Osteoclast rich osteopetrosis due to defects in the TCIRG1 gene

Correction of osteopetrosis in the neonate oc/oc murine model after lentiviral vector gene therapy and non-genotoxic conditioning

Introduction

Autosomal recessive osteopetrosis (ARO) is a rare genetic disease, characterized by increased bone density due to defective osteoclast function. Most of the cases are due to TCIRG1 gene mutation, leading to severe bone phenotype and death in the first years of life. The standard therapy is the hematopoietic stem cell transplantation (HSCT), but its success is limited by several constraints. Conversely, gene therapy (GT) could minimize the immune-mediated complications of allogeneic HSCT and offer a prompt treatment to these patients.

Methods

The Tcirg1-defective oc/oc mouse model displays a short lifespan and high bone density, closely mirroring the human condition. In this work, we exploited the oc/oc neonate mice to optimize the critical steps for a successful therapy.

Results

First, we showed that lentiviral vector GT can revert the osteopetrotic bone phenotype, allowing long-term survival and reducing extramedullary haematopoiesis. Then, we demonstrated that plerixafor-induced mobilization can further increase the high number of HSPCs circulating in peripheral blood, facilitating the collection of adequate numbers of cells for therapeutic purposes. Finally, pre-transplant non-genotoxic conditioning allowed the stable engraftment of HSPCs, albeit at lower level than conventional total body irradiation, and led to long-term survival and correction of bone phenotype, in the absence of acute toxicity.

Conclusion

These results will pave the way to the implementation of an effective GT protocol, reducing the transplant-related complication risks in the very young and severely affected ARO patients.

Different impact of short-term and long-term hindlimb disuse on bone homeostasis

Disuse osteoporosis is one of the major problems of bone health which commonly occurs in astronauts during long-term spaceflight and bedridden patients. However, the mechanisms underlying such mechanical unloading induced bone loss have not been fully understood. In this study, we employed hindlimb-unloading mice models with different length of tail suspension to investigate if the bone loss was regulated by distinct factors under different duration of disuse. Our micro-CT results showed more significant decrease of bone mass in 6W (6-week) tail-suspension mice compared to the 1W (1-week) tail-suspension ones, as indicated by greater reduction of BV/TV, Tb.N, B.Ar/T.Ar and Ct.Th. RNA-sequencing results showed significant effects of hindlimb disuse on cell locomotion and immune system process which could cause bone loss.Real-time quantitative PCR results indicated a greater number of bone formation related genes that were downregulated in short-term tail-suspension mice compared to the long-term ones. It is, thus, suggested while sustained hindlimb unloading continuously contributes to bone loss, molecular regulation of bone homeostasis tends to reach a balance during this process.

Rankl genetic deficiency and functional blockade undermine skeletal stem and progenitor cell differentiation

BACKGROUND

Skeletal Stem Cells (SSCs) are required for skeletal development, homeostasis, and repair. The perspective of their wide application in regenerative medicine approaches has supported research in this field, even though so far results in the clinic have not reached expectations, possibly due also to partial knowledge of intrinsic, potentially actionable SSC regulatory factors. Among them, the pleiotropic cytokine RANKL, with essential roles also in bone biology, is a candidate deserving deep investigation.

METHODS

To dissect the role of the RANKL cytokine in SSC biology, we performed ex vivo characterization of SSCs and downstream progenitors (SSPCs) in mice lacking Rankl (Rankl-/-) by means of cytofluorimetric sorting and analysis of SSC populations from different skeletal compartments, gene expression analysis, and in vitro osteogenic differentiation. In addition, we assessed the effect of the pharmacological treatment with the anti-RANKL blocking antibody Denosumab (approved for therapy in patients with pathological bone loss) on the osteogenic potential of bone marrow-derived stromal cells from human healthy subjects (hBMSCs).

