Background : The p53 (also known as TP53) tumor suppressor gene encodes for a nuclear phosphoprotein thought to regulate proliferation of normal cells. Most p53 mutations result in a nonfunctional protein that accumulates in tumor cell nuclei. These common mutations appear to be involved in the development and/or progression of several neoplastic diseases including human breast cancer. Purpose : Our purpose was to investigate the relationships between levels of mutant p53 protein expression, tumor cell proliferation rate, and clinical outcome in patients with node-negative breast cancer. Methods : Expression of mutant p53 protein was evaluated by frozen-section immunohistochemistry (IHC) and light microscopy in 700 breast cancers from axillary lymph node-negative patients with long-term follow-up (median, 54 months). The immunostaining signal was expressed as the sum of scores representing the proportion and staining intensity of negative and positive tumor cell nuclei (ranges, 0 and 2–8, respectively). Statistical comparisons were made between levels of p53 protein expression and disease-free survival, overall survival, and tumor proliferation rate expressed as the percentage of cells in the S phase (%S phase) as determined by flow cytometry. Results : Of the 700 tumors, 362 (52%) showed positive nuclear immunostaining (IHC score >0). Proliferation rates were significantly higher ( P = .0001) in positive tumors (median %S phase, 7.1%) than in negative tumors (4.1%). In a univariate cutpoint analysis, negative tumors (n = 388) versus low-positive tumors (IHC score = 2–6; n = 263) versus high-positive tumors (IHC score >6; n = 99) showed progressively reduced disease-free survival (80% versus 72% versus 58% at 5 years, respectively; P ≤.05 for all pairwise comparisons). Analogous results for overall survival were 88% versus 84% versus 74%; only the result for negative versus high positive tumors was significant ( P = .003). In a multivariate analysis, expression of p53 protein and high %S phase were independently associated with reduced disease-free survival ( P = .008 and .01, respectively). Conclusions : Expression of mutant p53 protein was associated with high tumor proliferation rate, early disease recurrence, and early death in node-negative breast cancer. Despite the strong direct correlation between accumulation of p53 protein and tumor proliferation rate, both factors were independently associated with poor prognosis, suggesting that p53 may have other biological functions in addition to cell-cycle regulation. Implications : This test, when combined with other prognostic factors, may enhance our ability to identify node-negative breast cancer patients at high risk for early disease recurrence and/or death, for whom the use of adjuvant chemotherapy is unequivocally justified. [J Natl Cancer Inst 85:200–206, 1993]

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