Erill N1, Colomer A1, Verdú M2, Román R1, Vidal A2, Condom E2, Banús JM3, Cordón-Cardo C1,2, Puig X1,2.
1BIOPAT. Grup Assistència, 2HISTOPAT Laboratoris & 3ICUN. Institut Català d’Urologia i Nefrologia. Barcelona (Spain).
Mutations of the TP53 tumor suppressor gene and nuclear accumulation of abnormal p53 proteins are common in colon and bladder cancer, and have been associated with clinical aggressiveness and poor response to specific therapies. However, there is no general agreement regarding the optimal technical approach to define the TP53 status in clinical samples. We undertook this study to define the concordance between the detection of TP53 mutations and altered patterns of p53 expression in colon and bladder carcinomas.
We studied 100 primary colorectal carcinomas and 76 transitional cell carcinomas of the bladder by immunohistochemistry (IHC; PAb 1801 or mAb DO-7) and PCR-SSCP (exons 4-8) followed by direct sequencing of shifted bands. The correlation between both techniques was evaluated by Fisher’s exact test. The receiver-operating-characteristics (ROC) curve and the logistic-regression analysis were used to estimate the accuracy of IHC when predicting TP53 mutations.
In colorectal cancer, mutations were detected in 91% of the tumors displaying a positive phenotype. We also found that 51% of the negative phenotype cases exhibited mutations. According to the ROC curve and logistic-regression, the cutoff value was set at 15% tumor cells displaying nuclear staining, value on which errors were mainly balanced. The accuracy of predicting mutations by IHC was 79.3%.
Regarding bladder cancer, mutations were identified in 78% of the immunopositive tumors, but only in two cases (10%) of the negative phenotype group. Within the group of non-mutated tumors, 9% showed positive scoring between 20% to 30%. The cutoff value was set at 30%. The accuracy in this case was 88%.
Data from this study confirms that, although there is a strong correlation between phenotype and genotype, both approaches sustain serious discrepancies according to the tumor type. It can be concluded, based on accuracy estimates, that p53 assessment in colorectal tumors must be performed using a combination of both IHC and molecular techniques, especially in cases with a negative phenotype since frequently carry nonsense or frameshift mutations. However, the low incidence of these alterations in bladder tumors may justify the use of only IHC, molecular assay being less informative, except for those low stage tumors on which TP53 status may determine its progression to invasive disease.