Autosomal Recessive Osteopetrosis Research Articles

SNX10 regulates osteoclastogenic cell fusion and osteoclast size in mice.

Bone-resorbing osteoclasts (OCLs) are formed by differentiation and fusion of monocyte precursor cells, generating large multi-nucleated cells. Tightly-regulated cell fusion during osteoclastogenesis leads to formation of resorption-competent OCLs, whose sizes fall within a predictable physiological range. The molecular mechanisms that regulate the onset of OCL fusion and its subsequent arrest are, however, largely unknown. We have previously shown that OCLs cultured from mice homozygous for the R51Q mutation in the vesicle trafficking-associated protein sorting nexin 10, a mutation that induces autosomal recessive osteopetrosis in humans and in mice, display deregulated and continuous fusion that generates gigantic, inactive OCLs. Fusion of mature OCLs is therefore arrested by an active, genetically-encoded, cell-autonomous, and SNX10-dependent mechanism. In order to directly examine whether SNX10 performs a similar role in vivo, we generated SNX10-deficient (SKO) mice and demonstrated that they display massive osteopetrosis and that their OCLs fuse uncontrollably in culture, as do homozygous R51Q SNX10 (RQ/RQ) mice. OCLs that lack SNX10 exhibit persistent presence of DC-STAMP protein at their periphery, which may contribute to their uncontrolled fusion. In order to visualize endogenous SNX10-mutant OCLs in their native bone environment we genetically labelled the OCLs of wild-type, SKO and RQ/RQ mice with EGFP, and then visualized the three-dimensional organization of resident OCLs and the pericellular bone matrix by two-photon, confocal, and second harmonics generation microscopy. We show that the volumes, surface areas and, in particular, the numbers of nuclei in the OCLs of both mutant strains were on average 2-6 fold larger than those of OCLs from wild-type mice, indicating that deregulated, excessive fusion occurs in the mutant mice. We conclude that the fusion of OCLs, and consequently their size, are regulated in vivo by SNX10-dependent arrest of fusion of mature OCLs.

Osteopetrosis in the pediatric patient: what the radiologist needs to know.

Osteopetrosis describes several types of rare sclerosing bone dysplasias of varying clinical and radiographic severity. The classic autosomal dominant subtype emerges most often in adolescence but can present from infancy through adulthood. The autosomal recessive osteopetrosis, or "malignant infantile osteopetrosis," presents in infancy with a grimmer prognosis, though the autosomal dominant forms (often mislabeled as "benign") actually can have life-threatening consequences as well. Often osteopetrosis is detected due to skeletal findings on radiographs performed to evaluate injury or as an incidental finding during evaluation for illness. Given the varied phenotypic severity and presentations at different ages, radiologists play an integral role in the care of these patients both in diagnosis and in clinical evaluation and monitoring. A deeper understanding of the underlying genetic basis of the disease can aid in the radiologist in diagnosis and in anticipation of unique complications. An overview of current clinical management is also discussed.

Autosomal Dominant Osteopetrosis (ADO) Caused by a Missense Variant in the TCIRG1 Gene.

Autosomal dominant osteopetrosis (ADO) is a rare genetic disorder resulting from impaired osteoclastic bone resorption. Clinical manifestations frequently include fractures, osteonecrosis (particularly of the jaw or maxilla), osteomyelitis, blindness, and/or bone marrow failure. ADO usually results from heterozygous missense variants in the Chloride Channel 7 gene (CLCN7) that cause disease by a dominant negative mechanism. Variants in the T-cell immune regulator 1 gene (TCIRG1) are commonly identified in autosomal recessive osteopetrosis but have only been reported in 1 patient with ADO. Here, we report 3 family members with a single heterozygous missense variant (p.Gly579Arg) in TCIRG1 who have a phenotype consistent with ADO. Three of 5 protein prediction programs suggest this variant likely inhibits the function of TCIRG1. This is the first description of adult presentation of ADO caused by a TCIRG1 variant. Similar to families with ADO from CLCN7 mutations, this variant in TCIRG1 results in marked phenotype variability, with 2 subjects having severe disease and the third having very mild disease. This family report implicates TCIRG1 missense mutations as a cause of ADO and demonstrates that the marked phenotypic variability in ADO may extend to disease caused by TCIRG1 missense mutations.

