Journal of Bone and Mineral Research最新文献

Mendelian randomization (MR) is a genetic epidemiological technique that uses genetic variation to infer causal relationships between modifiable exposures and outcome variables. Conventional observational epidemiological studies are subject to bias from a range of sources; MR analyses can offer an advantage in that they are less prone to bias as they use genetic variants inherited at conception as "instrumental variables", which are proxies of an exposure. However, as with all research tools, MR studies must be carefully designed to yield valuable insights into causal relationships between exposures and outcomes, and to avoid biased or misleading results that undermine the validity of the causal inferences drawn from the study. In this review, we outline Mendel's laws of inheritance, the assumptions and principles that underlie MR, MR study designs and methods, and how MR analyses can be applied and reported. Using the example of serum phosphate concentrations on liability to kidney stone disease we illustrate how MR estimates may be visualized and, finally, we contextualize MR in bone and mineral research including exemplifying how this technique could be employed to inform clinical studies and future guidelines concerning BMD and fracture risk. This review provides a framework to enhance understanding of how MR may be used to triangulate evidence and progress research in bone and mineral metabolism as we strive to infer causal effects in health and disease.

Pregnancy and lactation associated osteoporosis (PLO) is a rare presentation of early-onset osteoporosis characterized by low trauma, spontaneous fractures during late pregnancy/lactation. Herein, we report areal BMD (aBMD) by DXA and volumetric BMD (vBMD), microarchitecture and strength at the distal radius and tibia by HR-pQCT in 59 women with PLO - in comparison to both healthy premenopausal Controls (n = 28) and premenopausal women with idiopathic osteoporotic fractures not associated with pregnancy/lactation (Non-PLO IOP;n = 50). Women with PLO (aged 34 ± 6 yrs) had a more severe clinical presentation than Non-PLO IOP: 80% had vertebral and 92% had multiple fractures (P<.001). They had lower DXA aBMD at all sites vs Controls (all P<.001) and non-PLO IOP (all P<.05). By HR-pQCT, PLO had deficits in all radial/tibial density and most microarchitecture parameters, and lower bone strength than Controls (all P<.001). Compared to non-PLO IOP, PLO had lower total and trabecular density at radius and tibia (all P≤.01) and significant deficits in trabecular microstructure and cortical thickness at the radius only. We studied PLO subgroups with clinical factors potentially related to bone physiology: Within PLO, women with vertebral fractures had lower spine aBMD and higher tibial cortical porosity but were otherwise structurally similar to the nonvertebral group. Those with prior heparin exposure had larger bone size and trabecular area, and those with renal stones had smaller bone size and lower 1/3radius aBMD. We also compared groups based on postpartum timing: Recent PLO (n = 25) evaluated ≤12 M postpartum, before expected recovery of pregnancy/lactation bone loss, had significantly lower aBMD than Distant PLO (n = 34) evaluated >12 M postpartum. However, radial/tibial HR-pQCT measures did not differ, suggesting pre-existing and/or persistent structural deficits. This structural study increases our mechanistic understanding of the severe bone fragility presentation that characterizes PLO and also highlights areas of potential mechanistic heterogeneity that require additional investigation.

Achondroplasia is the most common form of human dwarfism caused by mutations in the FGFR3 receptor tyrosine kinase. Current therapy begins at two years of age and improves longitudinal growth but does not address the cranial malformations including midface hypoplasia and foramen magnum stenosis, which lead to significant otolaryngeal and neurologic compromise. A recent clinical trial found partial restoration of cranial defects with therapy starting at 3 months of age, but results are still inconclusive. The benefits of achondroplasia therapy are therefore controversial, increasing skepticism among the medical community and patients. We used a mouse model of achondroplasia to test treatment protocols aligned with human studies. Early postnatal treatment (from day 1) was compared to late postnatal treatment (from day 4, equivalent to ~5 months in humans). Animals were treated with the FGFR3 inhibitor infigratinib and the effect on skeleton was thoroughly examined. We show that premature fusion of the skull base synchondroses occurs immediately after birth and leads to defective cranial development and foramen magnum stenosis in the mouse model to achondroplasia. This phenotype appears significantly restored by early infigratinib administration when compared to late treatment, which provides weak to no rescue. In contrast, the long bone growth is similarly improved by both early and late protocols. We provide clear evidence that immediate postnatal therapy is critical for normalization of skeletal growth in both the cranial base and long bones and the prevention of sequelae associated with achondroplasia. We also describe the limitations of early postnatal therapy, providing a paradigm-shifting argument for the development of prenatal therapy for achondroplasia.

