Pascale Chavassieux

Denosumab is an anti-resorptive therapy that effectively reduces fracture risk and increases BMD in women with postmenopausal osteoporosis. In this study, we focused on the less well-investigated cortical remodeling process and its transition from erosion to formation after denosumab treatment, using a histomorphometric classification of intracortical pores. Cortical bone is more prominent in non-vertebral bones, where denosumab is less effective in fracture reduction. In iliac crest biopsies from the FREEDOM-study, where postmenopausal osteoporotic women were treated with placebo (n = 43) or denosumab (n = 43) for 2-3 yr, the cortical microstructure and remodeling stage and type of intracortical pores were analyzed. Cortical thickness was unaffected in the denosumab group vs placebo (P = .9). Mean pore diameter was significantly decreased in the denosumab group vs placebo (P < .002) with no change in the number of pores per tissue area (P = .83). However, the cortical porosity was not decreased in the denosumab vs placebo group (P < .077). When stratifying the pores according to their remodeling stage, the eroded pores had an increased contribution to the total pore area in the denosumab vs placebo group (P = .001). Eroded-formative and formative pores had a decreased contribution to the total pore area (P < .001). The contribution of quiescent pores was unchanged. The increased contribution of eroded pores and the concomitant reduced contribution of eroded-formative and formative pores indicate that the transition from erosion to formation is largely limited, while pores with ongoing bone formation at the time of initiation of denosumab treatment are refilled.

OI, or bone brittle disease, is characterized by increased mineralization of bone matrix independently of clinical severity. So, a beneficial effect of antiresorptive treatments such as bisphosphonates (BP) is questionable. We aim to compare the bone matrix characteristics before and after BP pamidronate (PAM). Fifty-eight children (9 ± 5 yr-old) with OI (Type I, III, IV, V, VI, XI, or unknown) received intravenous PAM for 2 yr and underwent transiliac bone biopsies before (n = 57) and after (n = 35) treatment. Compared with age-matched controls, untreated OI was characterized by cortical and cancellous rarefaction. Two years of PAM in OI patients significantly decreased bone remodeling activity, increased cortical thickness, improved the maturation of both organic and mineral matrix, and most of the nanomechanical properties, despite further increase in the degree of mineralization. Overall, in addition to a gain in bone mass, our results showed for the first time that PAM is able to increase the maturation of mineral crystals and collagen matrix contributing to its antifracture efficacy in OI patients.

Romosozumab, a humanized monoclonal antibody that binds and inhibits sclerostin, produces a marked increase in bone formation with a concomitant decreased bone resorption. This transient rise in bone formation in the first 2 months of treatment is mainly due to an increased modeling-based bone formation. This requires the recruitment and differentiation of osteoblasts, one possibility being a preferential switch in commitment of precursors to osteoblasts over adipocytes. The purpose of this study was to analyze the marrow adiposity in transiliac bone biopsies at months 2 or 12 from the FRAME biopsy sub-study in patients receiving romosozumab or placebo. The total adipocyte area, number, and density were measured on the total cancellous bone area. The size and shape at the individual adipocyte level were assessed including the mean adipocyte area, perimeter, min and max diameters, and aspect ratio. No significant difference in total adipocyte area, number, or density between placebo and romosozumab groups was observed at months 2 and 12, and no difference was observed between 2 and 12 months. After 2 or 12 months, romosozumab did not modify the size or shape of the adipocytes. No relationship between the adipocyte parameters and the dynamic parameters of bone formation could be evidenced. In conclusion, based on the analysis of a small number of biopsies, no effect of romosozumab on bone marrow adiposity of iliac crest was identified after 2 and 12 months suggesting that the modeling-based formation observed at month 2 was not due to a preferential commitment of the precursor to osteoblast over adipocyte cell lines but may result from a reactivation of bone lining cells and from a progenitor pool independent of the marrow adipocyte population.