The molecular spectrum of Turkish osteopetrosis and related osteoclast disorders with natural history (including a candidate gene, CCDC120).

Osteopetrosis and related osteoclastic disorders are a heterogeneous group of inherited diseases characterized by increased bone density. The aim of this study is to investigate the molecular spectrum and natural history of the clinical and radiological features of these disorders. 28 patients and 20 families were enrolled in the study; 20 of them were followed for a period of 1-16 years. Targeted gene analysis and whole-exome sequencing (WES) were performed. Biallelic mutations in CLCN7 or TCIRG1 were detected in three families, in TNFRSF11A or CA2 in two families, and in SNX10 in one family in the osteopetrosis group. A heterozygous variant of CLCN7 was also found in one family. In the osteopetrosis and related osteoclast disorders group, three different variants of CTSK were detected in five families with pycnodysostosis and the SLC29A3 variant causing dysosteosclerosis was detected in one family. In autosomal recessive osteopetrosis (ARO), a malignant infantile form, four patients died during follow-up, two of whom had undergone hematopoietic stem cell transplantation. Interestingly, all patients had osteopetrorachia of the long bone metaphyses in infancy, typical skeletal features such as Erlenmeyer flask deformity and bone-in-bone appearance that developed toward the end of early childhood. Two siblings with a biallelic missense mutation in CLCN7 and one patient with the compound heterozygous intronic TCIRG1 variant corresponded to the intermediate form of ARO (IARO); there was intrafamilial clinical heterogeneity in the family with the CLCN7 variant. One of two patients with IARO and distal tubular acidosis was found to have a large deletion in CA2. In one family, two siblings with a heterozygous mutation in CLCN7 were affected, whereas the father with the same mutation was asymptomatic. In WES analysis of three brothers from a family without mutations in osteopetrosis genes, a hemizygous missense variant in CCDC120, a novel gene, was found to be associated with high bone mass. This study extended the natural history of the different types of osteopetrosis and also introduced a candidate gene potentially causing osteopetrosis.

Modeling TCIRG1 Neutropenia By Utilizing Patient Derived Induced Pluripotent Stem Cells

Background: Severe congenital neutropenia (SCN) is a hereditary hematopoietic disorder characterized by recurrent infections and leukemic transformation. We previously identified a novel heterozygous missense mutation (R736S) in TCIRG1 causing SCN in a large multigenerational family (PMID: 24753205). We also identified a statistically significant correlation in individuals with lower ANC with rare missense variants in TCIRG1 in a large cohort of population-based genome screening (PMID: 27229898). Recently, we have discovered another TCIRG1 heterozygousvariant: R736C in two unrelated families with congenital neutropenia. Independently, Shinwari et al identified a novel TCIRG1 mutation, V52L, which was correlated with the neutropenic phenotype (PMID: 35573728). Homozygous mutations of TCIRG1 are causing autosomal recessive osteopetrosis (ARO), which is characterized by osteopetrosis and secondary hematological and neurological abnormalities. The Genome Aggregation Database (gnomAD) has no data about TCIRG1 R736S and R736C variants accruing in the healthy population. This is in contrast of numerous healthy individuals with a various heterozygous variants throughout the TCIRG1 gene. Aims: We aim to generate in-vitro model of TCIRG1 neutropenia by utilizing patient derived induced pluripotent stem cells (iPSCs) and characterize the hematological abnormalities caused by mutant TCIRG1 expression. We also aim to characterize the incidence of TCIRG1-associated neutropenia and the prevalence of TCIRG1 variants by sharing this new information within the scientific community through identifying more cases/families with this disease. Methods: Medical history, differential WBC counts and whole exome sequencing (WES) was used to identify the genetic basis of neutropenia in the family with R736S mutation. The R736C mutation in the new families also were identified by WES of the index cases. Medical history, blood counts and Sanger sequencing identified their affected relatives. iPSC lines of 4 affected individuals from 2 families harboring TCIRG1 R736S and R736C mutations and healthy volunteer were generated using standard methodology. S732F benign heterozygous TCIRG1 variant was generated by utilizing CRISPR/Cas9 knock-In gene editing using a healthy volunteer iPSC line. iPSCs hematopoietic differentiation was achieved by using STEMdiff Hematopoietic Kit. Resultant CD34+ cells were differentiated towards neutrophils using a published protocol (PMID: 35795780). Cell proliferation, survival and differentiation characteristics were measured by automated cell counter and flow cytometry, cell viability staining and surface markers analysis for granulocytic differentiation. Results: The discovery of these families came through the reputation and work of the Severe Congenital Neutropenia International Registry (SCNIR) and the expanding use of WES to identify the causes of hereditary disorders. With the identification of the index cases, we have identified several affected family members in each family. In all three families the severity of neutropenia is variable and correlates with the incidence of recurrent bacterial infections. Myeloid differentiation of CD34+ cells derived from iPSCs of 4 patients from two families with R736S and R736C mutations revealed maturation, survival and proliferation abnormalities compared to volunteer cells and cell line harboring benign S732F heterozygous TCIRG1 mutation. All patient cells had from 2 to 3-fold drop in cell proliferation and cell viability drop by over 70% for the patients with R736S and up to 90% with R736C mutation compared to the controls. The myeloid differentiation in the patients' cells, measured by flow cytometry and assessed by CD66b+/CD14+ and CD11b+/CD15+ markers was impaired by 38% and 23% respectively for the patients with R736S and 43% and 24% for the patients harboring R736C mutation. Conclusions: The pathogenesis of TCIRG1-associated neutropenia is currently unknown. Carriers of pathogenic variants from these families appear to have neutropenia with variable severity. Our preliminary studies show that patient iPSC derived hematopoietic cells closely recapitulate the neutropenic phenotype. Population based genomic studies; careful family histories and WES will identify new patients with this disorder.

