Short- and long-term reproducibility of the COMET assay for measuring DNA damage biomarkers in frozen blood samples of the EPIC-Heidelberg cohort.

The comet assay is widely used for quantification of genomic damage in humans. Peripheral blood derived mononuclear cells (PBMCs) are the most often used cell type for this purpose. Since the comet assay can be performed in an enhanced throughput format, it can be applied to large sample collections such as biobanks. The European Prospective Investigation into Cancer and Nutrition (EPIC) study is one of the largest existing prospective cohort studies, and the German Cancer Research Institute (DKFZ) in Heidelberg is a participating center with 25.000 frozen blood samples stored from around 25 years ago, enabling retrospective assessment of disease risk factors. However, experience with decades long frozen samples in the comet assay is so far missing. In Heidelberg, 800 study participants were re-invited twice between 2010 and 2012 to donate further blood samples. Here, we analyzed 299 Heidelberg-EPIC samples, compiled from frozen PBMC and buffy coat preparations selected from the different sampling time points. In addition, 47 frozen PBMC samples from morbidly obese individuals were included. For buffy coat samples, we observed a poor correlation between DNA damage in the same donors assessed at two sampling time points. Additionally, no correlation between DNA damage in buffy coat samples and PBMCs was found. For PBMCs, a good correlation was observed between samples of the same donors at the two time points. DNA damage was not affected by age and smoking status, but high BMI (>30; obesity) was associated with increased DNA damage in PBMCs. There was no indication for a threshold of a certain BMI for increased DNA damage. In conclusion, while 25 year-long stored buffy coat preparations may require adaptation of certain experimental parameters such as cell density and electrophoresis conditions, frozen PBMC biobank samples can be analyzed in the comet assay even after a decade of storage.

Effect of cryopreservation on DNA damage and DNA repair activity in human blood samples in the comet assay.

The comet assay is a commonly used method to determine DNA damage and repair activity in many types of samples. In recent years, the use of the comet assay in human biomonitoring became highly attractive due to its various modified versions, which may be useful to determine individual susceptibility in blood samples. However, in human biomonitoring studies, working with large sample numbers that are acquired over an extended time period requires some additional considerations. One of the most important issues is the storage of samples and its effect on the outcome of the comet assay. Another important question is the suitability of different blood preparations. In this study, we analysed the effect of cryopreservation on DNA damage and repair activity in human blood samples. In addition, we investigated the suitability of different blood preparations. The alkaline and FPG as well as two different types of repair comet assay and an in vitro hydrogen peroxide challenge were applied. Our results confirmed that cryopreserved blood preparations are suitable for investigating DNA damage in the alkaline and FPG comet assay in whole blood, buffy coat and PBMCs. Ex vivo hydrogen peroxide challenge yielded its optimal effect in isolated PBMCs. The utilised repair comet assay with either UVC or hydrogen peroxide-induced lesions and an aphidicolin block worked well in fresh PBMCs. Cryopreserved PBMCs could not be used immediately after thawing. However, a 16-h recovery with or without mitotic stimulation enabled the application of the repair comet assay, albeit only in a surviving cell fraction.