Identification of osteoporosis genes using family studies

Abstract

Osteoporosis is a multifactorial bone disease characterised by reduced bone mass and increased fracture risk. Family studies have made significant contribution in unravelling the genetics of osteoporosis. Yet, most of the underlying molecular and biological mechanisms remain unknown prompting the need for further studies. This review outlines the proper phenotyping and advanced genetic techniques in the form of high-throughput DNA sequencing used to identify genetic factors underlying monogenic osteoporosis in a family-based setting. The steps related to variant filtering prioritisation and curation are also described. From an evolutionary perspective, deleterious risk variants with higher penetrance tend to be rare as a result of negative selection. High-throughput sequencing (HTS) can identify rare variants with large effect sizes which are likely to be missed by candidate gene analysis or genome-wide association studies (GWAS) wherein common variants with small to moderate effect sizes are identified. We also describe the importance of replicating implicated genes, and possibly variants, identified following HTS to confirm their causality. Replication of the gene in other families, singletons or independent cohorts confirms that the shortlisted genes and/or variants are indeed causal. Furthermore, novel genes and/or variants implicated in monogenic osteoporosis require a thorough validation by means of in vitro and in vivo assessment. Therefore, analyses of families can continue to elucidate the genetic architecture of osteoporosis, paving the way for improved diagnostic and therapeutic strategies.

Keywords: family studies; fragility fractures; genetics; high-throughput sequencing; osteoporosis.

Figure 1

A list of benefits and risks that families offer when pursing genetic studies. Figure created using BioRender.

Figure 2

Proposed flowchart for identifying the genetic variants contributing to osteoporosis in a family study. Affected proband and relatives are recruited and subjected to deep phenotyping followed by genetic evaluation using HTS. A stepwise filtering scheme is applied with the aim of filtering out common, benign, and low-penetrance variants that are unlikely to be causal, retaining high-impact variants possibly residing in genes having a role in bone biology. Segregation of the variants in the entire pedigree is confirmed by Sanger sequencing. The shortlisted variants are tested in other independent families and/or population studies, and functionally validated using cells or animal models to confirm their role in bone metabolism and osteoporosis pathogenesis. ACTH, adrenocorticotropic hormone; ALP, alkaline phosphatase; ALT, alanine transaminase; AST, aspartate transaminase; DGP, deamidated gliadin peptide; E2, oestradiol; ESR, erythrocyte sedimentation rate; FSH, follicle-stimulating hormone; FT3, free T3; FT4, free T4; GGT, gamma-glutamyltransferase; HbA1c, glycosylated haemoglobin; HDL, high-density lipoprotein; IGF1, insulin-like growth factor 1; LH, luteinising hormone; LDL, low-density lipoprotein; PTH, parathyroid hormone; SHBG, sex hormone-binding globulin; TSH, thyroid-stimulating hormone; tTG, tissue transglutaminase; 25-OH Vitamin D, 25-hydroxy vitamin D. Figure created using BioRender.