Hematopoietic Stem Cell-Targeted Neonatal Gene Therapy with a Clinically Applicable Lentiviral Vector Corrects Osteopetrosis in oc/oc Mice

Molecular Heterogeneity of Osteopetrosis in India: Report of 17 Novel Variants

Osteopetrosis is a clinically and genetically heterogeneous group of inherited bone disorders that is caused by defects in osteoclast formation or function. Treatment options vary with the disease severity and an accurate molecular diagnosis helps in prognostication and treatment decisions. We investigated the genetic causes of osteopetrosis in 31 unrelated patients of Indian origin. Screening for the genetic variants was done by Sanger sequencing or next generation sequencing in 48 samples that included 31 samples from index patients, 16 from parents' and 1 chorionic villus sample. A total of 30 variants, including 29 unique variants, were identified in 26 of the 31 patients in the study. TCIRG1 was the most involved gene (n = 14) followed by TNFRSF11A (n = 4) and CLCN7 (n = 3). A total of 17 novel variants were identified. Prenatal diagnosis was done in one family and the foetus showed homozygous c.807 + 2T > G variant in TCIRG1. Molecular diagnosis of osteopetrosis aids in therapeutic decisions including the need for a stem cell transplantation and gives a possible option of performing prenatal diagnosis in affected families. Further studies would help in understanding the genetic etiology in patients where no variants were identified.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12288-023-01732-4.

Advances in Genetic Reprogramming: Prospects from Developmental Biology to Regenerative Medicine

The foundations of cell reprogramming were laid by Yamanaka and co-workers, who showed that somatic cells can be reprogrammed into pluripotent cells (induced pluripotency). Since this discovery, the field of regenerative medicine has seen advancements. For example, because they can differentiate into multiple cell types, pluripotent stem cells are considered vital components in regenerative medicine aimed at the functional restoration of damaged tissue. Despite years of research, both replacement and restoration of failed organs/ tissues have remained elusive scientific feats. However, with the inception of cell engineering and nuclear reprogramming, useful solutions have been identified to counter the need for compatible and sustainable organs. By combining the science underlying genetic engineering and nuclear reprogramming with regenerative medicine, scientists have engineered cells to make gene and stem cell therapies applicable and effective. These approaches have enabled the targeting of various pathways to reprogramme cells, i.e., make them behave in beneficial ways in a patient-specific manner. Technological advancements have clearly supported the concept and realization of regenerative medicine. Genetic engineering is used for tissue engineering and nuclear reprogramming and has led to advances in regenerative medicine. Targeted therapies and replacement of traumatized , damaged, or aged organs can be realized through genetic engineering. Furthermore, the success of these therapies has been validated through thousands of clinical trials. Scientists are currently evaluating induced tissue-specific stem cells (iTSCs), which may lead to tumour-free applications of pluripotency induction. In this review, we present state-of-the-art genetic engineering that has been used in regenerative medicine. We also focus on ways that genetic engineering and nuclear reprogramming have transformed regenerative medicine and have become unique therapeutic niches.

Metabolic bone disorders and the promise of marine osteoactive compounds

Metabolic bone disorders and associated fragility fractures are major causes of disability and mortality worldwide and place an important financial burden on the global health systems. These disorders result from an unbalance between bone anabolic and resorptive processes and are characterized by different pathophysiological mechanisms. Drugs are available to treat bone metabolic pathologies, but they are either poorly effective or associated with undesired side effects that limit their use. The molecular mechanism underlying the most common metabolic bone disorders, and the availability, efficacy, and limitations of therapeutic options currently available are discussed here. A source for the unmet need of novel drugs to treat metabolic bone disorders is marine organisms, which produce natural osteoactive compounds of high pharmaceutical potential. In this review, we have inventoried the marine osteoactive compounds (MOCs) currently identified and spotted the groups of marine organisms with potential for MOC production. Finally, we briefly examine the availability of in vivo screening and validation tools for the study of MOCs.

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.

