Infantile Malignant, Autosomal Recessive Osteopetrosis: The Rich and The Poor

Vacuolar Hþ -ATPase (V-ATPase), a multisubunit enzyme located at the ruffled border and in lysosomes of osteoclasts, is necessary for bone resorption. We previously showed that heterozygous mice with an R740S mutation in the a3 subunit of V-ATPase (þ/R740S) have mild osteopetrosis resulting from an $90% reduction in proton translocation across osteoclast membranes. Here we show that lysosomal pH is also higher in þ/R740S compared with wild-type (þ/þ) osteoclasts. Both osteoclast number and size were decreased in cultures of þ/R740S compared with þ/þ bone marrow cells, with concomitant decreased expression of key osteoclast markers (TRAP, cathepsin K, OSCAR, DC-STAMP, and NFATc1), suggesting that low lysosomal pH plays an important role in osteoclastogenesis. To elucidate the molecular mechanism of this inhibition, NFATc1 activation was assessed. NFATc1 nuclear translocation was significantly reduced in þ/R740S compared with þ/þ cells; however, this was not because of impaired enzymatic activity of calcineurin, the phosphatase responsible for NFATc1 dephosphorylation. Protein and RNA expression levels of regulator of calcineurin 1 (RCAN1), an endogenous inhibitor of NFATc1 activation and a protein degraded in lysosomes, were not significantly different between þ/R740S and þ/þ osteoclasts, but the RCAN1/NFATc1 ratio was significantly higher in þ/R740S versus þ/þ cells. The lysosomal inhibitor chloroquine significantly increased RCAN1 accumulation in þ/þ cells, consistent with the hypothesis that higher lysosomal pH impairs RCAN1 degradation, leading to a higher RCAN1/NFATc1 ratio and consequently NFATc1 inhibition. Our data indicate that increased lysosomal pH in osteoclasts leads to decreased NFATc1 signaling and nuclear translocation, resulting in a cell autonomous impairment of osteoclastogenesis in vitro. ß

Section: Introductionmentioning

confidence: 62%

The R740S mutation in the V-ATPase a3 subunit increases lysosomal pH, impairs NFATc1 translocation, and decreases in vitro osteoclastogenesis

Voronov

Ochotny

Jaumouillé

et al. 2012

“…In the last decade, it has been demonstrated to be genetically heterogeneous and it has been shown that the specific molecular defect defines subgroups with different prognostic and therapeutic implications. (15) Of note, to date, in approximately 25% of patients the specific genetic defect is not known, confirming that our understanding of bone biology related to this disorder is still incomplete.…”

Section: Discussionmentioning

confidence: 99%

SNX10 mutations define a subgroup of human autosomal recessive osteopetrosis with variable clinical severity

Pangrazio

Fasth

Sbardellati

et al. 2012

Self Cite

Human Autosomal Recessive Osteopetrosis (ARO) is a genetically heterogeneous disorder caused by reduced bone resorption by osteoclasts. In 2000, we found that mutations in the TCIRG1 gene encoding for a subunit of the proton pump (V-ATPase) are responsible for more than one-half of ARO cases. Since then, five additional genes have been demonstrated to be involved in the pathogenesis of the disease, leaving approximately 25% of cases that could not be associated with a genotype. Very recently, a mutation in the sorting nexin 10 (SNX10) gene, whose product is suggested to interact with the proton pump, has been found in 3 consanguineous families of Palestinian origin, thus adding a new candidate gene in patients not previously classified. Here we report the identification of 9 novel mutations in this gene in 14 ARO patients from 12 unrelated families of different geographic origin. Interestingly, we define the molecular defect in three cases of ''Västerbottenian osteopetrosis,'' named for the Swedish Province where a higher incidence of the disease has been reported. In our cohort of more than 310 patients from all over the world, SNX10-dependent ARO constitutes 4% of the cases, with a frequency comparable to the receptor activator of NF-kB ligand (RANKL), receptor activator of NF-kB (RANK) and osteopetrosisassociated transmembrane protein 1 (OSTM1)-dependent subsets. Although the clinical presentation is relatively variable in severity, bone seems to be the only affected tissue and the defect can be almost completely rescued by hematopoietic stem cell transplantation (HSCT). These results confirm the involvement of the SNX10 gene in human ARO and identify a new subset with a relatively favorable prognosis as compared to TCIRG1-dependent cases. Further analyses will help to better understand the role of SNX10 in osteoclast physiology and verify whether this protein might be considered a new target for selective antiresorptive therapies. ß

“…(1)(2)(3)(4) At least six genes have already been implicated in the pathogenesis of the disease. (5) In particular, the identification of mutations in the TNFSF11 (receptor activator of NF-kB ligand [RANKL], tumor necrosis factor-related activation induced cytokine [TRANCE]) gene in a form of osteoclast-poor ARO (6) has highlighted the presence of a subgroup of patients who cannot benefit from HSCT, because the mutated gene is specifically expressed by cells of the stromal compartment. (7,8) RANKL is a cytokine driving osteoclast progenitors to differentiate along the osteoclast lineage.…”

Section: Introductionmentioning

confidence: 99%

Osteopetrosis rescue upon RANKL administration to Rankl−/− mice: A new therapy for human RANKL-dependent ARO

Iacono

Blair

Poliani

et al. 2012

Self Cite

In the last decades the molecular basis of monogenic diseases has been largely unraveled, although their treatment has often remained unsatisfactory. Autosomal recessive osteopetrosis (ARO) belongs to the small group of genetic diseases that are usually treated with hematopoietic stem cell transplantation (HSCT). However, this approach is not effective in the recently identified form carrying mutations in the receptor activator of NF-kB ligand (RANKL) gene. In this subset, therapy replacement approach based on RANKL delivery has a strong rationale. Here we demonstrate that the systematic administration of RANKL for 1 month to Rankl À/À mice, which closely resemble the human disease, significantly improves the bone phenotype and has beneficial effects on bone marrow, spleen and thymus; major adverse effects arise only when mice are clearly overtreated. Overall, we provide evidence that the pharmacological administration of RANKL represents the appropriate treatment option for RANKL-deficient ARO patients, to be validated in a pilot clinical trial. ß