THE COMET ASSAY - WHAT CAN IT REALLY TELL US

A range of applications of the alkaline comet assay is covered, from i nvestigations of the physicochemical behaviour of DNA, through studies of cellular responses to DNA damage, to biomonitoring of human popula tions. The underlying principles of this assay are discussed, and new evidence presented which supports the concept of relaxation of superco iled loops, rather than alkaline unwinding, as the primary reason for comet tail formation. DNA-damaging agents that do not induce strand br eaks directly can be detected when cellular repair processes convert l esions to transient strand breaks; an approach is outlined here which maximises this effect and thus widens the scope of the assay. Purified repair enzymes, applied to DNA during the course of the comet assay p rocedure, greatly increase the sensitivity and specificity of the assa y; recent developments with formamidopyrimidine glycosylase (recognisi ng 8-OH-gua and other damaged purines) and uvrABC (for bulky lesions) are presented. The kinetics of cellular repair after low doses of oxid ative damage have been followed with this modified comet assay. Finall y, the successful measurement of biomarkers of oxidative damage in hum an populations establishes the comet assay as a valuable tool in molec ular epidemiology.