Unusual Cortical Phenotype After Hematopoietic Stem Cell Transplantation in a Patient With Osteopetrosis

The osteopetroses are a group of rare genetic diseases caused by osteoclast dysfunction or absence. The hallmark of osteopetrosis is generalized increased bone mineral density (BMD). However, the bone is fragile and fractures are common. Autosomal recessive osteopetrosis is usually a severe disorder and often life-threatening in childhood. We present male siblings with autosomal recessive osteopetrosis due to biallelic variants in TCIRG1 who survived childhood and underwent hematopoietic stem cell transplant (HSCT) in adulthood. One sibling died of posttransplant complications. After transplant, the other sibling had improvement of multiple clinical parameters, including some decline in BMD Z-scores by dual-energy X-ray absorptiometry (DXA) and cessation of fractures. However, spine quantitative computed tomography 11 years after transplant demonstrated an anvil pattern of sclerosis with BMD Z-score of +18.3. High-resolution peripheral quantitative computed tomography (HR-pQCT) of the tibia demonstrated near complete obliteration of the marrow space combined with an unusual cortical phenotype, suggesting extensive cortical porosity at the distal tibia. This case highlights that despite successful transplantation and subsequent improvement in clinical parameters, this patient continued to have significantly elevated bone density and decreased marrow space. Transplant-associated increased cortical porosity is multifactorial and occurs in two-thirds of non-osteopetrotic patients undergoing HSCT. This finding after transplant in osteopetrosis may suggest particular sensitivity of the cortical bone to resorptive activity of transplanted osteoclasts. The case also suggests HR-pQCT may be a useful modality for imaging and assessing the therapeutic effects on bone in individuals with osteopetrosis. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

SLC4A2 Deficiency Causes a New Type of Osteopetrosis

Osteopetrosis is a group of rare inherited skeletal disorders characterized by a marked increase in bone density due to deficient bone resorption. Pathogenic variants in several genes involved in osteoclast differentiation and/or function have been reported to cause osteopetrosis. Solute carrier family 4 member 2 (SLC4A2, encoding anion exchanger 2) plays an important role in osteoclast differentiation and function by exchange of Cl^- with HCO₃^- . Biallelic Slc4a2 loss-of-function mutations in mice and cattle lead to osteopetrosis with osteoclast deficiency; however, pathogenic SLC4A2 variants in humans have not been reported. In this study, we describe a patient with autosomal recessive osteopetrosis due to biallelic pathogenic variants in SLC4A2. We identified novel compound heterozygous variants in SLC4A2 (NM_003040.4: c.556G>A [p.A186T] and c.1658T>C [p.V553A]) by exome sequencing. The measurement of intracellular Cl^- showed that the variants decrease the anion exchange activity of SLC4A2. The impact of the variants on osteoclast differentiation was assessed by a gene knockout-rescue system using a mouse macrophage cell line, RAW 264.7. The Slc4a2-knockout cells show impaired osteoclastogenesis, which was rescued by the wild-type SLC4A2, but not by the mutant SLC4A2s. Immunofluorescence and pit assay revealed that the mutant SLC4A2s leads to abnormal podosome belt formation with impaired bone absorption. This is the first report on an individual affected by SLC4A2-associated osteopetrosis (osteopetrosis, Ikegawa type). With functional studies, we prove that the variants lead to SLC4A2 dysfunction, which altogether supports the importance of SLC4A2 in human osteoclast differentiation. © 2021 American Society for Bone and Mineral Research (ASBMR).

New Insights Into Osteoclast Biology

Osteoclasts are multinucleated cells that are characterized by their unique ability to resorb large quantities of bone. Therefore, they are frequently the target of therapeutic interventions to ameliorate bone loss. In an adult organism, osteoclasts derive from hematopoietic stem cells and differentiate into osteoclasts within a multistep process under the influence of macrophage colony‐stimulating factor (M‐CSF) and receptor activator of NF‐κB ligand (RANKL). Historically, the osteoclast life cycle has been defined as linear, whereby lineage‐committed mononuclear precursors fuse to generate multinucleated highly specialized and localized bone phagocytic cells, which then undergo apoptosis within weeks. Recent advances through lineage tracing, single cell RNA sequencing, parabiosis, and intravital imaging approaches have challenged this dogma, revealing they have greater longevity and the capacity to circulate and undergo cell recycling. Indeed, these new insights highlight that under homeostatic conditions very few incidences of osteoclast apoptosis occur. More importantly, as we revisit the formation and fate of the osteoclast, novel methods to target osteoclast biology in bone pathology and regeneration are emerging. This review briefly summarizes the historical life cycle of osteoclasts and highlights recent discoveries made through advanced methodologies, which have led to a paradigm shift in osteoclast biology. These findings are discussed in light of both existing and emerging bone targeted therapeutics, bone pathologies, and communication between osteoclasts and cells resident in bone or at distant sites. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Efficient generation of osteoclasts from human induced pluripotent stem cells and functional investigations of lethal CLCN7-related osteopetrosis

