Optimal bone fracture repair requires 24R,25-dihydroxyvitamin D3 and its effector molecule FAM57B2.

The biological activity of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] remains controversial, but it has been suggested that it contributes to fracture healing. Cyp24a1-/- mice, synthesizing no 24R,25(OH)2D3, show suboptimal endochondral ossification during fracture repair, with smaller callus and reduced stiffness. These defects were corrected by 24R,25(OH)2D3 treatment, but not by 1,25-dihydroxyvitamin D3. Microarrays with Cyp24a1-/- callus mRNA identified FAM57B2 as a mediator of the 24R,25(OH)2D3 effect. FAM57B2 produced lactosylceramide (LacCer) upon specific binding of 24R,25(OH)2D3. Fam57b inactivation in chondrocytes (Col2-Cre Fam57bfl/fl) phenocopied the callus formation defect of Cyp24a1-/- mice. LacCer or 24R,25(OH)2D3 injections restored callus volume, stiffness, and mineralized cartilage area in Cyp24a1-null mice, but only LacCer rescued Col2-Cre Fam57bfl/fl mice. Gene expression in callus tissue suggested that the 24R,25(OH)2D3/FAM57B2 cascade affects cartilage maturation. We describe a previously unrecognized pathway influencing endochondral ossification during bone repair through LacCer production upon binding of 24R,25(OH)2D3 to FAM57B2. Our results identify potential new approaches to ameliorate fracture healing.

Washing the Femoral Canal Results in More Predictable Seating of a Short, Tapered Femoral Stem.

BACKGROUND: It is critical that a femoral rasp be effective in preparing the proximal femur to accept the size and the geometry of the femoral implant at the time of total hip arthroplasty. Short, tapered femoral stems may be at greater risk because they require the preparation of a short femoral region without any reaming. We undertook a study to determine the effect on implant seating in femora that were prepared by rasping alone with those that were rasped and the canal was washed with saline at the time of cementless THA with a short, tapered femoral implant. METHODS: We retrospectively analyzed the preoperative, intraoperative, and radiographic data on 170 consecutive patients undergoing a primary THA using a short, taper, uncemented metaphyseal-filling stem. The femur was prepared using a rasp-only technique. In the initial 99 patients, the canal was rasped, but not washed (group 1). In the subsequent 71 patients, the canal was rasped and before implant insertion the canal was washed with 100 cc of normal saline to remove all loose cancellous bone (group 2). Intraoperatively, the distance between the calcar cut and the rasp and subsequently, the calcar cut and the implant was measured. We defined a difference of more than 2 mm between the seating of the rasp and the final implant as a clinically significant mismatch. RESULTS: Overall, a clinically significant mismatch occurred in 50% (49/99) of cases in group 1 and 15% (11/71) in group 2. Multivariate logistic regression analysis corrected for preoperative, intraoperative, and radiographic measurements showed that washing significantly decreased the mismatch between the rasp and the implant (odds ratio, 5.32; confidence interval, 2.10-13.73; P < .001). CONCLUSION: Although the present rasp design is sufficient to create the geometric space for this short, metaphyseal stem, it does not adequately remove the bone debris to ensure reproducible seating of the implant. Washing the femoral metaphysis with saline to remove bone debris, after rasping and before inserting the final implant, significantly decreased the mismatch between seating of the final rasp and the implant in this cementless short, metaphyseal-filling, taper design stem. Level of Evidence III.

The osteogenic cell surface marker BRIL/IFITM5 is dispensable for bone development and homeostasis in mice.

BRIL (bone-restricted IFITM-like), is a short transmembrane protein expressed almost exclusively in osteoblasts. Although much is known about its bone-restricted gene expression pattern and protein biochemical and topological features, little information is available for BRIL physiological function. Two autosomal dominant forms of osteogenesis imperfecta (OI) are caused by distinct, but recurrent mutations in the BRIL gene. Yet, the underlying mechanisms by which those mutations lead to OI are still poorly understood. A previous report indicated that BRIL knockout (KO) mice had bone deformities, shortened long bones, and reproductive problems. Here we generated and systematically analyzed the skeletal phenotype of a new global Bril KO/LacZ knockin mouse model. KO mice reproduced and thrived normally up to 12 month of age. The skeletal phenotype of KO and WT littermates was assessed at embryonic (E13.5 to E18.5) and postnatal (2 days, 3 weeks, 3 months and 8 months) time-points. Embryos from E13.5 through to E18.5 showed significant X-Gal staining in all skeletal elements without any apparent patterning anomalies. Although bone deformities were never observed at any postnatal ages, minor and transient differences were noted in terms of bone length and static uCT parameters, but not systematically across all ages and genders. These changes, however, were not accompanied by significant alteration in bone material properties as assessed by a 3-point bending test. In addition, no changes were detected in circulating serum markers of bone turnover (P1NP, CTX-I, and osteocalcin). Gene expression monitoring also revealed no major impact of the loss of BRIL. Further, when mice were challenged with a surgically-induced fracture in tibia, bones repaired equally well in the KO mice as compared to WT. Finally, we showed that BRIL C-terminus is not a bona fide binding site for calcium. In conclusion, our in depth analysis suggest that skeletal patterning, bone mass accrual and remodeling in mice proceeded independent of BRIL.

