ESCAPE FROM IN-VITRO AGING IN SV40 LARGE T-ANTIGEN-TRANSFORMED HUMAN-DIPLOID CELLS - A KEY EVENT RESPONSIBLE FOR IMMORTALIZATION OCCURS DURING CRISIS

A human diploid fibroblast strain MRC-5 was transfected with a replica tion origin-defective early region of SV40 containing the gene of larg e T antigen, and 48 clones of T antigen-transformed MRC-5 were isolate d. Although T antigen prolonged the lifespan of MRC-5, all the transfo rmed clones were still mortal. From two of the transformed MRC-5 clone s, eight independent immortalized clones were obtained in triple exper iments of immortalization. The transformed phenotypes of immortalized clones were widely varied and did not always retain those of the paren tal pre-immortalized clones. The immortalization occurred at the frequ ency of about 1-3 x 10(-7)/cell. From cell number and population doubl ing time of the immortalized clones, the immortalization was estimated to occur in or just before the crisis of parental mortal cells. The d ecreases of modal chromosome numbers and the loss of chromosomes 5 and 10 were found to be common in three independent immortalized clones e xamined. Thus, the escape from the cellular aging process seemed to be caused by certain genetic events including the loss of chromosomes at the end of lifespan in T antigen-transformed human diploid cells.