Human induced pluripotent stem cells (hiPSCs) hold great potential for modeling human diseases and the development of innovative therapeutic approaches. Here, we report on a novel, simplified differentiation method for forming functional osteoclasts from hiPSCs. The three-step protocol starts with embryoid body formation, followed by hematopoietic specification, and finally osteoclast differentiation. We observed continuous production of monocyte-like cells over a period of up to 9 weeks, generating sufficient material for several osteoclast differentiations. The analysis of stage-specific gene and surface marker expression proved mesodermal priming, the presence of monocyte-like cells, and of terminally differentiated multinucleated osteoclasts, able to form resorption pits and trenches on bone and dentine in vitro. In comparison to peripheral blood mononuclear cell (PBMC)-derived osteoclasts hiPSC-derived osteoclasts were larger and contained a higher number of nuclei. Detailed functional studies on the resorption behavior of hiPSC-osteoclasts indicated a trend towards forming more trenches than pits and an increase in pseudoresorption. We used hiPSCs from an autosomal recessive osteopetrosis (ARO) patient (BIHi002-A, ARO hiPSCs) with compound heterozygous missense mutations p.(G292E) and p.(R403Q) in CLCN7, coding for the Cl^- /H⁺ -exchanger ClC-7, for functional investigations. The patient's leading clinical feature was a brain malformation due to defective neuronal migration. Mutant ClC-7 displayed residual expression and retained lysosomal co-localization with OSTM1, the gene coding for the osteopetrosis-associated transmembrane protein 1, but only ClC-7 harboring the mutation p.(R403Q) gave strongly reduced ion currents. An increased autophagic flux in spite of unchanged lysosomal pH was evident in undifferentiated ARO hiPSCs. ARO hiPSC-derived osteoclasts showed an increased size compared to hiPSCs of healthy donors. They were not able to resorb bone, underlining a loss-of-function effect of the mutations. In summary, we developed a highly reproducible, straightforward hiPSC-osteoclast differentiation protocol. We demonstrated that osteoclasts differentiated from ARO hiPSCs can be used as a disease model for ARO and potentially also other osteoclast-related diseases. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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).

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.

Adult Osteosclerotic Metaphyseal Dysplasia With Progressive Osteonecrosis of the Jaws and Abnormal Bone Resorption Pattern Due to a LRRK1 Splice Site Mutation

Osteosclerotic metaphyseal dysplasia (OSMD) is a rare autosomal recessive sclerosing skeletal dysplasia. We report on a 34-year-old patient with sandwich vertebrae, platyspondyly, osteosclerosis of the tubular bones, pathologic fractures, and anemia. In the third decade, he developed osteonecrosis of the jaws, which was progressive in spite of repeated surgical treatment over a period of 11 years. An iliac crest bone biopsy revealed the presence of hypermineralized cartilage remnants, large multinucleated osteoclasts with abnormal morphology, and inadequate bone resorption typical for osteoclast-rich osteopetrosis. After exclusion of mutations in TCIRG1 and CLCN7 we performed trio-based exome sequencing. The novel homozygous splice-site mutation c.261G>A in the gene LRRK1 was found and co-segregated with the phenotype in the family. cDNA sequencing showed nearly complete skipping of exon 3 leading to a frameshift (p.Ala34Profs*33). Osteoclasts differentiated from the patient's peripheral blood monocytes were extremely large. Instead of resorption pits these cells were only capable of superficial erosion. Phosphorylation of L-plastin at position Ser5 was strongly reduced in patient-derived osteoclasts showing a loss of function of the mutated LRRK1 kinase protein. Our analysis indicates a strong overlap of LRRK1-related OSMD with other forms of intermediate osteopetrosis, but an exceptional abnormality of osteoclast resorption. Like in other osteoclast pathologies an increased risk for progressive osteonecrosis of the jaws should be considered in OSMD, an intermediate form of osteopetrosis. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

TNFRSF11A-Associated Dysosteosclerosis: A Report of the Second Case and Characterization of the Phenotypic Spectrum

Dysosteosclerosis (DOS) is a distinct form of sclerosing bone disease characterized by irregular osteosclerosis and platyspondyly. DOS is genetically heterogeneous; however, only five cases with SLC29A3 mutations and a single case with a splice-site mutation of TNFRSF11A have been reported, and TNFRSF11A is also a causal gene for osteopetrosis, autosomal recessive 7 (OP-AR7). Thus, the causal genes of DOS and their genotype-phenotype associations remain unclear. In this study, we examined a Japanese patient with DOS and found a novel variant in TNFRSF11A. The homozygous variant was a G to T transversion at the first nucleotide of exon 9 (c.784G>T). Although the variant was predicted to cause a stop codon mutation (p.E262*), in silico evaluation of the exonic splicing elements followed by RT-PCR for the patient-derived cells showed that it caused aberrant splicing due to the change in the exonic splicing element and produced two types of aberrant transcripts: One caused a premature stop codon (p.E262Vfs*17) leading to nonsense mutation-mediated mRNA decay; the other produced a protein with interstitial deletion (p.E262_Q279del). The effects of the mutation on five splicing isoforms of TNFRSF11A were different from those in OP-AR7, but comparable with those in the first DOS with the TNFRSF11A mutation. Thus, we identified the second case of DOS caused by the TNFRSF11A splice-site mutation and confirmed the novel disease entity. © 2019 American Society for Bone and Mineral Research.

