Current definition determines osteoporosis as a systemic skeletal disorder characterized by low bone mass and microarchitectural deterioration of bone tissue that result in reduced bone strength, bone fragility, and increased risk of fracture. The main clinical end points as well as complications of osteoporosis are skeletal fractures. The most common fracture sites are hip, spine and forearm although any bone can be affected (1).

Fracture risk is influenced by a number of factors, including bone mineral density (BMD), bone quality parameters and non-skeletal factors such as muscle strength, balance, cognition and cardiovascular function affecting the risk of falls (2). Each of these factors is itself under at least partial genetic control. BMD can be determined by dual energy X-ray absorptiometry (DXA) usually performed at the lumbar spine and hip. It was estimated that each standard deviation decrease in BMD from the age-adjusted mean was associated with a 2.3-fold increase in incidence of vertebral fractures and a 2.6-fold increase in hip fractures. Bone quality can be assessed by three-dimensional imaging modalities, such as peripheral computed tomography and high resolution magnetic resonance imaging, that allow to determine geometric parameters of the skeleton and to assess its microstructure (1).

Growing evidence indicates that fracture risk is influenced by a combination of genetic and environmental factors. Several twin and family studies suggest that between 50 and 85% of the variance in peak bone mass is genetically determined, depending on skeletal site and the age of the subjects studied (3, 4). It is also presumed that genes contribute significantly to variability in aging-related bone loss and other determinants of fracture risk, including femoral neck geometry, muscle strength, bone turnover, body mass index and age at menopause (1).

It is probably a multi-gene relation and no single gene was found to prevail over others in the determination of BMD and fracture risk. Studies of post-menopausal women and their first-degree relatives as well as twin studies (5) showed that the heritability of forearm fractures was about 25-50%, of hip fractures approximately 50% and of vertebral fractures 24% (6-8). In contrast, heritability study of elderly twins from Finland showed little evidence to suggest that fractures were heritable (9). These divergent results may be explained by the fact that the heritability of fracture decreases with age as environmental factors become more important. It was demonstrated in a large study of Swedish twins that the heritability of hip fractures was as high as 68% among persons under the age of 65 but dropped off rapidly with age to reach a value of almost zero by the eighth decade (6). It has been also shown that genetic component of low muscle mass, increasing the susceptibility to falls, was over 50% (4).

Early efforts to identify specific genes related to variation in BMD and fracture risk focused on identifying biologically motivated candidate genes and testing specific genotyped variants for association with BMD and/or fractures. The vitamin D receptor gene (VDR), the collagen type I alpha 1 gene (COLIA1) and estrogen receptor gene (ER) alpha have been most widely investigated and found to play a role in regulating BMD, but the effects were modest and together probably accounted for less than 5% of the heritable contribution to BMD. Candidate gene association studies are relatively easy to perform and a well validated method for the identification of genes responsible for monogenic diseases. However, they have low statistical power to detect genes having modest effects on BMD and fractures, and hence require family samples of several thousand people.

Demonstration of an association between a candidate gene and BMD or fractures does not necessarily mean that the gene is causally responsible for the effect observed, as there may be linkage disequilibrium with a nearby causal gene. Linkage disequilibrium refers to the phenomenon whereby genes lying close together tend to be inherited together (10).

With advances in genomic technology genome-wide association studies (GWAS) have been published. GWAS is an approach that involves scanning of the entire genome to identify novel genes with modest effects on complex diseases or traits. Array GWAS technologies are capable of analyzing thousands of polymorphisms distributed throughout the genome. Through the use of dense genotyping, large study samples, and replication studies to confirm results, these studies have led to the discovery of many genetic variants that have robust statistical evidence for association with various diseases. It has become possible to perform association studies on a genome-wide basis by analyzing a large number of closely spaced single-nucleotide polymorphisms (SNPs) spread randomly across the genome (11). The GWAS studies published to date disclosed more than 82 loci significantly associated with BMD, of which at least 16 were found to be associated also with fractures. The effect sizes of these loci are small, each accounting for less than 1% of the total variation in BMD (1, 12). Finding genes for fracture risk is likely to be more difficult than for BMD due to the complexity of the fracture phenotype. The vast majority of SNPs that have been associated with fracture have odds ratios for fracture of 1.11 or lower (13).