Experimental therapies for osteopetrosis

The medical treatment of osteopetrosis is an ongoing clinical problem. There are no effective and safer therapeutic approaches for all its forms. However, recent discoveries concerning the etiology and the pathogenesis of osteopetrosis, the development of dedicated cellular and animal models, and the advent of new technologies are paving the way for the development of targeted and safer therapies for both lethal and milder osteopetrosis. This review summarizes the huge effort and successes made by researchers to identify and develop new experimental approaches with this objective, such as the use of non-genotoxic myeloablation, gene correction of inducible Pluripotent Stem Cells (iPSCs), lentiviral-based gene therapy, protein replacement, prenatal treatment, osteoclast precursors transplantation and RNA Interference.

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.

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].

Target receptor identification and subsequent treatment of resected brain tumors with encapsulated and engineered allogeneic stem cells

Cellular therapies offer a promising therapeutic strategy for the highly malignant brain tumor, glioblastoma (GBM). However, their clinical translation is limited by the lack of effective target identification and stringent testing in pre-clinical models that replicate standard treatment in GBM patients. In this study, we show the detection of cell surface death receptor (DR) target on CD146-enriched circulating tumor cells (CTC) captured from the blood of mice bearing GBM and patients diagnosed with GBM. Next, we developed allogeneic "off-the-shelf" clinical-grade bifunctional mesenchymal stem cells (MSCBif) expressing DR-targeted ligand and a safety kill switch. We show that biodegradable hydrogel encapsulated MSCBif (EnMSCBif) has a profound therapeutic efficacy in mice bearing patient-derived invasive, primary and recurrent GBM tumors following surgical resection. Activation of the kill switch enhances the efficacy of MSCBif and results in their elimination post-tumor treatment which can be tracked by positron emission tomography (PET) imaging. This study establishes a foundation towards a clinical trial of EnMSCBif in primary and recurrent GBM patients.

Clinical presentation and analysis of genotype-phenotype correlations in patients with malignant infantile osteopetrosis

Malignant infantile osteopetrosis (MIOP) is the autosomal recessive, severe form of osteopetrosis. This rare genetic syndrome usually presents soon after birth and is often fatal if left untreated. Early diagnosis is key for proper management but clinical presentation is diverse, and oftentimes diagnosis may be challenging. In this study, we retrospectively collected data of genetic mutations and phenotypic characteristics at the initial presentation of 81 MIOP patients and analyzed genotype-phenotype correlations. The most common genetic mutation was in the TCIRG1 gene (n = 46, 56.8%), followed by SNX10 (n = 20, 25%). Other genetic mutations included RANK (n = 7, 8.7%), CLCN7 (n = 5, 6.2%) and CA2 (n = 3, 3.7%). More than half of the patients presented with growth retardation (n = 46, 56.8%). Twenty-one of the patients were blind (26%) and thirty-seven patients had other neurological deficits (45.7%) at the time of initial presentation. Most patients presented with hematological signs of bone marrow failure including anemia (n = 69, 85.2%) and thrombocytopenia (n = 33, 40.7%). Thrombocytopenia at initial presentation was significantly more prevalent in patients with mutations in the TCIRG1 gene (p = 0.036). Other phenotypic presenting features were not found to be significantly correlated to specific gene mutations. In conclusion, the initial presentation of MIOP is variable, but some features are common such as growth retardation, visual impairment, and cytopenias. High awareness of MIOP presenting signs is essential for prompt diagnosis of this challenging disease.

The Engineered MARCH8-Resistant Vesicular Stomatitis Virus Glycoprotein Enhances Lentiviral Vector Transduction

Lentiviral vectors are one of the most commonly used viral delivery systems for gene therapy. Vesicular stomatitis virus-G envelope glycoprotein (VSV G)-pseudotyped lentiviral vectors have been widely used in clinical studies for treatment of virus infections and genetic deficient diseases. However, the efficiency of lentiviral vector transduction has been long recognized as a limiting factor in clinical gene therapy application, especially in transducing hematopoietic stem cells. MARCH8 (membrane-associated RING-CH 8), an E3 ubiquitin ligase, has been reported to target and downregulate VSV G. Results in this study show that MARCH8 induces ubiquitination and lysosome degradation of VSV G, and knockout of MARCH8 in virus-producing cells increases lentiviral vector transduction by elevating the level of VSV G protein. We then engineered VSV G mutant that has the lysine residues in the cytoplasmic domain substituted for arginine, and showed that this G mutant resists degradation by MARCH8, and allows the enhancement of transduction efficiency of lentiviral vector particles than the parental VSV G protein. This engineered VSV G mutant thus further advances the lentiviral vector system as a powerful tool in gene therapy.

