activation by either mechano-stimulation or agonist stimulation by arachidonic acid or the TRPV4-specific agonist 4alpha-phorbol 12,13-didecanoate (4alphaPDD). This study thus defines a previously unknown mechanism, activation of a calcium-permeable TRP ion channel, in skeletal dysplasia pathogenesis.     

Mutations in a human homologue of Drosophila crumbs cause retinitis pigmentosa (RP12).

Retinitis pigmentosa (RP) comprises a clinically and genetically heterogeneous group of diseases that afflicts approximately 1.5 million people worldwide. Affected individuals suffer from a progressive degeneration of the photoreceptors, eventually resulting in severe visual impairment. To isolate candidate genes for chorioretinal diseases, we cloned cDNAs specifically or preferentially expressed in the human retina and the retinal pigment epithelium (RPE) through a novel suppression subtractive hybridization (SSH) method. One of these cDNAs (RET3C11) mapped to chromosome 1q31-q32.1, a region harbouring a gene involved in a severe form of autosomal recessive RP characterized by a typical preservation of the para-arteriolar RPE (RP12; ref. 3). The full-length cDNA encodes an extracellular protein with 19 EGF-like domains, 3 laminin A G-like domains and a C-type lectin domain. This protein is homologous to the Drosophila melanogaster protein crumbs (CRB), and denoted CRB1 (crumbs homologue 1). In ten unrelated RP patients with preserved para-arteriolar RPE, we identified a homozygous AluY insertion disrupting the ORF, five homozygous missense mutations and four compound heterozygous mutations in CRB1. The similarity to CRB suggests a role for CRB1 in cell-cell interaction and possibly in the maintenance of cell polarity in the retina. The distinct RPE abnormalities observed in RP12 patients suggest that CRB1 mutations trigger a novel mechanism of photoreceptor degeneration.     

Identification of the gene (BBS1) most commonly involved in Bardet-Biedl syndrome,a complex human obesity syndrome

Bardet-Biedl syndrome (BBS, OMIM 209900) is a genetic disorder with the primary features of obesity, pigmentary retinopathy, polydactyly, renal malformations, mental retardation and hypogenitalism. Individuals with BBS are also at increased risk for diabetes mellitus, hypertension and congenital heart disease. What was once thought to be a homogeneous autosomal recessive disorder is now known to map to at least six loci: 11q13 (BBS1), 16q21 (BBS2), 3p13 p12 (BBS3), 15q22.3 q23 (BBS4), 2q31 (BBS5) and 20p12 (BBS6). There has been considerable interest in identifying the genes that underlie BBS, because some components of the phenotype are common. Cases of BBS mapping ro BBS6 are caused by mutations in MKKS; mutations in this gene also cause McKusick-Kaufman syndrome (hydrometrocolpos, post-axial polydactyly and congenital heart defects). In addition, we recently used positional cloning to identify the genes underlying BBS2 (ref. 16) and BBS4 (ref. 17). The BBS6 protein has similarity to a Thermoplasma acidophilum chaperonin, whereas BBS2 and BBS4 have no significant similarity to chaperonins. It has recently been suggested that three mutated alleles (two at one locus, and a third at a second locus) may be required for manifestation of BBS (triallelic inheritance). Here we report the identification of the gene BBS1 and show that a missense mutation of this gene is a frequent cause of BBS. In addition, we provide data showing that this common mutation is not involved in triallelic inheritance.     

Mutations in SLC19A2 cause thiamine-responsive megaloblastic anaemia associated with diabetes mellitus and deafness.

Thiamine-responsive megaloblastic anaemia (TRMA), also known as Rogers syndrome, is an early onset, autosomal recessive disorder defined by the occurrence of megaloblastic anaemia, diabetes mellitus and sensorineural deafness, responding in varying degrees to thiamine treatment (MIM 249270). We have previously narrowed the TRMA locus from a 16-cM to a 4-cM interval on chromosomal region 1q23.3 (refs 3,4) and this region has been further refined to a 1.4-cM interval. Previous studies have suggested that deficiency in a high-affinity thiamine transporter may cause this disorder. Here we identify the TRMA gene by positional cloning. We assembled a P1-derived artificial chromosome (PAC) contig spanning the TRMA candidate region. This clarified the order of genetic markers across the TRMA locus, provided 9 new polymorphic markers and narrowed the locus to an approximately 400-kb region. Mutations in a new gene, SLC19A2, encoding a putative transmembrane protein homologous to the reduced folate carrier proteins, were found in all affected individuals in six TRMA families, suggesting that a defective thiamine transporter protein (THTR-1) may underlie the TRMA syndrome.     

