The second derivative of the dose-time-response surface as a measure of DNA repair kinetics in the comet assay

The comet assay is a sensitive and rapid method for detecting DNA damage and repair of damage in individual cells. It has been widely used in genetic toxicology, environmental biomonitoring, molecular and human epidemiology and clinical investigations, since it was first introduced by Ostling and Johanson in 1984 and independently modified by Singh et al. and Olive et al. There are still several issues that have to be resolved before the comet assay is accepted as a standard assay for detecting DNA damage/repair in a single cell. One of the major issues is the proper quantification of DNA repair kinetics. This article develops a new quantitative measure of the DNA repair kinetics which is represented in the dose-time-response surface. For measuring the DNA repair kinetics, we propose to use the second derivative, 2D, of the dose-time-response surface. This approach enables us to represent with a single number the DNA repair activity of cells exposed to ionizing radiation, by combining all the information in the experiment. The computation procedure includes the application of linear regression. In our application to the assessment of radiation sensitivity of patients we considered for the response four DNA damage parameters, i.e. the tail moment, the tail length, the tail DNA and the tail inertia for constructing the dose-time-response surface. Based on the data from 25 patients we observed that the second derivatives based on the tail moment and the tail DNA showed high correlation and that the tail inertia might express a somewhat different aspect of the DNA damage/repair. Copyright (C) 2003 John Wiley Sons, Ltd.