Topoisomerase II expression as a determinant of chromosomal radiosensitivity and possible susceptibility in breast cancer

Background

Elevated chromosomal radiosensitivity in lymphocytes of breast cancer patients is thought to be an indicator for the presence of one or more as yet unidentified genes of low penetrance that promote susceptibility to the disease in up to 60% of cases [1, 2]. One such gene may be TOPO2A, encoding for the DNA processing enzyme topoisomerase IIa. The involvement of topoisomerase IIa is predicted from the author's model for formation of chromatid breaks [3]. In the model the DNA double-strand break is not directly involved in the chromatid break, but acts as an initiator in a sequence of events leading to a chromatid break. It is thought that a chromatid break may be formed by a misjoining of chromatin ends during topoisomerase IIa decatenation of chromatids as the cell progresses through G₂ towards mitosis. Topoisomerase IIa is known to be vulnerable to perturbation by oxidative stress during the precise process of cutting and joining DNA strands [4].

Methods

Gamma-radiation-induced chromatid breaks are scored for chromatid breaks in colcemid-blocked chromosome spreads of metaphase HL60 and mitoxantrone-resistant variants: MX1 and MX2 cells with reduced topoisomerase II expression. Topoisomerase IIa expression levels were measured using western blotting. SiRNA was used to knock down expression in normal exponentially growing human cells that are irradiated with a low dose of γ-rays and scored for the presence of chromatid breaks. The chromatid break frequency and topoisomerase IIa expression (ELISA assay) are being compared in 3-day-stimulated peripheral blood T lymphocytes from a group of breast cancer patients and control individuals exposed to a low dose of γ-radiation.

Results

We show that chromatid radiosensitivity (based on the frequency of metaphase chromatid breaks in irradiated G₂ cells) is significantly lower in a topoisomerase IIa underexpressing variant cell lines [5], and preliminary results show that reducing expression with SiRNA also reduces chromatid radiosensitivity. In a pilot study we are currently comparing the chromatid radiosensitivity and topoisomerase IIa expression in stimulated peripheral blood lymphocytes of a group of Tayside breast cancer patients and a similar number of normal noncancer control individuals.

Conclusion

Our data support the hypothesis that topoisomerase IIa expression is a determinant of chromatid break frequency in irradiated G₂ cells, and thus could be an underlying cause of the observed variability of chromatid radiosensitivity among both sporadic breast cancer cases and normal control individuals.