Family-based whole-exome sequencing implicates a variant in lysyl oxidase like 4 in atypical femur fractures

Atypical femur fractures (AFFs) are rare adverse events associated with bisphosphonate use, having unclear pathophysiology. AFFs also cluster in families and have occurred in patients with monogenetic bone diseases sometimes without bisphosphonate use, suggesting an underlying genetic susceptibility. Our aim was to identify a genetic cause for AFF in a Caucasian family with 7 members affected by osteoporosis, including 3 siblings with bisphosphonate-associated AFFs. Using whole-exome sequencing, we identified a rare pathogenic variant c.G1063A (p.Gly355Ser) in lysyl oxidase like 4 (LOXL4) among 64 heterozygous rare, protein-altering variants shared by the 3 siblings with AFFs. The same variant was also found in a fourth sibling with a low-trauma femur fracture above the knee, not fulfilling all the ASBMR criteria of AFF and in 1 of 73 unrelated European AFF patients. LOXL4 is involved in collagen cross-linking and may be relevant for microcrack formation and bone repair mechanisms. Preliminary functional analysis showed that skin fibroblast-derived osteoblasts from the unrelated patient with the LOXL4 variant expressed less collagen type I and elastin, while osteogenic differentiation and mineralization were enhanced compared with 2 controls. In conclusion, this LOXL4 variant may underlie AFF susceptibility possibly due to abnormal collagen metabolism, leading to increased formation of microdamage or compromised healing of microcracks in the femur.