Candidate genes for osteoporosis were classified according to metabolic or hormonal pathways, which regulate BMD and bone quality, however, to date no gene has been definitively identified as a major gene.

A large collaborative study of more than 19,000 men and women identified 241 SNPs from 9 genes, which were significantly associated with lumbar spine BMD (230 SNPs), femoral neck BMD (100 SNPs), or both (89 SNPs). Among them 60 SNPs from 4 genes were also significantly associated with risk for fracture. The effect of these SNPs on fracture rate ranged between an odds ratio (OR) of 1.13 and 1.43 for the allele that was associated with decreased BMD. These results confirmed the highly polygenic nature underlying BMD variation and the role of several biological pathways influencing osteoporosis and fracture susceptibility (12, 14).

The Wnt/β-catenin signaling pathway, also called the canonical Wnt pathway, is crucially important for a variety of processes, including bone cell differentiation, proliferation, and apoptosis. Interactions of Wnt proteins with their receptors cause an accumulation of β-catenin in the cytoplasm and then in the nucleus where it participates in gene transcription. In the canonical Wnt pathway Wnt proteins bind Frizzled proteins and either lipoprotein receptor-related proteins 5 or 6 (LRP5 or LRP6). This results in the inhibition of glycogen synthase kinase 3-dependent phosphorylation of β-catenin, followed by the stabilization of β-catenin (4, 10). Following the transfer of LRP and Frizzled proteins into the nucleus of the pre-osteoblast and binding to transcription factor TCFS, proliferation and differentiation of this cell is induced. The LRP5 pathway was discovered to be a key regulator of bone mass mainly by influence on osteoblast proliferation and bone matrix deposition (15, 16).

The Rotterdam Study, that included 2995 participants, revealed that SNP of the gene involved in osteoblast differentiation through activation of Wnt/β-catenin signals localized on chromosome 16q24, was significantly associated with an increased risk of vertebral fractures evident on the spinal radiographs. Compared to non-carriers, the heterozygous carriers of the minor allele (C) had the OR = 1.7 for vertebral fractures, and the homozygous carriers OR = 5.8. The vertebral fracture SNP was not associated with either lumbar spine or femoral neck BMD (17).

LRP5 and LRP6 genes have been implicated to play a role in bone metabolism, and LRP5 was thought to be important for the establishment of peak bone mass (15, 18). A study of 7983 inhabitants of Rotterdam, aged > 55 years, showed that in men, the Ala1330Val polymorphism in the LRP5 gene was associated with significantly decreased BMD at the lumbar spine and femoral neck, reduced vertebral body size and femoral neck width, and a 60% increased risk for fragility fractures. Carriers of two risk alleles LRP5 1330Val and LRP6 1062Val had a 2.4 and 1.9 times higher risk for fragility and vertebral fractures, respectively, compared with men not carrying a risk allele. It was suggested that both SNPs account for one-tenth of the fracture cases in men, while in women carrying those risk alleles only nonsignificant trend for 30% higher risk for both fragility fractures and vertebral fractures was found (19).

Large multicenter study including more than 37,000 participants from Europe and North America showed, that the Val667Met (in the exon 9) and Ala1330Val (in the exon 18) polymorphisms of LRP5 gene were associated with an increased incidence of vertebral fractures by 26 and 12% respectively and an increased incidence of all fractures by 14 and 6% respectively. These data suggest that SNPs in LRP5 and LRP6 may affect the pathogenesis of osteoporosis through decreasing activity of the WNT/β-catenin pathway (20).