Despite a reduction in both incidence and mortality of colorectal cancer, partially due to rapidly increased use of endoscopy, colorectal cancer (CRC) still remains the 4th most common incident cancer and the 2nd most common cause of cancer death in the US. Thus, it is critical to develop new primary prevention strategies. High consumption of calcium (Ca) has been linked to reduced risks of colorectal adenoma and CRC; however, results have been inconsistent. NKCC2, encoded by SLC12A1, is the direct downstream effector of ROMK encoded by KCNJ1. Both NKCC2 and ROMK serve as driving forces for reabsorption of Ca and Mg. Homozygous rare mutations in SLC12A1 and KCNJ1 cause type I and type II hyperprostaglandin E syndrome, respectively, both of which are characterized by marked hypercalcinuria (calcium wasting) and hyperprostaglandin E2 (high levels of PGE2). Over the past 5 years, in an NIH R01 project (AT004660, PI: Dai), we have examined single nucleotide polymorphisms (SNPs) in SLC12A1,and KCNJ1 and 12 other candidate genes which are critically involved in Ca and Mg (re)absorption and regulation, for interaction with intakes of Ca, Mg or Ca/Mg ratio in relation to colorectal adenoma risk. In this two-phase study, we identified and replicated 2 SNPs in KCNJ1 and SLC12A1 that significantly interacted with Ca intake in relation to colorectal adenoma risk, particularly multiple/advanced adenoma. In joint analysis of 2 genes, we observed highest Ca intake was not associated with risk among those with no variant alleles in 2 genes, but was significantly related to 39% and 69% reduced adenoma risk, among those who carry variant allele(s) in 1 and 2 genes, respectively. The corresponding reduction in risk with advanced or multiple adenomas was 89% among those with variant alleles in both genes. We also found similar findings in a third independent set of hyperplastic polyp cases vs. controls. We expanded to 10 more candidate genes and identified and replicated 1 SNP in SLC8A1 interacting with Ca in relation to adenoma risk. 38% of the US population have at least 1 variant allele in any 2 of 3 genes, among whom high Ca intake was associated with a 70% reduced adenoma risk. These findings are novel and promising. However, 3 SNPs in the Ca-gene interactions are non-functional tags. Also, we only examined tagging SNPs with a minor allele frequencye 5%, whereas rare mutations in these genes are causally linked to diseases with Ca homeostasis dysfunction. Furthermore, it is unknown if these interactions are associated with incident CRC or adenoma recurrence or confer additional protection from CRC in individuals receiving endoscopic screenings. The proposed study based on the unique resources available from PLCO will prospectively address these important questions. We propose to conduct targeted deep resequencing in these 3 genes to examine functional and rare SNPs and their interactions with Ca intake. In addition, we will conduct a molecular epidemiologic study to prospectively understand the mechanisms underlying these Ca-gene interactions.