Publications

We have proposed 24-mo between-treatment difference (active-placebo) in mean percent change in total hip bone mineral density (%THBMD) be used to evaluate whether a drug will likely reduce fracture risk, such that a mean %THBMD change greater than the surrogate threshold effect (STE) would indicate fracture benefit. However, this approach does not consider trial size. Here, we investigate using the lower limit of the 95% confidence interval (LCL) of %THBMD as an alternative to the mean to account for the impact that trial size has on estimator uncertainty. We compared the performance of these two measures (mean and LCL of %THBMD) in indicating fracture risk reduction relative to the STE by simulating trials of various sizes (100, 250, 500, 750, and 1000) based on data re-sampling from existing large trials with THBMD at 24 mo; this included 11 studies with radiographic vertebral fracture, three studies with hip fracture, and five studies with all clinical fractures. We re-sampled the THBMD data from each study 1000 times with equal numbers in treatment groups to estimate the reliability of these measures in being consistent with the observed fracture risk reduction due to treatment. Concordance between the %THBMD-STE comparisons and observed fracture risk reduction generally converged at a sample size of 500 (250 per treatment group). For vertebral fracture using mean %THBMD, 9 of the 11 studies had ≥90% of trials consistent with the observed fracture risk reduction if the sample size exceeded 500, which decreased to 7 of the 11 studies using the LCL. For both hip and all clinical fracture, all included studies had ≥90% of trials consistent with the observed fracture risk reduction if the sample size exceeded 500, regardless of using mean or LCL. Overall, the %THBMD-STE comparisons were generally consistent with the original studies' fracture risk reduction observed.

At present, there are no FDA-approved orally-available bone anabolic agents to treat osteoporosis. PTH stimulates bone formation through an intracellular signaling cascade that involves the inhibition of salt-inducible kinase (SIK) isoforms 2 and 3. Therefore, direct small molecule SIK2/SIK3 inhibitors may represent a strategy to mimic PTH actions to treat different forms of osteoporosis. We previously described the synthesis and characterization of SK-124, a pharmacologic SIK2/SIK3 inhibitor that increases trabecular bone formation in eugonadal mice. However, the efficacy of this agent in osteoporosis mouse models remains unknown. Hypogonadism is an important cause of age-related bone loss. In this study, we investigated the therapeutic potential of SK-124 in a male hypogonadal bone loss model (orchiectomy, ORX) in BALB/c mice. Radiographic and histological analyses revealed that SK-124-treated ORX mice showed reduced bone loss compared to the vehicle-treated ORX mice. Serum bone turnover markers demonstrated that SK-124 treatment increased bone turnover, suggesting that SK-124 acts in a PTH-like manner in ORX mice. Bone RNA-sequencing analysis demonstrated novel pathways associated with increased bone formation in response to SK-124 treatment. These findings indicate that SK-124 prevents bone loss in a hypogonadal bone loss model and holds potential as an orally available therapeutic for treating osteoporosis due to testosterone deficiency.

Type 1 diabetes mellitus (T1D) is associated with a marked increase in fracture risk, a phenomenon not entirely explained by lower DXA-BMD. Emerging evidence suggests T1D may adversely affect bone microarchitecture, though findings are inconsistent. We aimed to characterize bone microarchitecture and estimated bone strength in adults with longstanding T1D. We enrolled 96 individuals with T1D (median HbA1c 7.0% [IQR 6.3,7.7], mean diabetes duration 46±10 years) and 57 individuals without diabetes, all aged >50 years. Assessments included areal BMD (aBMD) at the lumbar spine, femoral neck, and total hip via DXA, trabecular bone score (TBS), and high-resolution peripheral quantitative computed tomography (HR-pQCT) to evaluate volumetric BMD (vBMD), bone microarchitecture, and estimated failure load at the distal radius and tibia. Individuals with T1D were more likely to report prior history of fracture compared to controls (26% vs 4%, p<0.001). After adjusting for age, sex, height, and weight, aBMD and TBS did not differ between groups. HR-pQCT revealed modest cortical deficits in the T1D group, with largely preserved trabecular microarchitecture and no significant difference in estimated failure load compared to control participants. Within the T1D group, those who reported a prior fracture had lower spine aBMD and lower estimated strength at the tibia. Notably, individuals diagnosed with T1D at or before age 12 years had worse trabecular parameters at the radius than those diagnosed later, with no corresponding differences at the tibia and no differences seen on DXA. Retinopathy was associated with lower aBMD at the hip and femoral neck and with reductions in trabecular thickness, area, failure load, and stiffness at the tibia. The minor differences in bone microarchitecture observed in this study may partly contribute to the increased fracture risk among patients with T1D, though more research examining mediating factors both intrinsic and extrinsic to bone is needed.