Limb-girdle muscular dystrophy type 2G is caused by mutations in the gene encoding the sarcomeric protein telethonin

Autosomal recessive limb-girdle muscular dystrophies (AR LGMDs) are a genetically heterogeneous group of disorders that affect mainly the proximal musculature. There are eight genetically distinct forms of AR LGMD, LGMD 2A-H (refs 2-10), and the genetic lesions underlying these forms, except for LGMD 2G and 2H, have been identified. LGMD 2A and LGMD 2B are caused by mutations in the genes encoding calpain 3 (ref. 11) and dysferlin, respectively, and are usually associated with a mild phenotype. Mutations in the genes encoding gamma-(ref. 14), alpha-(ref. 5), beta-(refs 6,7) and delta (ref. 15)-sarcoglycans are responsible for LGMD 2C to 2F, respectively. Sarcoglycans, together with sarcospan, dystroglycans, syntrophins and dystrobrevin, constitute the dystrophin-glycoprotein complex (DGC). Patients with LGMD 2C-F predominantly have a severe clinical course. The LGMD 2G locus maps to a 3-cM interval in 17q11-12 in two Brazilian families with a relatively mild form of AR LGMD (ref. 9). To positionally clone the LGMD 2G gene, we constructed a physical map of the 17q11-12 region and refined its localization to an interval of 1.2 Mb. The gene encoding telethonin, a sarcomeric protein, lies within this candidate region. We have found that mutations in the telethonin gene cause LGMD 2G, identifying a new molecular mechanism for AR LGMD.     

A conserved sorting-associated protein is mutant in chorea-acanthocytosis

Chorea-acanthocytosis (CHAC, MIM 200150) is an autosomal recessive neurodegenerative disorder characterized by the gradual onset of hyperkinetic movements and abnormal erythrocyte morphology (acanthocytosis). Neurological findings closely resemble those observed in Huntington disease. We identified a gene in the CHAC critical region and found 16 different mutations in individuals with chorea-acanthocytosis. CHAC encodes an evolutionarily conserved protein that is probably involved in protein sorting.     

The nicotinic receptor beta 2 subunit is mutant in nocturnal frontal lobe epilepsy

Clustered attacks of epileptic episodes originating from the frontal lobe during sleep are the main symptoms of autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE, MIM 600513). Despite the clinical homogeneity, three forms of ADNFLE have been associated with chromosomes 20 (ENFL1; ref. 1), 15 (ENFL2; ref. 2) and 1 (ENFL3; ref. 3). Mutations of the gene encoding the neuronal nicotinic acetylcholine receptor alpha 4 subunit (CHRNA4 ) have been found in ADNFLE-ENFL1 families, but these mutations account for only a small proportion of ADNFLE cases. The newly identified locus associated with ENFL3 harbours several candidate genes, including CHRNB2 (ref. 8), whose gene product, the beta 2 nicotinic acetylcholine receptor (nAChR) subunit, co-assembles with the alpha 4 nAChR subunit to form the active receptor.     

Identification of the gene altered in Berardinelli-Seip congenital lipodystrophy on chromosome 11q13

Congenital generalized lipodystrophy, or Berardinelli-Seip syndrome (BSCL), is a rare autosomal recessive disease characterized by a near-absence of adipose tissue from birth or early infancy and severe insulin resistance. Other clinical and biological features include acanthosis nigricans, hyperandrogenism, muscular hypertrophy, hepatomegaly, altered glucose tolerance or diabetes mellitus, and hypertriglyceridemia. A locus (BSCL1) has been mapped to 9q34 with evidence of heterogeneity. Here, we report a genome screen of nine BSCL families from two geographical clusters (in Lebanon and Norway). We identified a new disease locus, designated BSCL2, within the 2.5-Mb interval flanked by markers D11S4076 and D11S480 on chromosome 11q13. Analysis of 20 additional families of various ethnic origins led to the identification of 11 families in which the disease cosegregates with the 11q13 locus; the remaining families provide confirmation of linkage to 9q34. Sequence analysis of genes located in the 11q13 interval disclosed mutations in a gene homologous to the murine guanine nucleotide-binding protein (G protein), gamma3-linked gene (Gng3lg) in all BSCL2-linked families. BSCL2 is most highly expressed in brain and testis and encodes a protein (which we have called seipin) of unknown function. Most of the variants are null mutations and probably result in a severe disruption of the protein. These findings are of general importance for understanding the molecular mechanisms underlying regulation of body fat distribution and insulin resistance.     

Mutations in the gene encoding mevalonate kinase cause hyper-IgD and periodic fever syndrome. International Hyper-IgD Study Group.

Hyperimmunoglobulinaemia D and periodic fever syndrome (HIDS; MIM 260920) is a rare, apparently monogenic, autosomal recessive disorder characterized by recurrent episodes of fever accompanied with lymphadenopathy, abdominal distress, joint involvement and skin lesions. All patients have high serum IgD values (>100 U/ml) and HIDS 'attacks' are associated with an intense acute phase reaction whose exact pathophysiology remains obscure. Two other hereditary febrile disorders have been described. Familial Mediterranean fever (MIM 249100) is an autosomal recessive disorder affecting mostly populations from the Mediterranean basin and is caused by mutations in the gene MEFV (refs 5,6). Familial Hibernian fever (MIM 142680), also known as autosomal dominant familial recurrent fever, is caused by missense mutations in the gene encoding type I tumour necrosis factor receptor. Here we perform a genome-wide search to map the HIDS gene. Haplotype analysis placed the gene at 12q24 between D12S330 and D12S79. We identified the gene MVK, encoding mevalonate kinase (MK, ATP:mevalonate 5-phosphotransferase; EC 2.7.1.36), as a candidate gene. We characterized 3 missense mutations, a 92-bp loss stemming from a deletion or from exon skipping, and the absence of expression of one allele. Functional analysis demonstrated diminished MK activity in fibroblasts from HIDS patients. Our data establish MVK as the gene responsible for HIDS.