TUMOR CELL-SPECIFIC LOSS OF P53 PROTEIN IN A UNIQUE IN-VITRO MODEL OFHUMAN BREAST-TUMOR PROGRESSION

Mutations of the p53 gene are the most frequent genetic lesion in brea st cancer. Here, we examined p53 expression in a unique in vitro model of tumor progression derived from a single breast cancer patient (21T series). While the normal mammary epithelial, fibroblast and mesothel ial cells derived from this patient expressed easily detectable functi onal p53 protein, the primary as well as metastatic tumor cell lines d emonstrated a lack of p53 protein synthesis. 21T tumor cells failed to exhibit G(1) cell cycle arrest upon exposure to gamma-irradiation, an d their growth was suppressed by transfection of a normal p53 cDNA, de monstrating a lack of p53-mediated function in these cells. No p53 gen e deletion or rearrangements were detectable. PCR and sequence analysi s of the entire coding region of p53 gene revealed a novel mutation, a n insertion of a single T within codon 33, which resulted in a frame-s hift and early termination. The same mutation was observed in all 21T tumor cell lines. These results demonstrate a tumor cell-specific loss of p53 protein due to a frame-shift mutation, and suggest that p53 lo ss may occur at a relatively early step in breast tumorigenesis before metastatic seeding or emergence of tumor heterogeneity. In addition, the availability of normal and tumor-derived epithelial cells with kno wn p53 sequences from a single breast cancer patient should facilitate understanding of the p53 regulation in mammary cells.