As human space exploration advances, understanding how different gravity levels affect skeletal muscle is critical for long-term health. Among the major organ systems, skeletal muscle is particularly sensitive to gravitational unloading, yet the gravity threshold required to maintain homeostasis remains unclear. Using the Multiple Artificial-gravity Research System aboard the International Space Station, mice were exposed to graded gravity levels, microgravity, 0.33g, 0.67g, and 1g, and their muscles were analyzed postflight. In the gravity-sensitive soleus, the cross-sectional area was preserved at 0.33g, while the slow-to-fast myofiber transition was partially suppressed at 0.33g and fully prevented at 0.67g. Functional measures, including forelimb grip strength and electrical impedance myography, indicated that 0.67g was sufficient to maintain muscle performance. Plasma metabolomics identified 11 metabolites with gravity-dependent changes, suggesting potential biomarkers for monitoring physiological adaptation. Collectively, these results identify 0.67g as a critical threshold for mitigating spaceflight-induced muscle atrophy and myofiber type transitions.

The increased hip fracture risk in individuals with type 1 (T1D) and type 2 (T2D) diabetes is not explained by areal BMD (aBMD), indicating that diabetes increases fracture risk through mechanisms independent of aBMD. To investigate, we used QCT to compare femoral strength, volumetric BMD (vBMD), and geometry in cadaveric femora from older adults with T1D (n = 23; 13 female) and T2D (n = 21; 11 female) to controls of similar age, sex, and race (n = 19; 11 female). While aBMD and vBMD measures were similar across groups, femoral strength was lower in the diabetic groups compared to controls. Geometric strength, based on external bone shape, was lower in T1D (-15%, p = .001) and T2D (-12%, p = .014) compared to controls. When combining geometry and density, femoral strength was significantly lower in T1D (-19%, p = .044). The strength-to-density ratio was also lower in T1D and T2D (p ≤ .013), indicating greater skeletal fragility in the diabetic groups beyond what is predicted by BMD. Diabetic groups had smaller bone size, including lower femoral neck volume (-8%, p ≤ .030), neck cross-sectional area (CSA) (-8%, p ≤ .030), and trochanter CSA (-7%, p ≤ .010). These findings suggest that lower femoral strength and smaller geometry contribute to elevated fracture risk in diabetes, warranting further study in larger populations.

Bone density and microarchitecture measurements from high-resolution peripheral computed tomography (HR-pQCT) are increasingly being used to predict fracture risk and to gain insight into the pathophysiology of skeletal fragility. Using the first-generation HR-pQCT scanner, we previously showed that extraosseous soft tissue can impact HR-pQCT measurements. Yet similar data is not available for the second-generation scanner. Thus, we aimed to determine the impact of increased soft tissue on bone density, microarchitecture and strength measurements acquired using the second-generation HR-pQCT scanner. We performed HR-pQCT scans on a hydroxyapatite phantom and in human volunteers (n = 12) with no soft tissue covering, and with a thin (0.5 cm) and thick (1 cm) layer of soft tissue surrounding the phantom or limb. We found that density values of the phantom were minimally affected by the thin layer of soft tissue. In contrast, with the thick (1 cm) layer of soft-tissue, bone density was significantly lower than baseline (no bolus), with greater deficits as the density of the rod increased (-1% to -3.6%, p < 0.01 for all). In human volunteers, soft tissue layering influenced measures of both cortical and trabecular microarchitecture at the distal tibia and radius, with larger differences observed in trabecular versus cortical measures and at the tibia compared to the radius. For example, at the tibia, Tt.BMD was lower than the baseline scan under both soft tissue layering conditions (thin: -1.3%, p < 0.001; thick: -2.4%, p = 0.003), while at the radius Tt.BMD was only significantly lower for the thick layer (-1.2%, p = 0.004). Ct.BMD followed a similar pattern, with slightly greater magnitude of BMD decline with increased soft tissue (thin: -1.5%, p < 0.001, thick -2.8%, p < 0.001 at the tibia) compared to Tt.BMD. Altogether our results indicate that HR-pQCT measurements at both the metaphyseal and diaphyseal sites must be interpreted carefully when comparing subjects with varying body composition, or when assessing longitudinal changes in individuals who experience marked changes in weight and/or body composition as true differences to these measures may be more or less extreme than they appear.

