The abasic site lesions in the human telomeric sequence d[TA(G3T2A)3G3]: A thermodynamic point of view

Background: The abasic sites represent one of the most frequent lesions of DNA and most of the events able to generate such modifications involve guanine bases. G-rich sequences are able to form quadruplex structures that have been proved to be involved in several important biological processes. Methods: In this paper, we report investigations, based on calorimetric, UV, CD and electrophoretic techniques, on 12 oligodeoxynucleotides analogues of the quadruplex forming human telomere sequence d[TA(G3T2A)3G3], in which each guanine has been replaced, one at a time, by an abasic site mimic. Results: Although all data show that the modified sequences preserve their ability to form quadruplex structures, the thermodynamic parameters clearly indicate that the presence of an abasic site decreases their thermal stability compared to the parent unmodified sequence, particularly if the replacement concerns one of the guanosines involved in the formation of the central G-tetrad. Conclusions: The collected data indicate that the effects of the presence of abasic site lesions in telomeric quadruplex structures are site-specific. The most dramatic consequences come out when this lesion involves a guanosine in the centre of a G-run. General significance: Abasic sites, by facilitating the G-quadruplex disruption, could favour the formation of the telomerase primer. Furthermore they could have implications in the pharmacological approach targeting telomere.