Three novel mutations in the ANK membrane protein cause craniometaphyseal dysplasia with variable conductive hearing loss

Craniometaphyseal dysplasia (CMD) is a rare, sclerosing skeletal disorder caused by mutations in ANKH, which encodes a putative pyrophosphate transporting membrane protein. Six distinct ANKH mutations have been described to date. We report here on three novel mutations in simplex patients with CMD. The c.1015T>C (p.Cys339Arg) mutation found in Patient A was associated with congenital facial palsy, early-onset conductive hearing loss, and a generalized undermodeling of the long bones. The c.1172T>C (p.Leu391Pro) mutation in Patient B was associated with facial palsy, progressive conductive hearing loss, and generalized undermodeling of tubular bones. A milder phenotype without cranial nerve affection was observed in Patient C, associated with a c.1001T>G (p.Leu334Arg) mutation. All affected residues lie in evolutionarily conserved sequence blocks. These additional cases and the associated mutations contribute to an improved appreciation of the variability of this rare skeletal dysplasia. (c) 2010 Wiley-Liss, Inc.

Lysosomal pathology and osteopetrosis upon loss of H+-driven lysosomal Cl- accumulation

During lysosomal acidification, proton-pump currents are thought to be shunted by a chloride ion (Cl-) channel, tentatively identified as ClC-7. Surprisingly, recent data suggest that ClC-7 instead mediates Cl-/proton (H+) exchange. We generated mice carrying a point mutation converting ClC-7 into an uncoupled (unc) Cl- conductor. Despite maintaining lysosomal conductance and normal lysosomal pH, these Clcn7(unc/unc) mice showed lysosomal storage disease like mice lacking ClC-7. However, their osteopetrosis was milder, and they lacked a coat color phenotype. Thus, only some roles of ClC-7 Cl-/H+ exchange can be taken over by a Cl- conductance. This conductance was even deleterious in Clcn7(+/unc) mice. Clcn7(-/-) and Clcn7(unc/unc) mice accumulated less Cl- in lysosomes than did wild-type mice. Thus, lowered lysosomal chloride may underlie their common phenotypes.

Heritable sclerosing bone disorders: presentation and new molecular mechanisms

Sclerosing bone disorders can be subdivided according to their clinical presentation, the primarily affected cell type, and the cellular pathways. Osteoclast-rich osteopetrosis and related disorders have been related in most cases to mutations in genes required for osteoclast function. More recently, osteoclast-poor forms of osteopetrosis have been described as being connected to factors that govern osteoclast differentiation. However, increased bone formation can also cause osteosclerosis. Camurati-Engelman disease and osteopoikilosis are both related transforming growth factor-beta signaling. Rare recessive or dominant sclerosing disorders, such as endosteal hyperostosis, sclerosteosis, van Buchem disease, high bone-mass syndrome, and osteopathia striata, are caused by mutations in genes involved in the Wnt pathway, which regulates osteoblast differentiation. Finally, a third entity, including Ghosal syndrome and pachydermoperiostosis, is related to mutations in genes of the eicosanoid pathway. Clinical aspects and the consequences for our understanding of bone biology are discussed.

Novel mutations in Indian patients with autosomal recessive infantile malignant osteopetrosis

BACKGROUND & OBJECTIVES

Although clinical reports have described infantile malignant autosomal recessive osteopetrosis (ARO) in Indian patients, no published data are available about the genetic causes of ARO in this population. We investigated the main genetic causes of ARO in eight Indian patients with early postnatal onset and the typical severe clinical course including visual impairment and anaemia.

METHODS

Mutation screening in the genes CLCN7 and TCIRG1 was done on genomic DNA from 8 affected individuals (diagnosed on the basis of clinical and haematological parameters and characteristic radiological changes of increased bone density) and their parents. In one family, after detection of both mutations in the proband, targeted mutation analysis was also done in chorionic villus samples for prenatal diagnosis.

