Many anticipate that application of findings in molecular genetics will help to achieve greater precision in defining high-risk populations that may benefit from chemopreventive interventions. We must recognize, however, that
genetic susceptibility, environmental factors, and complex gene-environment interactions are all likely to be risk determinants for most
cancers. Cohort studies of twins and
cancer indicate that having "identical" genes is generally not a very accurate predictor of
cancer incidence. Data from twin studies support the suggestion that environmental factors such as tobacco use significantly influence
cancer risk. The complexities of the genetic contribution to disease risk are exemplified by the development of
Duchenne muscular dystrophy in only one of monozygotic twin girls, hypothesized to be the result of X chromosome inactivation, with the distribution patterns of the X chromosome being skewed to the female X in the manifesting twin and to the male X in the normal twin. Evidence from transgenic and genetic-environmental studies in animals support the possibility of genetic-environmental interactions. Calorie restriction modifies
tumor expression in p53 knockout mice; a high-fat, low-
calcium, low-
vitamin D diet increases prepolyp
hyperplasia formation in Apc-mutated mice; and calorie restriction early in life influences development of
obesity in the genetically obese Zucker rat (fafa). Such environmental modulation of gene expression suggests that
chemoprevention has the potential to reduce risk for both environmentally and genetically determined
cancers. In view of the growing research efforts in
chemoprevention, the NCI has developed a Prevention Trials Decision Network (PTDN) to formalize the evaluation and approval process for large-scale
chemoprevention trials. The PTDN addresses large trial prioritization and the associated issues of minority recruitment and retention; identification and validation of
biomarkers as intermediate endpoints for
cancer; and chemopreventive agent selection and development. A comprehensive database is being established to support the PTDN's decision-making process and will help to determine which agents investigated in preclinical and early phase clinical trials should move to large-scale testing. Cohorts for large-scale
chemoprevention trials include individuals who are determined to be at high risk as a result of
genetic predisposition, carcinogenic exposure, or the presence of
biomarkers indicative of increased risk. Current large-scale trials in well-defined, high-risk populations include the
Breast Cancer Prevention Trial (
tamoxifen), the
Prostate Cancer Prevention Trial (
finasteride), and the N-(4-hydroxyphenyl)
retinamide (4-HPR)
breast cancer prevention study being conducted in Milan.
Biomarker studies will provide valuable information for refining the design and facilitating the implementation of future large-scale trials. For example, potential
biomarkers are being assessed at biopsy in women with
ductal carcinoma in situ (
DCIS). The women are then randomized to either placebo,
tamoxifen,
4-HPR, or
tamoxifen plus
4-HPR for 2-4 weeks, at which time surgery is performed and the
biomarkers reassessed to determine
biomarker modulation by the interventions. For
prostate cancer, modulation of
prostatic intraepithelial neoplasia (PIN) by
4-HPR and
difluoromethylornithine is being investigated; similar studies are being planned for
oltipraz,
dehydroepiandrosterone, and
vitamin E plus
selenomethionine. The validation of
biomarkers as
surrogate endpoints for
cancer incidence in high-risk cohorts will allow more agents to be evaluated in shorter studies that use fewer subjects to achieve the desired statistical power.