Chlorambucil induction of HsRad51 in B-cell chronic lymphocytic leukemia

Our previous studies with B-cell chronic lymphocytic leukemia (B-CLL) have suggested that one of the mechanisms of nitrogen mustard (NM) drug resistan ce is increased repair of drug-induced damage. We have postulated that reco mbination may play a crucial role in this process, The human homologue of R ad51, (HsRad51), has homology to the RecA protein in Escherichia coli, whic h is implicated in recombination repair and induction of DNA repair enzymes , In this report, we have examined the expression and distribution of HsRad 51 protein in lymphocytes from patients with B-CLL to see whether the expre ssion of HsRad51 is associated with NM damage to the malignant B lymphocyte s, specifically chlorambucil (CLB), which is the standard alkylating agent used to treat patients with B-CLL, We have analyzed the intracellular distr ibution of HsRad51 protein in these lymphocytes before and after treatment with CLB by immunofluorescence, In vitro CLB treatment induces Rad51 expres sion, as measured by increased immunopositive staining in all CLL samples. In the CLB-resistant CLL lymphocytes, there was a linear correlation betwee n induction of Rad51 protein at 5.4 mu M CLB and the in vitro LD50 dose of CLB, Surprisingly, although it has been reported that Rad51 is induced in S phase and only 10% of cells from cell lines expressed positive immunostain ing for Rad51, our CLL lymphocytes, which were not subjected to in vitro dr ug exposure, were 90% positive for Rad51, despite their nonproliferative st ate, which suggests that there is chronic activation of the protein, Our re sults suggest that CLB activates HsRad51-directed recombination repair and that this process may be important in NM drug-induced cytotoxicity.