CYP24A1-deficiency does not affect bone regeneration in distraction osteogenesis.

The putative biological activity of 24,25-(OH)2D remains unclear. Previous studies showed an increase in the circulating levels of this metabolite following fracture in chicks. Our laboratory has generated a mouse model deficient for the Cyp24a1 gene for studying the role of 24,25-(OH)2D. We set out to study the role of CYP24A1 and 24,25-(OH)2D in intramembranous bone formation during distraction osteogenesis in wild-type and Cyp24a1-deficient mice. Distraction osteogenesis was applied to mouse tibiae using a miniature external fixator apparatus. Histomorphometric parameters and gene expression differences between the mutant mice and the wild-type controls were measured using micro computed tomography and reverse-transcription quantitative PCR.There were no statistically significant differences between genotypes when bone volume/tissue volume ratios were calculated at mid distraction, end of distraction, mid consolidation, or end of consolidation. We measured reduced expression of the Col10a1 gene in the mutant vs. wild-type mice at mid distraction (0.4±0.1 vs. 1.0±0.2 respectively, p=0.01). Similarly, we measured a significantly lower expression of the osteogenic marker Atf4 in mutant vs. wild-type mice at end of distraction (0.7±0.1 vs. 1.0±0.1 respectively, p=0.01) and at mid consolidation (0.6±0.1 vs. 1.0±0.1 respectively, p=0.0003). These results suggest moderate and restricted differences in chondrogenesis and osteogenesis that did not affect the volume of bone produced following distraction. We conclude that CYP24A1 activity is not essential for intramembranous bone formation.

Lack of circulating pigment epithelium-derived factor is a marker of osteogenesis imperfecta type VI.

BACKGROUND: Osteogenesis imperfecta (OI) type VI is a rare autosomal recessive bone fragility disorder that is caused by inactivating mutations in SERPINF1, the gene that encodes pigment-epithelium derived factor (PEDF). Determining PEDF serum levels might facilitate the diagnosis of OI type VI. OBJECTIVE: The objective of the study was to assess whether lack of circulating PEDF is a specific marker of OI type VI and to evaluate whether PEDF serum levels are influenced by other metabolic bone diseases. MATERIALS AND METHODS: Serum PEDF concentrations were measured in 12 patients with OI type VI (aged 2.7-31 yr) as well as in 96 children and adolescents with OI types I, III, and IV; in 26 young patients with hypophosphatemic rickets; and in 19 healthy controls. RESULTS: Circulating PEDF was undetectable in all 12 patients with OI type VI but was measurable for the other 141 study participants. No significant differences in serum PEDF concentrations were found between the diagnostic groups other than OI type VI. Treatment with bisphosphonates (in OI types I, III, and IV) and with phosphate and calcitriol (in hypophosphatemic rickets) did not have a detectable influence on serum PEDF. In patients with OI types I, III, and IV, serum creatinine, body mass index z-score, and OI severity were significant predictors of PEDF serum levels. CONCLUSION: Determining PEDF serum concentration helps to diagnose OI type VI but does not seem to provide information on the activity of bone turnover or mineralization.

Serum 24,25-dihydroxyvitamin D concentrations in osteogenesis imperfecta: relationship to bone parameters.

BACKGROUND: Several studies suggest that 24,25-dihydroxyvitamin D [24,25(OH)2D] may have an effect on bone mass and metabolism. OBJECTIVE: We evaluated the relationship between serum 24,25(OH)2D levels and bone density and bone metabolism in children with a primary bone disorder-osteogenesis imperfecta (OI). MATERIALS AND METHODS: The study included 132 patients (age, 1.1 to 17.9 yr; 67 girls) with OI types I, III, or IV who had not received bisphosphonate treatment at the time of analysis. RESULTS: Serum 24,25(OH)2D levels were significantly higher in OI type III than in OI type I or IV. Serum 24,25(OH)2D concentrations were positively correlated with serum 25-hydroxyvitamin D (25OHD) levels and negatively correlated with serum PTH levels, and were not correlated with serum 1α,25-dihydroxyvitamin D [1,25(OH)2D]. The ratio between serum 24,25(OH)2D and 25OHD was negatively correlated with age and was independent of serum 25OHD concentrations. Regression analysis revealed that OI severity (P = 0.04), serum 25OHD levels (P < 0.001), and serum PTH concentrations (P = 0.045), but not age, gender, or serum 1,25(OH)2D, were independent predictors of serum 24,25(OH)2D levels. No correlation was found between serum 24,25(OH)2D levels or the ratio between serum 24,25(OH)2D and 25OHD and lumbar spine bone mineral density z-scores or bone marker levels (serum osteocalcin and urinary collagen type I N-telopeptide) after adjusting for OI type, age, and gender. CONCLUSION: Patients with more severe OI type had higher 24,25(OH)2D serum levels and higher serum 24,25(OH)2D to 25OHD ratios, suggesting an increased 25OHD-24-hydroxylase activity.