Biallelic COL3A1 mutations result in a clinical spectrum of specific structural brain anomalies and connective tissue abnormalities

Vascular Ehlers-Danlos syndrome (type IV) is an autosomal dominant disorder caused by heterozygous variants of COL3A1. We identified biallelic COL3A1 variants in two unrelated families. In a 3-year-old female with developmental delay the nonsense variant c.1282C>T, p.(Arg428*) was detected in combination the c.2057delC, p.(Pro686Leufs*105) frame shift variant. Both compound heterozygous variants were novel. This patient was born with bilateral clubfoot, joint laxity, and dysmorphic facial features. At the age of 2 years she developed an aneurysmal brain hemorrhage. Cerebral MRI showed a peculiar pattern of profound cerebral abnormalities including bilateral frontoparietal polymicrogyria of the cobblestone variant. In the second family, the two affected siblings were homozygous for the missense variant c.145C<G, p.(Pro49Ala) of COL3A1 and showed cobblestone-like cortical malformation, cerebellar cysts, and white matter abnormalities, developmental delay, and seizures. To date, three further families have been reported with biallelic variants of this gene and specific structural brain anomalies in all, and a severe Ehlers-Danlos syndrome phenotype in some. Bilateral frontoparietal polymicrogyria of the cobblestone variant, cerebellar microcysts, and abnormalities of the white matter characterize this brain phenotype and resemble neurological manifestations in individuals with autosomal recessive mutations in GPR56, which serves as a ligand of COL3A1. In concordance with the findings in knock out mice, the collagen III protein plays a role in the regulation of cortical development in addition to its well-known function in connective tissue formation.

Novel splice mutation in LRP4 causes severe type of Cenani-Lenz syndactyly syndrome with oro-facial and skeletal symptoms

Cenani-Lenz syndactyly syndrome (CLSS; MIM-212780) is a rare autosomal recessive limb malformation characterized by complete osseous fusion of all fingers and toes, disorganization of phalangeal elements and severe shortening of the radius and ulna. It is occasionally associated with renal hypoplasia, oro-facial defects, scoliosis of the thoracic spine, hearing loss, and genital anomalies. Here we describe a consanguineous Pakistani kindred with a severe form of CLSS characterized by complete syndactyly and disorganization of fingers, oligo-syndactyly of toes, shortening of limbs, frontal bossing, and hypoplasia/agenesis of left kidney. The affected individuals were additionally presented with short stature, cleft-lip and hypoplastic shoulder joint with restricted upper limb movement. A novel splice variant in LRP4 (c.316+1G > A) segregated with the phenotype in a five generations family. The mutation is predicted to add 29 non-native amino acids with a premature termination, resulting in approximately 90% length reduction of the wild-type transcript. These findings not only further expand the phenotypic variability of CLSS but also indicate that early truncated and loss-of-function mutations in LRP4 lead to a more severe CLSS phenotype.

Mutations in TGDS associated with additional malformations of the middle fingers and halluces: Atypical Catel-Manzke syndrome in a fetus

Pierre-Robin sequence, radial deviation, and ulnar clinodactyly of the index fingers due to an additional phalangeal bone, as well as heart defects are the key features of Catel-Manzke syndrome. Although mutations in TGDS were identified as the cause of this disorder, the pathogenetic mechanism remains unknown. Here, we report on a fetus with severe heart defect, nuchal edema, talipes, Pierre-Robin sequence, and bilateral deviation and clinodactyly of the index and middle fingers. Pregnancy was terminated at the 22nd week of gestation. Postmortem radiographs showed hypoplasia and V-shaped displacement of the second and third proximal phalanges of both hands as well as hypoplasia of the first metatarsals and the phalangeal bones of the halluces. The suggested diagnosis Catel-Manzke syndrome was confirmed by the detection of two compound heterozygous mutations in TGDS: The known variant c.298G>T; p.(Ala100Ser) and the so far undescribed variant c.895G>A; p.(Asp299Asn), located in the predicted substrate binding site of TGDS. This is the first report on the association of mutations in TGDS with additional anomalies of the middle fingers and halluces. We provide a detailed phenotypic characterization of the only fetus with molecularly confirmed Catel-Manzke syndrome, which is relevant for prenatal diagnosis. Our findings widen the phenotype spectrum caused by TGDS mutations and underline the phenotypic overlap with Temtamy preaxial brachydactyly syndrome. This improves our understanding of the prenatal development and the pathogenetic mechanism of Catel-Manzke syndrome.