Autosomal recessive osteopetrosis type I: description of pathogenic variant of TCIRG1 gene

Autosomal recessive osteopetrosis type I: description of pathogenic variant of TCIRG1 gene

Osteopetrorickets: two contradictory patterns-one unifying diagnosis.

A 5-month-old infant with bone findings on x-ray presented an apparent contradiction including findings of both diffusely dense bones and rickets in the context of a history and laboratory investigation that suggested leukemia. Next generation gene panel sequencing revealed a TCIRG1 mutation which is consistent with autosomal recessive osteopetrosis. The paradoxical x-ray findings underscore a recently elucidated mechanism for the pathogenesis of a TCIRG mutation. This case highlights the importance of recognizing this radiographic, seeming contradictory, association in the context of a confusing clinical presentation. Failure to recognize this pattern promptly may lead to a delay in diagnosis, thus potentially permanent organ failure.

Osteopetrosis: Discovery and early history of "marble bone disease".

Discovery in 1904 of the disorder initially called "marble bones", then in 1926 more appropriately referred to as "osteopetrosis", is attributed to Heinrich E. Albers-Schönberg (1865-1921), the first radiologist. He used the new technique of "Röntgenographie" to report in a young man the radiographic hallmarks of this osteopathy. Clinical descriptions of lethal forms of osteopetrosis had apparently been published earlier by others. In 1926, "osteopetrosis" (stony or petrified bones) replaced "marble bone disease" because the skeletal fragility resembled limestone more than marble. In 1936, despite fewer than 80 reported patients, a fundamental defect in hematopoiesis, secondarily impacting the entire skeleton, was hypothesized. By 1938, the signature histopathological finding of osteopetrosis was recognized -- persistence of unresorbed calcified growth plate cartilage. Also, it was apparent that besides lethal autosomal recessive osteopetrosis a less severe form was "handed down directly from generation to generation". In 1965, quantitative, but also qualitative, defects in osteoclasts became apparent. Here, I review the discovery and early understanding of osteopetrosis. Characterization of this disorder commencing at the beginning of the past century would support the aphorism of Sir William Osler (1849-1919): "Clinics Are Laboratories; Laboratories Of The Highest Order". As featured in this special issue of Bone, the osteopetroses would prove remarkably informative about the formation and function of the cells responsible for skeletal resorption.