Autosomal recessive osteopetrosis: mechanisms and treatments

Autosomal recessive osteopetrosis (ARO) is a severe inherited bone disease characterized by defective osteoclast resorption or differentiation. Clinical manifestations include dense and brittle bones, anemia and progressive nerve compression, which hamper the quality of patients' lives and cause death in the first 10 years of age. This Review describes the pathogenesis of ARO and highlights the strengths and weaknesses of the current standard of care, namely hematopoietic stem cell transplantation (HSCT). Despite an improvement in the overall survival and outcomes of HSCT, transplant-related morbidity and the pre-existence of neurological symptoms significantly limit the success of HSCT, while the availability of human leukocyte antigen (HLA)-matched donors still remains an open issue. Novel therapeutic approaches are needed for ARO patients, especially for those that cannot benefit from HSCT. Here, we review preclinical and proof-of-concept studies, such as gene therapy, systematic administration of deficient protein, in utero HSCT and gene editing.

Summary: Autosomal recessive osteopetrosis is a heterogeneous and rare bone disease for which effective treatments are still lacking for many patients. Here, we review the literature on clinical, preclinical and proof-of-concept studies.

Gene therapy for infantile malignant osteopetrosis: review of pre-clinical research and proof-of-concept for phenotypic reversal

Infantile malignant osteopetrosis is a devastating disorder of early childhood that is frequently fatal and for which there are only limited therapeutic options. Gene therapy utilizing autologous hematopoietic stem and progenitor cells represents a potentially advantageous therapeutic alternative for this multisystemic disease. Gene therapy can be performed relatively rapidly following diagnosis, will not result in graft versus host disease, and may also have potential for reduced incidences of other transplant-related complications. In this review, we have summarized the past sixteen years of research aimed at developing a gene therapy for infantile malignant osteopetrosis; these efforts have culminated in the first clinical trial employing lentiviral-mediated delivery of TCIRG1 in autologous hematopoietic stem and progenitor cells.

How we approach malignant infantile osteopetrosis

Malignant infantile osteopetrosis (MIOP) is a rare hereditary disorder characterized by excessive bone overgrowth due to a defect in bone marrow resorption by osteoclasts. In most cases, hematopoietic stem cell transplantation (HSCT) may correct bone metabolism but the rapidly progressing nature of this condition necessitates early diagnosis and prompt treatment to minimize irreversible cranial nerve damage. The management of patients with MIOP presents many unique challenges. In this review, the clinical management of patients with MIOP is discussed, including diagnosis, preparation for HSCT and special transplant considerations, management of unique HSCT complications, and long-term follow-up.

Pathobiologic Mechanisms of Neurodegeneration in Osteopetrosis Derived From Structural and Functional Analysis of 14 ClC-7 Mutants

ClC-7 is a chloride-proton antiporter of the CLC protein family. In complex with its accessory protein Ostm-1, ClC-7 localizes to lysosomes and to the osteoclasts' ruffled border, where it plays a critical role in acidifying the resorption lacuna during bone resorption. Gene inactivation in mice causes severe osteopetrosis, neurodegeneration, and lysosomal storage disease. Mutations in the human CLCN7 gene are associated with diverse forms of osteopetrosis. The functional evaluation of ClC-7 variants might be informative with respect to their pathogenicity, but the cellular localization of the protein hampers this analysis. Here we investigated the functional effects of 13 CLCN7 mutations identified in 13 new patients with severe or mild osteopetrosis and a known ADO2 mutation. We mapped the mutated amino acid residues in the homology model of ClC-7 protein, assessed the lysosomal colocalization of ClC-7 mutants and Ostm1 through confocal microscopy, and performed patch-clamp recordings on plasma-membrane-targeted mutant ClC-7. Finally, we analyzed these results together with the patients' clinical features and suggested a correlation between the lack of ClC-7/Ostm1 in lysosomes and severe neurodegeneration. © 2020 American Society for Bone and Mineral Research (ASBMR).