Vertebral compression fractures (VFs) and spinal degeneration are both common causes of back pain, particularly in older adults. Previous cross-sectional studies have shown a potential association between these entities, but there is limited evidence on the role of VFs in spinal degeneration. In this longitudinal study, we evaluated the association between prevalent VFs and the subsequent progression of facet joint osteoarthritis (FJOA) and intervertebral disc height narrowing (DHN), using data from the Framingham Heart Study Offspring and Third Generation Multi-Detector Computed Tomography study. Summary indices representing the total burden of each spinal parameter (VFs, DHN and FJOA) were calculated for each individual. We hypothesized that prevalent VFs are associated with worsening spinal degeneration. 370 (31%) of 1197 participants, had a baseline (prevalent) VF. The change in summary index of DHN over the follow-up period was significantly higher in those with versus without prevalent VF (difference in change in DHN 0.38, 95% CI 0.18 to 0.59, P<.001), but the change in summary index of FJOA was similar between those with and without prevalent VF. However, once adjusted for age, sex, cohort, smoking status, body mass index, and baseline DHN, the change in summary index of DHN did not differ by prevalent VF status. There was a modestly higher change in the FJOA summary index in those with prevalent VFs compared to those without in the fully adjusted model (difference in change in FJOA 0.62, 95% CI -0.01 to 1.24, P=.054), driven primarily by those with severe (grade 3) VF (difference in change in FJOA 4.48, 95% CI 1.99 to 6.97). Moreover, there was greater change in the summary index of FJOA with increasing severity of prevalent VF (linear trend P=.005). Beyond the established morbidity and mortality associated with VFs, our study suggests that VFs may also lead to worsening spine osteoarthritis.

Upon denosumab discontinuation, an observed overshoot phenomenon in bone turnover may occur, potentially leading to a reduction in bone mineral density and the occurrence of vertebral fractures. Several theories have been proposed to explain this phenomenon, one of which is that osteoclast precursors might be accumulating during treatment. Our aim was to study the effects of denosumab on osteoclast precursors in postmenopausal women. This cross-sectional observational study included 30 postmenopausal women with osteopenia or osteoporosis, divided into two groups: 15 treated with denosumab (mean duration 4 years, range 6 months-9 years) and 15 treatment-naïve controls. Peripheral blood mononuclear cells (PBMCs) were isolated from whole blood and were stained for CD14, MCSFR, CD11b and TNFRII. Osteoclast precursors (CD14+/MCSFR+, CD14+/CD11b + OR CD14+/TNFRII+) were identified with fluorescent activated cell sorting (FACS). The proportion of osteoclasts was determined by calculating their percentage of the total cell population in each whole blood sample. To confirm the expected suppression of bone turnover in the subjects treated with denosumab, we measured serum PINP, CTX and TRACP5b. Denosumab-treated patients exhibited a significantly higher count of CD14+/CD11b + osteoclast precursors compared to controls (median 4% vs 0.75%, P=.011). There was no correlation with the duration of treatment. Bone turnover markers were significantly lower in the group treated with denosumab than controls. Our findings indicate an increase in osteoclast precursors, which could explain the overshoot phenomenon observed after discontinuing denosumab.

Understanding bone accrual in adolescents may inform approaches to improve skeletal health and reduce adult fracture risk. We investigated the effect of HIV on bone mineral accrual assessed by peripheral Quantitative Computed tomography (pQCT). Children with HIV (CWH) on ART for ≥2 years, and children without HIV (CWOH), aged 8-16 years (n = 609), had tibial pQCT scans at 0 and 12 months. Linear regression estimated sex stratified differences in change (∆) and mean pQCT bone density (trabecular and cortical), size (total cross-sectional area [CSA]) and strength (SSI) between CWH and CWOH, adjusting for socio-economic status (SES) and orphanhood and incorporating an interaction term for baseline pubertal status (Tanner 1-2[pre/early] vs 3-5[mid/late]). Structural equation modelling tested whether baseline height-for-age-Z-scores (HAZ) mediate the effect of HIV on ∆bone outcomes. CWH were more likely than CWOH to be orphans (44% vs 7%), of lower SES (43% vs 27%) and be stunted (30% vs 8%); but similar in age. At baseline and follow up, CWH had lower trabecular density, CSA and SSI than CWOH. After adjustment, bone density and strength increased similarly in CWH and CWOH. CWH in mid/late puberty at baseline had greater 12 months increases in CSA than CWOH, particularly males (mean difference [31.3(95%CI:-3.1, 65.6) mm2 in mid/late puberty vs. -2.04(-23.8, 19.7) mm2 in pre/early puberty; interaction P-value = 0.013]. HAZ mediated the effect of HIV on ∆bone outcomes only in females as follows: indirect pathways from HIV to ∆trabecular density [-1.85(-3.5, -0.2) mg/cm3], ∆cortical density [-2.01(-3.9, -0.01) mg/cm3], ∆CSA [-2.59(-4.7, -0.5) mm] and ∆SSI [-18.36(-29.6, -7.2) mm3]. In conclusion, CWH show bone deficits at follow up. Investigations of bone mineral accrual earlier in life and post-puberty to peak bone mass are needed.

