Single-nucleotide polymorphisms in the P2X7 receptor gene are associated with post-menopausal bone loss and vertebral fractures

Abstract

The purinergic P2X7 receptor has a major role in the regulation of osteoblast and osteoclast activity and changes in receptor function may therefore affect bone mass in vivo. The aim of this study was to determine the association of non-synonymous single-nucleotide polymorphisms in the P2RX7 gene to bone mass and fracture incidence in post-menopausal women. A total of 1694 women (aged 45-58) participating in the Danish Osteoporosis Prevention Study were genotyped for 12 functional P2X7 receptor variants. Bone mineral density was determined at baseline and after 10 years. In addition, vertebral fracture incidence was documented at 10 years. We found that the rate of bone loss was clearly associated with the Arg307Gln amino acid substitution such that individuals heterozygous for this polymorphism had a 40% increased rate of bone loss. Furthermore, individuals carrying the Ile568Asn variant allele had increased bone loss. In contrast, the Gln460Arg polymorphism was associated with protection against bone loss. The Ala348Thr polymorphism was associated with a lower vertebral fracture incidence 10 years after menopause. Finally, we developed a risk model, which integrated P2RX7 genotypes. Using this model, we found a clear association between the low-risk (high-P2X7 function) alleles and low rate of bone loss. Conversely, high-risk (reduced P2X7 function) alleles were associated with a high rate of bone loss. In conclusion, an association was demonstrated between variants that reduce P2X7 receptor function and increased rate of bone loss. These data support that the P2X7 receptor is important in regulation of bone mass.

Figure 1

Relationship of P2X7 SNPs with changes in BMD. (a) Post-menopausal change in BMD in non-HRT individuals for the total loss-of-function SNP Arg307Gln. Individuals carrying the mutant allele have an increased rate of bone loss. This is significant for the total hip (TH) 10 years after menopause, and the same trend is present at the LS and FN (top panel). Furthermore, a similar trend is seen after the first 5 years after menopause (bottom panel). The number of individuals with the two genotypes was 860 GG and 16 GA after 5 years and 758 GG and 15 GA after 10 years. No study participants were homozygous for the variant A allele. (b) The presence of Arg307Gln variant, increases the rate of bone loss at the femoral neck (0–5 years) despite HRT treatment. (c) Changes in BMD associated with the Ile568Asn variant over 5 years post menopause. Individuals homozygous for the variant allele (AA) have significantly accelerated bone loss in the femoral neck, with the same trend seen at TH and LS. ^*P-value <0.05; LS, lumbar spine; FN, femoral neck; NS, not significant; TH, total hip.

Figure 2

Association of fracture incidence with Ala348Thr variant in non-HRT subjects. A χ²-test was performed to test for differences in fracture incidences between the three genotype groups 0–10 years post menopause. Significantly fewer fractures are associated with homozygous mutant (AA) group than the wild type (GG) or heterozygous (GA) group (P=0.04). The number of individuals with the genotype is shown below the bars. The number of individuals having sustained a fracture is shown above each bar.

Figure 3

The rate of post-menopausal bone loss at the LS is associated with P2X₇ functional groupings. In the absence of HRT treatment women who have one or more P2X₇ loss-of-function SNP (high-risk group) have significantly greater loss in BMD than women who have P2X₇ gain-of-function variants (low-risk group) and an intermediate group consisting of all other individuals. ^**P<0.01.