Nearly all immortal human cancer cells have high levels of telomerase and the inhibition of telomerase causes them to die. We work with human keratinocyte cultures because these cells form immortal cancers in vivo and because mouse models are not easy to use in studies of replicative senescence. We have a large panel of genetically characterised human squamous carcinoma lines which we use as targets for monochromosome transfer experiments. This enables us to validate the presence of mortality genes on particular chromosomes and because some of the chromosomes accidentally break and do not work, the analysis of these chromosomes with polymorphic markers permits the mapping of the mortality gene. Figure 1 Neoplastic human keratinocyte.
Senescence is triggered by progressive reduction in telomere length during DNA replication.
The presence of a mortality gene on chromosome 4 has been confirmed by the induction of mortal clones on the re-introduction of the wild type chromosome. We have now mapped this gene to a 1.5Mb region of the chromosome and are beginning to test the genes within this region for alterations in cancer cells. The de-regulation of telomerase in human squamous cancer is likely to be complex. Additionally, hTERT and the RNA telomerase enzyme subunit, hTR, are over-expressed and sometimes amplified. The de-regulation of telomerase in human squamous cancer is likely to be complex.