Cancer surveys最新文献

The probability of a mouse cell becoming fully transformed in vivo or in vitro is enormously greater than that of a human cell. The number of events in tumour progression is similar in rodent and human cells, and it is unlikely that the difference in neoplastic transformation frequency can be explained on the basis of gene mutation in oncogenes and tumour suppressor genes. Instead, it is proposed that mouse cells may be (a) more subject to destabilization of the karyotype, (b) have less efficient check point cell cycle controls after DNA is damaged and/or (c) have less stringent epigenetic controls of gene activity, based on DNA methylation. Much evidence exists that mouse or rat cells are less efficient in DNA repair, maintenance of DNA methylation and other aspects of DNA metabolism. These relate to the difference in longevity in these and other mammalian species. Ageing is likely to be due to the failure of cell and tissue maintenance. Long lived species invest more in various somatic maintenance mechanisms than do short lived ones, and this includes protection against neoplastic transformation. The future study of the basis of the difference between human and mouse or rat cells in resistance to transformation is likely to yield important insights into the sequential events in tumour progression.

In E coli, new spontaneous mutations can arise in bacteria that are non-dividing and in which there is little or no DNA synthesis. These mutations are almost invariably those that enable the cell to resume growth, a phenomenon that has been termed directed or adaptive mutation. Evidence is accumulating from studies with DNA repair deficient strains that damage produced by endogenous mutagens may be an important source of such mutations. A DNA lesion that can miscode can explain the apparent adaptive behaviour since if a "mutant" RNA transcript confers sufficient advantage that the cell is triggered into a cycling state, the ensuing round of DNA replication will be likely to fix the mutation by means of a DNA miscoding event. The most important lesion in this respect appears to be 8-oxoG, which can pair equally well with adenine or cytosine and so give rise to G to T transversions. It is responsible for almost half the G to T transversions arising in non-growing repair proficient bacteria. Alkylations contribute to the production of both transitions and transversions but only those at A:T base pairs are important in repair proficient bacteria. There is also a report of a lesion susceptible to UvrA,B,C dependent excision repair, but whether it is important in bacteria possessing excision repair has not been addressed. Data on mammalian cells are almost non-existent, but there is evidence that point mutations can occur in vivo in postmitotic neurons. The underlying assumption that there is little or no DNA synthesis in non-dividing bacteria has been challenged by recent data suggesting that there may be extensive cryptic DNA turnover.

Recent insights into the action of TP53 have uncovered signal transduction pathways that maintain genomic integrity. TP53 was the first gene demonstrated to be involved in these pathways, but mutation of several other genes can have a similar terminal effect. The characterization of these signal transduction pathways should provide further targets for the improvement of neoplastic diagnosis as well as therapeutic efficacy.

TGF beta represents the largest and most versatile cytokine family known in metazoans. The recent identification of transmembrane serine/threonine kinases as TGF beta family receptors represents a major milestone towards understanding how these factors elicit their varied responses. Genetic and biochemical evidence suggests a general model for the mechanism of activation of these receptors. In this model, the ligand acts as an adaptor, bringing a primary receptor kinase in contact with a second kinase, which becomes phosphorylated and thereby competent to propagate the signal to downstream components. Such a kinase cascade on the membrane defines a new variation in signal transduction. Although many details of this mechanism remain to be clarified, its combinatorial capacity may explain the multifunctional nature of its ligands. Several key genes have been identified whose regulation by these signals mediates cellular responses such as cell cycle arrest. This field is advancing at a fast pace, and the identity of components that carry the receptor signals to these genes should soon be unveiled. It is expected that these advances, as they unfold, will in turn clarify the role of miscreant TGF beta signalling in human diseases, cancer included.