Interferon Gamma-1b Does Not Increase Markers of Bone Resorption in Autosomal Dominant Osteopetrosis

In autosomal dominant osteopetrosis type 2 (ADO2) CLCN7 mutations cause impaired osteoclast function. Severe consequences include skeletal fragility despite high bone mass, osteomyelitis, osteonecrosis, bone marrow failure, and severe cranial nerve impingement. There is no effective medical treatment for ADO2. We recruited subjects with ADO2 into a 14-week, open-label, pilot clinical trial of interferon gamma-1b. Doses were titrated based on tolerability and if fasting serum C-telopeptide (CTX) was <25% above baseline at week 8, targeting doses of 100 µg/m² three times a week. The primary outcomes were change from baseline in CTX and N-telopeptide/creatinine ratio (NTX/Cr) at week 14. Secondary outcomes included changes in urine calcium/creatinine ratio, bone formation markers and tolerability. Nine adults and three children were recruited. Severe manifestations of ADO2 included histories of fractures (100%), osteomyelitis (16.7%), vision loss (50%), and anemia (58.3%). Baseline CTX and NTX/Cr were generally low-normal. Procollagen type I N-terminal propeptide was elevated or in the upper-normal range in 11 of 12 (91.6%) subjects. Elevations of aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) were common. One subject withdrew due to rash. Five subjects achieved doses of 50 µg/m² 3 days a week, while six reached the full dose of 100 µg/m² 3 days a week. Only 3 of 11 (27.3%) completing subjects achieved the primary outcome of increasing CTX ≥25% above baseline at week 14. The mean ± SD change from baseline in CTX at week 14 was +2.2% ± 43.2%, p = 0.86). Likewise, there was no significant change in NTX/Cr (mean change -2.1%, p = 0.81). Interferon gamma-1b was poorly tolerated. Most subjects had adverse events, and the Mental Health and Mental Component Scales of the SF-36v2 health survey declined slightly (p < 0.05). Over 14 weeks, interferon gamma-1b failed to significantly increase bone turnover markers in ADO2 and was poorly tolerated. Consequently, interferon gamma-1b is unlikely to be effective for decreasing bone mass in ADO2. © 2019 American Society for Bone and Mineral Research.

Unique Variant of NOD2 Pediatric Granulomatous Arthritis With Severe 1,25-Dihydroxyvitamin D-Mediated Hypercalcemia and Generalized Osteosclerosis

Pediatric granulomatous arthritis (PGA) refers to two formerly separate entities: autosomal dominant Blau syndrome (BS) and its sporadic phenocopy early-onset sarcoidosis (EOS). In 2001 BS and in 2005 EOS became explained by heterozygous mutations within the gene that encodes nucleotide-binding oligomerization domain-containing protein 2 (NOD2), also called caspase recruitment domain-containing protein 15 (CARD15). NOD2 is a microbe sensor in leukocyte cytosol that activates and regulates inflammation. PGA is characterized by a triad of autoinflammatory problems (dermatitis, uveitis, and arthritis) in early childhood, which suggests the causal NOD2/CARD15 mutations are activating defects. Additional complications of PGA were recognized especially when NOD2 mutation analysis became generally available. However, in PGA, hypercalcemia is only briefly mentioned, and generalized osteosclerosis is not reported, although NOD2 regulates NF-κB signaling essential for osteoclastogenesis and osteoclast function. Herein, we report a 4-year-old girl with PGA uniquely complicated by severe 1,25(OH)₂ D-mediated hypercalcemia, nephrocalcinosis, and compromised renal function together with radiological and histopathological features of osteopetrosis (OPT). The classic triad of PGA complications was absent, although joint pain and an antalgic gait accompanied wrist, knee, and ankle swelling and soft non-tender masses over her hands, knees, and feet. MRI revealed tenosynovitis in her hands and suprapatellar effusions. Synovial biopsy demonstrated reactive synovitis without granulomas. Spontaneous resolution of metaphyseal osteosclerosis occurred while biochemical markers indicated active bone turnover. Anti-inflammatory medications suppressed circulating 1,25(OH)₂ D, corrected the hypercalcemia, and improved her renal function, joint pain and swelling, and gait. Mutation analysis excluded idiopathic infantile hypercalcemia, type 1, and known forms of OPT, and identified a heterozygous germline missense mutation in NOD2 common in PGA (c.1001G>A, p.Arg334Gln). Thus, radiological and histological findings of OPT and severe hypercalcemia from apparent extrarenal production of 1,25(OH)₂ D can complicate NOD2-associated PGA. Although the skeletal findings seem inconsequential, treatment of the hypercalcemia is crucial to protect the kidneys. © 2018 American Society for Bone and Mineral Research.