Romosozumab treatment results in a transient early increase in bone formation and sustained decrease in bone resorption. Histomorphometric analyses revealed that the primary bone-forming effect of romosozumab is a transient early stimulation of modeling-based bone formation on cancellous and endocortical surfaces. Furthermore, preclinical studies have demonstrated that romosozumab may affect changes in the remodeling unit, resulting in positive bone balance. To further investigate the effects of romosozumab on bone balance, mo 12 (M12) and mo 2 (M2) (to analyze early effects) unpaired bone biopsies from the FRAME clinical trial were analyzed using remodeling site reconstruction to assess whether positive changes in bone balance on cancellous/endocortical surfaces may contribute to the progressive improvement in bone mass/structure and reduced fracture risk in osteoporotic women at high fracture risk. At M12, bone balance was higher with romosozumab vs placebo on cancellous (+6.1 vs +1.5 μm; P = .012) and endocortical (+5.2 vs −1.7 μm; P = .02) surfaces; higher bone balance was due to lower final erosion depth (40.7 vs 43.7 μm; P = .05) on cancellous surfaces and higher completed wall thickness (50.8 vs 47.5 μm; P = .037) on endocortical surfaces. At M2, the final erosion depth was lower on the endocortical surfaces (42.7 vs 50.7 μm; P = .021) and was slightly lower on the cancellous surfaces (38.5 vs 44.6 μm; P = .11) with romosozumab vs placebo. Sector analysis of early endocortical formative sites revealed higher osteoid thickness (29.9 vs 19.2 μm; P = .005) and mineralized wall thickness (18.3 vs 11.9 μm; P = .004) with romosozumab vs placebo. These evolving bone packets may reflect the early stimulation of bone formation that contributes to the increase in completed wall thickness at M12. These data suggest that romosozumab induces a positive bone balance due to its effects on bone resorption and formation at the level of the remodeling unit, contributing to the positive effects on bone mass, structure, and fracture risk. Romosozumab treatment has a dual effect on bone, adding new bone and reducing bone loss. In the FRAME clinical trial, romosozumab increased the bone mass and strength and reduced fracture risk in postmenopausal women with osteoporosis. Addition of new bone occurs early in treatment and rapidly on cancellous and endocortical bone surfaces where bone resorption is not ongoing. However, it remains unclear if romosozumab affects bone loss or gain in areas where bone resorption is ongoing (remodeling units), contributing to a further positive bone balance. Here, we examined whether changes at the remodeling unit occur early (2 mo) and/or late (12 mo) in treatment by using bone biopsies from patients treated with romosozumab or placebo in FRAME. At M2, a combination of lower bone resorption and higher bone gain on endocortical surfaces resulted in a positive bone balance with romosozumab vs placebo. At M12, the bone balance was positive with romosozumab vs placebo due to lower bone resorption on cancellous surfaces and greater bone gain on endocortical surfaces. This demonstrates that romosozumab induces a positive bone balance at remodeling units early in treatment, leading to overall gains observed later, contributing to the positive effects of romosozumab on bone mass and structure.

Purpose The main effect of anti-resorptive agents such as bisphosphonates is a reduction of bone resorption, with a consequent marked decrease of bone turnover. This post-hoc analysis investigated the changes of histomorphometric parameters of bone turnover after alendronate (ALN), according to the baseline turnover. Methods Ninety postmenopausal women underwent a transiliac bone biopsy before and after 6 (n = 44) or 12 (n = 46) months of treatment with ALN (70 mg/week). The dynamic parameters reflecting the bone formation and bone turnover were mineralizing surface (MS/BS; %), bone formation rate (BFR/BS; μm³/μm²/d), and activation frequency (Ac.f; /yr). Biochemical markers sPINP and the sCTX were assessed before treatment and after 3, 6, and 12 months. Subjects were divided into quartiles based on the baseline values of BFR/BS. Results At baseline, MS/BS and Ac.f were significantly different (p < 0.0001) among the BFR quartiles. sCTX and sP1NP were not significantly different among quartiles. After ALN treatment, MS/BS was not significantly different among quartiles but Ac.f remained significantly lower in the first quartile compared to the third and fourth ones (p < 0.03). The absolute value of the difference between pre- and post-treatment significantly correlated with the baseline BFR/BS but when expressed in percent of the baseline value, the magnitude of the diminutions of MS/BS, Ac.f, sCTX, and sP1NP was similar in the four baseline BFR quartiles. Conclusion The percentage response to ALN appeared independent of the baseline level of bone turnover. After treatment, the bone turnover tended to be similar in all BFR quartiles. Summary This analysis investigated the influence of baseline turnover measured by bone histomorphometry on the effect of alendronate. When expressed in percent of pre-treatment values, the decreases of histomorphometric parameters and biochemical markers of bone turnover were independent of the baseline turnover.

Despite the development of non-invasive methods, bone histomorphometry remains the only method to analyze bone at the tissue and cell levels. Quantitative analysis of transiliac bone sections requires strict methodologic conditions but since its foundation more 60 years ago, this methodology has progressed. Our purpose was to review the evolution of bone histomorphometry over the years and its contribution to the knowledge of bone tissue metabolism under normal and pathological conditions and the understanding of the action mechanisms of therapeutic drugs in humans. The two main applications of bone histomorphometry are the diagnosis of bone diseases and research. It is warranted for the diagnosis of mineralization defects as in osteomalacia, of other causes of osteoporosis as bone mastocytosis, or the classification of renal osteodystrophy. Bone biopsies are required in clinical trials to evaluate the safety and mechanism of action of new therapeutic agents and were applied to anti-osteoporotic agents such as bisphosphonates and denosumab, an anti-RANKL, which induces a marked reduction of the bone turnover with a consequent elongation of the mineralization period. In contrast, an increased bone turnover with an extension of the formation site is observed with teriparatide. Romosozumab, an anti-sclerostin, has a dual effect with an early increased formation and reduced resorption. Bone histomorphometric studies allow us to understand the mechanism of coupling between formation and resorption and to evaluate the respective role of bone modeling and remodeling. The adaptation of new image analysis techniques will help bone biopsy analysis in the future.