RESULTS

We found that, regardless of the ossification type of bone, osteochondral SSCs had a higher frequency and impaired differentiation along the osteochondrogenic lineage in Rankl-/- mice as compared to wild-type. Rankl-/- mice also had increased frequency of committed osteochondrogenic and adipogenic progenitor cells deriving from perivascular SSCs. These changes were not due to the peculiar bone phenotype of increased density caused by lack of osteoclast resorption (defined osteopetrosis); indeed, they were not found in another osteopetrotic mouse model, i.e., the oc/oc mouse, and were therefore not due to osteopetrosis per se. In addition, Rankl-/- SSCs and primary osteoblasts showed reduced mineralization capacity. Of note, hBMSCs treated in vitro with Denosumab had reduced osteogenic capacity compared to control cultures.

CONCLUSIONS

We provide for the first time the characterization of SSPCs from mouse models of severe recessive osteopetrosis. We demonstrate that Rankl genetic deficiency in murine SSCs and functional blockade in hBMSCs reduce their osteogenic potential. Therefore, we propose that RANKL is an important regulatory factor of SSC features with translational relevance.

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

CONTEXT

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.

CASE DESCRIPTION

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.

CONCLUSION

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.

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.

Unveiling the link between lactate metabolism and rheumatoid arthritis through integration of bioinformatics and machine learning

Rheumatoid arthritis (RA) is a persistent autoimmune condition characterized by synovitis and joint damage. Recent findings suggest a potential link to abnormal lactate metabolism. This study aims to identify lactate metabolism-related genes (LMRGs) in RA and investigate their correlation with the molecular mechanisms of RA immunity. Data on the gene expression profiles of RA synovial tissue samples were acquired from the gene expression omnibus (GEO) database. The RA database was acquired by obtaining the common LMRDEGs, and selecting the gene collection through an SVM model. Conducting the functional enrichment analysis, followed by immuno-infiltration analysis and protein-protein interaction networks. The results revealed that as possible markers associated with lactate metabolism in RA, KCNN4 and SLC25A4 may be involved in regulating macrophage function in the immune response to RA, whereas GATA2 is involved in the immune mechanism of DC cells. In conclusion, this study utilized bioinformatics analysis and machine learning to identify biomarkers associated with lactate metabolism in RA and examined their relationship with immune cell infiltration. These findings offer novel perspectives on potential diagnostic and therapeutic targets for RA.

Generation of an induced pluripotent stem cell line (SYSUSCi004-A) from a patient with Infantile Malignant Osteopetrosis

Infantile Malignant Osteopetrosis (IMO) is a rare, severe autosomal recessive form of osteopetrosis. Here, the peripheral blood mononuclear cells (PBMCs) extracted from a patient with IMO carrying a compound heterozygous mutation in T cell immune regulator 1, ATPase H + transporting V0 subunit a3 (TCIRG1) gene (c.242delC; c.1114C > T) were successfully reprogrammed using Sendai virus encoding the four Yamanaka factors. The generated hiPSCs, IMO-hiPSCs, displayed typical embryonic stem cell-like morphology and were verified by expression of pluripotency markers such as OCT4, SOX2, NANOG, TRA-1-60 and SSEA4, as well as in vivo and in vitro differentiation into derivatives of three germ layers.

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.

Loss of the Golgi-localized v-ATPase subunit does not alter insulin granule formation or pancreatic islet β-cell function

Delayed Golgi export of proinsulin has recently been identified as an underlying mechanism leading to insulin granule loss and β-cell secretory defects in type 2 diabetes (T2D). Because acidification of the Golgi lumen is critical for proinsulin sorting and delivery into the budding secretory granule, we reasoned that dysregulation of Golgi pH may contribute to proinsulin trafficking defects. In this report, we examined pH regulation of the Golgi and identified a partial alkalinization of the Golgi lumen in a diabetes model. To further explore this, we generated a β-cell specific knockout (KO) of the v0a2 subunit of the v-ATPase pump, which anchors the v-ATPase to the Golgi membrane. Although loss of v0a2 partially neutralized Golgi pH and was accompanied by distension of the Golgi cisternae, proinsulin export from the Golgi and insulin granule formation were not affected. Furthermore, β-cell function was well preserved. β-cell v0a2 KO mice exhibited normal glucose tolerance in both sexes, no genotypic difference to diet-induced obesity, and normal insulin secretory responses. Collectively, our data demonstrate the v0a2 subunit contributes to β-cell Golgi pH regulation but suggest that additional disturbances to Golgi structure and function contribute to proinsulin trafficking defects in diabetes.NEW & NOTEWORTHY Delayed proinsulin export from the Golgi in diabetic β-cells contributes to decreased insulin granule formation, but the underlying mechanisms are not clear. Here, we explored if dysregulation of Golgi pH can alter Golgi function using β-cell specific knockout (KO) of the Golgi-localized subunit of the v-ATPase, v0a2. We show that partial alkalinization of the Golgi dilates the cisternae, but does not affect proinsulin export, insulin granule formation, insulin secretion, or glucose homeostasis.