Osteopetrosis Revealed by Pseudarthrosis of a Neglected Subtrochanteric Fracture in an Adult: Case Report

First described in 1904, Osteopetrosis (OP) is a rare genetic disorder [1] affecting osteoclast activity resulting in decreased bone resorption and bone densification [2]. There are three types of osteopetrosis depending on the world of transmission: autosomal dominant OP, autosomal recessive OP, and intermediate autosomal OP [3]. Indeed, osteopetrosis is characterized by a hard but fragile bone with a narrow or absent medullary canal [4]. Consequently, the bone is so dense and fragile that it is subject to an increased risk of fractures and complications during surgery, which is why patients with OP are treated conservatively [5]. Furthermore, due to poor blood supply, patients with osteopetrosis have a high risk of infection and osteomyelitis, in addition to the risk of pseudarthrosis [6].

A novel compound heterozygous mutation of the CLCN7 gene is associated with autosomal recessive osteopetrosis.

Osteopetrosis is a genetic condition of the skeleton characterized by increased bone density caused by osteoclast formation and function defects. Osteopetrosis is inherited in the form of autosomal dominant and autosomal recessive manner. We report autosomal recessive osteopetrosis (ARO; OMIM 611490) in a Chinese case with a history of scarce leukocytosis, vision and hearing loss, frequent seizures, and severe intellectual and motor disability. Whole-exome sequencing (WES) followed by Sanger sequencing revealed novel compound heterozygous mutations in the chloride channel 7 (CLCN7) gene [c.982-1G > C and c.1208G > A (p. Arg403Gln)] in the affected individual, and subsequent familial segregation showed that each parent had transmitted a mutation. Our results confirmed that mutations in the CLCN7 gene caused ARO in a Chinese family. Additionally, our study expanded the clinical and allelic spectrum of the CLCN7 gene and enhanced the applications of WES technology in determining the etiology of prenatal diagnoses in fetuses with ultrasound anomalies.

Bone Turnover Marker for the Evaluation of Skeletal Remodelling in Autosomal Recessive Osteopetrosis after Haematopoietic Stem Cell Transplantation: A Case Report

Background: Autosomal recessive osteopetrosis (ARO) is a rare genetic disorder of bone metabolism, primarily affecting the remodelling function of osteoclasts. Haematopoietic stem cell transplantation (HSCT) is the first-line treatment for ARO. Traditional tools for the assessment of therapeutic response, such as measuring donor chimerism, do not provide information on bone remodelling. The use of bone turnover markers (BTMs) might be ideal. Here, we report a case of a paediatric ARO patient undergoing successful HSCT. Methods: For the evaluation of donor-derived osteoclast activity and skeletal remodelling throughout the transplantation, the bone resorption marker β-CTX (β-C-terminal telopeptide) was used. Results: The low baseline level of β-CTX markedly increased after transplantation and remained in the elevated range even after 3 months. Donor-derived osteoclast activity reached its new baseline level around the 50th percentile range after 5 months and proved to be stable during the 15-month follow-up time. The apparent increase of the baseline osteoclast activity after HSCT was in consonance with the radiographic improvement of the disease phenotype and the correction of bone metabolic parameters. Despite the successful donor-derived osteoclast recovery, craniosynostosis developed, and reconstructive surgery had to be performed. Conclusions: The use of β-CTX may be of aid in assessing osteoclast activity throughout the transplantation. Further studies could help to establish the extended BTM profile of ARO patients using the available osteoclast- and osteoblast-specific markers.

CLCN7, a gene shared by autosomal recessive and autosomal dominant osteopetrosis.

