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

Wei Zhou; Denise M van de Laarschot; Jeroen G J van Rooij; Marijke Koedam; Hanh H Nguyen; André G Uitterlinden; Peter R Ebeling; Rajesh V Thakker; Piet Geusens; Bram C J van der Eerden; Annemieke J M H Verkerk; M Carola Zillikens

doi:10.1093/jbmr/zjae175

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

J Bone Miner Res. 2024 Nov 1:zjae175. doi: 10.1093/jbmr/zjae175. Online ahead of print.

Affiliations

¹ Department of Internal Medicine, Erasmus MC, Erasmus University Medical Center, Rotterdam, the Netherlands.
² Department of Medicine, School of Clinical Sciences, Monash University, Clayton, Australia.
³ Academic Endocrine Unit, Radcliffe Department of Medicine, University of Oxford, Oxford, United Kingdom.
⁴ National Institute for Health Research Oxford Biomedical Research Centre, Oxford, United Kingdom.
⁵ Biomedical Research Institute, University Hasselt, Diepenbeek, Belgium.
⁶ Department of Internal Medicine, Maastricht University, Maastricht, Netherlands.

PMID: 39485938
DOI: 10.1093/jbmr/zjae175

Abstract

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 seven members affected by osteoporosis, including three 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 three 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 one 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 two 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.

Keywords: Atypical femur fractures; Bisphosphonates; Collagen; Family study; Loxl4; Osteoporosis; Whole exome sequencing.

Plain language summary

Atypical femur fractures (AFFs) are rare fractures of the upper leg that can occur in people using bisphosphonates for osteoporosis. Genetic factors may play a role. We studied a family with seven members affected by osteoporosis, including three siblings who developed AFFs after using bisphosphonates. In this family, a rare variant in the LOXL4 gene was identified as a potential cause for AFFs. This variant was also present in another sibling who had a different type of upper leg fracture, and in one of 73 unrelated European patients with AFFs. LOXL4 is involved in collagen cross-linking, a crucial bone formation process. The variant may impair collagen metabolism, leading to increased microdamage or compromised bone healing, which could increase the risk of AFFs.