Susceptibility and biomarker knowledge for improvement of environmental health

Use of biomarkers to elucidate genetic susceptibility to cancer

Genetic variations have generally been accepted to be the major mechanism for the differences observed in susceptibility to cancer. However, extensive investigations on genotype-disease associations have not produced consistent results. The inconsistency may be caused by many factors, such as improper study design, insufficient sample size, complexity of the traits under investigation, heterogeneity of the study subjects, incorrect assumptions about the underlying genetic architecture, misclassification of the disease, improper selection of potential alleles, and overinterpretation of the data. Besides these "traditional" factors, a recent problem is that the function of many variant genotypes is unknown, especially with regard to polymorphic DNA repair genes. Therefore, in addition to the genotype-disease relationship, it may be prudent to step back and investigate the fundamental role of genetic variation in the development of cancer. The latter studies may focus on understanding the genotype-exposure interactions and evaluating genotype-health risk associations. This review will emphasize the use of relevant biomarkers and polymorphic DNA repair genes for investigations. These data will be useful for a better understanding of the complexity of disease causation and development, developing new models for human disease, and identifying pathways for prevention of cancer.

Use of biomarkers to characterize functions of polymorphic DNA repair genotypes

Inheritance of variant DNA repair genes is believed to influence individual susceptibility to the development of environmental cancer. However, the validity of the belief is dependent upon understanding the functions of the variant genes. Consequently, a variety of studies have been conducted to investigate the functions of variant DNA repair genes, e.g. using biomarkers. These studies on several representative polymorphic DNA repair genes are reviewed in this report. From a general overview, it appears that the biomarker investigations did not provide consistent observations. However, from a more careful evaluation, it is clear that the inconsistencies are probably caused by the use of populations and biomarkers that are not appropriate for investigating the repair activities of the genes. For example, the use of cigarette smokers and patients may not generate precise information for this type of investigations because these conditions can modify the functions of the investigated genes. Thus, the use of healthy non-smokers would be more appropriate. Other problems with these studies includes the small sample size used and the fact that some of the biomarkers used, such as sister chromatid exchanges, are not appropriate because the mechanisms for formation of the biomarkers and their biological significance are unknown. Nevertheless, the following conclusions can be derived from the review of the various biomarker studies that have been published. XRCC1 194Trp, OGG1 326Cys and APE1 148Glu probably have limited alterations in repair activities compared to the wild-type genotypes. XRCC1 399Gln and XRCC3 241Met are deficient in the repair of X-ray-, but not UV-light-induced chromosome aberrations, therefore the variant genes are defective in base excision repair. XPD 312Asn and XPD 751Gln are deficient in the repair of UV-light- but not X-ray-induced chromosome aberrations, therefore they are defective in nucleotide excision repair.