Predictive value of <i>TP53</i> RNAscope^® <i>in situ</i> hybridization and p53 immunohistochemistry for <i>TP53</i> mutational status in canine diffuse large B-cell lymphoma

<i>TP53</i> mutations are associated with short survival and poor treatment response in canine diffuse large B-cell lymphoma (cDLBCL). The expression of <i>TP53</i> by RNAscope^® <i>in situ</i> hybridization and p53 by immunohistochemistry (IHC) was investigated in 37 formalin-fixed paraffin-embedded cDLBCL, to assess their correlation with <i>TP53</i> mutational status and to evaluate their prognostic value. <i>TP53</i> was detected in all samples by RNAscope^®. Ten of 37 (27%) cases expressed p53 by IHC, with highly variable percentage of positive cells. <i>TP53</i> RNAscope^® scores and p53 IHC results were not correlated. The expression of <i>TP53</i> by RNAscope^® was not influenced by its mutational status. Conversely, p53 IHC and <i>TP53</i> mutations were significantly associated. p53 IHC predicted <i>TP53</i> genetic mutations with high accuracy (97.3%). All <i>TP53-</i>mutated samples carrying missense mutations exhibited p53 expression by IHC, while all wild-type cases and a single case with frameshift insertion were negative. In univariable analysis, p53 IHC was associated with shorter time to progression (TTP) and lymphoma-specific survival (LSS). Nevertheless, in multivariable analysis, only treatment significantly affected TTP and LSS. These findings suggest p53 IHC is an accurate, cost-effective tool for predicting <i>TP53</i> mutations in cDLBCL, unlike <i>TP53</i> RNAscope^®, though its prognostic value requires further validation.