Osteoclast (OC) formation and bone resorption are regulated by several factors, including V-ATPase, Wnt/β-Catenin, and RANKL/RANK signaling. ATP6AP2, also known as the prorenin receptor (PRR), is an accessory subunit of V-ATPase and a regulator of Wnt/β-Catenin signaling. While the V-ATPase subunit ATP6AP1 is essential for osteoclast formation and function, the role of ATP6AP2 in OC-lineage cells is less clear. Here, we provide evidence that ATP6AP2 plays a negative role in osteoclastogenesis and function, contrasting with the positive role of ATP6AP1. Mice with conditional knockout (cKO) of ATP6AP2 in OCs (Atp6ap2LysM) exhibit trabecular bone loss, likely due to the increased osteoclastogenesis and activity, since bone formation rates are comparable to control mice. In vitro assays using bone marrow macrophages (BMMs) show that Atp6ap2LysM cultures have more RANKL-induced TRAP+ OC-like cells and increased bone resorptive activity. Further studies reveal that while RANKL signaling and V-ATPase activity are normal, in ATP6AP2 KO OCs, but not BMMs, have reduced basal levels of Wnt/β-Catenin pathway proteins, such as LRP5/6 and β-Catenin, compared to controls. Wnt3A treatment induces β-Catenin and suppresses osteoclast formation in both control and ATP6AP2 KO OC-lineage cells, indicating that Wnt/β-Catenin signaling negatively regulates OC-formation and operates independently of ATP6AP2. Overall, these results suggest that ATP6AP2 is critical for maintaining basal levels of LRP5/6 receptors and β-Catenin in osteoclasts, thus acting as a negative regulator of osteoclastogenesis and activation.

Patella fractures are not typically considered osteoporotic fractures. We compared bone mineral density (BMD) and microstructure in elderly women from a multiethnic population-based study in New York City with any history of a patella fracture (n = 27) to those without historical fracture (n = 384) and those with an adult fragility forearm fracture (n = 28) using dual energy x-ray absorptiometry (DXA) and high resolution peripheral quantitative computed tomography (HR-pQCT). Compared to those without fracture, women with patella fracture had 6.5% lower areal BMD (aBMD) by DXA only at the total hip (P=.007), while women with forearm fracture had lower aBMD at multiple sites and lower trabecular bone score (TBS), adjusted for age, body mass index (BMI), race and ethnicity (all P<.05). By HR-pQCT, adjusted radial total and trabecular (Tb) volumetric BMD (vBMD) and Tb number were 10-24% lower while Tb spacing was 12-23% higher (all P<.05) in the fracture groups versus women without fracture. Women with a forearm, but not a patella, fracture also had lower adjusted radial cortical (Ct) area and vBMD and 21.8% (P<.0001) lower stiffness vs. women without fracture. At the tibia, the fracture groups had 9.3-15.7% lower total and Tb vBMD (all P<.05) compared to the non-fracture group. Women with a forearm fracture also had 10.9, and 14.7% lower tibial Ct area and thickness versus those without fracture. Compared to women without fracture, tibial stiffness was 9.9 and 12% lower in the patella and forearm fracture groups, respectively (all P<.05). By HR-pQCT, the patella vs. forearm fracture group had 36% higher radial Tb heterogeneity (P<.05). In summary, women with patella fracture had Tb deterioration by HR-pQCT associated with lower tibial mechanical competence that was similar to those with fragility forearm fracture, a more universally accepted "osteoporotic" fracture. These data suggest patella fractures are associated with skeletal fragility and warrant skeletal evaluation.

Inactivation of 24-hydroxylase (CYP24A1) causes mild hypercalcemia in humans that becomes severe and life-threatening during pregnancy through unclear mechanisms. We studied Cyp24a1 null mice during pregnancy, lactation, and post-weaning. We hypothesized that Cyp24a1 nulls have a much greater increase in calcitriol during pregnancy and lactation, leading to markedly increased intestinal calcium absorption and reduced lactational bone loss. WT and Cyp24a1 null sisters were mated to Cyp24a1+/- males. Timepoints included baseline (BL), late pregnancy (LP), mid-lactation (ML), late lactation (LL), and weekly x4 weeks of post-weaning recovery (R1-4). Assessments included intestinal calcium absorption (IntCaAbs) by gavage of 45Ca, bone mineral content (BMC) by DXA, microCT of femurs, 3-point bending tests of tibias, serum hormones, serum and urine minerals, milk analysis, and intestinal gene expression. At LP, whole body BMC increased equally by ~12% in null and WT. Calcitriol was 2.5-fold higher in nulls vs WT, accompanied by 3-fold increased IntCaAbs, hypercalcemia, hypercalciuria, and 6.5-fold higher FGF23. PTH was suppressed in both. Twenty percent of null dams died during delivery but their serum calcium at LP did not differ from Cyp24a1 nulls that survived. At ML, calcitriol, IntCaAbs, and FGF23 declined in both genotypes but remained higher than BL values in Cyp24a1 nulls. By LL, nulls were still hypercalcemic vs WT, and had lost less mean whole body BMC (11% vs. 21%, P<.02), but by micro-CT there were no differences from WT in cortical or trabecular bone mass. Lactational losses in BMC, cortical thickness, and trabecular number were restored by R4 in both genotypes. In summary, ablation of Cyp24a1 increased IntCaAbs and caused hypercalcemia during pregnancy and lactation, late gestational mortality in some nulls, and reduced lactational BMC loss. Treating women with gestational hypercalcemia from CYP24A1 mutations should focus on reducing calcitriol or IntCaAbs, since increased bone resorption is not the cause.