Extending the Spectrum of Radiological Findings in Patients With Severe Osteopetrosis and Different Genetic Backgrounds

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

BACKGROUND

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.

METHODS

28 patients from 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.

RESULTS

Biallelic mutations in CLCN7 and TCIRG1 were detected in three families each, in TNFRSF11A and CA2 in two families each, and in SNX10 in one family in the osteopetrosis group. A heterozygous variant in CLCN7 was also found in one family. In the osteopetrosis and related osteoclast disorders group, three different variants in CTSK were detected in five families with pycnodysostosis and a 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 osteopetrorickets 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 novel splicing variants in intron 15 and 17 in TCIRG1 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.

CONCLUSION

This study extended the natural history of the different types of osteopetrosis and also introduced a candidate gene, CCDC120, potentially causing osteopetrosis.

Severe hypophosphataemia can be an early sign of osteopetrorickets: a case report

Osteopetrorickets is a rare complication of autosomal recessive ("malignant") osteopetrosis. Its prompt diagnosis is essential, because early suspicion of infantile osteopetrosis enables treatment with human stem cell transplantation, depending on the gene involved. It is important to identify not only the characteristic radiological changes of rickets, but also the coexistence of increased bone density, so as not to miss this very rare entity. Herein, a brief case report is presented.

MARBLE DISEASE (CASE REPORT)

We present clinical case of marble disease in 5 yo girl. The management of this child was made in Vinnitsia Regional Children's Hospital (Vinnitsia, Ukraine). CBC, X-ray of bones, bone marrow biopsy, genetical testing, MRI of the brain and CT of the skull were done during this period. Marble disease is a very rare disease with very serious consequences, the prevention of which requires timely diagnosis and treatment, namely the prevention of infectious complications and early allogenic transplantation of stem cells. As it is a genetically determined disease, it is not possible to prevent the development of osteopetrosis. Genetic screening and proper treatment will allow the patient to lead an almost normal life.

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.

Osteopetrosis associated with PLEKHM1 and SNX10 genes, both involved in osteoclast vesicular trafficking

The clinical and radiological variability seen in different forms of osteopetrosis, all due to impaired osteoclastic bone resorption, reflect many causal genes. Both defective differentiation of osteoclasts from hematopoietic stem cells as well as disturbed functioning of osteoclasts can be the underlying pathogenic mechanism. Pathogenic variants in PLEKHM1 and SNX10 can be classified among the latter as they impair vesicular transport within the osteoclast and therefore result in the absence of a ruffled border. Some of the typical radiological hallmarks of osteopetrosis can be seen, and most cases present as a relatively mild form segregating in an autosomal recessive mode of inheritance.

Osteoclast-poor osteopetrosis

Osteopetrosis (OPT) is a rare inherited bone disease characterized by a bone resorption defect, due to osteoclast malfunction (in osteoclast-rich, oc-rich, OPT forms) or absence (in oc-poor OPT forms). This causes severe clinical abnormalities, including increased bone density, lack of bone marrow cavity, stunted growth, macrocephaly, progressive deafness, blindness, hepatosplenomegaly, and severe anemia. The oc-poor subtype of OPT is ultra-rare in humans. It is caused by mutations in either the tumor necrosis factor ligand superfamily member 11 (TNFSF11) gene, encoding RANKL (Receptor Activator of Nuclear factor-kappa B [NF-κB] Ligand) which is expressed on cells of mesenchymal origin and lymphocytes, or the TNFRSF member 11A (TNFRSF11A) gene, encoding the RANKL functional receptor RANK which is expressed on cells of myeloid lineage including osteoclasts. Clinical presentation is usually severe with onset in early infancy or in fetal life, although as more patients are reported, expressivity is variable. Phenotypic variability of RANK-deficient OPT sometimes includes hypogammaglobulinemia or radiological features of dysosteosclerosis. Disease progression is somewhat slower in RANKL-deficient OPT than in other 'malignant' subtypes of OPT. While both RANKL and RANK are essential for normal bone turnover, hematopoietic stem cell transplantation (HSCT) is the treatment of choice only for patients with the RANK-deficient form of oc-poor OPT. So far, there is no cure for RANKL-deficient OPT.

