The Contribution of Deleterious Rare Alleles in ENPP1 and Osteomalacia Causative Genes to Atypical Femoral Fracture

Gene-based association analysis of a large patient cohort provides insights into genetics of atypical femur fractures

Several small genetic association studies have been conducted for atypical femur fracture (AFF) without replication of results. We assessed previously implicated and novel genes associated with AFFs in a larger set of unrelated AFF cases using whole exome sequencing (WES). We performed gene-based association analysis on 139 European AFF cases and 196 controls matched for bisphosphonate use. We tested all rare, protein-altering variants using both candidate gene and hypothesis-free approaches. In the latter, genes suggestively associated with AFFs (uncorrected p-values <.01) were investigated in a Swedish whole-genome sequencing replication study and assessed in 46 non-European cases. In the candidate gene analysis, PLOD2 showed a suggestive signal. The hypothesis-free approach revealed 10 tentative associations, with XRN2, SORD, and PLOD2 being the most likely candidates for AFF. XRN2 and PLOD2 showed consistent direction of effect estimates in the replication analysis, albeit not statistically significant. Three SNPs associated with SORD expression according to the GTEx portal were in linkage disequilibrium (R2 ≥ 0.2) with an SNP previously reported in a genome-wide association study of AFF. The prevalence of carriers of variants for both PLOD2 and SORD was higher in Asian versus European cases. While we did not identify genes enriched for damaging variants, we found suggestive evidence of a role for XRN2, PLOD2, and SORD, which requires further investigation. Our findings indicate that genetic factors responsible for AFFs are not widely shared among AFF cases. The study provides a stepping-stone for future larger genetic studies of AFF.

Towards understanding how bisphosphonate-dependent alterations to nutrient canal integrity can contribute to risk for atypical femoral fractures: Biomechanical considerations and potential relationship to a real-world analogy

Bisphosphonates are a class of drugs which have shown good efficacy in the treatment of post-menopausal osteoporosis, as well as a good safety profile. However, side-effects such as risk for atypical femoral fractures (AFF) have appeared, leading to a decline in use of the drugs by many patients who would benefit from the treatment. While patient characteristics have contributed to improved understanding of risk factors, the mechanisms involved that explain AFF risk have not appeared. Recently, the possibility that the mechanism(s) involved drug-induced modification of cells of the nutrient canals of the femur and subsequent compromise in the bone matrix has been published. The present Hypothesis article builds on the concept presented earlier and expands into biomechanical considerations. An analogy of the mechanisms involved to a real-life scenario is also presented. While this analogy has limitations, consideration of the biomechanical implications of progressive alterations to defects presented by compromised nutrient canal-bone matrix also presents potential relationships with AFF risk.

Are secondary effects of bisphosphonates on the vascular system of bone contributing to increased risk for atypical femoral fractures in osteoporosis?

Osteoporosis (OP) is a bone disease which affects a number of post-menopausal females and puts many at risk for fractures. A large number of patients are taking bisphosphonates (BPs) to treat their OP and a rare complication is the development of atypical femoral fractures (AFF). No real explanations for the mechanisms underlying the basis for development of where AFF develop while on BPs has emerged. The present hypothesis will discuss the possibility that part of the risk for an AFF is a secondary effect of BPs on a subset of vascular cells in a genetically at-risk population, leading to localized deregulation of the endothelial cell (EC)-bone cell-matrix units in nutrient channels/canals of the femur and increased risk for AFF. This concept of targeting ECs is consistent with location of AFF in the femur, the bilateral risk for occurrence of AFF, and the requirement for long term exposure to the drugs.

High frequency of heterozygous rare variants of the SLC34A1 and SLC9A3R1 genes in patients with atypical femur fracture

OBJECTIVE

Atypical femur fractures (AFFs) are rare fragility fractures originating at the lateral cortex of the femur, affecting the subtrochanteric or diaphyseal area of thebone with a transverse morphology. Occurrence of AFF is specifically associated with a small number of rare monogenic congenital metabolic bone disorders, such as hypophosphatasia, and with long-term treatment with antiresorptiondrugs. The exact pathogenesis of these fractures remains poorly understood and, except for cases of diagnosed HPP or other AFF-causing bone diseases, it is not possible to assess which patients are at higher riskof developing AFFs as a consequence of anti-resorption therapy.

DESIGN

We genetically screened 25 unrelated patients who had developed at least one AFF.

INTERVENTION

Genetic screening was performed through a nextgeneration sequencing analysis with a customized panel containing 76 human genes involved in the regulation of the mineralization processWe genetically screened 25 unrelated patients who had developed at least one AFF.

RESULTS

We found a relatively high frequency (32.0%) of heterozygous rare variants inthe SLC34A1 and SLC9A3R1 genes, two genes whose heterozygous inactivating mutations have been respectively associated with autosomal dominant hypophosphatemic nephrolithiasis/osteoporosis types 1 and 2 (NPHLOP1and NPHLOP2). Other heterozygous rare variants were found in the BMPR1B, CYP27B1, FBN1, MEPE, PIGO, and PHOSPHO1 genes, each in a single AFF case (4.0%).

CONCLUSIONS AND RELEVANCE

Our findings suggest that rarevariants of SLC34A1 and SLC9A3R1 could represent a possible genetic risk factor for the occurrence of AFFs. On the other hand, AFFs could represent an unsuspected clinical manifestation and/or an anti-resorption therapycorrelatedadverse event in patients with NPHLOP disorders.

Sclerostin is a promising therapeutic target for oral inflammation and regenerative dentistry

Sclerostin is the protein product of the SOST gene and is known for its inhibitory effects on bone formation. The monoclonal antibody against sclerostin has been approved as a novel treatment method for osteoporosis. Oral health is one of the essential aspects of general human health. Hereditary bone dysplasia syndrome caused by sclerostin deficiency is often accompanied by some dental malformations, inspiring the therapeutic exploration of sclerostin in the oral and dental fields. Recent studies have found that sclerostin is expressed in several functional cell types in oral tissues, and the expression level of sclerostin is altered in pathological conditions. Sclerostin not only exerts similar negative outcomes on the formation of alveolar bone and bone-like tissues, including dentin and cementum, but also participates in the development of oral inflammatory diseases such as periodontitis, pulpitis, and peri-implantitis. This review aims to highlight related research progress of sclerostin in oral cavity, propose necessary further research in this field, and discuss its potential as a therapeutic target for dental indications and regenerative dentistry.