ELEVATED RECOMBINATION IN IMMORTAL HUMAN-CELLS IS MEDIATED BY HSRAD51RECOMBINASE

Normal diploid cells have a limited replicative potential in culture, with progressively increasing interdivision time, Rarely, cell lines a rise which can divide indefinitely; like tumor cells, such ''immortal' ' lines display frequent chromosomal aberrations which may reflect hig h rates of recombination. Recombination frequencies within a plasmid s ubstrate were 3.5-fold higher in nine immortal human cell lines than i n six untransformed cell strains, Expression of HsRAD51, a human homol og of the yeast RAD51 and Escherichia coli red recombinase genes, was 4.5-fold higher in immortal cell lines than in mortal cells, Stable tr ansformation of human fibroblasts with simian virus 40 large T antigen prior to cell immortalization increased both chromosomal recombinatio n and the level of HsRAD51 transcripts by two- to fivefold, T-antigen induction of recombination was efficiently blocked by introduction of HsRAD51 antisense (but not control) oligonucleotides spanning the init iation codon, implying that HsRAD51 expression mediates augmented reco mbination, Since p53 binds and inactivates HsRAD51, T-antigen-p53 asso ciation may block such inactivation and liberate HsRAD51. Upregulation of HsRAD51 transcripts in T-antigen-transformed and other immortal ce lls suggests that recombinase activation can also occur at the RNA lev el and may facilitate cell transformation to immortality.