After the discovery of abundant v-ATPase complexes in the osteoclast ruffled membrane it was obvious that in parallel a negative counter-ion needs to be transported across this membrane to allow for efficient transport of protons into the resorption lacuna. While different candidate proteins were discussed the osteopetrosis phenotype of Clcn7 knockout mice suggested that the chloride/proton-exchanger ClC-7 might be responsible for transporting the negative charge. In the following, individuals with autosomal recessive osteopetrosis (ARO) were found to carry biallelic CLCN7 pathogenic variants. Shortly thereafter, heterozygous pathogenic variants were identified as the exclusive cause of autosomal dominant osteopetrosis type 2 (ADO2). Since in most cell types other than osteoclasts ClC-7 resides in late endosomes and lysosomes, it took some time until the electrophysiological properties of ClC-7 were elucidated. Whereas most missense variants lead to reduced chloride currents, several variants with accelerated kinetics have been identified. Evidence for folding problems is also known for several missense variants. Paradoxically, a heterozygous activating variant in ClC-7 was described to cause lysosomal alteration, pigmentation defects, and intellectual disability without osteopetrosis. The counter-intuitive 2 Cl-/H+ exchange function of ClC-7 was shown to be physiologically important for intravesicular ion homeostasis. The lysosomal function of ClC-7 is also the reason why individuals with CLCN7-ARO can develop a storage disorder and neurodegeneration, a feature that is variable and difficult to predict. Furthermore, the low penetrance of heterozygous pathogenic CLCN7 variants and the clinical variability of ADO2 are incompletely understood. We aim to give an overview not only of the current knowledge about ClC-7 and its related pathologies, but also of the scientists and clinicians that paved the way for these discoveries.

Carbonic Anhydrase II Activators in Osteopetrosis Treatment: A Review.

Osteopetrosis is a rare hereditary illness generated by failure in osteoclasts resulting in elevated bone densities. Patients with osteopetrosis possess several complications, like dental caries, earlier teeth loss, delayed eruption, malformed crowns and roots, and lamina dura thickening. Since deficiency of carbonic anhydrase II is a major cause behind osteopetrosis, carbonic anhydrase II activators have a large number of applications in osteopetrosis treatment. There is a lack of a comprehensive review on osteopetrosis, pathogenesis of dental abnormalities, and the role of carbonic anhydrase II activators in osteopetrosis treatment. To address this research gap, the authros perfomed a comprehensive review on osteopetrosis and its types, pathogenesis of dental abnormalities, and the role of carbonic anhydrase II activators in osteopetrosis treatment. A brief introduction to the pathogenesis of dental abnormalities and regeneration is provided in this survey. A discussion of types of osteopetrosis depending on genetic inheritance, such as autosomal dominant, autosomal recessive, and X-linked inheritance osteopetrosis, is presented in this survey. The paper also focuses on the importance of carbonic anhydrase II activators as a potential drug therapy for dental osteopetrosis. In addition, a brief note on the role of azole and fluconazole in treating osteopetrosis is given. Finally, future directions involving gene therapy for dental osteopetrosis are described.

A rare cause of anemia and hepatosplenomegaly-marble bone disease

Abstract Marble bone disease (“osteopetrosis”) is a rare, heritable disorder of the skeleton due to a defective function of the osteoclasts and characterized by increased bone density on radiographs. It is an uncommon disease with an unknown etiology. The presentation and severity of osteopetrotic conditions vary greatly, from neonatal onset with potentially fatal complications such as bone marrow failure (e.g., classic or “malignant” or autosomal recessive osteopetrosis) to the incidental discovery of osteopetrosis on radiographs (e.g., osteopoikilosis), susceptibility to infections, growth failure, and generalized osteosclerosis. If left untreated most of the cases of infantile form osteopetrosis die as a result of bone marrow suppression in the first decade of life. Normocytic anemia is caused either by hypoproliferation of hemopoietic tissue or increased destruction of red cells. Osteopetrosis is a rare cause of anemia. We are presenting here an unusual case that was diagnosed clinically and radiologically as malignant infantile osteopetrosis and was referred to us for the evaluation of anemia and hepatosplenomegaly.

Novel Mutation in T-Cell Immune Regulator 1: A Case of Adult-Onset Autosomal Recessive Osteopetrosis

Novel Mutation in T-Cell Immune Regulator 1: A Case of Adult-Onset Autosomal Recessive Osteopetrosis

Osteoclast rich osteopetrosis due to defects in the TCIRG1 gene.

