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The activity of cytochrome P4501A2 (CYP1A2) may be an important determinant of risk for certain cancers (1, 2). Phenotyping procedures such as the caffeine breath test (CBT) have advantages over genotyping and have been used to characterize CYP1A2 activity in epidemiologic studies (1). The large scale use of these methods, however, is limited by several factors including high cost, a complicated analytical procedure, and interference by dietary caffeine (1, 3).
CYP1A2 metabolizes certain polychlorinated biphenyls (PCBs, ubiquitous environmental contaminants) such as PCB118 and PCB105, but not others, such as PCB153 (4). Ayotte et al. (5) recently reported that the serum concentration ratio of PCB105/PCB153, and that for PCB118/PCB153 significantly decreased with CYP1A2 activity, measured using a CBT among 20 subjects. They hypothesized that these ratios maybe useful surrogate biomarkers of CYP1A2 activity (6); in this paper, we tested their hypothesis using data from a much larger study (7).
The study population consisted of Mohawk men and women from the Mohawk Nation at Akwesasne, a Native American community along the St. Lawrence River in New York, Ontario, and Quebec (7). Briefly, of the 111 Mohawk women who became pregnant between 1 April 1992 and 31 March 1995 and of the 139 Mohawk men who were husbands or close relatives of the women, 172 agreed to undergo a CBT. After exclusion of participants who had a history of heart disease, stroke, seizure disorders, uncontrolled hypertension, arrhythmia, hepatitis, jaundice or other types of liver disease, adverse reaction to caffeine, chemotherapy within the past 5 years, currently taking prescription medications (not including oral contraceptives) or who were currently breast-feeding, the CBT was administered to 103 persons. Information about demographic characteristics, height and weight, diet, and use of medications, alcohol, caffeine, and cigarettes was collected. Each participant signed an informed consent form.
A 20 mL non-fasting blood specimen was used for serum PCB analysis. Sixty-eight PCB congeners were determined with gas chromatography with electron-capture detection (7). The limits of detection (LOD) were 0.02 ng/g wet weight for PCB153, 0.01 ng/g for PCB118 and PCB 105.
CYP1A2 activity was measured with a CBT (7). Labeled caffeine (3-13C-methyl) was given in a dose of 3 mg/kg, up to a maximum of 200 mg. A 20-mL breath sample of expired air was collected immediately before and 30- and 60-min after ingestion of the labeled caffeine. After cryogenic purification of the CO2 present in the exhaled air samples, the 13CO2:12CO2 ratio was determined by differential gas-isotope ratio mass spectrometry (8). The excess 13C was calculated from the ratio found in the breath sample before and after ingestion of the substrate and expressed as the dose exhaled per hour.
When the level of a given PCB congener was below the LOD, half of the LOD value for that congener was imputed. PCB153 serum concentrations (pg/g wet weight) were divided by PCB118 or PCB105 to get ratios (PCB153/PCB118 and PCB153/PCB105) predicted to be directly proportional to CYP1A2 activity. Spearman correlations between CBT level and ratios were calculated. Multiple linear regression models of log10 transformed CBT were fitted, with adjustment for the potential confounders age, gender, body mass index (BMI), smoking, and usual consumption of alcohol and of caffeine.
While PCB 153 was detectable in the serum of 100 (97%) participants, detection frequencies were lower for PCB118 (33%) and PCB105 (10%). Unadjusted and lipid-adjusted median concentrations of the 3 PCB congeners (Table 1) were similar to levels in the United States general population (9). Given the lower proportion detected for PCB105, the ratio of PCB153/PCB105 was much higher than that for PCB153/PCB118.
CBT level was correlated with PCB153/PCB105 (r=0.22, p=0.02, n = 103), but not PCB153/PCB118 (r=0.12, p=0.22, n = 103). A stronger correlation was found between CBT and PCB153 (r=0.34, p=0.001) than with PCB118 (r=0.14, p=0.27) or PCB105 (r=0.13, p=0.20). In the regression models (Table 2), associations were not found between CBT level and PCB congener ratios. Current smoking and male sex, however, were associated with higher CBT values (p<0.05).
While Ayotte et al. (5) found Spearman correlations between CBT and PCB153/PCB118 (r=0.53, p=0.02) and PCB153/PCB105 (r=0.62, p=0.003), we found smaller associations in this larger study (r=0.12 and r=0.22, respectively). Ayotte et al. (5) found that PCB118 and PCB105 concentrations were inversely correlated with CBT levels, whereas Fitzgerald et al. (7) did not. We also note that Fitzgerald et al. (7) found a direct association between PCB153 and CBT, while Ayotte et al. (5) did not.
The inconsistent findings between the present study and the Ayotte et al. (5) study may be explained by differences in sample size, PCB exposure level, and race. The size of the Ayotte et al. (5) study may not have given enough statistical power to detect an association between CBT and PCB153, which may have relatively low potential to induce CYP1A2 and is little or not metabolized by CYP1A2. The subjects in the Ayotte et al. (5) study were more highly exposed (at least 15 fold higher than those in the present study) to PCBs through fish consumption. That the associations between CBT and PCB118 and PCB105 were found by Ayotte et al. (5) but not by Fitzgerald et al. (7) suggest that PCB118 and PCB105 can induce the CYP1A2 enzyme involved in their own metabolism only when present at higher levels or that the levels of these PCBs were so low in the present study that measuring the full extent of variability was not possible. In addition, the subjects in the Ayotte et al. (5) study were Caucasians and they may have higher CYP1A2 activity than do Asians and Native Americans (10). Furthermore, CYP1A2 activity may be more easily induced in Caucasians than in other populations such as African-Americans, Chinese, and we speculate, Native Americans (1).
In summary, PCB ratios were not correlated with CYP1A2 activity in this population. For subjects with greater PCB body burdens or different genotypes, however, the utility of PCB ratios as surrogate biomarker of CYP1A2 activity remains to be determined.
We thank the study participants, the Akwesasne Task Force on the Environment, and the following persons for their past and present help: A. Casey, M. Cayo, A. Jacobs, K. Jock, B. LaFrance, K. Langguth, T. Lauzon, F.H. Lickers, and P. Worswick. This work was supported in part by the National Institute of Environmental Health Sciences (grants 11256 and 2P42-ES04913), Agency for Toxic Substances and Disease Registry (grant H75/ATH298312), and the U.S. Environmental Protection Agency (grant 829391). This study was also supported in part by the Intramural Research Program of the National Institutes of Health, National Institute of Environmental Health Sciences