RESULTS

Six patients had mutations in TCIRG1 and two patients harboured mutations in CLCN7 gene. Three of the five different TCIRG1 mutations identified and both CLCN7 mutations were novel mutations. Except for the already known mutation p.Ile720del, all TCIRG1 mutations disrupt conserved splice consensus sequences or lead to premature stop codons. In contrast, both CLCN7 mutations only lead to missense changes of conserved amino acids. In a foetus harbouring TCIRG1 mutations osteopetrosis was visible radiologically at 23 wk of gestation.

INTERPRETATION & CONCLUSIONS

That the CLCN7 mutations provoke a phenotype as severe as the one caused by TCIRG1 loss of function suggests the affected residues to be crucial for the function of the ClC-7 chloride channel or chloride/proton-exchanger. Our data also show that ARO can manifest as early as in the second trimester of pregnancy.

Spinal cord atrophy in triple A syndrome associated with a novel compound heterozygous mutation

A 38-year-old male patient was admitted with slowly progressive spastic gait disturbance. Imaging revealed general spinal cord atrophy. Because of adrenal insufficiency, alacrima and achalasia, triple A syndrome was suspected. This is a case report of a triple A syndrome patient with a predominance of neurological features and a new heterozygous compound mutation in triple A syndrome gene.

Mutations in PYCR1 cause cutis laxa with progeroid features

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.

Impaired gastric acidification negatively affects calcium homeostasis and bone mass

Activation of osteoclasts and their acidification-dependent resorption of bone is thought to maintain proper serum calcium levels. Here we show that osteoclast dysfunction alone does not generally affect calcium homeostasis. Indeed, mice deficient in Src, encoding a tyrosine kinase critical for osteoclast activity, show signs of osteopetrosis, but without hypocalcemia or defects in bone mineralization. Mice deficient in Cckbr, encoding a gastrin receptor that affects acid secretion by parietal cells, have the expected defects in gastric acidification but also secondary hyperparathyroidism and osteoporosis and modest hypocalcemia. These results suggest that alterations in calcium homeostasis can be driven by defects in gastric acidification, especially given that calcium gluconate supplementation fully rescues the phenotype of the Cckbr-mutant mice. Finally, mice deficient in Tcirg1, encoding a subunit of the vacuolar proton pump specifically expressed in both osteoclasts and parietal cells, show hypocalcemia and osteopetrorickets. Although neither Src- nor Cckbr-deficient mice have this latter phenotype, the combined deficiency of both genes results in osteopetrorickets. Thus, we find that osteopetrosis and osteopetrorickets are distinct phenotypes, depending on the site or sites of defective acidification.

Loss-of-function mutations in ATP6V0A2 impair vesicular trafficking, tropoelastin secretion and cell survival

Autosomal recessive cutis laxa type 2 (ARCL2), a syndrome of growth and developmental delay and redundant, inelastic skin, is caused by mutations in the a2 subunit of the vesicular ATPase H+-pump (ATP6V0A2). The goal of this study was to define the disease mechanisms that lead to connective tissue lesions in ARCL2. In a new cohort of 17 patients, DNA sequencing of ATP6V0A2 detected either homozygous or compound heterozygous mutations. Considerable allelic and phenotypic heterogeneity was observed, with a missense mutation of a moderately conserved residue p.P87L leading to unusually mild disease. Abnormal N- and/or mucin type O-glycosylation was observed in all patients tested. Premature stop codon mutations led to decreased ATP6V0A2 mRNA levels by destabilizing the mutant mRNA via the nonsense-mediated decay pathway. Loss of ATP6V0A2 either by siRNA knockdown or in ARCL2 cells resulted in distended Golgi cisternae, accumulation of abnormal lysosomes and multivesicular bodies. Immunostaining of ARCL2 cells showed the accumulation of tropoelastin (TE) in the Golgi and in large, abnormal intracellular and extracellular aggregates. Pulse-chase studies confirmed impaired secretion and increased intracellular retention of TE, and insoluble elastin assays showed significantly reduced extracellular deposition of mature elastin. Fibrillin-1 microfibril assembly and secreted lysyl oxidase activity were normal in ARCL2 cells. TUNEL staining demonstrated increased rates of apoptosis in ARCL2 cell cultures. We conclude that loss-of-function mutations in ATP6V0A2 lead to TE aggregation in the Golgi, impaired clearance of TE aggregates and increased apoptosis of elastogenic cells.

