Disease relevance of rare VPS13B missense variants for neurodevelopmental Cohen syndrome.

Autosomal recessive Cohen syndrome is a neurodevelopmental disorder characterized by postnatal microcephaly, intellectual disability, and a typical facial gestalt. Genetic variants in VPS13B have been found to cause Cohen syndrome, but have also been linked to autism, retinal disease, primary immunodeficiency, and short stature. While it is well established that loss-of-function mutations of VPS13B cause Cohen syndrome, the relevance of missense variants for the pathomechanism remains unexplained. Here, we investigate their pathogenic effect through a systematic re-evaluation of clinical patient information, comprehensive in silico predictions, and in vitro testing of previously published missense variants. In vitro analysis of 10 subcloned VPS13B missense variants resulted in full-length proteins after transient overexpression. 6/10 VPS13B missense variants show reduced accumulation at the Golgi complex in the steady state. The overexpression of these 6/10 VPS13B missense variants did not rescue the Golgi fragmentation after the RNAi-mediated depletion of endogenous VPS13B. These results thus validate 6/10 missense variants as likely pathogenic according to the classification of the American College of Medical Genetics through the integration of clinical, genetic, in silico, and experimental data. In summary, we state that exact variant classification should be the first step towards elucidating the pathomechanisms of genetically inherited neuronal diseases.

Primary Hypertrophic Osteoarthropathy Mimicking Juvenile Idiopathic Arthritis: A Novel SLCO2A1 Mutation and Imaging Findings.

Primary hypertrophic osteoarthropathy (PHO), also known as pachydermoperiostosis, is a rare, multisystemic, autosomal recessive condition typically presenting with digital clubbing, osteoarthropathy, and various skin manifestations. Radiographs show distinctive periosteal reaction and thickening along the long bones. PHO is caused by homozygous mutations in the HPGD gene in chromosome 4q34.1 or the SLCO2A1 gene in 3q22.1q22.2. Here, we report on a 20-year-old male with enlarged and swollen joints with arthralgia, palmoplantar hyperhidrosis, and large hands and feet with marked digital clubbing. We also present radiographic, MRI, and ultrasonographic features of the case. These clinical and imaging findings were compatible with the diagnosis of PHO, and a novel homozygous mutation, c.576C>G, p.Ile192Met, was found in SLCO2A1.

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.

Cohen syndrome-associated protein COH1 physically and functionally interacts with the small GTPase RAB6 at the Golgi complex and directs neurite outgrowth.

Postnatal microcephaly, intellectual disability, and progressive retinal dystrophy are major features of autosomal recessive Cohen syndrome, which is caused by mutations in the gene COH1 (VPS13B). We have recently identified COH1 as a Golgi-enriched scaffold protein that contributes to the structural maintenance and function of the Golgi complex. Here, we show that association of COH1 with the Golgi complex depends on the small GTPase RAB6. RNAi-mediated knockdown of RAB6A/A' prevents the localization of COH1 to the Golgi complex. Expression of the constitutively inactive RAB6_T27N mutant led to an increased solubilization of COH1 from lipid membrane preparations. Co-IP experiments confirmed the physical interaction of COH1 with RAB6 that preferentially occurred with the constitutively active RAB6_Q72L mutants. Depletion of COH1 in primary neurons negatively interfered with neurite outgrowth, indicating a causal link between the integrity of the Golgi complex and axonal outgrowth. We conclude that COH1 is a RAB6 effector protein and that reduced brain size in Cohen syndrome patients likely results from impaired COH1 function at the Golgi complex, causing decreased neuritogenesis.

Expanded spectrum of exon 33 and 34 mutations in SRCAP and follow-up in patients with Floating-Harbor syndrome.

BACKGROUND: Floating-Harbor syndrome is a rare autosomal dominant short stature syndrome with retarded speech development, intellectual disability and dysmorphic facial features. Recently dominant mutations almost exclusively located in exon 34 of the Snf2-related CREBBP activator protein gene were identified to cause FHS. METHODS: Here we report the genetic analysis of 5 patients fulfilling the diagnostic criteria of FHS obtained by Sanger sequencing. All of them presented with short stature, speech delay as well as psychomotor delay and typical facial dysmorphism. Three patients showed a good response to growth hormone treatment. RESULTS: Two patients demonstrate novel, heterozygous de novo frameshift mutations in exon 34 (c.7396delA and c.7218dupT) leading to premature stop mutations in SRCAP (p.Val2466Tyrfs*9 and p.Gln2407Serfs*36, respectively). In two further patients we found already known SRCAP mutations in exon 34, c.7330C > T and c.7303C > T, respectively, which also lead to premature stop codons: p.Arg2444* and p.Arg2435*. In one patient, we identified a novel de novo stop mutation in exon 33 (c.6985C > T, p.Arg2329*) demonstrating that not all FHS cases are caused by mutations in exon 34 of SRCAP. CONCLUSIONS: Our data confirm a mutational hot spot in the final exon of SRCAP in the majority of FHS patients but also show that exon 33 of this gene can be affected.

