Discovering Interactions Among BRCA1 and Other Candidate Genes Associated with Sporadic Breast Cancer
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Discovering interactions among BRCA1 and other candidate genes associated with sporadic breast cancer Shaw-Hwa Lo†‡, Herman Chernoff‡§, Lei Cong†, Yuejing Ding†, and Tian Zheng† †Department of Statistics, Columbia University, New York, NY 10027; and §Department of Statistics, Harvard University, Cambridge, MA 02138 Contributed by Herman Chernoff, June 9, 2008 (sent for review March 17, 2008) Analysis of a subset of case-control sporadic breast cancer data, effects that are significantly higher than those generated by [from the National Cancer Institute’s Cancer Genetic Markers of random fluctuations. In other words, all of the 18 genes would Susceptibility (CGEMS) initiative], focusing on 18 breast cancer- be missed if only marginal methods were used. related genes with 304 SNPs, indicates that there are many inter- Second, we demonstrate how to carry out a gene-based esting interactions that form two- and three-way networks in analysis by treating each gene as a basic unit while incorporating which BRCA1 plays a dominant and central role. The apparent relevant information from all SNPs within that gene. Two interactions of BRCA1 with many other genes suggests the con- summary test scores are proposed to quantify the strength of jecture that BRCA1 serves as a protective gene and that some interactions for each pair of genes. The pairwise interactions can mutations in it or in related genes may prevent it from carrying out be extended easily. We also provide results using third-order this protective function even if the patients are not carriers of interactions. known cancer-predisposing BRCA1 mutations. The method of anal- Third, to establish statistical significance, we generate a large ysis features the evaluation of the effect of a gene by averaging the number of permutations of the dependent variable (case or effects of the SNPs covered by that gene. Marginal methods that control) to see how the measures of interaction for the real data test one gene at a time fail to show any effect. That may be related compare with those from the many permutations. to the fact that each of these 18 genes adds very little to the risk Finally, when these procedures are applied to the data, they of cancer. Analysis that relates the ratio of interactions to the lead to a number of interesting findings. It is shown that there are maximum of the first-order effects discovers significant gene pairs a substantial number of significant interactions that form a and triplets. network in which BRCA1 plays a dominant role. The interactions of BRCA1 with many of the other genes suggests the conjecture reast cancer (MIM 114480) has complex causes. Known that BRCA1 serves as a protective gene and that some mutations Bpredisposition genes explain Ͻ15% of the breast cancer in it or in related genes may prevent it from carrying out the cases. It is generally believed that most sporadic breast cancers protective function. STATISTICS are triggered by unknown combined effects, possibly because of a large number of genes and other risk factors, each adding a Results small risk toward cancer etiology. Progress in seeking breast None of the 18 marginal effects are significantly higher than cancer genes other than BRCA1 and BRCA2 has been slow and those generated by random permutations. This claim is made limited because the individual risk due to each gene is small. This based on comparisons between the observed 18 V values and the difficulty may be partly due to the fact that current methods rely values obtained from 1,000 permutations. The exact procedures largely on marginal information from genes studied one at a time carried out were described in steps 1 and 2 in Methods.Ofthe and ignore potentially valuable information because of the 18 genes, the most significant one is BARD1 with a P value ϭ GENETICS interaction among multiple loci. Because each responsible gene 0.053, which is slightly short of being significant at 0.05. It is also may have a small marginal effect in causing disease, it is likely noticed that BRCA1 has a very large P value (0.944), meaning that such methods will fail to capture many responsible genes by that the real effects of BRCA1, as well as those of the other genes studying a dataset where the disease may be due to a variety of in breast cancer, cannot be properly measured and reflected by different sources. The possible presence