Ostm1 Bifunctional Roles in Osteoclast Maturation: Insights From a Mouse Model Mimicking a Human OSTM1 Mutation

Ostm1 mutations are responsible for the most severe form of osteopetrosis in human and mice. To gain insight into Ostm1 cellular functions, we engineered a conditional in-frame deletion of the Ostm1 transmembrane domain and generated the first Ostm1 mouse model with a human mutation. Systemic targeting of Ostm1 loss of transmembrane domain produced osteopetrosis, as in the null Ostm1 gl/gl mouse. Significantly, conditional osteoclast targeting of Ostm1 resulted in similar osteopetrosis, thereby demonstrating that the intrinsic Ostm1 osteoclast deficiency is solely responsible for the mouse phenotype. Our analysis showed oversized osteoclasts with enhanced multinucleation associated with stimulation of intracellular calcium levels, of Nfatc1 nuclear re-localization, and of specific downstream Nfatc1 target genes, providing compelling evidence that Ostm1 is a negative regulator of preosteoclast fusion. Moreover, mature OCs with Ostm1 loss of transmembrane domain show appropriate levels of intracellular acidification but an altered distribution pattern, highlighting misregulation of endolysosome localization and dispersion. Consistently, the hydrolases tartrate-resistant acid phosphatase (TRAP) and cathepsin K (Ctsk) normally produced are sequestered within the osteoclasts and are not extracellularly secreted. These studies defined bifunctional roles for Ostm1 as a major regulator of preosteoclast cytoskeletal rearrangements toward cell multinucleation and of mature osteoclast intracellular lysosomal trafficking and exocytosis mechanism, both of which are essential for bone resorption. Importantly, these Ostm1 molecular and regulatory functions could serve as preclinical targets in this mouse model toward osteoclastogenic pathologies as osteoporosis and inflammation-induced bone loss. © 2018 American Society for Bone and Mineral Research.

Monocyte-Specific Knockout of C/ebpα Results in Osteopetrosis Phenotype, Blocks Bone Loss in Ovariectomized Mice, and Reveals an Important Function of C/ebpα in Osteoclast Differentiation and Function

CCAAT/enhancer-binding protein α (C/ebpα) is critical for osteoclastogenesis by regulating osteoclast (OC) lineage commitment and is also important for OC differentiation and function in vitro. However, the role of C/ebpα in postnatal skeletal development has not been reported owing to lethality in C/ebpα-/- mice from hypoglycemia within 8 hours after birth. Herein, we generated conditional knockout mice by deleting the C/ebpα gene in monocyte via LysM-Cre to examine its role in OC differentiation and function. C/ebpαf/f LysM-Cre mice exhibited postnatal osteopetrosis due to impaired osteoclastogenesis, OC lineage priming defects, as well as defective OC differentiation and activity. Furthermore, our ex vivo analysis demonstrated that C/ebpα conditional deletion significantly reduced OC differentiation, maturation, and activity while mildly repressing macrophage development. At the molecular level, C/ebpα deficiency significantly suppresses the expressions of OC genes associated with early stages of osteoclastogenesis as well as genes associated with OC differentiation and activity. We also identified numerous C/ebpα critical cis-regulatory elements on the Cathepsin K promoter that allow C/ebpα to significantly upregulate Cathepsin K expression during OC differentiation and activity. In pathologically induced mouse model of osteoporosis, C/ebpα deficiency can protect mice against ovariectomy-induced bone loss, uncovering a central role for C/ebpα in osteolytic diseases. Collectively, our findings have further established C/ebpα as a promising therapeutic target for bone loss by concurrently targeting OC lineage priming, differentiation, and activity. © 2017 American Society for Bone and Mineral Research.

Osteoblast-Derived Extracellular Vesicles Are Biological Tools for the Delivery of Active Molecules to Bone