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.

Sorting Nexin 10 as a Key Regulator of Membrane Trafficking in Bone-Resorbing Osteoclasts: Lessons Learned From Osteopetrosis

Bone homeostasis is a complex, multi-step process, which is based primarily on a tightly orchestrated interplay between bone formation and bone resorption that is executed by osteoblasts and osteoclasts (OCLs), respectively. The essential physiological balance between these cells is maintained and controlled at multiple levels, ranging from regulated gene expression to endocrine signals, yet the underlying cellular and molecular mechanisms are still poorly understood. One approach for deciphering the mechanisms that regulate bone homeostasis is the characterization of relevant pathological states in which this balance is disturbed. In this article we describe one such “error of nature,” namely the development of acute recessive osteopetrosis (ARO) in humans that is caused by mutations in sorting nexin 10 (SNX10) that affect OCL functioning. We hypothesize here that, by virtue of its specific roles in vesicular trafficking, SNX10 serves as a key selective regulator of the composition of diverse membrane compartments in OCLs, thereby affecting critical processes in the sequence of events that link the plasma membrane with formation of the ruffled border and with extracellular acidification. As a result, SNX10 determines multiple features of these cells either directly or, as in regulation of cell-cell fusion, indirectly. This hypothesis is further supported by the similarities between the cellular defects observed in OCLs form various models of ARO, induced by mutations in SNX10 and in other genes, which suggest that mutations in the known ARO-associated genes act by disrupting the same plasma membrane-to-ruffled border axis, albeit to different degrees. In this article, we describe the population genetics and spread of the original arginine-to-glutamine mutation at position 51 (R51Q) in SNX10 in the Palestinian community. We further review recent studies, conducted in animal and cellular model systems, that highlight the essential roles of SNX10 in critical membrane functions in OCLs, and discuss possible future research directions that are needed for challenging or substantiating our hypothesis.

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.

Skeletal Changes After Hematopoietic Stem Cell Transplantation in Osteopetrosis

Osteopetrosis is a rare skeletal dysplasia resulting from an osteoclast defect leading to increased bone mass and density. Hematopoietic stem cell transplantation can rescue the disease phenotype and prevent complications. However, little is known about the skeletal changes hematopoietic stem cell transplantation induces in patients with this disease. The purpose of this study was to describe the skeletal changes after hematopoietic stem cell transplantation in a retrospective cohort of patients diagnosed with osteopetrosis in one medical center over 13 years. For this purpose, all available epidemiological, hematological, biochemical, and radiographic data were collected and quantitatively analyzed. We found a significant early change in bone metabolism markers coinciding with hematopoietic recovery after stem cell transplantation. Hematopoietic stem cell transplantation induced a later significant improvement in both skeletal mineral distribution and morphology but did not lead to complete radiological normalization. Presumably, changes in bone metabolism, skeletal mineral distribution, and morphology were the result of renewed osteoclast function enabling bone remodeling. We propose that biochemical bone metabolism markers and radiological indices be routinely used to evaluate response to hematopoietic stem cell transplantation in patients with osteopetrosis. © 2020 American Society for Bone and Mineral Research.

Genetics of Osteopetrosis

PURPOSE OF REVIEW

The term osteopetrosis refers to a group of rare skeletal diseases sharing the hallmark of a generalized increase in bone density owing to a defect in bone resorption. Osteopetrosis is clinically and genetically heterogeneous, and a precise molecular classification is relevant for prognosis and treatment. Here, we review recent data on the pathogenesis of this disorder.

RECENT FINDINGS

Novel mutations in known genes as well as defects in new genes have been recently reported, further expanding the spectrum of molecular defects leading to osteopetrosis. Exploitation of next-generation sequencing tools is ever spreading, facilitating differential diagnosis. Some complex phenotypes in which osteopetrosis is accompanied by additional clinical features have received a molecular classification, also involving new genes. Moreover, novel types of mutations have been recognized, which for their nature or genomic location are at high risk being neglected. Yet, the causative mutation is unknown in some patients, indicating that the genetics of osteopetrosis still deserves intense research efforts.