Vacuolar H+-ATPase meets glycosylation in patients with cutis laxa

Glycosylation of proteins is one of the most important post-translational modifications. Defects in the glycan biosynthesis result in congenital malformation syndromes, also known as congenital disorders of glycosylation (CDG). Based on the iso-electric focusing patterns of plasma transferrin and apolipoprotein C-III a combined defect in N- and O-glycosylation was identified in patients with autosomal recessive cutis laxa type II (ARCL II). Disease-causing mutations were identified in the ATP6V0A2 gene, encoding the a2 subunit of the vacuolar H(+)-ATPase (V-ATPase). The V-ATPases are multi-subunit, ATP-dependent proton pumps located in membranes of cells and organels. In this article, we describe the structure, function and regulation of the V-ATPase and the phenotypes currently known to result from V-ATPase mutations. A clinical overview of cutis laxa syndromes is presented with a focus on ARCL II. Finally, the relationship between ATP6V0A2 mutations, the glycosylation defect and the ARCLII phenotype is discussed.

Gerodermia osteodysplastica is caused by mutations in SCYL1BP1, a Rab-6 interacting golgin

Gerodermia osteodysplastica is an autosomal recessive disorder characterized by wrinkly skin and osteoporosis. Here we demonstrate that gerodermia osteodysplastica is caused by loss-of-function mutations in SCYL1BP1, which is highly expressed in skin and osteoblasts. The protein localizes to the Golgi apparatus and interacts with Rab6, identifying SCYL1BP1 as a golgin. These results associate abnormalities of the secretory pathway with age-related changes in connective tissues.

Modelling neurofibromatosis type 1 tibial dysplasia and its treatment with lovastatin

BACKGROUND

Bowing and/or pseudarthrosis of the tibia is a known severe complication of neurofibromatosis type 1 (NF1). Mice with conditionally inactivated neurofibromin (Nf1) in the developing limbs and cranium (Nf1Prx1) show bowing of the tibia caused by decreased bone mineralisation and increased bone vascularisation. However, in contrast to NF1 patients, spontaneous fractures do not occur in Nf1Prx1 mice probably due to the relatively low mechanical load. We studied bone healing in a cortical bone injury model in Nf1Prx1 mice as a model for NF1-associated bone disease. Taking advantage of this experimental model we explore effects of systemically applied lovastatin, a cholesterol-lowering drug, on the Nf1 deficient bone repair.

METHODS

Cortical injury was induced bilaterally in the tuberositas tibiae in Nf1Prx1 mutant mice and littermate controls according to a method described previously. Paraffin as well as methacrylate sections were analysed from each animal. We divided 24 sex-matched mutant mice into a lovastatin-treated and an untreated group. The lovastatin-treated mice received 0.15 mg activated lovastatin by daily gavage. The bone repair process was analysed at three consecutive time points post injury, using histological methods, micro computed tomography measurements and in situ hybridisation. At each experimental time point, three lovastatin-treated mutant mice, three untreated mutant mice and three untreated control mice were analysed. The animal group humanely killed on day 14 post injury was expanded to six treated and six untreated mutant mice as well as six control mice.

RESULTS

Bone injury repair is a complex process, which requires the concerted effort of numerous cell types. It is initiated by an inflammatory response, which stimulates fibroblasts from the surrounding connective tissue to proliferate and fill in the injury site with a provisional extracellular matrix. In parallel, mesenchymal progenitor cells from the periost are recruited into the injury site to become osteoblasts. In Nf1Prx1 mice bone repair is delayed and characterised by the excessive formation and the persistence of fibro-cartilaginous tissue and impaired extracellular matrix mineralisation. Correspondingly, expression of Runx2 is downregulated. High-dose systemic lovastatin treatment restores Runx2 expression and accelerates new bone formation, thus improving cortical bone repair in Nf1Prx1 tibia. The bone anabolic effects correlate with a reduction of the mitogen activated protein kinase pathway hyper-activation in Nf1-deficient cells.