Extracellular vesicles (EVs) are newly appreciated regulators of tissue homeostasis and a means of intercellular communication. Reports have investigated the role of EVs and their cargoes in cellular regulation and have tried to fine-tune their biotechnological use, but to date very little is known on their function in bone biology. To investigate the relevance of EV-mediated communication between bone cells, we isolated EVs from primary mouse osteoblasts and assessed membrane integrity, size, and structure by transmission electron microscopy (TEM) and fluorescence-activated cell sorting (FACS). EVs actively shuttled loaded fluorochromes to osteoblasts, monocytes, and endothelial cells. Moreover, osteoblast EVs contained mRNAs shared with donor cells. Osteoblasts are known to regulate osteoclastogenesis, osteoclast survival, and osteoclast function by the pro-osteoclastic cytokine, receptor activator of nuclear factor κ-B ligand (Rankl). Osteoblast EVs were enriched in Rankl, which increased after PTH treatment. These EVs were biologically active, supporting osteoclast survival. EVs isolated from rankl^-/- osteoblasts lost this pro-osteoclastic function, indicating its Rankl-dependence. They integrated ex vivo into murine calvariae, and EV-shuttled fluorochromes were quickly taken up by the bone upon in vivo EV systemic administration. Rankl^-/- mice lack the osteoclast lineage and are negative for its specific marker tartrate-resistant acid phosphatase (TRAcP). Treatment of rankl^-/- mice with wild-type osteoblast EVs induced the appearance of TRAcP-positive cells in an EV density-dependent manner. Finally, osteoblast EVs internalized and shuttled anti-osteoclast drugs (zoledronate and dasatinib), inhibiting osteoclast activity in vitro and in vivo. We conclude that osteoblast EVs are involved in intercellular communication between bone cells, contribute to the Rankl pro-osteoclastic effect, and shuttle anti-osteoclast drugs, representing a potential means of targeted therapeutic delivery. © 2017 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

V-ATPases Containing a3 Subunit Play a Direct Role in Enamel Development in Mice

Vacuolar H⁺ -ATPases (V-ATPases) are ubiquitous multisubunit proton pumps responsible for organellar pH maintenance. Mutations in the a3 subunit of V-ATPases cause autosomal recessive osteopetrosis, a rare disease due to impaired bone resorption. Patients with osteopetrosis also display dental anomalies, such as enamel defects; however, it is not clear whether these enamel abnormalities are a direct consequence of the a3 mutations. We investigated enamel mineralization, spatiotemporal expression of enamel matrix proteins and the a3 protein during tooth development using an osteopetrotic mouse model with a R740S point mutation in the V-ATPase a3 subunit. Histology revealed aberrations in both crown and root development, whereas SEM analysis demonstrated delayed enamel mineralization in homozygous animals. Enamel thickness and mineralization were significantly decreased in homozygous mice as determined by μCT analysis. The expression patterns of the enamel matrix proteins amelogenin, amelotin, and odontogenic ameloblast-associated protein (ODAM) suggested a delay in transition to the maturation stage in homozygous animals. Protein expression of the a3 subunit was detected in ameloblasts in all three genotypes, suggesting that a3-containing V-ATPases play a direct role in amelogenesis, and mutations in a3 delay transition from the secretory to the maturation stage, resulting in hypomineralized and hypoplastic enamel. J. Cell. Biochem. 118: 3328-3340, 2017. © 2017 Wiley Periodicals, Inc.

Raine Syndrome (OMIM #259775), Caused By FAM20C Mutation, Is Congenital Sclerosing Osteomalacia With Cerebral Calcification (OMIM 259660)

In 1985, we briefly reported infant sisters with a unique, lethal, autosomal recessive disorder designated congenital sclerosing osteomalacia with cerebral calcification. In 1986, this condition was entered into Mendelian Inheritance In Man (MIM) as osteomalacia, sclerosing, with cerebral calcification (MIM 259660). However, no attestations followed. Instead, in 1989 Raine and colleagues published an affected neonate considering unprecedented the striking clinical and radiographic features. In 1992, "Raine syndrome" entered MIM formally as osteosclerotic bone dysplasia, lethal (MIM #259775). In 2007, the etiology emerged as loss-of-function mutation of FAM20C that encodes family with sequence similarity 20, member C. FAM20C is highly expressed in embryonic calcified tissues and encodes a kinase (dentin matrix protein 4) for most of the secreted phosphoproteome including FGF23, osteopontin, and other regulators of skeletal mineralization. Herein, we detail the clinical, radiological, biochemical, histopathological, and FAM20C findings of our patients. Following premortem tetracycline labeling, the proposita's non-decalcified skeletal histopathology after autopsy indicated no rickets but documented severe osteomalacia. Archival DNA revealed the sisters were compound heterozygotes for a unique missense mutation and a novel deletion in FAM20C. Individuals heterozygous for the missense mutation seemed to prematurely fuse their metopic suture and develop a metopic ridge sometimes including trigonocephaly. Our findings clarify FAM20C's role in hard tissue formation and mineralization, and show that Raine syndrome is congenital sclerosing osteomalacia with cerebral calcification. © 2016 American Society for Bone and Mineral Research.