Mutations in ATP6V1E1 or ATP6V1A Cause Autosomal-Recessive Cutis Laxa

Defects of the V-type proton (H⁺) ATPase (V-ATPase) impair acidification and intracellular trafficking of membrane-enclosed compartments, including secretory granules, endosomes, and lysosomes. Whole-exome sequencing in five families affected by mild to severe cutis laxa, dysmorphic facial features, and cardiopulmonary involvement identified biallelic missense mutations in ATP6V1E1 and ATP6V1A, which encode the E1 and A subunits, respectively, of the V₁ domain of the heteromultimeric V-ATPase complex. Structural modeling indicated that all substitutions affect critical residues and inter- or intrasubunit interactions. Furthermore, complexome profiling, a method combining blue-native gel electrophoresis and liquid chromatography tandem mass spectrometry, showed that they disturb either the assembly or the stability of the V-ATPase complex. Protein glycosylation was variably affected. Abnormal vesicular trafficking was evidenced by delayed retrograde transport after brefeldin A treatment and abnormal swelling and fragmentation of the Golgi apparatus. In addition to showing reduced and fragmented elastic fibers, the histopathological hallmark of cutis laxa, transmission electron microscopy of the dermis also showed pronounced changes in the structure and organization of the collagen fibers. Our findings expand the clinical and molecular spectrum of metabolic cutis laxa syndromes and further link defective extracellular matrix assembly to faulty protein processing and cellular trafficking caused by genetic defects in the V-ATPase complex.

A Novel ANO5 Mutation Causing Gnathodiaphyseal Dysplasia With High Bone Turnover Osteosclerosis

Gnathodiaphyseal dysplasia (GDD) is a rare skeletal syndrome that involves an osteopetrosis-like sclerosis of the long bones and fibrous dysplasia-like cemento-osseous lesions of the jawbone. Although the genetic analysis of the respective patients has revealed mutations in the ANO5 gene as an underlying cause, there is still no established consensus regarding the bone status of GDD patients. We report a new case of GDD in a 13-year-old boy with recurrent diaphyseal fractures of the femur, in whom we identified a novel de novo missense mutation in the ANO5 gene, causing a p.Ser500Phe substitution at the protein level. After confirming the presence of GDD-characteristic abnormalities within the jaw bones, we focused on a full osteologic assessment using dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses. We thereby identified increased trabecular bone mass accompanied by elevated serum markers of bone formation and bone resorption. The high turnover bone pathology was further confirmed through the analysis of an iliac crest biopsy, where osteoblast and osteoclast indices were remarkably increased. Taken together, our findings provide evidence for a critical and generalized role of anoctamin-5 (the protein encoded by the ANO5 gene) in skeletal biology. As it is reasonable to speculate that modifying the function of anoctamin-5 might be useful for therapeutically activating bone remodeling, it is now required to analyze its function at a molecular level, for instance in mouse models. © 2016 American Society for Bone and Mineral Research.

Disruption of the vacuolar-type H+-ATPase complex in liver causes MTORC1-independent accumulation of autophagic vacuoles and lysosomes

