Implications of delayed reproductive cell death (lethal mutations/genomic instability) for the interpretation of tissue responses

Genomic instability: potential contributions to tumour and normal tissue response, and second tumours, after radiotherapy

PURPOSE

Induced genomic instability generally refers to a type of damage which is transmissible down cell generations, and which results in a persistently enhanced frequency of de novo mutations, chromosomal abnormalities or lethality in a significant fraction of the descendant cell population. The potential contribution of induced genomic instability to tumour and normal tissue response, and second tumours, after radiotherapy, is explored.

RESULTS

The phenomenon of spontaneous genomic instability is well known in some rare genetic diseases (e.g. Gorlin's syndrome), and there is evidence in such cases that it can lead to a greater propensity for carcinogenesis (with shortened latency) which is enhanced after irradiation. It is unclear what role induced genomic instability plays in the response of normal individuals, but persistent chromosomal instability has been detected in vivo in lymphocytes and keratinocytes from irradiated normal individuals. Such induced genomic instability might play some role in tumour response in a subset of tumours with specific defects in damage response genes, but again its contribution to radiocurability in the majority of cancer patients is unclear. In normal tissues, genomic instability induced in wild-type cells leading to delayed cell death might contribute to more severe or prolonged early reactions as a consequence of increased cell loss, a longer time required for recovery, and greater residual injury. In tumours, induced genomic instability reflected in delayed reductions in clonogenic capacity might contribute to the radiosensitivity of primary tumours, and also to a lower incidence, longer latency and slower growth rate of recurrences and metastases.

CONCLUSIONS

The evidence which is reviewed shows that there is little information at present to support these propositions, but what exists is consistent with their expectations. Also, it is not yet clear to what extent mutations associated with genomic instability, particularly gene polymorphisms, or other low penetrant gene mutations, contribute to the recognized spectrum of normal tissue radiosensitivity amongst cancer patients, or in the general population. Tests for such genetic modifications may help in the search for more accurate prognostic markers of response, which hopefully could be used in addition to other strategies to further improve the outcome for cancer patients given radiotherapy.

The radiosensitivity of recurrent clones of an irradiated murine squamous cell carcinoma in the in vitro megacolony system

PURPOSE

To measure changes in spontaneous growth rate and radiation response in the progeny of irradiated squamous cell carcinoma cells.

MATERIALS AND METHODS

Murine SCC cells of the line AT478 were grown as epithelial megacolonies in vitro, using both the original line and two subsequent passages derived from a clone that had recurred after a high radiation dose. Radiosensitivity was evaluated in terms of local control following single dose irradiation of standard size megacolonies (0.8 cm2). In addition, original megacolonies were given a priming dose of 20 Gy and the recurrent clones arising in situ were retreated at three dose levels for analysis of curability.

RESULTS

A marked increase in radiosensitivity was observed in the megacolonies grown from irradiated progeny as compared to original megacolonies, reflected in a shift of the TCD50 from 24.5 to 16.5 Gy. Direct parameter estimation from the cure data suggested that the underlying change was a lowered number of clonogenic 'stem' cells rather than increased cellular sensitivity. A similar decrease in clonogen density was also apparent for the recurrent clones in situ. The change in megacolony curability was paralleled by a substantial growth retardation.

CONCLUSION

The data demonstrate persistent changes in the progeny of irradiated SCC tumour cells that affect both growth and radiosensitivity and are compatible with the expression of delayed reproductive death.

The link between low-LET dose-response relations and the underlying kinetics of damage production/repair/misrepair

PURPOSE

To review current opinion on the production and temporal evolution of low-LET radiobiological damage.

METHODS

Standard cell survival models which model repair/misrepair kinetics in order to quantify dose-response relations and dose-protraction effects are reviewed and interrelated. Extensions of the models to endpoints other than cell survival, to multiple or compound damage processing pathways, and to stochastic intercellular damage fluctuations are surveyed. Various molecular mechanisms are considered, including double strand breaks restitution and binary misrepair.

CONCLUSIONS

(1) Linking dose-response curves to the underlying damage production/processing kinetics allows mechanistic biological interpretations of observed curve parameters. (2) Various damage processing pathways, with different kinetics, occur. (3) Almost every current kinetic model, whether based on binary misrepair or saturable repair, leads at low or intermediate doses to the LQ (linear-quadratic) formalism, including the standard (generalized Lea-Catcheside) dependence on dose protraction. (4) Two-track (beta) lethal damage is largely due to dicentric chromosome aberrations, but one-track (alpha) lethal damage is largely caused by other mechanisms such as point mutations in a vital gene, small deletions, residual chromosome breaks, induced apoptosis, etc. (5) A major payoff for 50 years of radiobiological modelling is identifying molecular mechanisms which underly the broadly applicable LQ formalism.

Apoptosis and mitotic cell death: their relative contributions to normal-tissue and tumour radiation response

The target-cell theory of tissue responses is reviewed with reference to the radiosensitivity of proposed target cells in bone marrow, intestine, epidermis, and spermatogenesis. The difficulties in precisely identifying target cells using histological/cell marker criteria, and hence determining the role of their mode of death in tissue responses, are being circumvented to some extent by the recent use of mice deficient in gene products required for radiation-induced apoptosis. In this case cell death results from 'mitotic cell death' and e.g. in the case of p53, any remaining p53-independent apoptosis. In the p53 null mouse, cell survival levels are increased in bone marrow and intestine but decreased in the testis. Different interpretations, based on the lack of p53-dependent apoptosis or the lack of a permanently induced G1-arrest in the case of marrow fibroblasts, can be applied to the results for different cell types. Hence both apoptosis and mitotic cell death can variously be involved as contributing to target-cell sensitivity and hence to early reactions in these tissues after irradiation. It is still unclear whether, or how, the mode of cell death (apoptotic versus mitotic) determines the radiosensitivity and response of tumours. In experimental tumours, the levels of radiation-induced apoptosis have been shown to correlate both with the in vivo response to radiation and the degree of spontaneous apoptosis in the tumours. Measurements of spontaneous apoptosis in human tumours, however, have yielded conflicting data with high apoptotic levels significantly correlating with both good and poor prognosis in different studies. There is one report of a lack of relationship between intrinsic radiosensitivity and spontaneous apoptosis in cervical cancers. In contrast several studies have reported correlations between apoptosis levels and the degree of tumour cell proliferation. Tumour hypoxia has also been shown to increase apoptosis. These data suggest that tumour apoptosis may be a reflection of intrinsic radiosensitivity, tumour cell proliferation and tumour hypoxia. Its relative importance will probably be tumour type, size and stage related.