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Original Studies |
Indiana University School of Medicine (D.L.K., M.J.E., J.C.C., S.L.H., P.M.C., M.P., C.C.J., T.F.), Indianapolis, Indiana 46202; and Axys Pharmaceuticals, Inc. (P.A.M., P.P., M.E.C., L.A.R., G.J.), La Jolla, California 92037
Address all correspondence and requests for reprints to: Dr. Tatiana Foroud, Department of Medical and Molecular Genetics, Indiana University School of Medicine, 975 West Walnut Street, Indianapolis, Indiana 46202.
| Abstract |
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The highest LOD score attained in the expanded sample was 3.86 at chromosome 1q2123 with lumbar spine BMD. Chromosome 5q3335 gave a LOD score of 2.23 with femoral neck BMD. At chromosome 6p1112, the 464 Caucasian pairs achieved a LOD score of 2.13 with lumbar spine BMD. Markers within the 11q1213 region continued to support linkage to femoral neck BMD, although the peak LOD score was decreased to 2.16 in the sample of 595 sibling pairs. Our study is the largest genome screen to date for genes underlying variations in peak BMD and represents an important step toward identifying genes contributing to osteoporosis in the general population.
| Introduction |
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The role of genetic factors in the rate of bone loss is unclear (2, 3, 4). However, these data (5) as well as those reported by others (6, 7) have shown as much as 80% of the variability in peak BMD to be attributable to genetic factors. Gender and race are also important predictors of peak BMD, with men and African-Americans having higher average peak BMD than women and Caucasians, respectively (8). Several genes probably contribute and interact to produce variability in peak bone density. Identification of these genes will be important in understanding the underlying mechanisms of bone formation and maintenance and in providing molecular targets for future osteoporosis therapies.
To date, most efforts toward understanding the genetics of bone density
have focused on population-based case-control association studies
of candidate genes known to be involved in bone metabolism. These
include the vitamin D receptor (9, 10, 11), the type I collagen
1 gene
(12, 13), the estrogen receptor (14, 15), and interleukin-6 (16).
Positive and negative association results have been reported for each
of these candidate genes, and a significant role in BMD variability in
the general population for any or all of these genes remains highly
controversial (17). In a previous study we employed a candidate region
approach (18) and found linkage of BMD in a sample of healthy
premenopausal women to a candidate region at chromosome 11q1213, of
interest due to the mapping of several Mendelian bone density
phenotypes to this region (19, 20, 21).
Genome-wide linkage screens for genes underlying BMD variability have been conducted in humans (22, 23) and mice (24, 25, 26). Devoto et al. (22) studied 7 extended pedigrees ascertained via probands with low BMD and reported linkage to chromosome regions 1p, 2p, and 4q. Niu et al. (23) measured radial BMD in 153 Chinese sibling pairs and reported linkage to chromosomes 2p and 13q. All of the mouse studies cited above found linkage to regions homologous to human chromosome 11q1213 as well as other chromosomal regions. Identification of the genes underlying BMD variation in animal models will provide novel or known genes that can be evaluated for possible effects on BMD in humans.
We report a genome-wide linkage screen with BMD in 429 healthy Caucasian, premenopausal sister pairs, ascertained without regard to bone density. We also performed additional genotyping of a larger sample of Caucasian and African-American sister pairs for markers in the regions of interest (LOD >1.85) identified in the genome screen.
| Experimental Subjects |
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BMD was measured by dual energy x-ray absorptiometry (DPXL, Lunar Corp., Madison, WI) at the lumbar (L2L4) spine, trochanter, femoral neck, and Wards triangle. Sisters were measured on the same machine. Height and weight were obtained for each sister, along with oral contraceptive and smoking (pack-years) history. Stepwise regression analysis was employed using these four variables along with age to identify significant covariates (P < 0.05) with BMD. Race-specific regression residuals, representing covariate-adjusted BMD values, were computed and used in all subsequent analyses.
| Materials and Methods |
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A total of 270 markers were genotyped for 429 Caucasian sister pairs and 1 of their parents, where available. Markers were selected from the ABI/Prism genome screening set (ABI, Foster City, CA), with additional public markers selected to fill large intermarker gaps. The average spacing of the markers was 12.9 centimorgans (cM). All markers were highly informative dinucleotide or tetranucleotide repeat polymorphisms, with heterozygosity greater than 0.70. PCRs for each marker were performed separately, and products were combined before gel electrophoresis. Data were collected using 373A or 377 automated DNA sequencers and genotyped using the Genescan Analysis and Genotyper software (Perkin-Elmer Corp., Norwalk, CT). Genotypes were normalized to control for gel to gel size variation, binned, coded, and checked for Mendelian inheritance.
The marker genotype data were used to verify the full sibling relationships among the subjects using the computer programs RELATIVE (28) and RELPAIR (29). Half-sibling pairs, identified based on aberrant IBD allele sharing, were eliminated from further analyses.