Synonymous Mutations Add a Layer of Complexity in the Diagnosis of Human Osteopetrosis

Autosomal recessive osteopetroses (AROs) are rare, genetically heterogeneous skeletal diseases with increased bone density that are often lethal if left untreated. A precise molecular classification is relevant for the patient's management, because in some subgroups hematopoietic stem cell transplantation (HSCT), which is the only curative therapy, is contraindicated. In two unrelated ARO patients, the molecular analysis revealed the presence of a synonymous variant in known ARO genes, namely in the TCIRG1 gene in one patient and in the CLCN7 in the other patient, predicted to impact on the splicing process. In the latter case, sequencing of the transcript confirmed the splicing defect, whereas in the former, for whom an RNA sample was not available, the defect was reconstructed in vitro by the minigene technology. These results strongly suggest that these synonymous changes were responsible for the disease in our patients. Our findings are novel with respect to ARO and add to the few reports in literature dealing with different diseases, underlining the importance of cDNA analysis for the correct assessment of exonic changes, even when exome sequencing is performed. In particular, we highlight the possibility that at least in some cases ARO is due to synonymous changes, erroneously considered clinically silent, in the genes already described in literature, and suggest carefully reevaluating the sequencing results of these genes when mutations are not found at a first analysis. In addition, with respect to the CLCN7 gene, we suggest that synonymous variants might also contribute to the large spectrum of severity typical of CLCN7-dependent osteopetrosis through more subtle, but not negligible, effects on protein availability and functionality. © 2016 American Society for Bone and Mineral Research.

Transplantation of Haploidentical TcRaß-Depleted Hematopoietic Cells Allows for Optimal Timing and Sustained Correction of the Metabolic Defect in Children With Infantile Osteopetrosis

In osteopetrosis, osteoclast dysfunction can lead to deafness, blindness, bone marrow failure, and death. Hematopoietic cell transplantation (HCT) is currently the only curative treatment, but outcome remains disappointing. Although a rapid progression toward HCT is detrimental to prevent further progress of disease manifestations, 70% of cases lack an HLA-matched sibling and require alternative stem cell sources. We present two cases of osteopetrosis that successfully received an HCT with haploidentical TcRαβ-depleted cells from one of the parents. These cases showed no further disease progression, had restoration of functional osteoclasts, and illustrate this approach to enable prompt HCT with ready available parental donors and rapid and sustained hematological, including osteoclast, recovery. © 2016 American Society for Bone and Mineral Research.

Characterization of a Relatively Malignant Form of Osteopetrosis Caused by a Novel Mutation in the PLEKHM1 Gene

Osteopetrosis (OMIM: 611497), literally "stone bone," is a group of inherited bone disorders characterized by increased skeletal mass due to defective osteoclast function. A patient who reported a history of frequent fractures, weakness and fatigue was admitted to our hospital in 2011. The patient presented with the typical features of osteopetrosis: fractures after minor trauma, early tooth loss, anemia, hepatosplenomegaly, and a generalized increase in bone mineral density (BMD). Aside from his father's complaint of excessive tooth loss, his mother, two sisters, son, and daughter were healthy. Blood samples of the family members were drawn for genetic analyses. The entire coding region and adjacent splice sites of the pleckstrin homology domain-containing family M (with RUN domain) member 1 (PLEKHM1) gene were sequenced. One mutation, a heterozygous deletion mutation in exon 11 (c.3051_3052delCA), was identified in the patient but not in his relatives. The mutation leads to a translation product with a highly impaired Rubicon homology domain. Co-immunoprecipitation and immunofluorescence analyses using HEK293 cells showed that overexpression of a PLEKHM1 CA-deletion mutant resulted in a dramatic decrease in the interaction between PLEKHM1 and the small GTPase Rab7 compared to wild-type PLEKHM1. The normal processes of endocytosis and autophagy were disturbed in cells expressing the mutant (transfected HEK293 and U937 cells), as indicated by epidermal growth factor receptor (EGFR) degradation and an altered LC3-I/II ratio, respectively, which may lead to a defect in osteoclast function. A four-year follow-up study of the patient showed that the PLEKHM1-dependent osteopetrosis was relatively malignant, with significant symptoms of pancytopenia and hepatosplenomegaly. © 2016 American Society for Bone and Mineral Research.

Bone Mineral Density and Microarchitecture in Patients With Autosomal Dominant Osteopetrosis: A Report of Two Cases

The aim of this case study is to describe changes in areal bone mineral density (aBMD) by dual-energy X-ray absorptiometry (DXA) scan, as well as volumetric bone density and microarchitecture by high-resolution peripheral quantitative computed tomography (HR-pQCT) in two patients with autosomal dominant osteopetrosis (ADO) and compare with 20 healthy subjects. We describe a 44-year-old male patient with six low-impact fractures since he was age 16 years, and a 32-year-old female patient with four low-impact fractures on her past history. Radiographic changes were typical of ADO. Consistent with the much higher aBMD, total volumetric BMD (average bone density of the whole bone, including trabecular and cortical compartments) at distal radius and tibia (HR-pQCT) was more than twice the mean values found in healthy subjects in both patients. Trabecular number and thickness were higher, leading to an evident increase in trabecular bone volume to tissue volume. Also, an enormous increase in cortical thickness was found. Most important, a great heterogeneity in bone microstructure of the affected patients was evident on HR-pQCT images: islets of very dense bone were interposed with areas with apparent normal density. The increase in aBMD, volumetric BMD, and most indices of trabecular and cortical bone, associated with the great heterogeneity on bone tridimensional microarchitecture, reflect the accumulation of old and fragile bone randomly distributed along the skeleton. These alterations in bone microstructure probably compromise bone quality, which might justify the high prevalence of low-impact fractures in patients with ADO, despite abnormally elevated BMD.

