Mutational and Epigenetic Spectrum of Colorectal Cancers in African Americans

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associated with an earlier onset of CRC (51 vs. 62, p=0.01) and with previous cancer (25%, p=0.06). The age association was also observed in NHW CRCs without APC mutations. These APC- CRCs were also associated with lower overall mutation burden, and with chromosomally stable tumors. Three of these CRCs had loss-of-function mutations in BCL9L. Analysis of the other cancer driver genes identified in the TCGA showed that, whereas mutations in TP53, KRAS, SMAD4, and FBWX7 occurred at similar frequencies in AAs and NHWs, mutations in ACVR1B, EDNRB, FAM123B, GPC6, KIAA1804, NRAS, SMAD2, SOX9, CTNNB1, and TCF7L2 were less frequent in AAs than in NHWs. In total, only 3 mutations in 3 CRCs were identified in these 10 genes in comparison to 101 mutations in 79 NHW CRCs in the TCGA (p=10-5). Examination of other possible cancer driver genes showed that somatic mutations in AXIN2, CHD5, FLT3, GLI2, SMO, and TOP1 were significantly more common in AAs compared to NHW CRCs (p=2 x 10-7). By copy number analysis, the overall pattern of gains and losses and the frequencies of events per chromosome arm were similar between AA and NHW CRCs, indicating that copy number variation does not explain the under-representation of known driver genes in AA CRCs. Thus, other driver genes or epigenetic changes drive AA CRC. By methylation analysis, APC- CRCs displayed higher overall levels of DNA methylation where a few hundred highly specific CpG sites were differentially methylated between APC+ and APC- groups. Conclusions: We have identified a subtype of CRC that is over-represented in AAs and it is associated with younger age of diagnosis, lack of APC mutation, microsatellite and chromosome stability, lower mutation burden, and a distinct methylation profile

GENETIC POLYMORPHISMS IN FATTY ACID METABOLISM MODIFY THE ASSOCIATION BETWEEN DIETARY N3:N6 INTAKE AND RISK OF ULCERATIVE COLITIS Ashwin Ananthakrishnan, Hamed Khalili, Mingyang Song, Paul Lochhead, James Richter, Andrew T. Chan Introduction: Diet plays an important role in the pathogenesis of inflammatory bowel diseases (IBD; Crohn's disease (CD), ulcerative colitis (UC). In particular, high intake of n3 polyunsaturated fatty acids (PUFA) and low intake of n-6 PUFA have been associated with a reduced risk of UC. Heterogeneity in diet effects on disease risk suggests underlying genetic variation in PUFA metabolism may modify this risk. There have been no prior prospective studies examining this. Methods: This was a case-control study nested within two prospective cohorts, Nurses Health Study (NHS) (1984 - 2010) and NHS II (1989 2011). Dietary information was prospectively collected through a validated food frequency questionnaire and updated every 4 years. Diagnosis of IBD was confirmed through medical record review by gastroenterologists blinded to exposure status. Study participants provided blood or buccal cells for genotyping. Confirmed IBD cases were matched 1:2 to controls by age, menopausal status, month of blood draw, and fasting status. Subjects were genotyped for single nucleotide polymorphisms (SNPs) at CYP4F3, FADS1, and FADS2 loci involved in PUFA metabolism. Conditional logistic regression models examined the association between n3:n6 PUFA intake ratio and individual fatty acids and risk of CD and UC. Results: Our study included 101 CD and 139 UC patients matched to 495 controls. There was no difference in the mean body mass index, or smoking status between the three groups. There was also no difference in total calorie consumption or intake of individual macronutrients between the three groups. On multivariable analysis, high intake of n3:n6 PUFA (> median) demonstrated a trend towards reduced risk of UC (Odds ratio (OR) 0.71, 95% confidence interval (CI) 0.47 - 1.09, p=0.11). This association demonstrated a statistically significant interaction with a SNP at CYP4F3 (rs4646904). High n3:n6 PUFA intake was associated with a reduced risk of UC in individuals with the GG/AG genotype (OR 0.57, 95% CI 0.32 - 0.99) but not those with the AA genotype (OR 0.95, 95% CI 0.47 - 1.93) (p-interaction = 0.049). No interactions were noted with SNPs at FADS1 or FADS2, or for Crohn's disease. Conclusion: The association between dietary n3:n6 PUFA intake and risk of UC is modified by a genetic variant at CYP4F3. Further gene-environment studies of the association between diet and IBD risk are warranted.

