O6 alkylguanine-DNA alkyltransferase activity in human myeloid cells.

Abstract

The association between alkylating agent exposure and acute nonlymphocytic leukemia in humans indicates that myeloid cells may be particularly susceptible to mutagenic damage. Alkylating agent mutagenesis is frequently mediated through formation and persistence of a particular DNA base adduct, O6alkylguanine, which preferentially mispairs with thymine rather than cytosine, leading to point mutations. O6alkylguanine is repaired by O6alkylguanine-DNA alkyltransferase (alkyltransferase), a protein that removes the adduct, leaving an intact guanine base in DNA. We measured alkyltransferase activity in myeloid precursors and compared it with levels in other cells and tissues.In peripheral blood granulocytes, monocytes, T lymphocytes, and B lymphocytes, there was an eightfold range of activity between individuals but only a twofold range in the mean activity between cell types. Normal donors maintained stable levels of alkyltransferase activity over time. In bone marrow T lymphocytes and myeloid precursors, there was an eightfold range of alkyltransferase activity between donors. Alkyltransferase activity in the two cell types was closely correlated in individual donors, r = 0.69, P < 0.005, but was significantly higher in the T lymphocytes than the myeloid precursors, P < 0.05. Liver contained the highest levels of alkyltransferase of all tissues tested. By comparison, small intestine contained 34%, colon 14%, T lymphocytes 11%, brain 11%, and myeloid precursors 6.6% of the activity found in liver. Thus, human myeloid precursors have low levels of Oalkylguanine-DNA alkyltransferase compared with other tissues. Low levels of this DNA repair protein may increase the susceptibility of myeloid precursors to malignant transformation after exposure to certain alkylating agents.

Promoter hypermethylation of MGMT is associated with protein loss in gastric carcinoma

“…In colon and ovarian cancers, increased expression of MGMT has been shown to be related to tumor progression. 22,27 In our study, the protein expression levels of MGMT were significantly associated with tumor stage, but promoter hypermethylation of the MGMT gene was not. Our data suggests that MGMT may be upregulated during tumor progression.…”

Down-regulation of DNA repair in human CD34+progenitor cells corresponds to increased drug sensitivity and apoptotic response

“…Low levels of MGMT expression have been correlated with the development of methylnitrosourea-induced thymic lymphoma/leukemia, and it has been speculated that the low MGMT expression measured in primary human myeloid leukemic cells may be an etiologic factor in the induction of secondary leukemias following chemotherapy with O 6 -alkylating agents. 36,37 Our data suggest that insufficient DNA repair may be a more widespread etiologic factor in human leukemias and may also be operative in the development of secondary leukemias after therapy with alkylating drugs such as nitrogen mustard, cyclophosphamide, chlorambucil, platinum derivatives, or the epipodophyllotoxins etoposide and teniposide. [38][39][40] To our knowledge, we have shown here for the first time that human CD34 ϩ progenitor cells have a grossly reduced overall repair capacity for a variety of drug-induced DNA lesions.…”

Sequential administration of varying doses of dacarbazine and fotemustine in advanced malignant melanoma

“…In contrast, in the present study this occurred in 40% and 53% of the patients receiving sequential 800 mg m2 DTIC and lOO mg m2 fotemustine. A more extensive marrow toxicity was seen in the schedule using 800 mgm2 of DTIC and one possible explanation for this might be due to a more extensive ATase depletion of the already low levels of ATase in the marrow (Gerson et al, 1985) resulting in an increased sensitivity to fotemustine or subsequent doses of DTIC. In this context, ATase-deficient murine haematopoietic stem cells transfected with and expressing bacterial ATase genes are highly resistant to the toxic effects of methylating and chloroethylating agents strongly suggesting that endogenous ATase expression would protect against the haematological effects of these agents (Jelinek et al, 1988) and hence ATase depletion would result in sensitisation.…”