Annexin A2 mediates apical trafficking of renal Na⁺-K⁺-2Cl⁻ cotransporter.

The furosemide-sensitive Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) is responsible for urine concentration and helps maintain systemic salt homeostasis. Its activity depends on trafficking to, and insertion into, the apical membrane, as well as on phosphorylation of conserved N-terminal serine and threonine residues. Vasopressin (AVP) signaling via PKA and other kinases activates NKCC2. Association of NKCC2 with lipid rafts facilitates its AVP-induced apical translocation and activation at the surface. Lipid raft microdomains typically serve as platforms for membrane proteins to facilitate their interactions with other proteins, but little is known about partners that interact with NKCC2. Yeast two-hybrid screening identified an interaction between NKCC2 and the cytosolic protein, annexin A2 (AnxA2). Annexins mediate lipid raft-dependent trafficking of transmembrane proteins, including the AVP-regulated water channel, aquaporin 2. Here, we demonstrate that AnxA2, which binds to phospholipids in a Ca(2+)-dependent manner and may organize microdomains, is codistributed with NKCC2 to promote its apical translocation in response to AVP stimulation and low chloride hypotonic stress. NKCC2 and AnxA2 interact in a phosphorylation-dependent manner. Phosphomimetic AnxA2 carrying a mutant phosphoacceptor (AnxA2-Y24D-GFP) enhanced surface expression and raft association of NKCC2 by 5-fold upon low chloride hypotonic stimulation, whereas AnxA2-Y24A-GFP and PKC-dependent AnxA2-S26D-GFP did not. As the AnxA2 effect involved only nonphosphorylated NKCC2, it appears to affect NKCC2 trafficking. Overexpression or knockdown experiments further supported the role of AnxA2 in the apical translocation and surface expression of NKCC2. In summary, this study identifies AnxA2 as a lipid raft-associated trafficking factor for NKCC2 and provides mechanistic insight into the regulation of this essential cotransporter.

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.

Mutations in the prostaglandin transporter encoding gene SLCO2A1 cause primary hypertrophic osteoarthropathy and isolated digital clubbing.

Digital clubbing is usually secondary to different acquired diseases. Primary hypertrophic osteoarthropathy (PHO) is a rare hereditary disorder with variable digital clubbing as the most prominent feature, subperiosteal new bone formation, and arthropathy. Recently, mutations in the 15-hydroxy-prostaglandin dehydrogenase (15-PGDH) encoding gene HPGD were found to cause PHO. Here, we identified three unrelated families with different mutations in the prostaglandin transporter (PGT) encoding gene SLCO2A1 which presumably result in reduced metabolic clearance by 15-PGDH due to diminished cellular uptake of prostaglandin E(2) (PGE(2)) by mutant PGT. In two consanguineous families, homozygous mutations, an intragenic deletion that results in frameshift and a missense mutation, are associated with a severe PHO phenotype. In a third family, a heterozygous carrier of a stop mutation presents with isolated digital clubbing. Thus, our study further supports the importance of PGE(2) metabolism in the pathogenesis of digital clubbing and PHO.

Cohen syndrome-associated protein, COH1, is a novel, giant Golgi matrix protein required for Golgi integrity.

Loss-of-function mutations in the gene COH1, also known as VPS13B, lead to autosomal recessive Cohen syndrome. However, the cellular distribution and function of the encoded protein COH1 (3997 amino acids), which lacks functional homologies to other mammalian proteins, have remained enigmatic. We show here that COH1 is a peripheral Golgi membrane protein that strongly co-localizes with the cis-Golgi matrix protein GM130. Consistent with its subcellular localization, COH1 depletion using RNAi causes fragmentation of the Golgi ribbon into ministacks. Disruption of Golgi organization observed in fibroblasts from Cohen syndrome patients suggests that Golgi dysfunction contributes to Cohen syndrome pathology. In conclusion, our findings establish COH1 as a Golgi-associated matrix protein required for Golgi integrity.

Identification of FOXP1 deletions in three unrelated patients with mental retardation and significant speech and language deficits.

Mental retardation affects 2-3% of the population and shows a high heritability.Neurodevelopmental disorders that include pronounced impairment in language and speech skills occur less frequently. For most cases, the molecular basis of mental retardation with or without speech and language disorder is unknown due to the heterogeneity of underlying genetic factors.We have used molecular karyotyping on 1523 patients with mental retardation to detect copy number variations (CNVs) including deletions or duplications. These studies revealed three heterozygous overlapping deletions solely affecting the forkhead box P1 (FOXP1) gene. All three patients had moderate mental retardation and significant language and speech deficits. Since our results are consistent with a de novo occurrence of these deletions, we considered them as causal although we detected a single large deletion including FOXP1 and additional genes in 4104 ancestrally matched controls. These findings are of interest with regard to the structural and functional relationship between FOXP1 and FOXP2. Mutations in FOXP2 have been previously related to monogenic cases of developmental verbal dyspraxia. Both FOXP1 and FOXP2 are expressed in songbird and human brain regions that are important for the developmental processes that culminate in speech and language.

