Expression of p21 and mutant p53 gene products in residual prostatic tumor cells after radical radiotherapy

BACKGROUND: In a previous study, sextant core biopsies revealed residual tumor in the prostate in 37/55 investigated patients, with an average of 6.8 years after external beam radiation therapy (RRT). More than half of the biopsies exhibited Ki-67 and PCNA proliferation activity. METHODS: The present study aims at further characterizing residual tumor cells post-RRT by investigating whether the tumor cells exhibit immunohistochemical expression of p21 and mutant p53 gene products, which reflect the state of cell cycle regulatory mechanisms. RESULTS: Positive p53 staining was observed in 11% and p21 positivity in 47% of biopsies. The proportion of positively stained cells was low for both antigens. The staining patterns point to the existence of wild-type p53-dependent, as well as alternative pathways for p21 protein induction. CONCLUSIONS: A low proportion of tumor cells exhibited p53 protein accumulation post-RRT. G1 arrest, as assessed by p21 immunoexpression, was demonstrated in a low percentage of tumor cells in < 50% of post-RRT biopsies, indicating that the vast majority of residual tumor cells following RRT escape the G1/S checkpoint control and propagate into S-phase, presumably with a maintained malignant potential.