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Report

DBy way of an introduction to the Symposium, an overview of latest developments in the role of telomerase in cellular immortalization and cancer was presented by R F Newbold. The importance of transcriptional depression of the gene encoding the catalytic sub-unit of human telomerase (hTERT) in immortalization and clonal evolution, and in multi-step cancer development, was highlighted. The mechanism by which various effectors of replicative cell senescence (p16, RB1, p53 and critically shortened telomeres) act as tumour suppressive mechanisms was discussed, and reasons for the widely different propensities of human versus rodent cells explained at the molecular level.

etails of latest advances in understanding the regulation of expression of human telomerase were described by J Lingner, who had recently used a highly sensitive quantitative reverse transcriptase PCR method to show conclusively that the gene encoding hTERT is differentially regulated in normal cells, and in cancer, at the level of transcription. Furthermore, a powerful telomerase repressor gene (previously located on chromosome-3 by RFN) was shown to repress telomerase by a transcriptional silencing mechanism. Regions within the hTERT genomic sequence had recently been identified (by the Swiss team) that exhibited a 'closed' chromatin conformation in telomerase-negative normal human cells and a relaxed state in telomerase-positive cancer cells. Moreover, chromosome-3 induced telomerase repression was accompanied by a rapid switch to the latter, 'silenced', hTERT chromatin form.

Latest studies on the influence of telomeres on DNA repair, specifically the non-homologous end-joining (NHEJ) of double-strand DNA breaks (DSBs) were summarized by M Blasco. Using an experimental approach based

on the construction of 'knockout' mice lacking DSB proteins (Ku-70/80; DNA-PKcs) normally located as a complex (DNA-PK) at the mammalian telomere, it was shown that deficiency results in an increased frequency of telomeric fusions suggesting a protective role of the DNA-PK complex at the telomere. A correlation between telomere length and organismal sensitivity to ionizing radiation (IR) in mammals was also described. This was not thought to be due to defective DSB repair, but rather via interference with the detection of short telomeres as DNA breaks by other proficient DNA repair machinery. Additional aspects of the role of telomere maintenance in radiosensitivity were discussed by P Slijepcevic, who again described an association between IR-induced clastogenicity and telomere length, suggesting that telomere maintenance and cellular responses to IR are linked. The idea that telomere dynamics and repair of DSBs are also associated was strengthened by the demonstration that scid (severe combined immunodeficiency) mice (defective in DNA-PKcs which are involved in DSB repair) have exceptionally long telomeres.

A major application, for mechanistic studies of carcinogenesis, of the capacity of cloned hTERT cDNA expression vectors to immortalize a wide range of human cells at high frequency, was the focus of the final presentation of the Symposium by A Riches. Newly immortalized cell lines were derived in this way from retinal pigment epithelial cells and human breast epithelium. Such lines were transformable to anchorage independence and (in the former case) to malignancy, following repeated exposure to IR. Molecular cytogenetic analysis of tumorigenic clones revealed common alterations involving chromosomes 10 and 13, that may well have mechanistic significance.

                                                                                                                                                                                                      

For further information

Robert F Newbold DSc (Lond)  FIBiol  FRCPath FRSA
Brunel University
Uxbridge, UB8 3PH, UK
Tel: 44-1895-203090
Fax: 44-1895-274348

email:  rnewbold@atlas.co.uk