The Study Of Relationship Between Human Genetics And Osteoporotic Risk Factor-Compression Strength Index

Osteoporosis is one of the most popular old people's diseases. Along with aging population, osteoporosis as a degenerative disease or a complication, has become a social health issue. It is getting great attention world widely. A lot of geriatrists are working in this area. The most serious result of osteoporosis is osteoporotic fracture.Osteoporosis is characterized by low bone mineral density (BMD) and deterioration of skeletal microarchitecture, leading to increased risk of fragility fracture. Low BMD is recognized as a major risk factor for osteoporotic fracture (OF). BMD is a quantitative trait determined by genetic and environmental factors, with heritability estimates ranging from 50% to 90%. A large number of studies used BMD as the major surrogate phenotype for osteoporotic fracture in the genetic studies and clinical works, et al. Many epidemiologic studies have indicated that BMD, as a major phenotype predicting fracture risk, has population- and ethnic-specificity. Therefore, studies clearly demonstrate that BMD, by itself, falls short of capturing the entire variation in fracture risk. In clinical work and genetic studies, the properly use of a surrogate is the key of disease diagnosis and studying pathogenesis. Compression strength index (CSI) is another independent risk factor for OF under strong genetic determination. Since CSI measures the capacity of withstanding compressive forces proportional to body weight along the main femoral neck axis, it may provide more information than BMD alone in assessing the risk of hip fracture.The purpose of this study is:to investigate the characteristics of compression strength index (CSI), a new parameter for measuring bone strength at the hip, in Caucasian and Chinese populations; to compare CSI with hip BMD and femoral neck width (FNW) in predicting the risk of hip fracture; by using genome-wide association analysis and genome-wide linkage analysis, looking for the fracture related gene or locus with CSI, and their potential function in osteoporosis and fracture risk.Materials and Methods1. Materials Samples:Two large samples were used in this study, including a Chinese Han sample from Xian'an province and Hu'nan province, recruited by related department in Xi'an Jiaotong University and Hunan Normal University, and a US Caucasian sample of Turopean origin, recruited by myself and some group members in University of Missouri-Kansas City.2. MethodsMeasurements:All the subjects were measured for hip BMD and FNW using the DEXA machine Hologic QDR4500. CSI was calculated as CSI=(BMD×FNW)/Weight.In the GWA study, genotyping with the Affymetrix arrays was performed at the Vanderbilt Microarray Shared Resource at Vanderbilt University Medical Center.In the linkage study, all the subjects were genotyped with 410 microsatellite markers from the Marshfield screening set 14 by Marsh Weld Center for Medical Genetics (Marshfield, WI, USA). These markers have an average population heterozygosity of 0.75 and were spaced, on the average, 8.9 cM apart.Statistical analyses:Regular ANOVA and regression analyses were conducted using the SPSS statistical package.In the linkage study, variance component linkage analyses for quantitative traits were performed using SOLAR (Sequential Oligogenic Linkage Analysis Routines).All GWA statistical analyses were performed using various statistical tests implemented in HelixTree 5.3.1 (Golden Helix, Bozeman, MT, USA).ResultsThe innovation in this study is that this is the first study investigating basic characteristics of CSI and comparing CSI with BMD and FNW in predicting hip fracture in populations of different ethnicities, and this is the first study using CSI as a surrogate to search the fracture related gene or locus.Our results showed as follows:1. An important finding in this study is that Chinese had higher CSI values than Caucasians, which is consistent with the observed lower hip fracture rates in Chinese compared to Caucasians in previous epidemiology studies. This finding may explain the discrepancy observed between BMD and hip fracture rates in Caucasian and Chinese populations. In addition, in the Chinese sample, after adjusting for covariates, CSI was the only bone phenotype that was significantly associated with the odds ratios for hip fracture. Our findings suggest that CSI may be a better predictor of incident hip fracture risk than FNK BMD or FNW. Moreover, the age-related decrease in CSI, as shown in this study in both Caucasians and Chinese, is consistent with the observation of higher hip fracture risk in the elderly.2. CSI, calculated as CSI=(BMD×FNW)/Weight, measures the ability of withstanding compressive forces proportional to body weight along the main femoral neck axis. Hip fracture is often a direct consequence of this compressive force and thus, CSI may represent a more direct and reliable measure of hip bone quality and strength than other commonly used phenotypic traits (e.g. BMD and FNW). Given the advantages of CSI in assessing risk of hip fracture and its strong genetic basis, it may serve as a useful complementary phenotype to BMD for gene identification for osteoporotic fractures at the hip. In this study, we report the first genome-wide association studies (GWAS) for CSI variation. Using Affymetrix 500 K arrays, we successfully genotyped and analyzed an independent cohort containing 1,000 homogeneous unrelated Caucasian subjects, including 501 females and 499 males. We identified a gene, HS3ST1, that was associated with CSI at the genome-wide significance level in subjects. HS3ST1 gene possesses both heparan sulfate glucosaminyl 3-O-sulfotransferase activity, anticoagulant heparan sulfate conversion activity, and is a rate limiting enzyme for synthesis of anticoagulant heparan. Our findings strongly support the importance of the HS3ST1 gene to CSI, and the function of HS3STl in osteoporosis need to be further studied.3. For a sample of pedigrees that contain 4,498 subjects from 451 families, we performed the first genome-wide linkage studies (GWLS) using 410 microsatellite markers to identify genomic regions that may contain quantitative trait loci (QTL) of CSI. The variance component linkage analysis for QTL was performed by using SOLAR. Several potentially important genomic regions (20p11.23?6q22.33?2q36.1) were identified. Some of the regions we identified are supported by earlier findings. In addition, to improve the power of WGLS, we conduct genomic convergence of WGLS results with our GWAS data here. We identified a gene, ARFGEF2, that was associated with CSI at the genome-wide significance level in subjects. Haplotype analyses using the Haploview program and our own genotype data indicated that strong LD exists among 8 single-nucleotide polymorphisms (SNPs) in ARFGEF2 gene, which form a single haplotype block. As a whole, the block is also strongly associated with CSI, with the p value approaching the genome-wide significance level. ARFGEF2 gene may relate to severe malformation of the cerebral cortex and recurrent infections, and the function of ARFGEF2 gene in osteoporosis need to be further studied.ConclusionsOur study for the first time suggests that, in comparison to BMD and FNW, CSI is a much improved parameter in predicting hip fracture, in particular when assessing fracture risk across different ethnic groups. We performed the first GWAS and GWLS for CSI variation. We identified two genes, HS3ST1 and ARFGEF2, which were associated with CSI. Our findings may have significant clinical implications.