Quantitative linkage analysis
Multipoint quantitative linkage analysis was performed for BMD at each skeletal site using the maximum likelihood variance components method as implemented in the computer package Mapmaker/SIBS (30). LOD scores were computed at 1-cM intervals along each autosome, using all possible sibling pairs formed from families larger than two sisters. Observed allele frequencies in the individuals genotyped for the genome screen were used. Marker order and map positions were obtained from the Marshfield electronic database (http://www.marshmed.org/genetics/).
Interesting linkage findings (LOD >1.85) in the initial genome screen sample of 429 Caucasian sister pairs were further pursued in an expanded sample of 464 Caucasian and 131 African-American sister pairs. Markers in these chromosomal regions of interest were selected for genotyping in the expanded sample. Due to the relatively small size of the African-American sample and the possibility of differences in effect of a particular QTL between racial groups, the Caucasians were analyzed both alone and jointly with the African-American sample for the follow-up analysis of interesting chromosomal regions. For the joint Caucasian/African-American analysis, race-specific allele frequencies were used by coding each marker allele differently for Caucasian and African-American individuals as described previously (18).
| Results |
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| Discussion |
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We also found suggestive evidence for linkage of femoral neck BMD to chromosome 5q, with a multipoint LOD score of 2.23 near D5S422. Potential candidate loci mapping to this region include the structural genes for osteonectin, believed to be responsible for the bone-specific calcification of collagen, and platelet-derived growth factor receptor-ß. Weaker evidence of linkage was found on chromosomes 6p, with a LOD score of 2.13 for lumbar spine BMD. The region of 6p implicated in spine BMD variation contains the gene for bone morphogenic protein-6, and may contain bone morphogenic protein-5 as well, although the latter locus is not well mapped relative to the microsatellite markers employed in our study. The evidence for linkage of spine BMD to chromosome 22q, observed in our initial genome scan, declined substantially (from a LOD score of 2.13 to 0.99) with genotyping of additional Caucasian and African-American sister pairs and probably represents a false positive linkage result.
The maximum LOD score on chromosome 11q in the Caucasian subset of our data for femoral neck BMD is 1.55, compared to 2.79 in our previous report (18). Since that report, 84 additional Caucasian sister pairs have been ascertained and genotyped for the current genome scan. This newly collected sample has greater evidence of linkage with the distal marker D11S937 and little evidence of linkage to D11S987 or D11S1313, the adjacent markers that provided the greatest linkage evidence in our initial report. Inclusion of the African-American sister pairs moves the linkage peak toward D11S935. This region of chromosome 11q (D11S935-D11S937) encompasses 34 cM. Inconsistent localization of the linkage finding in various subsets of our data within this broad region may indicate heterogeneity among our study group for genetic factors underlying BMD variability and/or the presence of 2 loci in this region of chromosome 11 affecting bone density. The identification of the genes for the 3 Mendelian bone-related phenotypes recently mapped to this region of 11q (19, 20, 21) will provide candidate genes that can be evaluated for their contribution to normal variation in bone density. In addition, linkage of BMD in other species to regions homologous to human chromosome 11q1213 has been reported in a large sample of baboons (33), as well as by Klein et al. (24) in a study of peak total body BMD in mouse.
With the exception of the finding on chromosome 11q, none of the regions identified showed evidence of linkage with LOD scores above 1.0 to BMD at skeletal sites other than the one providing the primary linkage finding. Thus, in our sample we failed to detect loci (apart from the 11q locus) with substantial effect on BMD at multiple sites or across the entire skeleton. Each of the loci detected on chromosomes 1, 5, and 6 appears to affect primarily spine or hip BMD, not both of these skeletal sites.
Our collection of 595 sister pairs is the largest sample collected to date specifically to identify genes influencing BMD. This sample has power to detect chromosomal regions with evidence for linkage, as demonstrated by our finding on chromosome 1, where the maximum LOD score was 3.8. Our own simulation studies have produced a mean LOD score of 2.0 with Mapmaker/SIBS for a sample of similar size and structure to this one, for a gene accounting for 25% of the phenotypic variance and an overall trait heritability of 80%. All linkage results presented here are near or above this mean LOD score, thus demonstrating the power of our sample to detect QTLs of moderate to large effect.
We report linkage to several broad chromosomal regions and are pursuing saturation mapping with additional markers in these regions. We are in the process of collecting 600 additional sister pairs (400 Caucasian and 200 African-American). This greatly enlarged sample and increased marker density will enable us to confirm the linkage findings we reported here as well as to map the QTLs in these regions with greater accuracy for subsequent positional cloning and increase our power to detect additional loci of smaller effect contributing to bone density. Peak bone density is one of the best predictors of fracture risk, and elucidation of the genes contributing to normal variation in peak BMD will probably provide molecular targets for future therapeutic interventions designed to prevent osteoporosis and associated fractures.
| Acknowledgments |
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| Footnotes |
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Received December 2, 1999.
Revised March 29, 2000.
Revised May 11, 2000.
Accepted May 19, 2000.
| References |
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1 gene. Nat Genet. 14:203205.[CrossRef][Medline]
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