Discovery that mutations in TCIRG1 (also known as Atp6i) gene are responsible for most instances of autosomal recessive osteopetrosis (ARO) heralded a new era for comprehension and treatment of this phenotypically heterogeneous rare bone disease. TCIRG1 encodes the a3 subunit, an essential isoform of the vacuolar ATPase proton pump involved in acidification of the osteoclast resorption lacuna and in secretory lysosome trafficking. TCIRG1 defects lead to inefficient bone resorption by nonfunctional osteoclasts seen in abundance on bone marrow biopsy, delineating this ARO as 'osteoclast-rich'. Presentation is usually in early childhood and features of extramedullary haematopoiesis (hepatosplenomegaly, anaemia, thrombocytopenia) due to bone marrow fibrosis, and cranial nerve impingement (blindness in particular). Impaired dietary calcium uptake due to high pH causes the co-occurrence of rickets, described as "osteopetrorickets". Osteoclast dysfunction leads to early death if untreated, and allogeneic haematopoietic stem cell transplantation is currently the treatment of choice. Studies of patients as well as of mouse models carrying spontaneous (the oc/oc mouse) or targeted disruption of Atp6i (TCIRG1) gene have been instrumental providing insight into disease pathogenesis and development of novel cellular therapies that exploit gene correction.

Novel TCIRG1 Mutations Causing Congenital Neutropenia

Background: Severe congenital neutropenia (SCN) is a bone marrow failure disorder leading to severe neutropenia related infections and leukemia. More than 30 different genes associated with this disorder have been identified. We previously identified a novel heterozygous missense mutation (R736S) of TCIRG1 as the probable cause for SCN in a large multigenerational family through exome sequencing (Makaryan et al; Hum Mutat. 2014 Jul;35(7):824-7). We also identified a statistically significant correlation of rare missense variants of TCIRG1 with lower ANC in three cohorts from the NHLBI Grand Opportunity Exome Sequencing Project (GO ESP; Rosenthal et al; Genet Epidemiol. 2016 Sep;40(6):470-4). Recently, we have discovered a novel TCIRG1 variant (R736C) at the same position in a new family with congenital neutropenia. TCIRG1 encodes subunit a3, a 116-kD component (OC116) of V-ATPases, vacuolar proton pumps which mediate H+ transport through membranes. Homozygous loss-of-function mutations of TCIRG1 are the most common cause of autosomal recessive osteopetrosis (ARO). Mutations R736S and R736C occur at an amino acid residue critical for proton translocation. There are no reports in the literature of individuals with heterozygous mutations at this locus. This is in contrast to the numerous healthy individuals with various heterozygous variants throughout the TCIRG1 gene. Parents of ARO patients harboring homozygous TCIRG1 mutations are considered healthy with no musculoskeletal or hematopoietic abnormalities. Pathogenesis of TCIRG1-associated neutropenia is largely unknown. Hypothesis: We hypothesize that certain heterozygous TCIRG1 variants, like R736S and R736C, may cause SCN through specific molecular pathways. Methods: Family and medical history, differential WBC counts and whole exome sequencing/Identity by Decent (IBD) analysis were used to identify the genetic basis of neutropenia in the previously reported family. Sanger sequencing was used to confirm the findings of exome sequencing and to identify other affected individuals. The R736C mutation was identified by whole exome sequencing with further site-specific analysis of 35 genes. Affected relatives were identified by Sanger sequencing. The Polyphen 2 online prediction tool was used to predict the impact of both substitutions on TCIRG1 function. Results: Discovery and details of the original family were described in our previous report: (Makaryan et al; Hum Mutat. 2014 Jul;35(7):824-7). The second family was discovered in Taiwan, where the primary physician contacted SCNIR about a young patient with neutropenia who had a novel TCIRG1 mutation. Shortly thereafter, we identified two more affected family members. In both families the severity of recurrent bacterial infections is variable, roughly correlating with the severity of the neutropenia. For example, the grandmother of the index patient in the second family was not aware that she had neutropenia. The novel R736C TCIRG1 variant is located in the same position as the previous R736S variant. Arginine in this position is evolutionary highly conserved and both these substitutions cause a major change in amino acid size and properties. Polyphen-2 analysis predicts the effects of both mutations "probably damaging” (score 1.0). Conclusions: Understanding the genetic, epidemiological and molecular basis of TCIRG1-associated neutropenia is important for improving the diagnosis and treatment of this form of congenital neutropenia. The pathogenesis of TCIRG1-associated neutropenia is unknown. It is very important to emphasize that carriers of pathogenic variants from both families have neutropenia with variable severity. The report of this new family suggests that there may be other patients and families with neutropenia caused by mutations in TCIRG1. Population based genome studies, careful family histories and whole genome/exome sequencing will be highly valuable for identifying and helping new patients with this disorder.

