1a. Objectives (from AD-416)
1. To define the role that availability of each of the one-carbon nutrients plays in determining the risk of common cancers and to determine the nature of the interactions between these nutrients in this regard. 2. To determine how dietary antagonists (such as alcohol) of one-carbon nutrients further modify the effects of these nutrients in determining cancer risk. 3. To determine how genetic make-up, and other select factors, interact with intake of these nutrients in determining cancer risk.
1b. Approach (from AD-416)
Appropriate alterations in dietary and nutritional habits have an important role to play in cancer prevention. The nutrients involved in one-carbon metabolism (methionine, choline and the B-vitamins, folate, B12, B6, and B2) have drawn considerable attention in this regard and are the focus of this laboratory. Our mission is to examine the complex roles that these nutrients play in modifying metabolic and genetic pathways that lead to human carcinogenesis and thereby define the means by which cancers can be nutritionally prevented. The program of research emphasizes how these dietary compounds interact with genetic background to modify molecular and signaling pathways which alter the development of cancers and to examine how other exogenous factors, such as alcohol consumption also play a role. The laboratory focuses on colorectal cancer and breast cancer and utilizes animal studies, cell culture studies, and human studies to accomplish our research goals.
3. Progress Report
We have projects designed to determine which of the B-Vitamins, and related nutrients, modify cancer risk, what underlying cellular pathways mediate these effects, and how factors such as genetic background, age, and alcohol consumption further modulate these relationships. Our studies have included those conducted in two genetically engineered strains of mice, the first of which is predisposed to develop intestinal tumors and the second strain which enables us to monitor activity of a pro-cancerous biochemical pathway in the cell (the Wnt signaling pathway). We succeeded in demonstrating that a mild degree of dietary depletion of B vitamins accelerates tumorigenesis and that the Wnt signaling cascade is concurrently upregulated in the colon. Other animal studies have moved ahead as well. We have progressed in characterizing the biochemical and molecular characteristics of the mouse that has a genetic deletion of a pivotal gene in folate, which will be used to examine the mechanism by which a common variant in the corresponding human gene modifies cancer risk. We have shown that enzyme activity is impaired in the intestine but not in the liver, which was the goal in creating this animal model. We have continued to generate preliminary observations in a unique mouse model of colon carcinogenesis in order to define the paradoxical cancer-promoting effect of excessive folic acid administration. Lastly, we have pursued experiments that are designed to examine whether maternal intake of B-vitamins during pregnancy impacts intestinal tumorigenesis in the offspring and we have demonstrated that it requires supplemental levels of these nutrients to fully suppress tumorigenesis in the offspring. We continue to conduct clinical studies as well. We have completed two collaborative studies in which ten volunteers took folic acid supplements for two months, and ten more were subjected to a folate depletion diet for two months. At baseline and at the 1 and 2 month time points we took colonic biopsies to examine gene expression and select molecular anomalies relevant to cancer. We have completed data analysis from the supplementation study: interesting alterations in genes relevant to folate metabolism, immunity, and inflammation were observed. We have also examined breast folates in a group of 200 women undergoing routine breast reduction surgery and shown that epigenetic modification of a cancer-preventive gene in the breast inversely correlates with tissue folate levels. Another clinical study examined common gene variants in DNA repair genes that we have previously shown to be associated with abnormal DNA in several thousand women who either had breast cancer, or who were matched controls, and we found that these variants were not predictive of breast cancer. Finally, we measured tissue folate concentrations in 800 colonic biopsies from a case-control study of normal subjects and those with colonic polyps. We observed that high folate levels in the tissue are associated with a reduced risk of having those types of polyps that are most likely to evolve into cancer. For publications related to this project, see parent project #1950-51000-074-00D.
1. Higher concentrations of B-vitamins in the human breast are associated with increased activity of an anti-cancer gene ARS-funded researchers from Tufts University in Boston, MA examined some of the factors that determine how readily an anti-cancer gene called p16 is expressed in the human breast since adequate expression of this gene is critical in preventing the evolution of cancer. Higher tissue levels of the B-vitamin folate concurred with less inhibition of expression of the p16 gene. Abstinence from alcohol, prior pregnancy, and no family history of breast cancer were each predictive of higher breast folate levels. This provides insight into how adequate amounts of folate in the diet might convey protection against breast cancer.
2. Providing mother mice with supplemental levels of 1-carbon vitamins during pregnancy suppresses the risk of developing intestinal cancer in the offspring. ARS-funded researchers from Tufts University in Boston, MA conducted a study in mice that demonstrated that maternal intake of B-vitamins during pregnancy is an important determinant of the risk of developing intestinal cancer in offspring. They demonstrated that it required levels of B-vitamin intake that far exceeded the basal requirement of the adult mouse to optimally suppress tumorigenesis in the offspring. These observations should assist in the future design of optimal diets during pregnancy.
3. Common variants in human genes that determine blood uracil levels do not predict breast cancer risk. ARS-funded researchers from Tufts University in Boston, MA have previously shown that some common variants in genes that are involved in the cellular disposal of an undesirable building block of DNA, uracil, predict how much uracil is inadvertently incorporated into DNA. A large case-control study of breast cancer was therefore examined and it was found that these common variants (‘polymorphisms’) did not predict the risk of breast cancer. This was true even when folate status, which partly determines uracil levels, was taken into account. In order to efficiently prevent common cancers it is helpful to identify those in the population who have a genetically-based hazard of developing those cancers. This study showed that four gene variants that were candidate risk factors did not, in fact, predict the likelihood of breast cancer and therefore we have shown that they are not suitable indicators of cancer risk.