Whole Exome Sequencing to Identify Genetic Predisposition to Atypical Femoral Fractures in Women Using Bisphosphonates for Osteoporosis

In 2010, a Task Force convened by the American Society for Bone and Mineral Research (ASBMR) investigated an apparent association of atypical subtrochanteric and diaphyseal femoral fractures (AFFs) with long-term bisphosphonate (BP) treatment given for the important and common disorder osteoporosis (OP).(1,2) Because the investigators had reported(2) in 2009 that the prodromal lesion for AFFs in OP resembles the femoral pseudofractures encountered in the rare adult form of hypophosphatasia (HPP),(3-5) they recommended sequencing the tissue non-specific alkaline phosphatase (TNSALP) gene of OP AFF patients to determine if mutations or polymorphisms in TNSALP genetically predispose to OP AFFs.(2)

In 2012, the investigators reported a 55-year-old woman treated for four years with BPs for presumed OP who then suffered simultaneous atraumatic bilateral AFFs.(4) Upon sequencing her TNSALP, a heterozygous mutation changing an arginine (Arg) to a histidine (His) (c.212 G>A, p.Arg71His) was discovered that the investigators had documented in the investigators' large cohort of HPP patients. She had been undiagnosed with HPP although her pre-BP serum ALP was persistently low (26 U/L, Nl 32 - 116 U/L). The investigators then recruited and sequenced TNSALP for 24 new OP AFF patients given BPs, and identified a second OP AFF patient carrying a TNSALP defect.(3) These two OP AFF patients with TNSALP defects support the investigators' hypothesis (below) that high-impact rare genetic variants in TNSALP, and perhaps other genes, can predispose to OP AFFs.(4) Furthermore, the investigators reported in 2009(2) that the prodromal lesions of OP AFFs can resemble the femoral pseudofractures seen in another, but more prevalent, heritable metabolic bone disease, X-linked hypophosphatemia (XLH). This is the most common genetic cause of osteomalacia,(6) and is inherited as an X-linked dominant trait caused by deactivating mutations in the PHEX gene.

Osteoporosis (OP) is a complex disorder likely involving the effects of multiple low-impact, common changes in the human genome that alter bone remodeling and/or mineralization.(7-9) The investigators propose that high-impact, rare, genetic variants predispose some OP patients to AFFs. Treatment with BPs could engender OP AFFs. The investigators' hypothesis: High-impact rare variants (i.e., mutations) that occur in genes/proteins that regulate pyrophosphate/phosphate homeostasis or BP metabolism predispose to OP AFFs and are unmasked in OP patients given BPs. Identification of these variants will guide OP therapies, perhaps on an individual basis (i.e., "personalized medicine"),(9) and reduce the incidence of OP AFFs.

The investigators will identify high-impact rare genetic variants using whole exome sequencing in two patient groups: 1) women using BPs for OP and have had one or more AFF, 2) women using BPs for OP but have not had an AFF. The investigators will focus on genes/proteins that: i) regulate pyrophosphate/phosphate effects, ii) others that regulate BP metabolism, and iii) have been associated with OP. The investigators will use gene burden analysis to determine whether there is an excess of novel or rare genetic variants for the group with AFFs.