The vacuolar-type H⁺-translocating ATPase (v-H⁺-ATPase) has been implicated in the amino acid-dependent activation of the mechanistic target of rapamycin complex 1 (MTORC1), an important regulator of macroautophagy. To reveal the mechanistic links between the v-H⁺-ATPase and MTORC1, we destablilized v-H⁺-ATPase complexes in mouse liver cells by induced deletion of the essential chaperone ATP6AP2. ATP6AP2-mutants are characterized by massive accumulation of endocytic and autophagic vacuoles in hepatocytes. This cellular phenotype was not caused by a block in endocytic maturation or an impaired acidification. However, the degradation of LC3-II in the knockout hepatocytes appeared to be reduced. When v-H⁺-ATPase levels were decreased, we observed lysosome association of MTOR and normal signaling of MTORC1 despite an increase in autophagic marker proteins. To better understand why MTORC1 can be active when v-H⁺-ATPase is depleted, the activation of MTORC1 was analyzed in ATP6AP2-deficient fibroblasts. In these cells, very little amino acid-elicited activation of MTORC1 was observed. In contrast, insulin did induce MTORC1 activation, which still required intracellular amino acid stores. These results suggest that in vivo the regulation of macroautophagy depends not only on v-H⁺-ATPase-mediated regulation of MTORC1.

A de novo 1q23.3-q24.2 deletion combined with a GORAB missense mutation causes a distinctive phenotype with cutis laxa

Gerodermia osteodysplastica is a recessive segmental progeroid disorder mainly characterized by wrinkled skin, generalized connective tissue weakness, infantile onset osteoporosis and normal intelligence. Coding mutations in GORAB, localized on chromosome 1q24.2, are the cause of this disease. 1q24 deletions underlie a spectrum of disorders with intellectual disability and ear abnormalities as phenotypic hallmarks. Here we report on an individual from Azerbaijan originating from a non-consanguineous couple showing short stature, cutis laxa, frequent fractures, facial dysmorphism, cup-shaped ears and intellectual disability. Sanger sequencing of GORAB revealed the seemingly homozygous missense mutation p.Ser175Phe. This mutation was detected in a heterozygous state in the clinically unaffected mother, but was absent in the healthy father. We performed copy-number investigations by high-resolution array-CGH and PCR approaches and found an ~6 Mb de novo deletion spanning 1q23.3-q24.2 in the affected boy. This novel combination of genetic defects very well explains the phenotype that goes beyond the usual presentation of gerodermia osteodysplastica. Our data provide new insight into the phenotypic spectrum of 1q23-q25 deletions and shows that the combination with another pathogenic allele can lead to more severe clinical manifestations.

The progressive ankylosis protein ANK facilitates clathrin- and adaptor-mediated membrane traffic at the trans-Golgi network-to-endosome interface

Dominant or recessive mutations in the progressive ankylosis gene ANKH have been linked to familial chondrocalcinosis (CCAL2), craniometaphyseal dysplasia (CMD), mental retardation, deafness and ankylosis syndrome (MRDA). The function of the encoded membrane protein ANK in cellular compartments other than the plasma membrane is unknown. Here, we show that ANK localizes to the trans-Golgi network (TGN), clathrin-coated vesicles and the plasma membrane. ANK functionally interacts with clathrin and clathrin associated adaptor protein (AP) complexes as loss of either protein causes ANK dispersion from the TGN to cytoplasmic endosome-like puncta. Consistent with its subcellular localization, loss of ANK results in reduced formation of tubular membrane carriers from the TGN, perinuclear accumulation of early endosomes and impaired transferrin endocytosis. Our data indicate that clathrin/AP-mediated cycling of ANK between the TGN, endosomes, and the cell surface regulates membrane traffic at the TGN/endosomal interface. These findings suggest that dysfunction of Golgi-endosomal membrane traffic may contribute to ANKH-associated pathologies.

The Diagnostic Dilemma of Cutis Laxa: A Report of Two Cases with Genotypic Dissimilarity

Cutis laxa is a heterogeneous group of diseases, with loose, wrinkled skin folds and hyperelasticity of the skin. There are overlapping of clinical features of the group of syndrome associated with cutis laxa, including congenital cutis laxa, wrinkly skin syndrome and gerodermia osteodysplastica. All these conditions present a challenge to the clinician. Thus, molecular diagnosis is the only way to resolve these phenotypically similar conditions. We hereby describe two Indian patients with wrinkled skin and mild craniofacial dysmorphic features who had molecular confirmation of autosomal recessive cutis laxa.