236 POLYMORPHISMS IN HIGH ORDER CHROMATIN REMODELER SA1 AS A BIOLOGICAL BASIS FOR RACIAL DISPARITIES IN COLORECTAL CANCER Mart DeLa Cruz, Richard Sherva, Sanjib Chowdhury, Hemant K. Roy Background: African Americans (AA) suffer disproportionately from CRC with a ~25% higher incidence and ~50% higher mortality than Caucasians. While socio-economic factors are undoubtedly contributory, there are profound genetic differences in CRC between the races with several oncogenes only mutated in blacks (Guda et al., PNAS 2015). Our group has identified the cohesin SA1 as a modulator of high order chromatin and hence gene expression in early colon carcinogenesis. Cohesin binding sites are known to be mutated in most CRCs (Kaiten et al., Nature Gen 2015). Intriguingly, SA1 loss was much more significant in AA rather than Caucasians (Cancer Prev Res 2016). The etiology of this preferential downregulation has not been elucidated. We hypothesized that single nucleotide polymorphism's (SNP's) might be one of the major factors in the racial disparities in SA1 expression during induction of CRC. Methods: To explore the role of SA1 variants and race in CRC pathogenesis, we selected 27 SNPs based on eQTL potential, impact on protein function, and haplotype tagging. Taqman SNP genotyping was performed using genomic DNA from 96 patients (negative colonoscopies from rectal biopsies =43, adenoma from rectal biopsies =38 and CRC tumor specimen =14). Logistic and linear regression models were used to test for associations between SA1 SNPs, CRC risk, and SA1 expression. Interaction tests were used to determine whether the effect of these SNPs differed between AAs and Caucasians. Results: Two SNPs showed nominally significant association with CRC risk (compared to adenoma and normal patients) after adjusting for race, including rs1681818 (OR=6.56; p=0.022). Additionally, 5 SNPs showed association with SA1 expression. The presence of each copy of the rs13069695 (C) alleles was associated with an 81% decrease in SA1 expression (p=0.018) whereas rs34992220 (C) alleles were associated with a 77% increase in the expression of SA1. Interaction tests showed 4 SNP altered SA1 expression within the AA patient cohort. The main effect of rs34149860 (G) allele predicted a marginally significant [p=0.068] 223% decrease in SA1 expression, while the same allele was associated with an additional 209% [p=0.023] decrease in AAs. Conclusions: We for the first time, show SA1 SNPs have racial predilection providing a biological basis into the preferential loss of the SA1 tumor suppressor gene in AAs and hence higher incidence of CRC. The importance of this finding is underscored by the emerging understanding of the integral role of SA1 and other cohesins in gene expression and hence carcinogenesis. Aside from biological insights, this may provide a novel tool to risk stratify AAs hence personalize screening strategies.

234 TREATMENT OF FAMILIAL ADENOMATOUS POLYPOSIS PATIENTS WITH APC STOP CODON MUTATION BY READ-THROUGH Revital Kariv, Michal Caspi, Naomi Fliss Isakov, Rina Arbsfeld Background: Read-through of genetic nonsense mutations has been proven effective in mice models and in clinical studies for Cystic Fibrosis and Duchene muscular dystrophy and can lead to the expression of a full length protein. Based on previous studies showing that both aminoglycoside and macrolide antibiotics can read-through adenomatous polyposis coli (APC) nonsense mutations, we have initiated a clinical trial for Erythromycin treatment in familial polyposis (FAP) patients that result from premature termination of the APC protein product. Design and methods: Study population included FAP patients with APC nonsense mutation, including children. This is a prospective, open label interventional study with oral Erythromycin 500mg BID (in chidren- dose was weight adjusted) for 4 months. The study includes a baseline as well as mid- and post- treatment colonoscopies, in which a meticulous polyp count and measurements were conducted. In addition, polyps were analyzed at various stages for Wnt target genes expression and were subjected to APC sequencing. Results: We have recruited 6 patients. Four patients have ended treatment protocol, in which cumulative polyp size and total number decreased throughout the treatment period in 77%, 54.8%, 12.5%, 32.7% and 71.42%, 5%, 28.6%, 37.25%, respectively. APC sequencing demonstrates an interesting decrease in somatic mutation percentage (second hit) in polyp samples throughout the treatment. Wnt pathway molecular analysis revealed a decrease in C-MYC, AXIN and Cyclin D gene expression (by qPCR) throughout the treatment period for individual polyps at variable degrees. The studied drug was well tolerated and we observed no side effects including ECG. Both sequencing and qPCR results raise the interesting and novel possibility that restoring the expression of APC reduces the oncogenic potential of the polyp by elimination of specific cells which contain the APC mutated form and show increased expression of Wnt target genes. Conclusions: In this pilot study, initial results point to molecular, somatic and clinical effects of Erythromycin treatment, which may indicate that APC read-through is feasible for FAP patients.