The bone-forming agent romosozumab is a monoclonal antibody that inhibits sclerostin, leading to increased bone formation and decreased resorption. The highest levels of bone formation markers in human patients are observed in the first 2 months of treatment. Histomorphometric analysis of bone biopsies from the phase 3 FRAME trial (NCT01575834) showed an early significant increase in bone formation with concomitant decreased resorption. Preclinical studies demonstrated that most new bone formation following romosozumab treatment was modeling-based bone formation (MBBF). Here we analyzed bone biopsies from FRAME to assess the effect of 2 months of romosozumab vs placebo on the surface extent of MBBF and remodeling-based bone formation (RBBF). In FRAME, postmenopausal women aged ≥55 years with osteoporosis were randomized 1:1 to 210 mg romosozumab or placebo SC QM for 12 months, followed by 60 mg denosumab SC Q6M for 12 months. Participants in the bone biopsy substudy received quadruple tetracycline labeling and underwent transiliac biopsies at month 2. A total of 29 biopsies were suitable for histomorphometry. Using fluorescent microscopy, bone formation at cancellous, endocortical, and periosteal envelopes was classified based on the appearance of underlying cement lines as modeling (smooth) or remodeling (scalloped). Data were compared using the Wilcoxon rank-sum test, without multiplicity adjustment. After 2 months, the median percentage of MBBF referent to the total bone surface was significantly increased with romosozumab vs placebo on cancellous (18.0% vs 3.8%; p = 0.005) and endocortical (36.7% vs 3.0%; p = 0.001), but not on periosteal (5.0% vs 2.0%; p = 0.37) surfaces, with no significant difference in the surface extent of RBBF on all three bone surfaces. These data show that stimulation of bone formation in the first 2 months of romosozumab treatment in postmenopausal women with osteoporosis is predominately due to increased MBBF on endocortical and cancellous surfaces.

Bisphosphonates (BPs) are the most widely used drugs for the treatment of osteoporosis but prolonged use of BPs might increase the risk of atypical femur fracture (AFF). There are only a few studies that address the bone material quality in patients on long-term BP treatment with or without AFFs. We analyzed 52 trans-iliac bone biopsies from patients on long-term BP therapy with (n = 26) and without (n = 26) AFF. At the microscopic level, the degree of mineralization of bone (DMB) was assessed on whole bone by X-ray digitized microradiography while microhardness by Vickers microindentation, and bone matrix characteristics by Fourier transform infrared microspectroscopy (FTIRM) (mineral/organic ratio, mineral maturity and crystallinity, and collagen maturity) were measured at random focal areas. The AFF patients were treated longer than non-AFF patients (9.7 ± 3.3 years versus 7.9 ± 2.7 years). As expected, bone remodeling was low in both groups, without difference between them. The AFF group had significantly higher DMB in cortical bone (+2.9%, p = .001), which remained so after adjusting for treatment duration (p = .007), and showed a trend in cancellous bone (+1.6%, p = .05). Consistent with higher DMB, heterogeneity index (HI) was lower in the AFF than in the non-AFF group, illustrating lower heterogeneity of mineralization in the AFF group. A significant positive correlation between the duration of treatment and DMB in cortical bone was found in AFF, and not in the non-AFF group. Microhardness and bone matrix characteristics were similar between groups. We conclude that the AFF group had a duration-dependent increase in DMB leading to a significantly higher DMB than the non-AFF. Because BPs have high affinity to bone mineral and lining the walls of the osteocyte lacunae, the accumulation of matrix-bound BPs in AFF could lead to inhibition of the osteocyte cytoskeleton blunting their response to mechanical strains, a hypothesis to be further investigated. © 2021 American Society for Bone and Mineral Research (ASBMR).

Osteogenesis Imperfecta (OI) is a primary bone fragility disorder with an estimated prevalence of 1/15 000 births. The majority of OI cases are inherited in an autosomal‐dominant manner, while five to ten percent have recessive or X‐linked inheritance. Up to now, approximately 5% of OI cases remain without mutation demonstrated, supporting the involvement of other genes in the disease spectrum. By whole exome sequencing, we identified a homozygous variant (c.2T>C) in CCDC134 gene in three patients from two unrelated families, with severe bone fragility which did not respond to bisphosphonate treatment, short stature, gracile long bones with pseudarthroses, but no dentinogenesis imperfecta. CCDC134 encodes a secreted protein widely expressed, implicated in the regulation of some mitogen‐activated protein kinases (MAPK) signaling pathway. Western blot and immunofluorescence analyses confirmed the absence of CCDC134 protein in patient cells, compared to controls. Furthermore, we demonstrated that CCDC134 mutations are associated with increased Erk1/2 phosphorylation, decreased OPN mRNA and COL1A1 expression and reduced mineralization in patient osteoblasts compared to controls. These data support that CCDC134 is a new gene involved in severe progressive deforming recessive osteogenesis imperfecta (type III).