Key features and clinical variability of COG6-CDG

The conserved oligomeric Golgi (COG) complex consists of eight subunits and plays a crucial role in Golgi trafficking and positioning of glycosylation enzymes. Mutations in all COG subunits, except subunit 3, have been detected in patients with congenital disorders of glycosylation (CDG) of variable severity. So far, 3 families with a total of 10 individuals with biallelic COG6 mutations have been described, showing a broad clinical spectrum. Here we present 7 additional patients with 4 novel COG6 mutations. In spite of clinical variability, we delineate the core features of COG6-CDG i.e. liver involvement (9/10), microcephaly (8/10), developmental disability (8/10), recurrent infections (7/10), early lethality (6/10), and hypohidrosis predisposing for hyperthermia (6/10) and hyperkeratosis (4/10) as ectodermal signs. Regarding all COG6-related disorders a genotype-phenotype correlation can be discerned ranging from deep intronic mutations found in Shaheen syndrome as the mildest form to loss-of-function mutations leading to early lethal CDG phenotypes. A comparison with other COG deficiencies suggests ectodermal changes to be a hallmark of COG6-related disorders. Our findings aid clinical differentiation of this complex group of disorders and imply subtle functional differences between the COG complex subunits.

Recurrent De Novo Mutations Affecting Residue Arg138 of Pyrroline-5-Carboxylate Synthase Cause a Progeroid Form of Autosomal-Dominant Cutis Laxa

Progeroid disorders overlapping with De Barsy syndrome (DBS) are collectively denoted as autosomal-recessive cutis laxa type 3 (ARCL3). They are caused by biallelic mutations in PYCR1 or ALDH18A1, encoding pyrroline-5-carboxylate reductase 1 and pyrroline-5-carboxylate synthase (P5CS), respectively, which both operate in the mitochondrial proline cycle. We report here on eight unrelated individuals born to non-consanguineous families clinically diagnosed with DBS or wrinkly skin syndrome. We found three heterozygous mutations in ALDH18A1 leading to amino acid substitutions of the same highly conserved residue, Arg138 in P5CS. A de novo origin was confirmed in all six probands for whom parental DNA was available. Using fibroblasts from affected individuals and heterologous overexpression, we found that the P5CS-p.Arg138Trp protein was stable and able to interact with wild-type P5CS but showed an altered sub-mitochondrial distribution. A reduced size upon native gel electrophoresis indicated an alteration of the structure or composition of P5CS mutant complex. Furthermore, we found that the mutant cells had a reduced P5CS enzymatic activity leading to a delayed proline accumulation. In summary, recurrent de novo mutations, affecting the highly conserved residue Arg138 of P5CS, cause an autosomal-dominant form of cutis laxa with progeroid features. Our data provide insights into the etiology of cutis laxa diseases and will have immediate impact on diagnostics and genetic counseling.

GORAB Missense Mutations Disrupt RAB6 and ARF5 Binding and Golgi Targeting

Gerodermia osteodysplastica is a hereditary segmental progeroid disorder affecting skin, connective tissues, and bone that is caused by loss-of-function mutations in GORAB. The golgin, RAB6-interacting (GORAB) protein localizes to the Golgi apparatus and interacts with the small GTPase RAB6. In this study, we used different approaches to shed more light on the recruitment of GORAB to this compartment. We show that GORAB best colocalizes with trans-Golgi markers and is rapidly displaced upon Brefeldin A exposition, indicating a loose association with Golgi membranes. A yeast two-hybrid screening revealed a specific interaction with the small GTPase ADP-ribosylation factor (ARF5) in its active, GTP-bound form. ARF5 and RAB6 bind to GORAB via the same internal Golgi-targeting RAB6 and ARF5 binding (IGRAB) domain. Two GORAB missense mutations identified in gerodermia osteodysplastica patients fall within this IGRAB domain. GORAB carrying the mutation p.Ala220Pro had a cytoplasmic distribution and failed to interact with both RAB6 and ARF5. In contrast, the p.Ser175Phe mutation displaced GORAB from the Golgi compartment to vesicular structures and selectively impaired ARF5 binding. Our findings indicate that the IGRAB domain is crucial for the Golgi localization of GORAB and that loss of this localization impairs its physiological function.