A novel homozygous splice site variant in the CLCN7 causes osteopetrosis

ObjectivesOsteopetrosis is a monogenic disorder represented by disturbed osteoclast resorption or osteoclastogenesis differentiation. Clinical symptoms are intensive and brittle bones, recurrent fractures, thrombocytopenia, impaired immune function, optic nerve compression, and anemia. Several osteopetrosis-causing genes have been identified and reported. MethodsThe present study describes two consanguineous Saudi families segregating a severe autosomal recessive osteopetrosis disease. A single proband (II-2) in family A and two probands (II-2; II-4) in family B exhibited increased bone density, multiple fractures, teeth abnormalities, bilateral optic atrophy with nystagmus, and progressive blindness. DNA of the affected individuals was exposed to whole-exome sequencing (WES) and Sanger sequencing. Further, reverse transcriptase-polymerase chain reaction (RT-PCR) and western blotting analysis were done to investigate the impact of the identified mutation. ResultsWES revealed a novel homozygous splice site variant (c.739-18G > A) in theCLCN7gene on chromosome 16p13.3, segregating perfectly with the disorder phenotype. RT-PCR resulted in the retention of a 50 bp sequence of intron 8 in the mutated sequence. As a result, this variant resulted in a large size exon 9 compared to the wild type.In addition, the western blot revealed the heteromeric form of ClC-7 disappeared in the patient’s fibroblasts versus the control, indicating identified variant pathogenicity. ConclusionThe present research provides certain proof that homozygous variants in the CLCN7 gene are responsible for intense osteopetrosis disorders with diverse phenotypes. These findings have significant implications for decisions regarding the clinical therapeutic regimen, prognosis assessment, and antenatal diagnosis.

OSTM1 pleiotropic roles from osteopetrosis to neurodegeneration.

Autosomal recessive osteopetroses (ARO) are rare genetic skeletal disorders of high clinical and molecular heterogeneity with an estimated frequency of 1:250,000 worldwide. The manifestations are diverse and although individually rare, the various forms contribute to the prevalence of a significant number of affected individuals with considerable morbidity and mortality. Among the ARO classification, the most severe form is the autosomal recessive-5 (OPTB5) osteopetrosis (OMIM 259720) that results from homozygous mutation in the OSTM1 gene (607649). OSTM1 mutations account for approximately 5% of instances of autosomal recessive osteopetrosis and lead to a highly debilitating form of the disease in infancy and death within the first few years of life (Sobacchi et al., 2013) [1].

Identification of a novel TNFRSF11A gene variant in a rare case of familial osteopetrosis autosomal recessive type 7

Osteopetrosis is a group of rare genetic disorders characterized by defective osteoclast differentiation that leads to abnormally dense bones resulting in frequent fractures in patients. Among the various types of osteopetrosis, based on inheritance patterns, the autosomal recessive (ARO) type is the least frequently encountered. Among the AROs, a particular sub-type, known as autosomal recessive osteopetrosis type 7 (ARO7), is characterized by osteoclast-poor osteopetrosis with hypo-gammaglobulinemia. This disease is caused by abnormalities in the receptor activator of nuclear factor kappa-Β (RANK) protein, which is encoded by the TNFSFR11A gene. We describe a clinically diagnosed case of ARO7, the product of a consanguineous marriage, who presented with clinical features typical of ARO7 including elevated bone density, optic nerve atrophy and macrocephaly. Whole exome sequencing revealed the presence of a previously unreported homozygous pathogenic frameshift variation c.1527_1528insG (p.Ser511ValfsX20) in exon 9 of the TNFRSF11A gene in the proband, which was confirmed by Sanger sequencing and was found in heterozygous state in both the parents.. Neurosurgical intervention performed to relieve optic canal decompression included bifrontal craniotomy with orbitofrontal Bandau advancement and cranial remodelling procedures. This was followed by hematopoietic stem cell transplantation, which is standard of care for ARO7 patients, from an unrelated donor. However, the patient eventually developed aspiration pneumonia with sepsis coupled with pulmonary haemorrhage and respiratory failure that proved to be fatal. Despite the unfortunate outcome, this study highlights the need for timely genetic diagnosis and improved treatment options for patients with ARO7.