CONCLUSION

Our data suggest the potential usefulness of lovastatin, a drug approved by the US Food and Drug Administration in 1987 for the treatment of hypercholesteraemia, in the treatment of Nf1-related fracture healing abnormalities. The experimental model presented here constitutes a valuable tool for the pre-clinical stage testing of candidate drugs, targeting Nf1-associated bone dysplasia.

Geroderma osteodysplasticum hereditaria and wrinkly skin syndrome in 22 patients from Oman

Excessive skin wrinkling and cutis laxa are seen in many genetic conditions and overlapping features can make a clinical diagnosis difficult. Here we report on 22 Omani patients from 11 consanguineous families with the diagnosis of wrinkly skin syndrome (WSS, OMIM 278250) or geroderma osteodysplasticum hereditaria (GO, OMIM 231070). The WSS phenotype evolves during early childhood and includes a generalized and excessive skin wrinkling, dental problems, herniae, foot deformities, hip dislocations, growth retardation, and a large anterior fontanelle. The facial gestalt is characterized by a broad nasal bridge, hypertelorism, and downslanting palpebral fissures. We were unable to differentiate between WSS and cutis laxa with growth and developmental delay (CLGDD, OMIM 219200) suggesting that both can be considered as one entity. Distinct hallmarks of GO were skin wrinkling limited to the dorsum of hands and feet and to the abdomen, normal fontanelles, maxillary hypoplasia, bowed long bones, and osteopenia with frequent fractures. In contrast to the attenuation of the skin phenotype with age in WSS, adult patients with GO appeared prematurely aged. A serum sialotransferrin type 2 pattern was found in all four WSS patients tested. Apolipoprotein CIII (a marker for O-glycosylation) was normal suggesting that WSS is frequently associated with a N-protein glycosylation defect, probably at the level of processing (CDG-II). All four investigated GO patients showed normal sialotransferrin patterns. The known loci for cutis laxa and WSS on 2q31, 5q23-q31, 7q11, 11q13, and 14q32 were excluded. We suggest that WSS and GO are distinct entities with overlapping features.

Ion channels and transporters in osteoclasts

The resorbing osteoclast is an exceptional cell that secretes large amounts of acid through the coupled activity of a v-type H+-ATPase and a chloride channel that both reside in the ruffled membrane. Impairment of this acid secretion machinery by genetic mutations can abolish bone resorption activity, resulting in osteopetrotic phenotypes. Another key feature of osteoclasts is the transport of high amounts of calcium and phosphate from the resorption lacuna to the basolateral plasma membrane. Evidence exists that this occurs in part through entry of these ions into the osteoclast cytosol. Handling of such large amounts of a cellular messenger requires elaborate mechanisms. Membrane proteins that regulate osteoclast calcium homeostasis and the effect of calcium on osteoclast function and survival are therefore the second main focus of this review.

Impaired glycosylation and cutis laxa caused by mutations in the vesicular H+-ATPase subunit ATP6V0A2

We identified loss-of-function mutations in ATP6V0A2, encoding the a2 subunit of the V-type H+ ATPase, in several families with autosomal recessive cutis laxa type II or wrinkly skin syndrome. The mutations result in abnormal glycosylation of serum proteins (CDG-II) and cause an impairment of Golgi trafficking in fibroblasts from affected individuals. These results indicate that the a2 subunit of the proton pump has an important role in Golgi function.