Diagnostic algorithms in Charcot-Marie-Tooth neuropathies: experiences from a German genetic laboratory on the basis of 1206 index patients

We present clinical features and genetic results of 1206 index patients and 124 affected relatives who were referred for genetic testing of Charcot-Marie-Tooth (CMT) neuropathy at the laboratory in Aachen between 2001 and 2012. Genetic detection rates were 56% in demyelinating CMT (71% of autosomal dominant (AD) CMT1/CMTX), and 17% in axonal CMT (24% of AD CMT2/CMTX). Three genetic defects (PMP22 duplication/deletion, GJB1/Cx32 or MPZ/P0 mutation) were responsible for 89.3% of demyelinating CMT index patients in whom a genetic diagnosis was achieved, and the diagnostic yield of the three main genetic defects in axonal CMT (GJB1/Cx32, MFN2, MPZ/P0 mutations) was 84.2%. De novo mutations were detected in 1.3% of PMP22 duplication, 25% of MPZ/P0, and none in GJB1/Cx32. Motor nerve conduction velocity was uniformly <38 m/s in median or ulnar nerves in PMP22 duplication, >40 m/s in MFN2, and more variable in GJB1/Cx32, MPZ/P0 mutations. Patients with CMT2A showed a broad clinical severity regardless of the type or position of the MFN2 mutation. Out of 75 patients, 8 patients (11%) with PMP22 deletions were categorized as CMT1 or CMT2. Diagnostic algorithms are still useful for cost-efficient mutation detection and for the interpretation of large-scale genetic data made available by next generation sequencing strategies.

Homozygous and compound-heterozygous mutations in TGDS cause Catel-Manzke syndrome

Catel-Manzke syndrome is characterized by Pierre Robin sequence and a unique form of bilateral hyperphalangy causing a clinodactyly of the index finger. We describe the identification of homozygous and compound heterozygous mutations in TGDS in seven unrelated individuals with typical Catel-Manzke syndrome by exome sequencing. Six different TGDS mutations were detected: c.892A>G (p.Asn298Asp), c.270_271del (p.Lys91Asnfs(∗)22), c.298G>T (p.Ala100Ser), c.294T>G (p.Phe98Leu), c.269A>G (p.Glu90Gly), and c.700T>C (p.Tyr234His), all predicted to be disease causing. By using haplotype reconstruction we showed that the mutation c.298G>T is probably a founder mutation. Due to the spectrum of the amino acid changes, we suggest that loss of function in TGDS is the underlying mechanism of Catel-Manzke syndrome. TGDS (dTDP-D-glucose 4,6-dehydrogenase) is a conserved protein belonging to the SDR family and probably plays a role in nucleotide sugar metabolism.

Effective diagnosis of genetic disease by computational phenotype analysis of the disease-associated genome

Less than half of patients with suspected genetic disease receive a molecular diagnosis. We have therefore integrated next-generation sequencing (NGS), bioinformatics, and clinical data into an effective diagnostic workflow. We used variants in the 2741 established Mendelian disease genes [the disease-associated genome (DAG)] to develop a targeted enrichment DAG panel (7.1 Mb), which achieves a coverage of 20-fold or better for 98% of bases. Furthermore, we established a computational method [Phenotypic Interpretation of eXomes (PhenIX)] that evaluated and ranked variants based on pathogenicity and semantic similarity of patients' phenotype described by Human Phenotype Ontology (HPO) terms to those of 3991 Mendelian diseases. In computer simulations, ranking genes based on the variant score put the true gene in first place less than 5% of the time; PhenIX placed the correct gene in first place more than 86% of the time. In a retrospective test of PhenIX on 52 patients with previously identified mutations and known diagnoses, the correct gene achieved a mean rank of 2.1. In a prospective study on 40 individuals without a diagnosis, PhenIX analysis enabled a diagnosis in 11 cases (28%, at a mean rank of 2.4). Thus, the NGS of the DAG followed by phenotype-driven bioinformatic analysis allows quick and effective differential diagnostics in medical genetics.

