Seymour Garte

Association between CYP1A1 genotype, mRNA expression and enzymatic activity in humans.

Genetic susceptibility factors may play a role in determining adverse effects of exposure to environmental toxins. As a preliminary step to a molecular epidemiological study in a population exposed to 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD), we investigated 20 healthy Caucasian volunteers with a set of putative susceptibility markers including a CYP1A1 Msp I restriction fragment length genetic polymorphism (RFLP), CYP1A1 mRNA expression, and ethoxyresorufin-O-deethylase (EROD) activity in cultured and mitogen-activated blood lymphocytes. Both basal (p = 0.008) and induced (p = 0.0001) EROD activity was significantly higher among persons with a mutation in one or both alleles of the CYP1A1 gene (variant CYP1A1 genotype). Induction in vitro by TCDD significantly increased EROD activity in both variant and wild-type CYP1A1 subjects; however, the absolute increase was greater in subjects with variant genotypes. An additive interaction between genotype and TCDD induction was suggested. Expression of CYP1A1 mRNA, both basal and induced, did not vary significantly across the genotypes.

Monitoring low benzene exposure : comparative evaluation of urinary biomarkers, influence of cigarette smoking and genetic polymorphisms

Benzene is a human carcinogen and an ubiquitous environmental pollutant. Identification of specific and sensitive biological markers is critical for the definition of exposure to low benzene level and the evaluation of the health risk posed by this exposure. This investigation compared urinary trans,trans-muconic acid (t,t-MA), S-phenylmercapturic acid, and benzene (U-benzene) as biomarkers to assess benzene exposure and evaluated the influence of smoking and the genetic polymorphisms CYP2E1 (RsaI and DraI) and NADPH quinone oxidoreductase-1 on these indices. Gas station attendants, urban policemen, bus drivers, and two groups of controls were studied (415 subjects). Median benzene exposure was 61, 22, 21, 9 and 6 μg/m3, respectively, with higher levels in workers than in controls. U-benzene, but not t,t-MA and S-phenylmercapturic acid, showed an exposure-related increase. All the biomarkers were strongly influenced by cigarette smoking, with values up to 8-fold higher in smokers compared with nonsmokers. Significant correlations of the biomarkers with each other and with urinary cotinine were found. A possible influence of genetic polymorphism of CYP2E1 (RsaI and/or DraI) on t,t-MA and U-benzene in subjects with a variant allele was found. Multiple linear regression analysis correlated the urinary markers with exposure, smoking status, and CYP2E1 (RsaI; R2 up to 0.55 for U-benzene). In conclusion, in the range of investigated benzene levels (<478 μg/m3 or <0.15 ppm), smoking may be regarded as the major source of benzene intake; among the study indices, U-benzene is the marker of choice for biomonitoring low-level occupational and environmental benzene exposure.

Effect of genotype on steady-state CYP1A1 gene expression in human peripheral lymphocytes

We have analyzed the steady-state levels of cytochrome P-450 1A1 (CYP1A1) mRNA in peripheral blood lymphocytes of 177 individuals with various CYP1A1 genotypes using a quantitative reverse transcriptase-polymerase chain reaction technique that makes use of a homologous internal standard for accurate quantitation. We found no effects of ethnicity, age, or smoking status on CYP1A1 gene expression in this population. We did see a significant 2-fold increase in the mean level of CYP1A1 mRNA in women compared with men for both Caucasians and African Americans. We observed no effect of the African American-specific polymorphism (CYP1A1(*)3) on expression of the gene. However, we found a significant 3-fold decrease in expression associated with the homozygous MspI restriction fragment length polymorphism (CYP1A1(*)2A/(*)2A).

Deletion of parental GST genes as a possible susceptibility factor in the etiology of infant leukemia.

Infant leukemia below the age of 12 months is a rare disease that exhibits a high frequency of 11q23 rearrangements. We assessed the presence of polymorphisms in several metabolic genes in 23 families of infants diagnosed with leukemia under 12 months of age in Italy. When polymorphism frequencies were calculated within families, frequencies of GST gene deletions were significantly higher than expected only among the parents of infants without the 11q23 rearrangement. These data suggest that the deletion of GST genes in parents may affect the risk of infant leukemia through a pathway independent of the MLL gene.