Multiple roles for neurofibromin in skeletal development and growth

Neurofibromatosis type 1 (NF1) is a prevalent genetic disorder primarily characterized by the formation of neurofibromas, café-au-lait spots and freckling. Skeletal abnormalities such as short stature or bowing/pseudarthrosis of the tibia are relatively common. To investigate the role of the neurofibromin in skeletal development, we crossed Nf1flox mice with Prx1Cre mice to inactivate Nf1 in undifferentiated mesenchymal cells of the developing limbs. Similar to NF1 affected individuals, Nf1(Prx1) mice show bowing of the tibia and diminished growth. Tibial bowing is caused by decreased stability of the cortical bone due to a high degree of porosity, decreased stiffness and reduction in the mineral content as well as hyperosteoidosis. Accordingly, osteoblasts show an increase in proliferation and a decreased ability to differentiate and mineralize in vitro. The reduction in growth is due to lower proliferation rates and a differentiation defect of chondrocytes. Abnormal vascularization of skeletal tissues is likely to contribute to this pathology as it exerts a negative effect on cortical bone stability. Furthermore, Nf1 has an important role in the development of joints, as shown by fusion of the hip joints and other joint abnormalities, which are not observed in neurofibromatosis type I. Thus, neurofibromin has multiple essential roles in skeletal development and growth.

Lysosomal storage disease upon disruption of the neuronal chloride transport protein ClC-6

Mammalian CLC proteins function as Cl(-) channels or as electrogenic Cl(-)/H(+) exchangers and are present in the plasma membrane and intracellular vesicles. We now show that the ClC-6 protein is almost exclusively expressed in neurons of the central and peripheral nervous systems, with a particularly high expression in dorsal root ganglia. ClC-6 colocalized with markers for late endosomes in neuronal cell bodies. The disruption of ClC-6 in mice reduced their pain sensitivity and caused moderate behavioral abnormalities. Neuronal tissues showed autofluorescence at initial axon segments. At these sites, electron microscopy revealed electron-dense storage material that caused a pathological enlargement of proximal axons. These deposits were positive for several lysosomal proteins and other marker proteins typical for neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disease. However, the lysosomal pH of Clcn6(-/-) neurons appeared normal. CLCN6 is a candidate gene for mild forms of human NCL. Analysis of 75 NCL patients identified ClC-6 amino acid exchanges in two patients but failed to prove a causative role of CLCN6 in that disease.

Detection of novel skeletogenesis target genes by comprehensive analysis of a Runx2(-/-) mouse model

Runx2 is an essential factor for skeletogenesis and heterozygous loss causes cleidocranial dysplasia in humans and a corresponding phenotype in the mouse. Homozygous Runx2-deficient mice lack hypertrophic cartilage and bone. We compared the expression profiles of E14.5 wildtype and Runx2(-/-) murine embryonal humeri to identify new transcripts potentially involved in cartilage and bone development. Seventy-one differentially expressed genes were identified by two independent oligonucleotide-microarray hybridizations and quantitative RT-PCR experiments. Gene Ontology analysis demonstrated an enrichment of the differentially regulated genes in annotations to terms such as extracellular, skeletal development, and ossification. In situ hybridization on E15.5 limb sections was performed for all 71 differentially regulated genes. For 54 genes conclusive in situ hybridization results were obtained and all of them showed skeletal expression. Co-expression with Runx2 was demonstrated for 44 genes. While 41 of the 71 differentially expressed genes have a known role in bone and cartilage, we identified 21 known genes that have not yet been implicated in skeletal development and 9 entirely new transcripts. Expression in the developing skeleton was demonstrated for 21 of these genes.

Tracheal atresia as part of an exceptional combination of malformations

A case of a premature infant with tracheal atresia together with an exceptional combination of congenital abnormalities that partially corresponds to the TACRD and VACTERL associations is presented. Tracheal atresia was not detectable in the prenatal ultrasound due to lacking of the typical diaphragmatic and pulmonary findings because of the esophago-tracheal fistula, and therefore the resuscitation team was not prepared for this severe airway complication. After prolonged resuscitation efforts were terminated and the newborn expired after birth. Even without typical warning signs physicians have to be aware of tracheal atresia and airway obstruction if VACTERL or TACRD associations are diagnosed.