Severe congenital cutis laxa with cardiovascular manifestations due to homozygous deletions in ALDH18A1

Autosomal recessive cutis laxa (ARCL) type 2 constitutes a heterogeneous group of diseases mainly characterized by lax and wrinkled skin, skeletal anomalies, and a variable degree of intellectual disability. ALDH18A1-related ARCL is the most severe form within this disease spectrum. Here we report on the clinical and molecular findings of two affected individuals from two unrelated families. The patients presented with typical features of de Barsy syndrome and an overall progeroid appearance. However, the phenotype was highly variable including cardiovascular involvement in the more severe case. Investigation of a skin biopsy of one patient revealed not only the typical alterations of elastic fibers, but also an altered structure of mitochondria in cutaneous fibroblasts. Using conventional sequencing and copy number analysis we identified a frameshift deletion of one nucleotide and a microdeletion affecting the ALDH18A1 gene, respectively, in a homozygous state in both patients. Expression analysis in dermal fibroblasts from the patient carrying the microdeletion showed an almost complete absence of the ALDH18A1 mRNA resulting in an absence of the ALDH18A1 protein. So far, only 13 affected individuals from seven unrelated families suffering from ALDH18A1-related cutis laxa have been described in literature. Our findings provide new insights into the clinical spectrum and show that beside point mutations microdeletions are a possible cause of ALDH18A1-ARCL.

Clinical and biochemical features guiding the diagnostics in neurometabolic cutis laxa

Patients with cutis laxa (CL) have wrinkled, sagging skin with decreased elasticity. Skin symptoms are associated with variable systemic involvement. The most common, genetically highly heterogeneous form of autosomal recessive CL, ARCL2, is frequently associated with variable metabolic and neurological symptoms. Progeroid symptoms, dysmorphic features, hypotonia and psychomotor retardation are highly overlapping in the early phase of these disorders. This makes the genetic diagnosis often challenging. In search for discriminatory symptoms, we prospectively evaluated clinical, neurologic, metabolic and genetic features in our patient cohort referred for suspected ARCL. From a cohort of 26 children, we confirmed mutations in genes associated with ARCL in 16 children (14 probands), including 12 novel mutations. Abnormal glycosylation and gyration abnormalities were mostly, but not always associated with ATP6V0A2 mutations. Epilepsy was most common in ATP6V0A2 defects. Corpus callosum dysgenesis was associated with PYCR1 and ALDH18A1 mutations. Dystonic posturing was discriminatory for PYCR1 and ALDH18A1 defects. Metabolic markers of mitochondrial dysfunction were found in one patient with PYCR1 mutations. So far unreported white matter abnormalities were found associated with GORAB and RIN2 mutations. We describe a large cohort of CL patients with neurologic involvement. Migration defects and corpus callosum hypoplasia were not always diagnostic for a specific genetic defect in CL. All patients with ATP6V0A2 defects had abnormal glycosylation. To conclude, central nervous system and metabolic abnormalities were discriminatory in this genetically heterogeneous group, although not always diagnostic for a certain genetic defect in CL.

Missense variant in CCDC22 causes X-linked recessive intellectual disability with features of Ritscher-Schinzel/3C syndrome

Ritscher-Schinzel syndrome (RSS)/3C (cranio-cerebro-cardiac) syndrome (OMIM#220210) is a rare and clinically heterogeneous developmental disorder characterized by intellectual disability, cerebellar brain malformations, congenital heart defects, and craniofacial abnormalities. A recent study of a Canadian cohort identified homozygous sequence variants in the KIAA0196 gene, which encodes the WASH complex subunit strumpellin, as a cause for a form of RSS/3C syndrome. We have searched for genetic causes of a phenotype similar to RSS/3C syndrome in an Austrian family with two affected sons. To search for disease-causing variants, whole-exome sequencing (WES) was performed on samples from two affected male children and their parents. Before WES, CGH array comparative genomic hybridization was applied. Validation of WES and segregation studies was done using routine Sanger sequencing. Exome sequencing detected a missense variant (c.1670A>G; p.(Tyr557Cys)) in exon 15 of the CCDC22 gene, which maps to chromosome Xp11.23. Western blots of immortalized lymphoblastoid cell lines (LCLs) from the affected individual showed decreased expression of CCDC22 and an increased expression of WASH1 but a normal expression of strumpellin and FAM21 in the patients cells. We identified a variant in CCDC22 gene as the cause of an X-linked phenotype similar to RSS/3C syndrome in the family described here. A hypomorphic variant in CCDC22 was previously reported in association with a familial case of syndromic X-linked intellectual disability, which shows phenotypic overlap with RSS/3C syndrome. Thus, different inactivating variants affecting CCDC22 are associated with a phenotype similar to RSS/3C syndrome.