Successful complete oral rehabilitation of a patient with osteopetrosis with extensive pre-treatments, bone grafts, dental implants and fixed bridges: a multidisciplinary case report

BACKGROUND

Osteopetrosis comprises a group of inherited disorders that are rare and result in abnormal bone structure. Bone remodeling is extremely inhibited because osteoclasts are nonfunctional or lacking. This condition causes overgrowth of bone with disappearance of the bone marrow, leading to aplastic anemia; obstruction of nerve passages in the skull leads to blindness and often hearing impairment. In most cases, osteopetrosis results in oral complications such as tooth deformation, hypomineralization, and delayed or absent tooth eruption. The only curative treatment is hematopoietic stem cell transplantation (HSCT). The main treatment of the oral complications during childhood and adolescence consists in protecting the erupted teeth against caries disease through prophylactic treatment aimed at optimal oral hygiene through frequent regular dental visits throughout life. Many patients with osteopetrosis require major oral rehabilitation to treat complications of the disease. Improved results of HSCT increase the likelihood that dental professionals will encounter patients with osteopetrosis.

CASE PRESENTATION

In this case report, we show that individuals with osteopetrosis who have severe oral complications can be treated successfully if they are treated for osteopetrosis at an early age. The boy had his dental care in pedodontics, and regular multidisciplinary meetings were held for future treatment planning. At the age of 15, he was then referred for rehabilitation. The initial evaluations revealed no further growth in the alveolar bone. The rehabilitation was done stepwise, with extraction of malformed and malpositioned teeth. Initially, the patient received a removable partial denture followed by reconstruction of the width of the alveolar process, titanium implants, temporary fixed bridges, and finally screw-retained titanium-ceramic bridges with titanium frames for the upper and lower jaws.

CONCLUSIONS

The three-year follow-up after loading indicated a stable marginal bone level and optimal oral hygiene as a result of frequent professional oral hygiene care. The patient showed no signs of symptoms from the temporomandibular joint and has adapted to the new jaw relation without any functional or phonetical issues.

Case report: Gene mutations and clinical characteristics of four patients with osteopetrosis

Osteopetrosis is characterized by increased bone density caused by decreased osteoclasts or dysfunction of their differentiation and absorption properties, usually caused by biallelic variants of the TCIRG1(OMIM:604592)and CLCN7(OMIM:602727) genes. Herein, the clinical, biochemical, and radiological manifestations of osteopetrosis in four Chinese children are described. Whole-exome sequencing identified compound heterozygous variants of the CLCN7 and TCIRG1 genes in these patients. In Patient 1, two novel variants were identified in CLCN7:c.880T > G(p.F294V) and c.686C > G(p.S229X). Patient 2 harbored previously reported a single gene variant c.643G > A(p.G215R) in CLCN7. Patient 3 had a novel variant c.569A > G(p.N190S) and a novel frameshift variant c.1113dupG(p.N372fs) in CLCN7. Patient 4 had a frameshift variant c.43delA(p.K15fs) and variant c.C1360T in TCIRG1, resulting in the formation of a premature termination codon (p.R454X), both of which were reported previously. Our results expand the spectrum of identified genetic variation in osteopetrosis and provide a deeper understanding of the relations between genotype and clinical characteristics of this disorder.