Extracts of chronic lymphocytic leukemia lymphocytes have a high level of DNA repair activity fo O6-methylguanine.

Extracts of peripheral lymphocytes from six individuals with chronic lymphocytic leukemia (CLL) were assayed for the ability to remove O6-methylguanine (O6MeGua) from exogenous DNA. The O6MeGua-removing activity in CLL lymphocytes, predominantly B cells, was approximately 7-fold higher than in B lymphocytes of normal individuals and about 2-fold higher than in the unstimulated T type cells of normal persons. The activity measured in extracts of lymphocytes from three blood relatives was in the upper range of the normal distribution. Over 80% of the removal of O6MeGua was accomplished by the transfer of the methyl group to cysteine moieties of acceptor proteins in a stoichiometric reaction. If one assumes one acceptor group per acceptor protein, the calculated number of acceptor molecules per CLL lymphocyte falls between 91,000 and 220,000. Thus CLL lymphocytes do not show lower O6MeGua-removing activity, in contrast to many tumor cell strains or transformed cell lines, which are reported to have a deficient methyl excision repair phenotype (Mer-). Instead, the CLL lymphocytes act as if they have a super-Mer+ phenotype.