Mutation and killing

One possibility is that there is differential susceptibility to both mutation and killing among the cells. If, for example, there are two populations of cells, the high-dose data reflect the mutation rate of the component more resistant to killing. If these cells are also resistant to mutation, the resistance of the total cell population is underestimated. Another possibility is that there is a mutation-repair mechanism that is turned on at high doses. Deviations from either of these causes can usually be detected by the shape and slope of the curve at high doses for example, linear extrapolation from high-dose... 

If the relevant physical lesions are formed in direct proportion to dose, and if no dose-dependent processes are involved in the conversion of initial lesions to biological hits, then the hit function will be linear. More generally, H x) could be some more complex function that, nonetheless, can always be represented by an infinite power series in x with no constant term (since there can be no induced hits for zero dose). We denote the expected number of lethal and mutational hits at dose x by Hk x) and Hm(x), respectively. Thus, on the basis of single-event Poisson statistics, population homogeneity, and stochastic independence of mutation and killing, we can write... 

In practice, the kinetic pattern can sometimes be guessed from the shapes of S x) and M x) for the dose range studied. Thus, the experimentally determined value of x can be substituted directly into the inferred form of K(a ) to see whether the observed and predicted yield maxima agree. Any marked disagreement between the observed and predicted yield maxima would mean either that the kinetic pattern was not inferred correctly because the dose range studied was too limited or that the assumptions of population homogeneity and/or stochastic independence of mutation and killing do not hold for the system under consideration. 

On the basis of the definition of mutation frequency given in equation (6), nonlinearities in frequency curves would be expected to arise only from the existence of multilesion mutation-induction processes. However, nonlinearities in mutation-frequency curves can arise if mutation and killing are not stochastically independent processes as assumed in Section 3 and in writing equation (6). We can generalize the formalism of Section 3 and 4 to allow for stochastic dependence of mutation and killing in the following way ... 

Stochastic independence of mutation and killing implies that 5(a ) = 1. For linear hit functions in equation (52), 8(x) = kim/ki = 6, a constant. Even for nonlinear hit functions, we would expect, on general radiobiological grounds, that 8 x) would be at most a slowly varying function of x that could be approximated adequately by a constant 8. 

FIGURE 8. Effect of stochastic dependence of mutation and killing (6 1) superimposed on a basically linear response pattern Lk,Lm). Mutation-frequency curves calculated from equation (55) for mi = 10 and Ai = 0.1 (ergs/mm ) for various values of h. The curves for 6 > 1 lie below those for 6 < 1 merely because a single, fixed value of mi was used to calculate all curves. 

The linkage of formal description and mechanistic model consists in working out the relationships between the formal coefficients of equations (7) and (8) and whatever parameters are introduced to define explicitly the functions on the right-hand side of equations (61) and (62). In establishing this link, it is immaterial whether we introduce the assumption of stochastic independence of mutation and killing if we consider that 5(x) in equation (52) can be approximated by a constant 5. 

Real cell populations are often heterogeneous and may consist of two or more subpopulations that differ in mutability and sensitivity. The formal expressions presented here could be applied to each subpopulation and computer methods then used to work out the possible total population responses. Clearly, it would be desirable to obtain data in synchronized systems (which should eliminate one prominent source of population heterogeneity that exists in many systems), and also in populations deliberately rendered heterogeneous by mixing strains with different sensitivities to mutation and killing. 

Mutation-induction data in unicellular systems can be described mathematically within the framework of single-event Poisson statistics. This formal description can be linked to various mechanistic models for mutation and killing. Such mathematical links between formalism and mechanism enable one to make use of the quantitative details of the dose-response relationships in drawing general inferences regarding the macromolecular processes involved in mutation and killing. 

A variety of mammalian cell culture systems can be used to detect mutations induced by chemical substances. The L5178Y mouse l)nnphoma line, measuring mutation at the TK locus, is preferred. TK is an important enz)une involved in DNA synthesis. Cells are exposed to the test substance at various concentrations, in the presence and absence of a metabolic activation system, for a suitable period of time, and then subcultured to assess cytotoxicity and to allow phenotypic expression prior to mutant selection. Cells deficient in TK because of a forward mutation are resistant to the cytotoxic effects of pyrimidine analogues (antimetabolites), such as trifluorothymidine (TFT). This is because the antimetabolites cannot be incorporated into cellular nucleotides and kill the cell through inhibition of cellular metabolism. After treatment, cells are grown in a medium containing TFT mutant cells can proliferate in the presence of TFT, whereas normal cells containing TK are killed. This allows the detection of an increase in mutant... 

HIV-specific CD8+ function may result from continued mutation and high levels of viral turnover, which gradually diminishes the capability of the cell to recognize the genetic sequences of the virus due to killing of some of the CD8 T cells repertoire by HIV itself. 

Stricharts Is it clear that the cells that express the mutation are the ones that are dying Are they releasing high concentrations of glutamate locally and killing the cells that are responding ... 

A.A. van Zeeland, A.T. Natarajan, E.A.M. Verdegaal-Immerzeel, A.R. Filon (1980). Photoreactivation of UV-induced cell killing, chromosome aberrations, sister chromatid exchanges, mutations and pyrimidine dimers in Xenopus laevis fibroblasts. Mol. Gen. Gen., 180,495-500. 

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