Neonatal High Bone Mass With First Mutation of the NF-κB Complex: Heterozygous De Novo Missense (p.Asp512Ser) RELA (Rela/p65)

Heritable disorders that feature high bone mass (HBM) are rare. The etiology is typically a mutation(s) within a gene that regulates the differentiation and function of osteoblasts (OBs) or osteoclasts (OCs). Nevertheless, the molecular basis is unknown for approximately one-fifth of such entities. NF-κB signaling is a key regulator of bone remodeling and acts by enhancing OC survival while impairing OB maturation and function. The NF-κB transcription complex comprises five subunits. In mice, deletion of the p50 and p52 subunits together causes osteopetrosis (OPT). In humans, however, mutations within the genes that encode the NF-κB complex, including the Rela/p65 subunit, have not been reported. We describe a neonate who died suddenly and unexpectedly and was found at postmortem to have HBM documented radiographically and by skeletal histopathology. Serum was not available for study. Radiographic changes resembled malignant OPT, but histopathological investigation showed morphologically normal OCs and evidence of intact bone resorption excluding OPT. Furthermore, mutation analysis was negative for eight genes associated with OPT or HBM. Instead, accelerated bone formation appeared to account for the HBM. Subsequently, trio-based whole exome sequencing revealed a heterozygous de novo missense mutation (c.1534_1535delinsAG, p.Asp512Ser) in exon 11 of RELA encoding Rela/p65. The mutation was then verified using bidirectional Sanger sequencing. Lipopolysaccharide stimulation of patient fibroblasts elicited impaired NF-κB responses compared with healthy control fibroblasts. Five unrelated patients with unexplained HBM did not show a RELA defect. Ours is apparently the first report of a mutation within the NF-κB complex in humans. The missense change is associated with neonatal osteosclerosis from in utero increased OB function rather than failed OC action. These findings demonstrate the importance of the Rela/p65 subunit within the NF-κB pathway for human skeletal homeostasis and represent a new genetic cause of HBM.

Lenz-Majewski hyperostotic dwarfism with hyperphosphoserinuria from a novel mutation in PTDSS1 encoding phosphatidylserine synthase 1

Lenz-Majewski hyperostotic dwarfism (LMHD) is an ultra-rare Mendelian craniotubular dysostosis that causes skeletal dysmorphism and widely distributed osteosclerosis. Biochemical and histopathological characterization of the bone disease is incomplete and nonexistent, respectively. In 2014, a publication concerning five unrelated patients with LMHD disclosed that all carried one of three heterozygous missense mutations in PTDSS1 encoding phosphatidylserine synthase 1 (PSS1). PSS1 promotes the biosynthesis of phosphatidylserine (PTDS), which is a functional constituent of lipid bilayers. In vitro, these PTDSS1 mutations were gain-of-function and increased PTDS production. Notably, PTDS binds calcium within matrix vesicles to engender hydroxyapatite crystal formation, and may enhance mesenchymal stem cell differentiation leading to osteogenesis. We report an infant girl with LMHD and a novel heterozygous missense mutation (c.829T>C, p.Trp277Arg) within PTDSS1. Bone turnover markers suggested that her osteosclerosis resulted from accelerated formation with an unremarkable rate of resorption. Urinary amino acid quantitation revealed a greater than sixfold elevation of phosphoserine. Our findings affirm that PTDSS1 defects cause LMHD and support enhanced biosynthesis of PTDS in the pathogenesis of LMHD.

Altered hematopoietic stem cell and osteoclast precursor frequency in cathepsin K null mice

