**2.2.3 Psoralen derivatives**

102 Selected Topics in DNA Repair

a base-flipping where the (+)-*anti*-B[*a*]P-N2-dG bulky adduct is located in the minor groove and the opposite cytosine is positioned in the major groove (Cosman et al., 1993). The precise orientation of this highly carcinogenic 10S(+)-*trans-anti*-B[*a*]P-N2-dG adduct depends on the sequence surrounding the target guanine (Cai et al., 2010). DNA is untwisted at 5'- CGG\*C sites where a large bend is induced in the DNA helix, but not at 5'-CG\*GC sequences where, conversely, DNA helix is destabilized in its portion orientated 5' to the lesion (Rodríguez et al., 2007). Such differences result in different protein/DNA recognition and repair activities (see 3.4). Thermal destabilization was also observed using 14R(+)-*transanti*-DB[*a*,*l*]P-N2-dG adduct (Zheng et al., 2010) or *14S*(-)-*trans-anti*-DB[*a,l*]P-N6-dA adducts

Fig. 4. 3D orientation of (+)-*anti-*BPDE [mmdbId:52106] and the psoralen derivative HMT [mmdbId:52343] and structure of some DNA alkylators that destabilize the DNA helix.

The hormone-derived genotoxic compound, 4-OHEN, derives from equilin and equilenin, two equine oestrogens present in hormone substitution therapies used to prevent the uncomfortable effects of menopauses but are also thought to increase breast cancer incidence in the population of hormonally-treated women (Rossouw et al., 2002). Its orthoquinone form is cytotoxic and genotoxic (Pisha et al., 2001) through the formation of bulky DNA lesions at dA, dC and dG (but not at T residues) (Kolbanovskiy et al., 2005) which were detected in both cell culture and breast cancer biopsies from patients treated with hormone substitution therapies (Embrechts et al., 2003). 4-OHEN derived from the intermediate catechol 4-hydroxyequilenin which was generated from a rapid conversion of both equilin and equilenin in the organism to four stereo-isomers differently affecting the 3D-structure of the DNA helix (Ding et al., 2007). For adducts on cytosine, the *syn*- or *anti*conformations of the bulky rings of 4-OHEN point along the major or the minor groove (Ding et al., 2005). Interestingly, alkylation at dA or dC residues is associated with a strong

whereas *14R*(+) isomer stabilizes the ds-DNA (Cai et al., 2011).

Psoralen is a chemotherapeutic agent known to cause DNA inter-strand crosslinks (ICLs) upon absorption of two photons from UVA irradiation at 365 nm, preferentially at 5′-TA and to a lesser extend at 5′-AT dinucleotides. This activity was the basis for use of psoralen and UVA exposure (PUVA therapy) to treat cutaneous diseases like psoriasis, vitiligo, atopic dermatitis or cutaneous T cell lymphomas. However, such treatment increased the risk of squamous and basal cell carcinomas (Teicher, 1996). Psoralen-induced ICLs are classically used models for DNA repair of ICLs. The psoralen derivative 4'-(hydroxymethyl)-4,5',8 trimethylpsoralen (HMT) (Figure 4) evidenced DNA destabilization by mono-addition of a psoralen residue to both thymines (one on each strand) of 5'-GGGTACCC sequence.
