18. Genomic instability and telomere length, radiation sensitivity of human mammary epithelial cells in culture

Abstract: Human mammary epithelial cells derived from normal breast tissue from three women subjected to spontaneous reduction were analysed. These cells may spontaneously overcome the normal proliferation limits by loosing expression of p16 protein. The karyotype evolution of HMECs was followed by means of an exhaustive cytogenetic analysis. As an overall result, there was an increase in the frequency of aberrations per cell throughout the culture. Anna Genesca's results confirm that the individual chromosome arms carrying the shortest telomeres are the ones involved in fusions. Moreover, the analysis of the specific chromosomes with the shortest telomeres in post-selection HMECs derived from the three women provided evidence that the set of chromosome arms showing preferential eroded telomeres depends on each individual. This variability may be responsible for the observed karyotype differences among tumors of the same type. Most importantly, this partner identified sister chromatid fusion as the first event in generating genomic instability in HMECs. During HMEC growth, double strand breaks are generated by both fused chromosomes as well as individual chromosomes with fused chromatids entering BFB cycles. These broken extremities are able to join other broken ends or eroded telomeres, producing massive instability at the latter passages of the cell culture.

Evidence is accumulating that telomeres may be involved in cellular as well as organismal responses to ionising radiation. Anna Genesca's laboratory analysis identified dysfunctional telomeres joined to radiation-induced DSBs in HMECs, providing the first conclusive evidence of this type of aberration in primary human cells. In addition, telomere-DSB aberrations increased with radiation exposure and are probably responsible for the increased radiation sensitivity of HMECs with short telomeres. From these results, it seems reasonable to conclude that, given a radiation-induced DSB in a chromosome, the probabilities of correct rejoining are dramatically different in cells with normal telomere length in comparison to cells with critically short telomeres.

In cells with normal telomere length, the broken chromosome can only rejoin another broken end, with a high probability of restoring the original chromosome if the number of breaks coexisting in that cell is low. On the contrary, in cells with critically short telomeres, there may be several uncapped chromosomes to which the broken end can join, thus making the restoration of the original chromosome more unlikely. As a consequence, higher radiation sensitivity is observed in HMECs with short telomeres in comparison to normal telomere length HMECs. Provided that it is known that HMECs shorten telomeres with population doublings in vitro (age in vivo) and inactivate expression of p16INK4a both in vitro and in the mammary gland, it might be important to re-evaluate the risks facing women of advancing age when exposed to radiation for diagnostic purposes.

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