Background: Inherited susceptibility is an important contributor to colorectal cancer risk, and rare variants in key genes or pathways could account in part for the missing proportion of colorectal cancer heritability.Methods: We conducted an exome-wide association study including 2,327 cases and 2,966 controls of European ancestry from three large epidemiologic studies. Single variant associations were tested using logistic regression models, adjusting for appropriate study-specific covariates. In addition, we examined the aggregate effects of rare coding variation at the gene and pathway levels using Bayesian model uncertainty techniques.Results: In an exome-wide gene-level analysis, we identified ST6GALNAC2 as the top associated gene based on the Bayesian risk index (BRI) method [summary Bayes factor (BF) BRI ¼ 2604.23]. A rare coding variant in this gene, rs139401613, was the top associated variant (P ¼ 1.01 Â 10 -6 ) in an exome-wide single variant analysis. Pathway-level association analyses based on the integrative BRI (iBRI) method found extreme evidence of association with the DNA repair pathway (BF iBRI ¼ 17852.4), specifically with the nonhomologous end joining (BF iBRI ¼ 437.95) and nucleotide excision repair (BF iBRI ¼ 36.96) subpathways. The iBRI method also identified RPA2, PRKDC, ERCC5, and ERCC8 as the top associated DNA repair genes (summary BF iBRI ≥ 10), with rs28988897, rs8178232, rs141369732, and rs201642761 being the most likely associated variants in these genes, respectively.Conclusions: We identified novel variants and genes associated with colorectal cancer risk and provided additional evidence for a role of DNA repair in colorectal cancer tumorigenesis.Impact: This study provides new insights into the genetic predisposition to colorectal cancer, which has potential for translation into improved risk prediction.

1523 Background: Mutation carrier prediction models are clinically useful tools for identifying candidates for genetic counseling and testing. Consensus guidelines recommend germline genetic testing for those with a carrier probability (CP) of approximately 5% or higher. However, prediction models may perform less well among racial/ethnic minorities. Our hypothesis is that pathogenic mutations (PM) are identifiable in a clinically meaningful fraction of racially/ethnically diverse patients with a CP of < 5%. Methods: We conducted a multicenter prospective clinical trial of patients undergoing cancer-risk assessment using a 25 gene panel, which include APC, ATM, BARD1, BMPR1A, BRCA1, BRCA2, BRIP1, CDH1, CDK4, CDKN2A, CHEK2, EPCAM, MLH1, MSH2, MSH6, MUTYH, NBN, PALB2, PMS2, PTEN, RAD51C, RAD51D, SMAD4, STK11, TP53. Patients were recruited from August 2014 to November 2016 at three centers. Patients were enrolled if they met standard clinical criteria for genetic testing or were predicted to have a ≥2.5% probability of inherited cancer susceptibility using validated prediction models. We evaluated the CP of patients with a PM in BRCA1, BRCA2, and/or a mismatch repair (MMR) gene using the following models: (1) BRCApro, (2) MMRpro and (3) PREMM1,2,6. Results: Of 2000 patients enrolled in this cohort, 80.6% are female (n = 1612). Regarding race/ethnicity, the cohort is 40.1% Non-Hispanic White (n = 802), 37.4% Hispanic (n = 748), 11.5% Asian (n = 230), 3.9% Black (n = 78), and 7.1% Other (n = 142). Among 241 (12.1%) patients who tested positive for a pathogenic mutation, 76 (31.5%) patients had a BRCA1 or BRCA2 mutation. Of those, 52 (68.4%) patients had a BRCApro CP of < 5%. Thirty-eight (15.8%) patients had a pathogenic mutation in an MMR gene: 19 (50.0%) had an MMRpro CP of < 5%, while 13 (34.2%) had a PREMM1,2,6 CP of < 5%. The racial/ethnic distribution of BRCA1/2 or MMR mutation carriers is similar to that of the whole cohort. Conclusions: In a diverse cohort of patients undergoing 25-gene multiple-gene panel testing, half or more carriers of BRCA1/2 or MMR mutations had a CP of < 5%, the consensus guideline-recommended cutoff for genetic testing. These results support a lower threshold for genetic testing guidelines. Clinical trial information: NCT02324062.

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