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The Genom of Homo sapiens.pdf

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400 GOLDSTEIN, CAVALLERI, AND AHMADIAt least part <strong>of</strong> the discrepancy between the results isdue to the way in which blocks were used in the identification<strong>of</strong> the tSNPs. Gabriel et al. estimated the totalnumber <strong>of</strong> common haplotypes residing in blocks <strong>of</strong> highLD using their 51 regions and extrapolated the resultsacross the genome, then calculated the total number <strong>of</strong>tags needed to tag these block-derived haplotypes usingthe diversity-based haplotype-tagging method (D. ClaytonWeb site: http://www-gene.cimr.cam.ac.uk/clayton/).Goldstein et al., however, chose to ignore the underlyingblock structure <strong>of</strong> LD and selected tSNPs across each <strong>of</strong>the entire regions they studied, thus capitalizing on longrangeassociations that would be ignored in the blockbasedidentification <strong>of</strong> tags. <strong>The</strong>y also used the haplotyper 2 criterion (Weale et al. 2003), which appears to increasetagging efficiency (see above). It would seem likely thatmuch <strong>of</strong> the difference between the two estimates has todo with whether tSNPs are selected within blocks, butmore detailed comparisons <strong>of</strong> different methods <strong>of</strong> selectingtSNPs are still required.COSMOPOLITAN TAGSA major goal <strong>of</strong> the HapMap project is to provide themeans by which a minimum set <strong>of</strong> tSNPs can be ascertainedand used in future association studies. However, thequestion remains as to how well tSNPs from one populationcan represent variation in others. It is already clear thatselecting tags in a population from one geographic region(e.g., Europe) and applying them to another (e.g., EastAsia) can result in a substantial decrease in performance(Weale et al. 2003). It is possible, however, that tSNPs canbe selected which are specifically designed to be cosmopolitanwithout requiring nearly as many tSNPs as thesum <strong>of</strong> those required for each constituent population.For example, a recent study <strong>of</strong> 105 genes identified thehaplotype-tagging SNPs (htSNPs) necessary to representall the observed haplotypes in population samples fromAfrica and Europe (Sebastiani et al. 2003). <strong>The</strong>y foundthat the htSNPs identified in Europe were largely a subset<strong>of</strong> those identified in Africa—a total <strong>of</strong> 538 and 379htSNPs were found to represent all the observed haplotypesin the African and European samples, respectively,and 360 <strong>of</strong> the European htSNPs (95%) were also observedin the Africans. Recent analyses from our lab alsoshow that only a 10–15% increase in the number <strong>of</strong> tSNPsfrom a northern European population (CEPH) is sufficientto tag the common genetic variation in the Japanese(K.R. Ahmadi et al., unpubl.). Overall, these results indicatethat although the patterns and magnitude <strong>of</strong> LD canbe markedly different across diverse populations, largelydue to population history, it may be possible to identify aset <strong>of</strong> tags that work adequately in multiple human populationgroups without excessive increases in the number<strong>of</strong> markers that must be typed.CONCLUSIONSMap-based studies incorporating tagging <strong>of</strong>fer an economical,yet powerful, tool to identify common variationcontributing to complex disease. With the inevitable publication<strong>of</strong> dense SNP maps covering large genomic regions,typed in large cohorts from different global populations,tagging SNP design will become more refined asthe issues raised here and elsewhere are addressed withempirical data. Although considerable work remains tobe done to optimize the selection and use <strong>of</strong> tSNPs, weexpect relatively efficient and standardized approaches tobecome available soon. We are therefore already enteringan era in which the constraints for genetic associationstudies relate to the phenotype. <strong>The</strong> complexity <strong>of</strong> the geneticcontrol <strong>of</strong> common disease and responses to theirtreatment means that genetic association studies will needto be carried out in very large population cohorts with detailedinformation not only about disease outcome, butalso about intermediate phenotypes.REFERENCESBotstein D. and Risch N. 2003. Discovering genotypes underlyinghuman phenotypes: Past successes for Mendelian disease,future approaches for complex disease. Nat. Genet. (suppl.)33: 228.Chakravarti A., Buetow K.H., Antonarakis S.E., Waber P.G.,Boehm C.D., and Kazazian H.H. 1984. Nonuniform recombinationwithin the human β-globin gene cluster. Am. J. Hum.Genet. 36: 1239.Chapman J.M., Cooper J.D., Todd J.A., and Clayton D.G. 2003.Detecting disease associations due to linkage disequilibrium:A class <strong>of</strong> tests and the determinants <strong>of</strong> statistical power.Hum. Hered. 56: 18.Daly M.J., Rioux J.D., Schaffner S.F., Hudson T.J., and LanderE.S. 2001. High-resolution haplotype structure in the humangenome. Nat. Genet. 29: 229.Dean M., Carrington M., Winkler C., Huttley G.A., Smith M.W.,Allikmets R., Goedert J.J., Buchbinder S.P., Vittingh<strong>of</strong>f E.,Gompert E., Donfield S., Vlahov D., Kaslow R., Saah A, RinaldoC., Detels R., and O’Brien S.J. 1996. Genetic restriction<strong>of</strong> HIV-1 infection and progresssion to AIDS by a deletionallele <strong>of</strong> the CKR5 structural gene Hemophilia Growth andDevelopment Study, Multicenter AIDS Cohort Study, MulticenterHemophila Cohort Study, San Francisco City Cohort,ALIVE Study (erraturm in Science [1996] 274: 1069). Science273: 1856.Gabriel S.B., Schaffner S.F., Nguyen H., Moore J.M., Roy J.,Blumenstiel B., Higgins J., DeFelice M., Lochner A., FaggartM., Liu-Cordero S.N., Rotimi C., Adeyemo A., Cooper R.,Ward R., Lander E.S., Daly M.J., and Altshuler D. 2002. <strong>The</strong>structure <strong>of</strong> haplotype blocks in the human genome. Science296: 2225.Glazier A.M., Nadeau J.H., and Aitman T.J. 2002. Finding genesthat underlie complex traits. Science 298: 2345.Goldstein D.B., Ahmadi K.R., Weale M.E., and Wood N.W.2003. <strong>Genom</strong>e scans and candidate gene approaches in thestudy <strong>of</strong> common diseases and variable drug responses.Trends Genet. 19: 615.Hirschhorn J.N., Lohmueller K., Byrne E., and Hirschhorn K.2002. A comprehensive review <strong>of</strong> genetic association studies.Genet. Med. 4: 45.International HapMap Consortium. 2003. <strong>The</strong> InternationalHapMap Project. Nature 426: 789.Jeffreys A.J., Kauppi L., and Neumann R. 2001. Intensely punctatemeiotic recombination in the class II region <strong>of</strong> the majorhistocompatibility complex. Nat. Genet. 29: 217.Jeffreys A.J., Ritchie A., and Neumann R. 2000. High resolutionanalysis <strong>of</strong> haplotype diversity and meiotic crossover in thehuman TAP2 recombination hotspot. Hum. Mol. Genet. 9:725.Johnson G.C., Esposito L., Barratt B.J., Smith A.N., Heward J.,Di Genova G., Ueda H., Cordell H.J., Eaves I.A., Dudbridge

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