On time dependency of frequency and distribution of chromatid aberrations induced by mutagens with delayed effects. An interpretation

Abstract

This paper provides a theoretical analysis of pecularities of both the frequency and intrachromosomal distribution of chromatid aberrations observed in the first post-treatment mitosis and induced by clastogenic agents showing delayed effects ( S-phase dependent clastogens), as functions of recovery time. The theoretical deductions are based on the following facts: (1) DNA is the target of clastogen action. Lesions induced by clastogens showing delayed effects (e.g. mono- and polyfunctional alkylating agents, ultraviolet light) give rise to aberrations only after interference with the process(es) associated with DNA replication. (2) DNA replication occurs asynchronously with respect to the local involvement in replication of different chromatin regions and according to a highly ordered pattern. (3) Lesions may be removed from DNA (or otherwise modified) by repair processes prior to replication. The removal of lesions from DNA is a time-dependent function. Several possibilities are analysed (i.e. random or non-random distribution of DNA lesions, uniform or locally differing capacities of pre-replicative repair of lesions, uniform or locally differing rates of DNA synthesis) and the frequencies and distribution patterns of chromosome structural changes, as expressed in form of aberration yield-time curves, have been discussed. The theory presented in this paper offers a simple interpretation both of variations of aberration frequency and aberration distribution in dependence on the cell's position within the cell cycle during induction of lesions. It is shown that the intrachromosomal aberration distribution is non-random even if random distribution of lesions and uniform repair rates between chromosome regions replicating at different time periods during S are assumed. Non-random aberration distributions are a necessary consequence of at least two factors: (a) the temporal replication pattern, and (b) the repair activities acting prior to replication. Random distribution of aberrations is only to be expected for the most simplified situation (random distribution of lesions along the DNA and equal transformation probabilities of a given kind of lesion into aberrations) when no loss of lesions prior to replication takes place (no pre-replicative repair) and cells treated with the mutagen during G1 are analysed.