Polymorphisms in the CLCN7 gene modulate bone density in postmenopausal women and in patients with autosomal dominant osteopetrosis type II

CONTEXT

Genetic factors are important determinants of bone mineral density (BMD). The fact that mutations in the ClC-7 chloride channel cause autosomal dominant osteopetrosis (ADOII) make the CLCN7 gene an attractive candidate for the regulation of bone density.

OBJECTIVE

The objective of the study was to investigate the association between polymorphisms in the CLCN7 gene and BMD in postmenopausal women and with clinical variability in ADOII.

DESIGN

This was a genetic association study using five single-nucleotide polymorphisms and a variable number tandem repeat (VNTR) polymorphism in the CLCN7 gene.

PARTICIPANTS

A total of 425 postmenopausal women aged 64 +/- 7 yr participated in the study. We also investigated an ADOII family with low penetrance comprising 18 mutation carriers.

MAIN OUTCOME MEASURE(S)

In our postmenopausal cohort, individual single-nucleotide polymorphism genotypes and haplotypes were analyzed for association with BMD at the lumbar spine and the femoral neck and with the bone resorption marker deoxypyridinoline (D-Pyr/Crea). The same polymorphisms on the nonmutated CLCN7 allele were investigated for association with the variability of the ADOII phenotype.

RESULTS

Analysis by multiple linear regression revealed a significant association between the ss genotype of the VNTR and higher Z-score values (P = 0.029). The haplotype 4, which comprises the long allele of the VNTR, was found to be significantly associated with lower femoral neck Z-score values (P = 0.011). Furthermore, we found an association of the ss genotype of the VNTR with lower levels of the bone resorption marker D-Pyr/Crea (P = 0.015), whereas haplotype 4 was associated with higher D-Pyr/Crea levels (P = 0.039). In the ADOII family, we could demonstrate that haplotype 3, which contains the s-allele of the VNTR, is associated with a slightly higher probability that mutation carriers develop osteopetrosis (P = 0.029). In both cases the association seems largely to be driven by the VNTR genotype but is further strengthened if surrounding polymorphisms are added to the analysis.

CONCLUSION

We observed a significant association of CLCN7 polymorphisms with the variance of BMD and bone resorption marker levels in postmenopausal women and with the variability of the ADOII phenotype.

Cannabinoid receptor type 2 gene is associated with human osteoporosis

Osteoporosis is one of the most common degenerative diseases. It is characterized by reduced bone mineral density (BMD) with an increased risk for bone fractures. There is a substantial genetic contribution to BMD, although the genetic factors involved in the pathogenesis of human osteoporosis are largely unknown. Mice with a targeted deletion of either the cannabinoid receptor type 1 (Cnr1) or type 2 (Cnr2) gene show an alteration of bone mass, and pharmacological modification of both receptors can regulate osteoclast activity and BMD. We therefore analyzed both genes in a systematic genetic association study in a human sample of postmenopausal osteoporosis patients and matched female controls. We found a significant association of single polymorphisms (P = 0.0014) and haplotypes (P = 0.0001) encompassing the CNR2 gene on human chromosome 1p36, whereas we found no convincing association for CNR1. These results demonstrate a role for the peripherally expressed CB2 receptor in the etiology of osteoporosis and provide an interesting novel therapeutical target for this severe and common disease.

