The comet assay for human biomonitoring: effect of cryopreservation on DNA damage in different blood cell preparations

This study was designed within the frame of the COST Action hCOMET 15132 (Working Group 6), with the aim of comparing different peripheral blood cell preparations for their feasibility in human biomonitoring studies, using the comet assay for the evaluation of DNA damage. Basal levels of strand breaks/ALS and formamidopyrimidine DNA glycosylase (Fpg) - sites, and H₂O₂ (500 μM)-induced strand breaks, were measured in whole blood, peripheral blood mononuclear cells - lymphocytes and monocytes - and buffy coat; in fresh and 1, 4 and 12 weeks-frozen samples. The comparison among the fresh preparations showed that the basal levels of DNA damage were all very low and similar in the three samples. Frozen whole blood samples stored in cryostraws without cryoprotection showed similar basal levels of DNA damage as fresh samples, indicating that this preparation, often chosen for biobanks, resists efficiently freezing/thawing artifacts. However, long-term storage of frozen buffy coat samples in cryostraws and with no cryopreservative did not appear feasible. Storage up to 3 months of frozen cryoprotected peripheral blood mononuclear cells induced small increases in basal strand breaks and no other statistically significant modification. Altogether, this study suggests that whole blood could be the most suitable sample to be used to perform comet assay in human epidemiological biomonitoring for genotoxicity assessment in frozen samples, such as those stored in biobanks.