237 EFFECT OF ERADICATION OF HELICOBACTER PYLORI ON GASTRIC CANCER PREVENTION IN HEALTHY ASYMPTOMATIC POPULATION Suh Eun Bae, Kee Don Choi, Jaewon Choe, Hye-Sook Chang, Hwoon-Yong Jung OBJECTIVES: We evaluated whether eradication of Helicobacter pylori infection would reduce the incidence of gastric cancer in healthy asymptomatic population. METHODS: We performed retrospective cohort study in 38984 asymptomatic individuals, who underwent health screening exams including upper endoscopy and H. pylori test more than twice from 2005 to 2016 at health screening & promotion center of Asan medical center, Seoul, Korea. We excluded 413 individuals, including 286 followed-up <1 year, 106 with gastric dysplasia or cancer in initial upper endoscopy, and 21 under 18 years old. We investigated the demographic characteristics, and incidence of gastric cancer among the three groups, those without H. pylori infection (Hp negative group, n=14282, 37.0%), those with H. pylori eradication (eradicated group, n=4535, 11. 8%), and those without H. pylori eradication (non-eradicated group, n=19754, 51.2 %). RESULTS: Overall, 135 (0.3%) individuals had gastric cancers during a median 6.4 years (range, 317-4010 days). Median time to cancer occurrence was 5.5 years (range, 3173717 days). When we analyzed the incidence of gastric cancer using the Kaplan Meier method, the cumulative incidence of gastric cancer differed significantly among the three groups. There were no significant differences in the cumulative incidence of gastric cancer between the eradicated group and Hp negative group in a multivariate Cox proportional hazard model (hazard ratio (HR) 1.51; 95% CI, 0.69-3.29; p=0.302). However, the cumulative incidence of gastric cancer in the non-eradicated group was significantly higher in the Hp negative (HR 4.12; 95% CI 2.34-7.24; p<0.001) and eradicated groups (HR 2.73; 95% CI 1.50-4.98; p=0.001) in multivariate analysis. Age (HR 1.04; 95% CI 1.02-1.06; p<0.001), smoking (HR 1.74; 95% CI 1.22-2.47; p=0.002), gastric cancer family history (HR 1.80;

235 MUTATIONAL AND EPIGENETIC SPECTRUM OF COLORECTAL CANCERS IN AFRICAN AMERICANS Rosa M. Xicola, Zarko Manojlovic, Gaius J. Augustus, Sonia S. Kupfer, John Carpten, Nathan A. Ellis, Xavier Llor Background: African Americans (AAs) have the highest incidence and mortality rates for colorectal cancer (CRC) of all US populations. They present with more right-sided, microsatellite stable tumors and are diagnosed at earlier ages compared to non-Hispanic whites (NHWs). To gain insight into these trends, we conducted a comprehensive molecular analysis of AA CRCs. Methods: Exome sequence data from 45 microsatellite stable tumor-normal pairs were analyzed. In addition, we obtained copy number data from 33 and methylation data from 11 of the 45 tumors. Results were compared to data from The Cancer Genome Atlas (TCGA). We used Fisher exact and chi square tests to compare frequencies and paired ttest with permutations for differential methylation. Results: Two of the 45 tumors contained POLE mutations and they had very high mutational loads. In the remaining 43, the mutation frequency was 180 nucleotide variants per tumor, comparable to loads reported by the TCGA. Interestingly, only 27 (63%) contained loss-of-function mutations in APC compared to 80% of TCGA NHW CRCs (p=0.05). APC mutation-negative CRCs (APC-)(N=16) were

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