Vacuolar H+-ATPase d2 subunit: molecular characterization, developmental regulation, and localization to specialized proton pumps in kidney and bone

The ubiquitous multisubunit vacuolar-type proton pump (H+- or V-ATPase) is essential for acidification of diverse intracellular compartments. It is also present in specialized forms at the plasma membrane of intercalated cells in the distal nephron, where it is required for urine acidification, and in osteoclasts, playing an important role in bone resorption by acid secretion across the ruffled border membrane. It was reported previously that, in human, several of the renal pump's constituent subunits are encoded by genes that are different from those that are ubiquitously expressed. These paralogous proteins may be important in differential functions, targeting or regulation of H+-ATPases. They include the d subunit, where d1 is ubiquitous whereas d2 has a limited tissue expression. This article reports on an investigation of d2. It was first confirmed that in mouse, as in human, kidney and bone are two of the main sites of d2 mRNA expression. d2 mRNA and protein appear later during nephrogenesis than does the ubiquitously expressed E1 subunit. Mouse nephron-segment reverse transcription-PCR revealed detectable mRNA in all segments except thin limb of Henle's loop and distal convoluted tubule. However, with the use of a novel d2-specific antibody, high-intensity d2 staining was observed only in intercalated cells of the collecting duct in fresh-frozen human kidney, where it co-localized with the a4 subunit in the characteristic plasma membrane-enhanced pattern. In human bone, d2 co-localized with the a3 subunit in osteoclasts. This different subunit association in different tissues emphasizes the possibility of the H+-ATPase as a future therapeutic target.

Loss of the chloride channel ClC-7 leads to lysosomal storage disease and neurodegeneration

ClC-7 is a chloride channel of late endosomes and lysosomes. In osteoclasts, it may cooperate with H(+)-ATPases in acidifying the resorption lacuna. In mice and man, loss of ClC-7 or the H(+)-ATPase a3 subunit causes osteopetrosis, a disease characterized by defective bone resorption. We show that ClC-7 knockout mice additionally display neurodegeneration and severe lysosomal storage disease despite unchanged lysosomal pH in cultured neurons. Rescuing their bone phenotype by transgenic expression of ClC-7 in osteoclasts moderately increased their lifespan and revealed a further progression of the central nervous system pathology. Histological analysis demonstrated an accumulation of electron-dense material in neurons, autofluorescent structures, microglial activation and astrogliosis. Like in human neuronal ceroid lipofuscinosis, there was a strong accumulation of subunit c of the mitochondrial ATP synthase and increased amounts of lysosomal enzymes. Such alterations were minor or absent in ClC-3 knockout mice, despite a massive neurodegeneration. Osteopetrotic oc/oc mice, lacking a functional H(+)-ATPase a3 subunit, showed no comparable retinal or neuronal degeneration. There are important medical implications as defects in the H(+)-ATPase and ClC-7 can underlie human osteopetrosis.

A molecular pathogenesis for transcription factor associated poly-alanine tract expansions

Poly-alanine (Ala) tract expansions in transcription factors have been shown to be associated with human birth defects such as malformations of the brain, the digits, and other structures. Expansions of a poly-Ala tract from 15 to 22 (+7)-29 (+14) Ala in Hoxd13, for example, result in the limb malformation synpolydactyly in humans and in mice [synpolydactyly homolog (spdh)]. Here, we show that an increase of the Ala repeat above a certain length (22 Ala) is associated with a shift in the localization of Hoxd13 from nuclear to cytoplasmic, where it forms large amorphous aggregates. We observed similar aggregates for expansion mutations in SOX3, RUNX2 and HOXA13, pointing to a common mechanism. Cytoplasmic aggregation of mutant Hoxd13 protein is influenced by the length of the repeat, the level of expression and the efficacy of degradation by the proteasome. Heat shock proteins Hsp70 and Hsp40 co-localize with the aggregates and activation of the chaperone system by geldanamycin leads to a reduction of aggregate formation. Furthermore, recombinant mutant Hoxd13 protein forms aggregates in vitro demonstrating spontaneous misfolding of the protein. We analyzed the mouse mutant spdh, which harbors a +7 Ala expansion in Hoxd13 similar to the human synpolydactyly mutations, as an in vivo model and were able to show a reduction of mutant Hoxd13 and, in contrast to wt Hoxd13, a primarily cytoplasmic localization of the protein. Our results provide evidence that poly-Ala repeat expansions in transcription factors result in misfolding, degradation and cytoplasmic aggregation of the mutant proteins.