Expanded circulating hematopoietic stem/progenitor cells as novel cell source for the treatment of TCIRG1 osteopetrosis

Allogeneic hematopoietic stem cell transplantation is the treatment of choice for autosomal recessive osteopetrosis caused by defects in the TCIRG1 gene. Despite recent progress in conditioning, a relevant number of patients are not eligible for allogeneic stem cell transplantation because of the severity of the disease and significant transplant-related morbidity. We exploited peripheral CD34+ cells, known to circulate at high frequency in the peripheral blood of TCIRG1-deficient patients, as a novel cell source for autologous transplantation of gene corrected cells. Detailed phenotypical analysis showed that circulating CD34+ cells have a cellular composition that resembles bone marrow, supporting their use in gene therapy protocols. Transcriptomic profile revealed enrichment in genes expressed by hematopoietic stem and progenitor cells (HSPCs). To overcome the limit of bone marrow harvest/ HSPC mobilization and serial blood drawings in TCIRG1 patients, we applied UM171-based ex-vivo expansion of HSPCs coupled with lentiviral gene transfer. Circulating CD34+ cells from TCIRG1-defective patients were transduced with a clinically-optimized lentiviral vector (LV) expressing TCIRG1 under the control of phosphoglycerate promoter and expanded ex vivo. Expanded cells maintained long-term engraftment capacity and multi-lineage repopulating potential when transplanted in vivo both in primary and secondary NSG recipients. Moreover, when CD34+ cells were differentiated in vitro, genetically corrected osteoclasts resorbed the bone efficiently. Overall, we provide evidence that expansion of circulating HSPCs coupled to gene therapy can overcome the limit of stem cell harvest in osteopetrotic patients, thus opening the way to future gene-based treatment of skeletal diseases caused by bone marrow fibrosis.

Genomic Medicine: Lessons Learned From Monogenic and Complex Bone Disorders

Current genetic studies of monogenic and complex bone diseases have broadened our understanding of disease pathophysiology, highlighting the need for medical interventions and treatments tailored to the characteristics of patients. As genomic research progresses, novel insights into the molecular mechanisms are starting to provide support to clinical decision-making; now offering ample opportunities for disease screening, diagnosis, prognosis and treatment. Drug targets holding mechanisms with genetic support are more likely to be successful. Therefore, implementing genetic information to the drug development process and a molecular redefinition of skeletal disease can help overcoming current shortcomings in pharmaceutical research, including failed attempts and appalling costs. This review summarizes the achievements of genetic studies in the bone field and their application to clinical care, illustrating the imminent advent of the genomic medicine era.

Generation of gene-corrected functional osteoclasts from osteopetrotic induced pluripotent stem cells

Background

Infantile malignant osteopetrosis (IMO) is an autosomal recessive disorder characterized by non-functional osteoclasts and a fatal outcome early in childhood. About 50% of patients have mutations in the TCIRG1 gene.

Methods

IMO iPSCs were generated from a patient carrying a homozygous c.11279G>A (IVS18+1) mutation in TCIRG1 and transduced with a lentiviral vector expressing human TCIRG1. Embryoid bodies were generated and differentiated into monocytes. Non-adherent cells were harvested and further differentiated into osteoclasts on bovine bone slices.

Results

Release of the bone resorption biomarker CTX-I into the media of gene-corrected osteoclasts was 5-fold higher than that of the uncorrected osteoclasts and 35% of that of control osteoclasts. Bone resorption potential was confirmed by the presence of pits on the bones cultured with gene-corrected osteoclasts, absent in the uncorrected IMO osteoclasts.

Conclusions

The disease phenotype was partially corrected in vitro, providing a valuable resource for therapy development for this form of severe osteopetrosis.