Low dose exposure to carcinogens and metabolic gene polymorphisms.

Metabolic gene polymorphisms encode for enzymes which are involved in both metabolism and conjugation of environmental as well endogenous compounds. Some of the products of the metabolic process are carcinogens. Two main categories of metabolic genes are known: Phase I genes, which include CYP1A1, CYP2E1, CYP2D6, and Phase II genes, such as GSTM1, GSTT1, NAT2. Polymorphisms have been described in these genes, with different frequencies according to ethnicity and geographic area.1 Several case-control studies have been conducted to study the association between metabolic gene polymorphisms and cancer of various sites, with special focus on lung and bladder cancer. It has been suggested that these genes play a role in cancer risk only when they interact with environmental exposure, since the substrates of their gene products are xenobiotic chemicals or their metabolites.2 This form of gene-environment interaction (GEI) has been described as “Type 2” GEI by Khoury3 and Ottman.4 According to this model, the presence or absence of the genetic risk factor is irrelevant for disease causation, if there is no exposure to an environmental agent. When the dose of environmental exposure (such as smoking) is analyzed with respect to metabolic susceptibility gene polymorphisms, two patterns are seen. In one case, a low exposure-gene (LEG) effect is observed, in which a decreasing degree of interaction occurs as a function of increasing exposure dose. A high exposure-gene (HEG) effect is observed when there is an increased degree of interaction as a function of exposure dose.2

Metabolic Susceptibility Genes — Phase 1

When one speaks of metabolism in the context of biochemical toxicology, one means a quite different thing than the normal definition of metabolism in the context of the cellular processes of energy production and use. Normal cellular metabolism is used to produce energy or cellular constituents, and generally the substrates for normal metabolic reactions are food molecules. In contrast, the metabolism of toxicants which use toxic materials as their substrates do not produce energy, but instead usually require the input of significant energy. They do not produce anything except safety for the cell.

Major Cancer Genes

The role of major cancer genes in human cancer is vast subject that has been the theme of a multitude of reviews and books. The best most recent volume dealing with this topic is Vogelstein and Kinzler (1998). The subject is difficult as it is, and problems with inconsistent nomenclature only compound the amount of confusion related to the field. In this chapter, and the rest of the book, the standardized convention for naming genes and the proteins they code for is followed: genes are written in lower case and italicized, while their protein products are written with an initial capital (eg. the c-myc oncogene codes for the Myc protein). There are of course several exceptions to this rule, depending on usage in the literature, and in these cases the common usage will be given.

Gene-Environment Interaction

One of the most profound topics for experimental and theoretical research into carcinogenesis and other human diseases, has been given the term gene-environment interaction (GEI). Of course one of the themes that has been repeatedly stressed in this book is the fact that environmental and genetic factors frequently interact and that disease occurrence is usually a product of such interactions. But it is one thing to say that both genes and environmental agents are important as causative agents, and another to say that such factors must undergo some sort of definitive interactive process in the causative mechanisms. Once we accept that gene environment interaction takes place, and that this interaction may often be more than simply the simultaneous presence of different risk factors, than we must address the question of how these interactions occur, both qualitatively and quantitatively.

Mechanisms of Carcinogenesis

In order to understand the mechanisms of carcinogenesis, research is required in many fields such as chemistry, pathology, biochemistry, genetics and molecular biology. While a great deal about carcinogenesis has been learned, we also know that the process is enormously complex, occurs by means of different pathways, and will require considerably more efforts to completely be unraveled.

Principles of Inheritance

Genetics as a field of science is worth years of study to obtain a minimum comprehension. In modern times, with the technical advances of molecular biology, genetics has become one of the most challenging and difficult fields of biology to fully master. Clearly then, no attempt will be made here to be thorough or comprehensive. Fortunately, as it happens, the main discoveries resulting from the application of new techniques to genetic research have served mainly to confirm (and greatly extend) the general theoretical conclusions reached by the great thinkers of many decades ago, although it is true that many of the details have been quite unexpected. To understand the basic ideas behind inheritance of genetic susceptibility to cancer therefore, one needs only to understand some of these basic ideas.