A modified yeast assay used on archival samples of localized prostate cancer tissue improves the detection of p53 abnormalities and increases their predictive value

Abstract

To determine the frequency and predictive value of p53 mutations in localized prostate cancer, comparing the accuracy of detection using immunohistochemistry (IHC) with a modified yeast assay, on archival tissue samples. Prostate cancer tissue was obtained from 98 patients who had >/= 2 years of clinical follow-up after radical prostatectomy. DNA sequencing was used to verify the presence of p53 mutations in samples that were immunopositive or that gave evidence for p53 alterations using the yeast assay. The IHC and yeast findings were compared with patient outcome to determine the predictive value of these two test types. Fifty-five tumours (57%) were immunopositive, and 58 (59%) were positive using the yeast assay. Sequence-confirmed p53 mutations occurred in 44 (45%) cases. The IHC protocol generated 49% (27/55) false-positive and 36% (15/42) false-negative results, and was 65% sensitive and 50% specific, with an overall accuracy of 57%. The yeast assay resulted in 24% (14/58) false-positive results with a specificity of 74% and an accuracy of 86%. When the p53 status of these patients was correlated with their clinical outcome, patients who had sequence-confirmed p53 mutations had a 2.6-fold greater failure rate (P = 0.026) and a 2.5-fold greater risk of dying from prostate cancer (P = 0.05). Notably, mutations in exon 6 predicted a six-fold increase in treatment failure (P = 0.043) and a 5.3-fold increase in the chance of dying from prostate cancer (P = 0.009). Abnormal yeast-assay findings gave similar predictive results to those obtained for DNA sequencing, while immunopositivity did not correspond to patient outcome. Mutations of p53 occurred in 45% of localized prostate cancers. These alterations have important prognostic implications. The yeast assay was more accurate for detecting p53 mutations than the IHC protocol used and, unlike IHC, the results of the yeast assay were predictive of patient outcome.