Alterations of DNA repair in melanoma cell lines resistant to cisplatin, fotemustine, or etoposide

Resistance to chemotherapy is a common phenomenon in malignant melanoma. In order to assess the role of altered DNA repair in chemoresistant melanoma, we investigated different DNA repair pathways in one parental human melano ma line (MeWo) and in sublines of MeWo selected in vitro for drug resistanc e against four commonly used drugs (cisplatin, fotemustine, etoposide, and vindesine). Host cell reactivation assays with the plasmid pRSVcat were use d to assess processing of different DNA lesions. With ultraviolet-irradiate d plasmids, no significant differences were found, indicating a normal (nuc leotide excision) repair of DNA photoproducts. With singlet oxygen-treated plasmid, the fotemustine- and cisplatin-resistant lines exhibited a signifi cantly increased (base excision) repair of oxidative DNA damage. With fotem ustine-treated plasmid, the fotemustine-resistant subline did not exhibit a n increased repair of directly fotemustine-induced DNA damage. Similar resu lts were obtained with cisplatin-induced DNA crosslinks in the cisplatin-re sistant line. The fotemustine- and etoposide-resistant sublines have been s hown to exhibit a reduced expression of genes involved in DNA mismatch repa ir. We used a ''host cell microsatellite stability assay'' with the plasmid pZCA29 and found a 2.0-fold to 2.5-fold increase of microsatellite framesh ift mutations (p less than or equal to 0.002) in the two resistant sublines . This indicates microsatellite instability, the hallmark of an impaired DN A mismatch repair. The increased repair of oxidative DNA damage might media te an increased chemoresistance through an improved repair of drug-induced DNA damage. In contrast, a reduced DNA mismatch repair might confer resista nce by preventing futile degradation of newly synthesized DNA opposite alky lation damage, or by an inability to detect such damage and subsequent inab ility to undergo DNA-damage-induced apoptosis.