There is evidence that variation in the detection, signaling, and repair of DNA damage (from internal and external exposures and intrinsic instability of DNA) contributes to human cancer risk. To estimate steady state capacity of base excision repair to rectify DNA damage, we quantified single strand breaks and abasic sites in DNA of untreated cells using the alkaline Comet Assay. We assayed exponentially growing lymphoblastoid cell lines derived from a cohort of radiologic technologists who had been diagnosed with breast cancer plus another malignancy (breast-plus; n=21), early onset breast cancer (EOB, age < 35; n=38), thyroid cancer (n=68), or were cancer-free (n=47). Coefficients of variation for 21 blinded replicates ranged from 4 to 26%. Komet4(TM) software provided measures of Comet tail length, % DNA in tail (tail DNA) and Comet distributed moment (CDM). Individual parameters were summarized as the geometric mean of 100 cells. There was no effect on Comet values for survival time in cases (median time since diagnosis: 16 yrs, range 2-46 yrs) or attained age in controls. Logistic regression was used to calculate odds ratios (OR) and 95% Confidence Intervals (CI), adjusted for age at blood draw and cell viability in culture, with category cut-points determined from the control distribution. Trend tests were based on the underlying continuous values. Comet tail length (above vs. below the 75th percentile) was associated with large increased risks for the breast-plus (OR=68, CI 7.1-13), EOB (OR=5.0, CI 1.7-15), and thyroid cancer (OR=13.1, CI 4.8-36) groups. For tail DNA and CDM (in tertiles), risks among breast-plus increased with increasing DNA damage: third vs. the first tertile for tail DNA, OR=5.7 (CI, 1.1-31.0; p-trend, 0.02) and third vs. the first tertile for CDM, OR=3.4 (CI, 0.7-15.4; p-trend, 0.15). The results for EOB were mixed with no clear patterns. Thyroid cancer risks were slightly elevated, with ORs for the third vs. first tertiles of tail DNA and CDM of 1.7 (CI, 0.6-4.5; p-trend 0.24) and 1.6 (CI, 0.6-4.4; p-trend 0.40), respectively. The pattern of results suggests that reduced capacity to repair DNA damage is a risk factor for some cancers in this cohort. Comet tail length, indicating the density of abasic sites and strand breaks, showed the strongest association with cancer among the three DNA damage parameters evaluated. Future studies will evaluate the inter-relationships with occupational radiation exposure, cancer therapy, DNA damage values and cancer risk. In addition, we plan to evaluate these markers as a screen for the functional impact of polymorphisms in DNA repair and relevant gene pathways. Work in part performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]