Selective elimination of human lymphoid cells with unstable chromosome aberrations by p53-dependent apoptosis

Alterations in p53 expression are associated with genomic instability, presumably because loss of p53 leads to an inability to eliminate damaged and therefore potentially unstable cells by apoptosis or by induced cell cycle block. We tested this hypothesis by examining the influence of apoptosis on X-ray-induced chromosome aberration frequency in two isogenic human B-lymphoblastoid cell lines; TK6, which is sensitive to the induction of apoptosis, and WI-L2-NS, a p53 mutant resistant to apoptosis induction. While TK6 was more sensitive than WI- L2-NS cells to the cytotoxic effects of X-rays, it showed fewer induced chromosome aberrations. Inhibition of apoptosis in TK6 cells with phorbol 12-myristate 13-acetate (PMA) resulted in X-ray-induced aberration frequencies similar in magnitude to WI-L2-NS. The results support the hypothesis that apoptosis acts to selectively remove damaged cells. The reduction in aberration frequency associated with apoptosis was seen primarily for unstable types of aberrations; acentric chromosome fragments and dicentric chromosomes. There was no effect on the induced frequency of balanced translocations, the stable counterpart to dicentrics. The failure to remove cells with unstable types of aberrations is consistent with the genomic instability that accompanies loss of p53 activity.