Identifying genetic factors underlying osteoporosis and fragility fractures in Malta

Abstract

Osteoporosis is a complex metabolic bone disease with a strong genetic background. Family-based studies have identified significant candidate genes implicated in bone pathophysiology. A two-generation Maltese family with primary osteoporosis at the spine or hip was recruited, with the aim of identifying known or novel genes and gene variants contributing to osteoporosis pathogenesis. A total of fifteen relatives were recruited with ages ranging from 28 to 74 years. Whole genome sequencing was performed on twelve relatives and a number of filtering schemes together with in silico modelling were applied to narrow down the list of potentially causal variants. Five missense variants segregating in a dominant inheritance pattern were shortlisted, all of which had an alternative allele frequency of ≤1% in the 1000Genome project. The variants identified were ADAMTS20 Tyr1364Asn (rs138035327), BMP1 Arg727Gln (rs368615556), SELP Tyr726Cys (rs754086574), TGF-β2 Pro379Leu (rs773943154) and TRIM45 Ser339Pro (rs146244405). Replication of ADAMTS20 Tyr1364Asn and TGF-β2 Pro379Leu variants in a case-control collection of 1045 Maltese postmenopausal women was performed to determine association with bone mineral density (BMD), fracture risk and other bone-related phenotypes. Heterozygosity for the ADAMTS20 Tyr1364Asn variant was associated with an increased risk of low total serum calcium levels compared to the homozygosity for the reference allele. Although heterozygosity for the TGF-β2 Pro379Leu variant was higher in research subjects with a low BMD and fracture history, numbers were too small to infer any statistical association. In conclusion, results suggest that identified variants, alone or in combination, could be disease-causing in the family, and possibly at the population level, requiring replication across larger collections. ADAMTS20 encodes a protease enzyme that cleaves aggrecan, required for cartilage and bone formation. TGF-β2 plays a role in bone remodelling by stimulating matrix protein synthesis. The two genes indirectly interact via the TGF-β pathway. Functional follow-up of the variants is warranted to determine their effect on bone physiology, particularly BMD and calcium homeostasis.