Telomeres play the key protective role at chromosomes. Many studies indicate that loss of telomere function causes activation of DNA damage response. Here, we review evidence supporting interdependence between telomere maintenance and DNA damage response and present a model in which these two pathways are combined into a single mechanism for protecting chromosomal integrity. Proteins directly involved in telomere maintenance and DNA damage response include Ku, DNA-PKcs, RAD51D, PARP-2, WRN and RAD50/MRE11/NBS1 complex. Since most of these proteins participate in the repair of DNA double-strand breaks (DSBs), this was perceived by many authors as a paradox, given that telomeres function to conceal natural DNA ends from mechanisms that detect and repair DSBs. However, we argue here that the key function of one particular DSB protein, Ku, is to prevent or control access of telomerase, the enzyme that synthesises telomeric sequences, to both internal DSBs and natural chromosomal ends. This view is supported by observations that Ku has a high affinity for DNA ends; it acts as a negative regulator of telomerase and that telomerase itself can target internal DSBs. Ku then directs other DSB repair/telomere maintenance proteins to either repair DSBs at internal chromosomal sites or prevent uncontrolled elongation of telomeres by telomerase. This model eliminates the above paradox and provides a testable scenario in which the role of DSB repair proteins is to protect chromosomal integrity by balancing repair activities and telomere maintenance. In our model, a close association between telomeres and different DNA damage response factors is not an unexpected event, but rather a logical result of chromosomal integrity maintenance activities.

Cells defective in BRCA1 show genomic instability as evidenced by increased radiosensitivity, the presence of chromosomal abnormalities and the loss of heterozygosity at many loci. Reported chromosomal abnormalities in BRCA1 deficient cells include dicentric chromosomes. Dicentric chromosomes, in some cases, may arise as a result of end-to-end chromosome fusions, which represent signatures of telomere dysfunction. In this study we examined BRCA1 deficient human and mouse cells for the presence of chromosomal aberrations indicative of telomere dysfunction. We identified a lymphoblastoid cell line, GM14090, established from a BRCA1 carrier that showed elevated levels of dicentric chromosomes. Molecular cytogenetic analysis revealed that these dicentric chromosomes result from end-to-end chromosome fusions. The frequency of end-to-end chromosome fusions did not change after exposure of GM14090 cells to bleomycin but we observed elevated levels of chromosomal abnormalities involving interactions between DNA double strand breaks and uncapped telomeres in this cell line. We observed similar chromosomal abnormalities involving telomeres in the breast cancer cell line, HCC1937, homozygous for BRCA1 mutation. Finally, we analyzed mouse embryonic stem cells lacking functional Brca1 and observed the presence of telomere dysfunction following exposure of these cells to bleomycin. Our results reveal cytogenetic evidence of telomere dysfunction in BRCA1 deficient cells.

BackgroundRecent studies suggest that BRCA2 affects telomere maintenance. Interestingly, anti cancer treatments that involve BRCA2 and telomerase individually are currently being explored. In the light of the above recent studies their combinatorial targeting may be justified in the development of future treatments. In order to investigate effects of BRCA2 that can be explored for this combinatorial targeting we focused on the analysis of recombination rates at telomeres by monitoring T-SCEs (Telomere Sister Chromatid Exchanges).ResultsWe observed a significant increase in T-SCE frequencies in four BRCA2 defective human cell lines thus suggesting that BRCA2 suppresses recombination at telomeres. To test this hypothesis further we analyzed T-SCE frequencies in a set of Chinese hamster cell lines with or without functional BRCA2. Our results indicate that introduction of functional BRCA2 normalizes frequencies of T-SCEs thus supporting the notion that BRCA2 suppresses recombination at telomeres. Given that ALT (Alternative Lengthening of Telomeres) positive cells maintain telomeres by recombination we investigated the effect of BRCA2 depletion in these cells. Our results show that this depletion causes a dramatic reduction in T-SCE frequencies in ALT positive cells, but not in non-ALT cells.ConclusionBRCA2 suppresses recombination at telomeres in cells that maintain them by conventional mechanisms. Furthermore, BRCA2 depletion in ALT positive cells reduces high levels of T-SCEs normally found in these cells. Our results could be potentially important for refining telomerase-based anti-cancer therapies.

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