Transport activity and presence of ClC-7/Ostm1 complex account for different cellular functions

Loss of the lysosomal ClC-7/Ostm1 2Cl(-)/H(+) exchanger causes lysosomal storage disease and osteopetrosis in humans and additionally changes fur colour in mice. Its conversion into a Cl(-) conductance in Clcn7(unc/unc) mice entails similarly severe lysosomal storage, but less severe osteopetrosis and no change in fur colour. To elucidate the basis for these phenotypical differences, we generated Clcn7(td/td) mice expressing an ion transport-deficient mutant. Their osteopetrosis was as severe as in Clcn7(-/-) mice, suggesting that the electric shunt provided by ClC-7(unc) can partially rescue osteoclast function. The normal coat colour of Clcn7(td/td) mice and their less severe neurodegeneration suggested that the ClC-7 protein, even when lacking measurable ion transport activity, is sufficient for hair pigmentation and that the conductance of ClC-7(unc) is harmful for neurons. Our in vivo structure-function analysis of ClC-7 reveals that both protein-protein interactions and ion transport must be considered in the pathogenesis of ClC-7-related diseases.

CLCN7 and TCIRG1 mutations differentially affect bone matrix mineralization in osteopetrotic individuals

Osteopetrosis is an inherited disorder of impaired bone resorption, with the most commonly affected genes being CLCN7 and TCIRG1, encoding the Cl(-) /H(+) exchanger CLC-7 and the a3 subunit of the vacuolar H(+) -ATPase, respectively. We and others have previously shown that the disease is frequently accompanied by osteomalacia, and that this additional pathology is also found in Tcirg1-deficient oc/oc mice. The remaining question was whether osteoid enrichment is specifically associated with TCIRG1 inactivation, or whether CLCN7 mutations would also cause skeletal mineralization defects. Here we describe a complete osteologic assessment of one family carrying a novel mutation in CLCN7 (D145G), which impairs the activation and relaxation kinetics of the CLC-7 ion transporter. The two siblings carrying the mutation in the homozygous state displayed high bone mass, increased serum levels of bone formation markers, but no impairment of calcium homeostasis when compared to the other family members. Most importantly, however, undecalcified processing of an iliac crest biopsy from one of the affected children clearly demonstrated a pathological increase of trabecular bone mass, but no signs of osteomalacia. Given the potential relevance of these findings we additionally performed undecalcified histology of iliac crest biopsies from seven additional cases with osteopetrosis caused by a mutation in TNFRSF11A (n=1), CLCN7 (n=3), or TCIRG1 (n=3). Here we observed that all cases with TCIRG1-dependent osteopetrosis displayed severe osteoid accumulation and decreased calcium content within the mineralized matrix. In contrast, there was no detectable bone mineralization defect in the cases with TNFRSF11A-dependent or CLCN7-dependent osteopetrosis. Taken together, our analysis demonstrates that CLCN7 and TCIRG1 mutations differentially affect bone matrix mineralization, and that there is a need to modify the current classification of osteopetrosis.

