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Two Genetic Variants Found to be Associated with Increased Risk for Osteoporosis and Fracture
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Large-Scale Analysis of Association Between LRP5 and LRP6 Variants and Osteoporosis
JAMA
March 19, 2008
ABSTRACT
Context: Mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene cause rare syndromes characterized by altered bone mineral density (BMD). More common LRP5 variants may affect osteoporosis risk in the general population.
Objective: To generate large-scale evidence on whether 2 common variants of LRP5 (Val667Met, Ala1330Val) and 1 variant of LRP6 (Ile1062Val) are associated with BMD and fracture risk.
Design and Setting: Prospective, multicenter, collaborative study of individual-level data on 37 534 individuals from 18 participating teams in Europe and North America. Data were collected between September 2004 and January 2007; analysis of the collected data was performed between February and May 2007. Bone mineral density was assessed by dual-energy x-ray absorptiometry. Fractures were identified via questionnaire, medical records, or radiographic documentation; incident fracture data were available for some cohorts, ascertained via routine surveillance methods, including radiographic examination for vertebral fractures.
Main Outcome Measures: Bone mineral density of the lumbar spine and femoral neck; prevalence of all fractures and vertebral fractures.
Results
The Met667 allele of LRP5 was associated with reduced lumbar spine BMD (n = 25 052 [number of participants with available data]; 20-mg/cm2 lower BMD per Met667 allele copy; P = 3.3 x 10-8), as was the Val1330 allele (n = 24 812; 14-mg/cm2 lower BMD per Val1330 copy; P = 2.6 x 10-9).
Similar effects were observed for femoral neck BMD, with a decrease of 11 mg/cm2 (P = 3.8 x 10-5) and 8 mg/cm2 (P = 5.0 x 10-6) for the Met667 and Val1330 alleles, respectively (n = 25 193).
Findings were consistent across studies for both LRP5 alleles. Both alleles were associated with vertebral fractures (odds ratio [OR], 1.26; 95% confidence interval [CI], 1.08-1.47 for Met667 [2001 fractures among 20 488 individuals] and OR, 1.12; 95% CI, 1.01-1.24 for Val1330 [1988 fractures among 20 096 individuals]).
Risk of all fractures was also increased with Met667 (OR, 1.14; 95% CI, 1.05-1.24 per allele [7876 fractures among 31 435 individuals)]) and Val1330 (OR, 1.06; 95% CI, 1.01-1.12 per allele [7802 fractures among 31 199 individuals]).
Effects were similar when adjustments were made for age, weight, height, menopausal status, and use of hormone therapy.
Fracture risks were partly attenuated by adjustment for BMD. Haplotype analysis indicated that Met667 and Val1330 variants both independently affected BMD. The LRP6 Ile1062Val polymorphism was not associated with any osteoporosis phenotype. All aforementioned associations except that between Val1330 and all fractures and vertebral fractures remained significant after multiple-comparison adjustments.
Conclusions: Common LRP5 variants are consistently associated with BMD and fracture risk across different white populations. The magnitude of the effect is modest. LRP5 may be the first gene to reach a genome-wide significance level (a conservative level of significance [herein, unadjusted P < 10-7] that accounts for the many possible comparisons in the human genome) for a phenotype related to osteoporosis.
COMMENT
In this large-scale multicenter collaborative study, we obtained evidence that genetic variation of the LRP5 gene is associated with both BMD and fracture risk. The magnitude of the effects was modest but very consistent across studies. The effect size was 14 to 20 mg/cm2 for lumbar spine and 8 to 11 mg/cm2 at the femoral neck, which approximately corresponds to a 0.15-SD difference at both sites. Based on the general acceptance that a 1-SD reduction in bone mass doubles the fracture rate,66 an increase of fracture risk of about 15% to 20% is expected. This is similar to the observed effects on fracture, although adjustment for BMD only partly reduced the increase in fracture risk. This could raise the possibility of effects on bone quality, bone dimension, or other nonskeletal determinants of fracture, but also could be due to error in measurement of BMD. Further work will be required to address this point.
