Identification of rare genetic variants in familial forms and unrelated cases of bisphosphonates-associated atypical femur fracture.

We performed next generation sequencing in two affected-sibling pairs of atypical femur fractures (AFF) and in unrelated cases of AFF to identify genetic variants of bisphosphonates (BP)-associated AFF. A whole exome sequencing (WES) was performed in two sisters with BP-associated AFF and their healthy brother naïve to BP treatment (family A). After bioinformatic filtering, the intrafamilial segregation was analysed. Then, we performed targeted sequencing of 62 genes, including 36 genes containing variants predicted to be damaging and segregating with the phenotype in both sisters of the family A, and 26 candidate genes for osteogenesis imperfecta (OI), hypophosphatasia and the mevalonate pathway. The targeted sequencing was performed on the family A, and on 47 unrelated participants and another affected sibling pair (family B) from the Quebec AFF Registry. 100 healthy controls recruited in same geographic area than patients were genotyped for rare variants. Sixty-three rare and deleterious variants were detected by the WES and shared by the two affected sisters of the family A. Among those variants, a rare likely pathogenic variant (p.Leu3fs) of the WNT1 gene, already linked to OI and early onset osteoporosis, was also shared by a third individual, an unrelated case with BP-associated AFF. The pair of siblings of family B carried a novel variant (p.Glu1323fs) in the COL1A2 gene, linked to OI. One unrelated case had a novel variant in the FDFT1 gene, involved in the mevalonate pathway. These rare variants were not found in 100 healthy controls. Some BP-associated AFFs may occur in the setting of clinically undiagnosed underlying genetic disorders predisposing to osteoporosis and fractures, such as OI.