Human O 6 -alkylguanine-DNA alkyltransferase (hAGT) expression increases mutations and cytotoxicity following exposure to 1,3-butadiene diepoxide (BDO) and hAGT-DNA cross-links are formed in the presence of BDO. We have used hAGT mutants to investigate the mechanism of crosslink formation and genotoxicity. Formation of a hAGT-DNA conjugate in vitro was observed with C145S and C145A mutant proteins but was considerably diminished with the C145A/C150S double mutant confirming that cross-linking primarily involves either of these two cysteine residues, which are located in the active site pocket of the protein. Cross-link formation by BDO occurred both via (a) an initial reaction of BDO with hAGT followed by attack of the reactive hAGT complex on DNA, and (b) the initial reaction of BDO with DNA followed by a reaction between hAGT and the DNA adduct. These results differ from those with 1,2-dibromoethane (DBE) where Cys 145 is the only site of attachment and pathway (b) does not occur. The complex formed between hAGT at Cys 145 and BDO was very unstable in aqueous solution. However, the BDO-hAGT complex at Cys 150 exhibited stability for more than 1 h. The effect of hAGT and mutants on BDO-induced genotoxicity was studied in E. coli using the forward assay to rifampicin resistance. Both mutations and cell killing were greatly increased by wild type hAGT and there was a smaller but significant effect with the C145A mutant. The R128A mutant and R128A/C145A and C145A/C150S double mutants were ineffective supporting the hypothesis that the formation of hAGT-DNA cross-links is responsible for the enhanced genotoxicity detected in this biological system. In the absence of hAGT, there were

In 2010, a new entity, characterized by the classical signs and symptoms of Kaposi sarcoma herpesvirus–associated multicentric Castleman’s disease (KSHV-MCD) in the absence of pathologic evidence of MCD, was described in individuals living with HIV. This syndrome was named KSHV inflammatory cytokine syndrome (KICS). It carries mortality rates of up to 60%. To date, there are no standard therapies. Treatment regimens studied in clinical trials for MCD disease are used in cases of KICS.

The primary function of O 6 -alkylguanine-DNA alkyltransferase (AGT) is to maintain genomic integrity in the face of damage by both endogenous and exogenous alkylating agents. However, paradoxically, bacterial and mammalian AGTs have been shown to increase cytotoxicity and mutagenicity of dihaloalkanes and other bis-electrophiles when expressed in bacterial cells. We have extended these studies to mammalian cells using CHO cells that lack AGT expression and CHO cells stably transfected with a plasmid that expresses human AGT. The cytotoxicity of 1,2-dibromoethane, dibromomethane and epibromohydrin was significantly increased by the presence of AGT but cytotoxicity of butadiene diepoxide was not affected. Mutations caused by these agents were assessed using hypoxanthine-guanine phosphoribosyltransferase (HPRT) as a reporter gene. There was a small (c. 2-3-fold) but statistically significant AGT-mediated increase in mutations caused by 1,2-dibromoethane, dibromomethane and epibromohydrin. Analysis of the mutation spectrum induced by 1,2-dibromoethane showed that the presence of AGT also altered the types of mutations with an increase in total base substitution mutants due to a rise in transversions at both G:C and A:T sites. AGT expression also led to mutations arising from the transcribed strand, which were not seen in cells lacking AGT. Although the frequency of deletion mutations was decreased by AGT expression, the formation of large deletions (≥3 exons) was increased. This work demonstrates that interaction of AGT with some bis-electrophiles can cause mutagenicity and diminished cell survival in mammalian cells. It is consistent with the hypothesis that DNA-AGT cross-links, which have been characterized in experiments with purified AGT protein and such bis-electrophiles, can be formed in mammalian cells.