Several previous reports have suggested that the association between genetic variation of the LRP5 gene and BMD might be stronger in men compared with women.22, 25 We could not find such a sex difference. In fact, for fractures we found a slightly stronger effect for women as compared with men, although power was lower to detect effects for men.
LRP5 may be involved in the establishment of peak bone mass6 and to a lesser extent involved in bone loss. Bone mineral density is substantially affected by age-related bone loss at older ages, so differences in BMD between LRP5 genotype groups might become smaller with age.25 In our study there was no clear influence of age on the magnitude of the association between LRP5 variants and BMD or fracture. For femoral neck BMD, differences between the Ala1330Val genotypes were larger in premenopausal women compared with postmenopausal women, which could indicate that the effect of LRP5 variants is largely seen on peak bone mass. However, this was not observed for lumbar spine BMD and the Ala1330Val variant or with the Val667Met polymorphism for any of the outcomes. Even with such large-scale evidence, the presence or absence of interaction effects should be interpreted very cautiously.
The 2 polymorphisms in LRP5 are each strongly associated with BMD. Although these polymorphisms are in strong linkage disequilibrium, the risk alleles were separated in 2 haplotypes: haplotype 2, carrying the common Val667 and the Val1330 risk allele, and haplotype 3, carrying risk alleles for both Met667 and Val1330. Haplotypes 2 and 3 were both associated with BMD while haplotype 3 was more strongly associated, which suggests that both variants have distinct effects. However, we cannot exclude that the polymorphisms are in linkage disequilibrium with 1 or more other causative polymorphisms rather than having an effect themselves.
The 2 studied LRP5 variants are situated in different domains of the protein. The Val667Met polymorphism is localized at the top of the third propeller module in the receptor extracellular domain. This domain is thought to be involved in binding of the Wnt-inhibitor Dkk1, so perhaps binding efficacy of this inhibitor is changed in the Met667 variant. The Ala1330Val polymorphism lies within a second low-density lipoprotein (LDL) receptor domain of LRP5. The function of this region in LRP5 is unknown, but similar domains in the LDL receptor domain interact with the propeller domains.67 Therefore, variations in the LDL receptor domains, such as Ala1330Val, may still alter protein function. Indeed, a recent report showed in vitro that Wnt-signaling capacity of the LRP5 Val1330 variant was decreased compared to the Ala1330 variant.25
The strengths of our consortium analysis include the very large sample size, consistency across cohorts, lack of publication bias within the consortium due to its prospective design, and analysis of individual-level data, which allows standardized statistical analyses across participating teams.
In particular, we focused on validation of genotyping to minimize genotyping errors and aimed at standardized definitions for the outcomes. Limitations arise due to ascertainment of fractures, which differed across participating studies. This could introduce some unavoidable heterogeneity in the analyses. Another potential limitation is due to missing data in some cohorts. In addition, our results might not pertain to Asian and/or African populations, since we only examined white populations.
Our findings demonstrate that the modest effects of common genetic variations in complex diseases can be effectively addressed through large consortia and coordinated, standardized analysis. Such effects might be missed by smaller and potentially underpowered individual studies. This prospective collaborative study with individual level-data of 37 534 participants shows an effect of LRP5 genetic variation on both BMD and risk of fracture. While some other common variants have been associated previously with osteoporosis phenotypes with large-scale evidence,17-19 this may be the first time that an association in this field crosses the threshold of genome-wide statistical significance (P < 10-7). Given the large number of polymorphisms that can be tested in the human genome, it has been argued that to fully account for all these possible comparisons (regardless of whether all of them are made), a very conservative threshold is needed.62-64 Although the magnitude of the effect was modest, the effect was very consistent in different populations and independent of sex or age. This suggests a role for LRP5 in determining BMD and fracture risk throughout life in the general population. Although any single marker explains only a small portion of the phenotype risk, identification of several such osteoporosis risk variants may eventually help in improving clinical prediction. Single genetic risk variants such as LRP5 variants may also offer useful insights about mechanisms and pathways that may be useful in drug development.
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