Chromosome Instability and Cancer

Jeffrey L. Schwartz, The University of Washington

Howard Liber, Colorado State University

Helen H. Evans, Case Western Reserve University

The induction of genome instability by radiation is an important early event in tumor formation. The long-term goal of these studies is to identify mechanisms involved in this process. One potential target of radiation-induced instability is telomere metabolism. The evidence for the essential role of telomeres in carcinogenesis is growing. We have developed a cell model for radiation-induced chromosome instability where a key initial event is chromosome fusion at telomeres. In our model, radiation exposure is associated with both telomere shortening and chromosome fusions. Our working hypothesis is that normal time-dependent variations in telomere size can lead to telomeres shortening to some critical length where they become susceptible to fusion through non-homologous end-joining processes. Cells with short telomeres are normally eliminated through TP53-dependent apoptosis. Radiation exposure could affect the signaling pathway that connects telomere metabolism to apoptosis thereby facilitating the survival of cells with short telomeres. Alternatively, prior radiation exposure could affect recombination activity in cells and that in turn leads to both telomere shortening and chromosome fusions. To distinguish between effects related to apoptosis and those reflecting changes in recombination activity, we have genetically tagged a single telomere in our cell model, and are now following changes at this locus over time in cell clones where either apoptosis is modified by eliminating TP53 expression or over-expressing bcl-2, or recombination is modified by genetic elimination of non-homologous end-joining or homologous recombination. The results of our study will aid in understanding the nature of the target for radiation-induced chromosome instability.

Figure 6. Analysis of chromosome fusions.

Chromosomes were hybridized with PNA probes for centromeres (green) and telomeres (red).

The arrows in A and C indicate the enlarged chromosomes in B and D. Some fusions do not show telomere signals at the fusion site (A, B), while others do (C, D).

Supported by US Department of Energy and National Institutes of Health