of many genes responsible its marginal effect alone. It is the interactive effect that reveals for different subgroups of cancer patients may reduce the power of the roles of some of these genes in breast cancer. See supporting current methods to detect genes partly responsible for some forms information (SI) Fig. S1. of breast cancer. It is believed that methods effective in extracting In seeking pairwise interactions among the 18 genes, both interactive information from data should be developed. Mean-ratio and Quantile-ratio methods were implemented. What should be done when marginal effects are too weak to Although the Mean-ratio method using (M, R) is more conser- be detected? Our methods use interactive information from vative than the Quantile-ratio method using (M, Q), it still multiple sites as well as marginal information, They provide identified 16 of 153 pairs as significant gene pairs at the 0.05 level power to detect interactive genes. To test this claim and to that are connected with breast cancer etiology with an estimated demonstrate the practical value of these methods in real appli- cations, we apply them to an important study: a subset of a large dataset collected from a case-control sporadic breast cancer Author contributions: S.-H.L. and H.C. designed research; S.-H.L., H.C., L.C., Y.D., and T.Z. study, focusing on gene–gene-based analysis. This partial dataset performed research; S.-H.L. and H.C. contributed new reagents/analytic tools; S.-H.L., H.C., comprises 18 genes with 304 SNP markers. The application L.C., Y.D., and T.Z. analyzed data; and S.-H.L., H.C., and T.Z. wrote the paper. results in a number of scientific findings. The authors declare no conflict of interest. The message of this article is fourfold. First, if marginal Freely available online through the PNAS open access option. methods fail, more powerful methods that take into account ‡To whom correspondence may be addressed. E-mail: [email protected] or interactive information can be used effectively. We apply our [email protected]. proposed methods to this dataset to illustrate the detection of the This article contains supporting information online at www.pnas.org/cgi/content/full/ interactions between genes. We point out that in our findings, 0805242105/DCSupplemental. none of the 18 selected genes show any detectable marginal © 2008 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0805242105 PNAS ͉ August 26, 2008 ͉ vol. 105 ͉ no. 34 ͉ 12387–12392 Downloaded by guest on September 24, 2021 Table 1. Significance of the gene pairs identified by the Mean-ratio and Quantile-ratio methods with P values estimated by the curve and rank methods Mean-ratio method Quantile-ratio method Pair no. Gene pair Curve P Rank P Gene pair Curve P Rank P 1 ESR1 BRCA1 0.017 Յ0.001 ESR1 BRCA1 0.013 0.001 2 BRCA1 PHB 0.026 0.040 BRCA1 PHB 0.029 0.073 3 KRAS2 BRCA1 0.002 0.006 KRAS2 BRCA1 0.002 0.004 4 SLC22A18 BRCA1 0.032 0.072 SLC22A18 BRCA1 0.019 0.079 5 RAD51 BRCA1 0.052 0.090 RAD51 BRCA1 0.005 0.032 6 RB1CC1 SLC22A18 0.024 0.026 ESR1 SLC22A18 0.033 0.016 7 CASP8 KRAS2 0.043 0.038 RB1CC1 SLC22A18 0.009 0.008 8 CASP8 SLC22A18 0.042 0.048 CASP8 KRAS2 0.038 0.036 9 PIK3CA BRCA1 0.030 0.048 CASP8 SLC22A18 0.021 0.012 10 PIK3CA ESR1 0.047 0.032 PIK3CA BRCA1 0.014 0.049 11 PIK3CA RB1CC1 0.047 0.051 PIK3CA ESR1 0.021 0.005 12 PIK3CA SLC22A18 0.025 0.036 PIK3CA RB1CC1 0.044 0.053 13 BRCA1 CHEK2 0.016 0.031 CASP8 PIK3CA 0.007 0.009 14 BARD1 BRCA1 0.032 0.057 BRCA1 CHEK2 0.007 0.022 15 BARD1 ESR1 0.044 0.025 BARD1 BRCA1 0.003 0.015 16 BARD1 TP53 0.019 0.019 BARD1 ESR1 0.017 0.003 17 BARD1 TP53 0.015 0.010 18 BARD1 SLC22A18 0.056 0.063 CASP8 ESR1 0.071 0.048 CASP8 ESR1 0.066 0.031 BARD1 KRAS2 0.055 0.036 ESR1 KRAS2 0.145 Յ0.001 ESR1 KRAS2 0.103 Յ0.001 ESR1 PPM1D 0.252 0.021 ESR1 PPM1D 0.348 Յ0.001 The gene pairs are listed in the same order as in Figs. 3 and 4. false discovery rate (FDR) of 50%. Of these 16 pairs, three pairs, Functional interaction between BRCA1 and RAD51 has been (BRCA1, ESR1), (BARD1, ESR1) and (BRCA1, KRAS2) are identified and suggested to play a role during the S phase of the significant even under more stringent criteria. The estimated P cell cycle, which is of critical importance for genome integrity values are 0.001, 0.003 and 0.004; see Table 1. Four pairs are (4). Levy-Lahad et al. (5) found evidence suggesting that a significant at level 0.01, and the remaining pairs are significant mutation at RAD51 modified breast cancer risk among BRCA2 at the 0.05 level by using the rank method.