Familial hemophagocytic lymphohistiocytosis type 5 (FHL-5) is caused by mutations in Munc18-2 and impaired binding to syntaxin 11.

Rapid intracellular transport and secretion of cytotoxic granules through the immunological synapse requires a balanced interaction of several proteins. Disturbance of this highly regulated process underlies familial hemophagocytic lymphohistiocytosis (FHL), a genetically heterogeneous autosomal-recessive disorder characterized by a severe hyperinflammatory phenotype. Here, we have assigned FHL-5 to a 1 Mb region on chromosome 19p by using high-resolution SNP genotyping in eight unrelated FHL patients from consanguineous families. Subsequently, we found nine different mutations, either truncating or missense, in STXBP2 in twelve patients from Turkey, Saudi Arabia, and Central Europe. STXBP2 encodes syntaxin binding protein 2 (Munc18-2), involved in the regulation of vesicle transport to the plasma membrane. We have identified syntaxin 11, a SNARE protein mutated in FHL-4, as an interaction partner of STXBP2. This interaction is eliminated by the missense mutations found in our FHL-5 patients, which leads to a decreased stability of both proteins, as shown in patient lymphocytes. Activity of natural killer and cytotoxic T cells was markedly reduced or absent, as determined by CD107 degranulation. Our findings thus identify a key role for STXBP2 in lytic granule exocytosis.

HPGD mutations cause cranioosteoarthropathy but not autosomal dominant digital clubbing.

Cranio-osteoarthropathy, clinically classified as a variant of primary hypertrophic osteoarthropathy, is a very rare autosomal-recessive condition characterized by delayed closure of the cranial sutures and fontanels, digital clubbing, arthropathy, and periostosis. Recently, mutations in the gene HPGD, which encodes the NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase, were reported in four families affected with primary hypertrophic osteoarthropathy and one family with autosomal-recessive isolated nail clubbing. We report the clinical and molecular findings in four patients from two families affected with cranio-osteoarthropathy and one family with isolated, autosomal dominant digital clubbing. Genome-wide homozygosity mapping identified a locus for cranio-osteoarthropathy harboring the HPGD gene in one affected family. We detected two novel homozygous mutations in HPGD in these families: a missense mutation affecting the NAD(+) binding motif and a frameshift mutation. The clinical presentation in our patients was variable. Digital clubbing and hyperhidrosis were present in all cases. Delayed closure of the cranial sutures and fontanels, periostosis, and arthropathy were not consistent clinical features. No HPGD mutation was detected in a familial case of autosomal dominant isolated digital clubbing. The failure to identify any mutation in a family with an autosomal dominant type of isolated digital clubbing suggests that HPGD is not the major gene for this condition.

Expanded mutational spectrum in Cohen syndrome, tissue expression, and transcript variants of COH1.

Cohen syndrome is characterised by mental retardation, postnatal microcephaly, facial dysmorphism, pigmentary retinopathy, myopia, and intermittent neutropenia. Mutations in COH1 (VPS13B) have been found in patients with Cohen syndrome from diverse ethnic origins. We have carried out mutation analysis in twelve novel patients with Cohen syndrome from nine families. In this series, we have identified 13 different mutations in COH1, twelve of these are novel including six frameshift mutations, four nonsense mutations, two splice site mutations, and a one-codon deletion. Since different transcripts of COH1 have been reported previously, we have analysed the expression patterns of COH1 splice variants. The transcript variant NM_152564 including exon 28b showed ubiquitous expression in all examined human tissues. In contrast, human brain and retina showed differential splicing of exon 28 (NM_017890). Moreover, analysis of mouse tissues revealed ubiquitous expression of Coh1 homologous to human NM_152564 in all examined tissues but no prevalent alternative splicing.

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.

Allelic heterogeneity in the COH1 gene explains clinical variability in Cohen syndrome.

Cohen syndrome is a rare autosomal recessive disorder with a variable clinical picture mainly characterized by developmental delay, mental retardation, microcephaly, typical facial dysmorphism, progressive pigmentary retinopathy, severe myopia, and intermittent neutropenia. A Cohen syndrome locus was mapped to chromosome 8q22 in Finnish patients, and, recently, mutations in the gene COH1 were reported in patients with Cohen syndrome from Finland and other parts of northern and western Europe. Here, we describe clinical and molecular findings in 20 patients with Cohen syndrome from 12 families, originating from Brazil, Germany, Lebanon, Oman, Poland, and Turkey. All patients were homozygous or compound heterozygous for mutations in COH1. We identified a total of 17 novel mutations, mostly resulting in premature termination codons. The clinical presentation was highly variable. Developmental delay of varying degree, early-onset myopia, joint laxity, and facial dysmorphism were the only features present in all patients; however, retinopathy at school age, microcephaly, and neutropenia are not requisite symptoms of Cohen syndrome. The identification of novel mutations in COH1 in an ethnically diverse group of patients demonstrates extensive allelic heterogeneity and explains the intriguing clinical variability in Cohen syndrome.