Formation and loss of O6-methyldeoxyguanosine in human leucocyte DNA following sequential DTIC and fotemustine chemotherapy

Abstract

There is increasing evidence to indicate that O6-methyldeoxyguanosine (O6-MedG) formation in DNA is a critical cytotoxic event following exposure to certain anti-tumour alkylating agents and that the DNA repair protein O6-alkylguanine-DNA alkyltransferase (ATase) can confer resistance to these agents. We recently demonstrated a wide inter-individual variation in the depletion and subsequent regeneration of ATase in human peripheral blood lymphocytes following sequential DTIC (400 mg m-2) and fotemustine (100 mg m-2) treatment, with the nadir ATase activity occurring approximately 4 h after DTIC administration. We have now measured the formation and loss of O6-methyldeoxyguanosine (O6-MedG) in the DNA of peripheral leucocytes of eight patients receiving this treatment regimen. O6-MedG could be detected within 1 h and maximal levels occurred approximately 3-5 h after DTIC administration. Following the first treatment cycle, considerable inter-individual variation was observed in the peak O6-MedG levels, with values ranging from 0.71 to 14.3 mumol of O6-MedG per mol of dG (6.41 +/- 5.53, mean +/- s.d.). Inter- and intra-individual variation in the extent of O6-MedG formation was also seen in patients receiving additional treatment cycles. This may be a consequence of inter-patient differences in the capacity for metabolism of DTIC to release a methylating intermediate and could be one of the determinants of clinical response. Both the pretreatment ATase levels and the extent of ATase depletion were inversely correlated with the amount of O6-MedG formed in leucocyte DNA when expressed either as peak levels (r = -0.59 and -0.75 respectively) or as the area under the concentration-time curve (r = -0.72 and -0.73 respectively). One complete and one partial clinical response were seen, and these occurred in the two patients with the highest O6-MedG levels in the peripheral leucocyte DNA, although the true significance of this observation has yet to be established.

Elevated oxidative stress and DNA damage and repair levels in urinary bladder carcinomas associated with schistosomiasis

“…The very clear differences indicate that the close correlation between the levels of DNA alkylation in normal and tumor tissue despite the very large difference in the levels of DNA damage might be partially correlated to the individual differences in modes of alkylating agent metabolism due to genetic polymorphism and/or DNA repair pathways. 55,57 Previous studies with urinary bladder 54 and other tissues 58,59 have indicated that higher levels of the DNA repair protein (i.e., O 6 -alkylguanine DNA alkyltransferase) may result in lower levels of O 6 -methylguanine in DNA. Therefore, the change towards a malignant phenotype could possibly be attributed in part to the differences in DNA repair levels.…”

Development and Application of a Sensitive and Rapid Immunoassay for the Quantitation of N7-Methyldeoxyguanosine in DNA Samples

“…DNA was isolated from rat pyloric tissues as previously described (25) using phenol and chloroform but with the proteinase K digestion carried out at 4°C. DNA purity was assessed by UV absorbance at 260 and 280 nm using a Shimadzu UV-160 spectrophotometer, and samples were considered to be pure if the 260 nm/280 nm ratio was between 1.7 and 1.9.…”

DNA Adducts: Endogenous and Induced

“…Whereas all wbc DNA samples contained (apparently) detectable levels of 80HG, in 2 studies it was reported that a limited number of individuals had no detectable bulky adducts (37,52) (11,65). Similarly, levels of bulky DNA adducts in smokers ranged from 1.9 to 25.3 (lung) and from <0.1 to 21.9 (liver) adducts/101 dN (11,65 (40), and in liver DNA, 80HG levels were several orders of magnitude higher than bulky DNA adducts and other adducts arising from endogenous processes such as lipid peroxidation (8,48 (58) and sperm DNA (20) and also bulky DNA adducts in stomach tissue (18 (41).…”

“…For example, Figure 5 shows the variation in the formation and persistence of 06-MeG in wbc DNA following treatment of 3 individuals with the same doses of the methylating agent dacarbazine (DTIC) and fotemustine (41 (19,49).…”