Dedicator of Cytokinesis 7 (DOCK7) has recently emerged as a regulator of skeletal homeostasis, but existing Dock7 mutant models harbor only global mutations and are incompatible with tissue-specific deletion studies. We previously generated a Dock7-floxed allele in which exons 3-4 are flanked by LoxP sites. To validate the utility of this allele for future conditional strategies, we globally deleted exons 3-4 to generate Dock7 em2/em2 mice and characterized their skeletal phenotype. Dock7 em2/em2 mice exhibited a diluted coat color and white belly spot, consistent with spontaneous Dock7 mutations. Bone microarchitecture was assessed in 21-week-old males and females. Global deletion of Dock7 exons 3-4 resulted in a 30-37% reduction in trabecular bone volume in the distal femur and L5 vertebrae. Cortical bone thickness was unchanged in both sexes; however, male Dock7 em2/em2 mice displayed reduced total femoral area, whereas females showed increased medullary area. These data suggest altered appositional bone growth with mutation of Dock7. To assess osteoblast function, bone marrow stromal cells (BMSCs) were differentiated in vitro. Dock7 em2/em2 BMSCs exhibited reduced mineralization and decreased Bglap expression, indicating attenuated osteoblast differentiation. These findings demonstrate that Dock7 exons 3-4 are required for normal trabecular bone acquisition and osteoblast function. Loss of these exons disrupts DOCK7 activity, supporting the Dock7 em2/em2 line as a valid loss-of-function model. The Dock7 em2/em2 mouse provides a foundation for future tissue-specific deletion studies to define the cellular roles of DOCK7 in regulating bone formation and trabecular architecture.

PURPOSE: This study investigated the effect of bone metastasis on the biomechanical environment of human vertebrae in patients with metastatic spine disease through the metric of load-to-strength ratio (LSR). Specifically, we compared the patients' LSRs to age and sex-similar noncancer controls from the Framingham Heart Study.

METHODS: Derived from clinical CT data of 135 metastatic spine disease patients planned for radiotherapy and 246 normative controls from the Framingham Heart Study, individualized spinal musculoskeletal models and vertebral strength estimates were used to compute level-specific LSR under natural standing and three weight-holding conditions (standing + weight, flexion + weight, and lateral bending + weight).

RESULTS: Adjusted for age, BMI, and spinal region, osteosclerotic and mixed lesion vertebrae had higher strength than osteolytic and control vertebrae. The musculoskeletal models suggested breast, prostate, and male lung cancer patients had higher compressive vertebral loading, and female lung cancer patients had lower compressive vertebral loading than controls. Male patients had higher standardized LSRs in natural standing, while female patients had lower LSRs for all activities than controls. Independent of sex, vertebrae with osteosclerotic and mixed bone metastasis had lower LSRs than controls, while, for osteolytic bone lesions, males had higher and females lower LSRs than controls. Vertebrae with no observed lesion on CT had higher LSRs than controls in males and lower LSRs in females.

DISCUSSION: Our findings highlighted that primary cancer and lesion type differentially affected task-specific vertebral loading and strength, thus modifying the vertebral LSRs. Sex-mediated differences in LSRs between FHS controls and vertebrae with no observed metastatic lesions suggest that considering the latter as "normal" should be taken with care. Our initial assessment supports further examination of whether vertebral LSR measurements are associated with vertebral risk and, if so, what threshold values indicate risk.

LEVEL OF EVIDENCE: 3.