Neonatal progeroid variant of Marfan syndrome with congenital lipodystrophy results from mutations at the 3' end of FBN1 gene

We report a 16-year-old girl with neonatal progeroid features and congenital lipodystrophy who was considered at birth as a possible variant of Wiedemann-Rautenstrauch syndrome. The emergence of additional clinical signs (marfanoid habitus, severe myopia and dilatation of the aortic bulb) lead to consider the diagnosis of the progeroid variant of Marfan syndrome. A de novo donor splice-site mutation (c.8226+1G>A) was identified in FBN1. We show that this mutation leads to exon 64 skipping and to the production of a stable mRNA that should allow synthesis of a truncated profibrillin-1, in which the C-terminal furin cleavage site is altered. FBN1 mutations associated with a similar phenotype have only been reported in four other patients. We confirm the correlation between marfanoid phenotype with congenital lipodystrophy and neonatal progeroid features (marfanoid-progeroid-lipodystrophy syndrome) and frameshift mutations at the 3' end of FBN1. This syndrome should be considered in differential diagnosis of neonatal progeroid syndromes.

Multiscale, converging defects of macro-porosity, microstructure and matrix mineralization impact long bone fragility in NF1

Bone fragility due to osteopenia, osteoporosis or debilitating focal skeletal dysplasias is a frequent observation in the Mendelian disease Neurofibromatosis type 1 (NF1). To determine the mechanisms underlying bone fragility in NF1 we analyzed two conditional mouse models, Nf1Prx1 (limb knock-out) and Nf1Col1 (osteoblast specific knock-out), as well as cortical bone samples from individuals with NF1. We examined mouse bone tissue with micro-computed tomography, qualitative and quantitative histology, mechanical tensile analysis, small-angle X-ray scattering (SAXS), energy dispersive X-ray spectroscopy (EDX), and scanning acoustic microscopy (SAM). In cortical bone of Nf1Prx1 mice we detected ectopic blood vessels that were associated with diaphyseal mineralization defects. Defective mineral binding in the proximity of blood vessels was most likely due to impaired bone collagen formation, as these areas were completely devoid of acidic matrix proteins and contained thin collagen fibers. Additionally, we found significantly reduced mechanical strength of the bone material, which was partially caused by increased osteocyte volume. Consistent with these observations, bone samples from individuals with NF1 and tibial dysplasia showed increased osteocyte lacuna volume. Reduced mechanical properties were associated with diminished matrix stiffness, as determined by SAM. In line with these observations, bone tissue from individuals with NF1 and tibial dysplasia showed heterogeneous mineralization and reduced collagen fiber thickness and packaging. Collectively, the data indicate that bone fragility in NF1 tibial dysplasia is partly due to an increased osteocyte-related micro-porosity, hypomineralization, a generalized defect of organic matrix formation, exacerbated in the regions of tensional and bending force integration, and finally persistence of ectopic blood vessels associated with localized macro-porotic bone lesions.

ClC-7 expression levels critically regulate bone turnover, but not gastric acid secretion

Mutations in the 2Cl(-)/1H(+)-exchanger ClC-7 impair osteoclast function and cause different types of osteoclast-rich osteopetrosis. However, it is unknown to what extent ClC-7 function has to be reduced to become rate-limiting for bone resorption. In osteoclasts from osteopetrosis patients expression of the mutated ClC-7 protein did not correlate with disease severity and resorption impairment. Therefore, a series of transgenic mice expressing ClC-7 in osteoclasts at different levels was generated. Crossing of these mice with Clcn7(-/-) mutants rescued the osteopetrotic phenotype to variable degrees. One resulting double transgenic line mimicked human autosomal dominant osteopetrosis. The trabecular bone of these mice showed a reduction of osteoblast numbers, osteoid, and osteoblast marker gene expression indicative of reduced osteoblast function. In osteoclasts from these mutants ClC-7 expression levels were 20 to 30% of wildtype levels. These reduced levels not only impaired resorptive activity, but also increased numbers, size and nucleus numbers of osteoclasts differentiated in vitro. Although ClC-7 was expressed in the stomach and PTH levels were high in Clcn7(-/-) mutants loss of ClC-7 did not entail a relevant elevation of gastric pH. In conclusion, we show that in our model a reduction of ClC-7 function by approximately 70% is sufficient to increase bone mass, but does not necessarily enhance bone formation. ClC-7 does not appear to be crucially involved in gastric acid secretion, which explains the absence of an osteopetrorickets phenotype in CLCN7-related osteopetrosis.