Cathepsin K (CatK) is a lysosomal cysteine protease necessary for bone resorption by osteoclasts (OCs), which originate from myeloid hematopoietic precursors. CatK-deficient (CatK(-/-) ) mice show osteopetrosis due to defective resorption by OCs, which are increased in number in these mice. We investigated whether genetic ablation of CatK altered the number of hematopoietic stem cells (HSCs) and OC precursor cells (OCPs) using two mouse models: CatK(-/-) mice and a knock-in mouse model in which the CatK gene (ctsk) is replaced by cre recombinase. We found that CatK deletion in mice significantly increased the number of HSCs in the spleen and decreased their number in bone marrow. In contrast, the number of early OCPs was unchanged in the bone marrow. However, the number of committed CD11b(+) OCPs was increased in the bone marrow of CatK(-/-) compared to wild-type (WT) mice. In addition, the percentage but not the number of OCPs was decreased in the spleen of CatK(-/-) mice relative to WT. To understand whether increased commitment to OC lineage in CatK(-/-) mice is influenced by the bone marrow microenvironment, CatK(Cre/+) or CatK(Cre/Cre) red fluorescently labeled OCPs were injected into WT mice, which were also subjected to a mid-diaphyseal femoral fracture. The number of OCs derived from the intravenously injected CatK(Cre/Cre) OCPs was lower in the fracture callus compared to mice injected with CatK(+/Cre) OCPs. Hence, in addition to its other effects, the absence of CatK in OCP limits their ability to engraft in a repairing fracture callus compared to WT OCP.

CLCN7 and TCIRG1 mutations differentially affect bone matrix mineralization in osteopetrotic individuals

Osteopetrosis is an inherited disorder of impaired bone resorption, with the most commonly affected genes being CLCN7 and TCIRG1, encoding the Cl(-) /H(+) exchanger CLC-7 and the a3 subunit of the vacuolar H(+) -ATPase, respectively. We and others have previously shown that the disease is frequently accompanied by osteomalacia, and that this additional pathology is also found in Tcirg1-deficient oc/oc mice. The remaining question was whether osteoid enrichment is specifically associated with TCIRG1 inactivation, or whether CLCN7 mutations would also cause skeletal mineralization defects. Here we describe a complete osteologic assessment of one family carrying a novel mutation in CLCN7 (D145G), which impairs the activation and relaxation kinetics of the CLC-7 ion transporter. The two siblings carrying the mutation in the homozygous state displayed high bone mass, increased serum levels of bone formation markers, but no impairment of calcium homeostasis when compared to the other family members. Most importantly, however, undecalcified processing of an iliac crest biopsy from one of the affected children clearly demonstrated a pathological increase of trabecular bone mass, but no signs of osteomalacia. Given the potential relevance of these findings we additionally performed undecalcified histology of iliac crest biopsies from seven additional cases with osteopetrosis caused by a mutation in TNFRSF11A (n=1), CLCN7 (n=3), or TCIRG1 (n=3). Here we observed that all cases with TCIRG1-dependent osteopetrosis displayed severe osteoid accumulation and decreased calcium content within the mineralized matrix. In contrast, there was no detectable bone mineralization defect in the cases with TNFRSF11A-dependent or CLCN7-dependent osteopetrosis. Taken together, our analysis demonstrates that CLCN7 and TCIRG1 mutations differentially affect bone matrix mineralization, and that there is a need to modify the current classification of osteopetrosis.

Hereditary osteopetrosis of the rabbit. III. Pathologic observations; skeletal abnormalities

Gross and microscopic observations on the skeleton of rabbits with hereditary osteopetrosis have been described. All the bones were invariably and similarly affected. The outstanding abnormalities included the persistence of spongy bone and the presence of fibrous tissue together with the failure of development of a marrow cavity and medullary marrow. The amount of hemopoietic tissue, which was present only in comparatively small foci, was greatly reduced. The character of the lesions and their development as determined by examination of material at various stages of the disease suggested that the essential defect concerned the mesenchymal cell. The pathologic features observed generally resembled those of human osteopetrosis as did the manifestations of the disease during life (1, 2).

Hereditary osteopetrosis of the rabbit. IV. Pathologic observations; general features

The results of postmortem examination of cases of hereditary osteopetrosis of the rabbit together with histologic observations on organs and tissues other than the skeleton have been described. The principal findings were, first, those associated with the characteristic progressive anemia of the disease, such as extramedullary foci of hemopoietic tissue, lymphoid hyperplasia, and the occurrence of hemosiderin in the liver, spleen, and lymph nodes. There was a widespread tissue distribution of intense phosphatase staining and of fine calcium deposition, as would be expected in the circumstances of the profound skeletal abnormality (3). In advanced cases with established growth retardation, malnutrition, and deterioration, the tissues generally showed a decreased glycogen content. The large amount of parathyroid tissue found in both early and late cases suggested a state of hyperparathyroidism. Low serum calcium, high serum phosphorus and phosphatase levels (2), and a predominately osteoblastic reaction (3) were suggestive of hypoparathyroidism. The possibility that an involvement of the parathyroid glands was a basic or primary condition of the disease is discussed. Evidence of a disturbance of other endocrine glands was shown by the predominately acidophilic staining reaction of the colloid of the thyroid, an enlargement of the adrenals in which both cortex and medulla participated, and the tendency toward a basophilia of the anterior lobe of the pituitary. It was pointed out that before an explanation of the part played by the parathyroid glands in this disease could be made, other data, including particularly embryological studies, must be available. Similarly, an interpretation of other endocrine gland changes must await additional information.

STUDIES IN OSTEOPETROSIS

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