YAP1 and β-Catenin Immunohistochemistry as a Surrogate Marker for GTF2I-Mutant Type A/AB Thymomas

Thymomas are rare thymic epithelial tumors classified by the World Health Organization into type A/AB thymomas, which commonly harbor GTF2I mutations and behave indolently, and type B thymomas and thymic carcinomas, in which these mutations are less common. Type A and AB thymomas are uniquely enriched for a recurrent somatic hotspot mutation in GTF2I (p.L424H); yet this gene is rarely included in clinical sequencing panels, limiting its diagnostic utility. YAP1, the principal effector of the Hippo signaling pathway, and β-catenin, the central transcriptional effector of the Wnt pathway, have emerging roles in thymoma biology; however, their relationship to GTF2I mutation status and histologic subtype has not been systematically characterized. We analyzed the TCGA thymoma dataset and an institutional cohort of 38 thymic epithelial tumors to evaluate YAP1 and β-catenin immunohistochemistry (IHC) as surrogate markers for GTF2I mutation status and histologic classification. In the TCGA dataset, YAP1 and CTNNB1 mRNA expression were markedly elevated in type A/AB thymomas relative to type B and carcinoma subtypes, and GTF2I-mutant tumors exhibited significantly higher YAP1 mRNA expression than GTF2I-wildtype tumors. Targeted next-generation sequencing of our institutional cohort confirmed enrichment of the canonical GTF2I p.L424H hotspot in indolent subtypes. By IHC, nuclear YAP1 positivity and cytoplasmic β-catenin localization were both significantly more frequent in indolent thymomas. Cytoplasmic β-catenin demonstrated high specificity (94%) for indolent histology, supporting its use in diagnostically challenging cases such as the type A versus B3 distinction on small biopsies. YAP1 IHC showed a high negative predictive value for GTF2I mutations, such that a YAP1-negative result reliably excludes a GTF2I-mutant tumor. These findings implicate crosstalk between Hippo and Wnt signaling in GTF2I-mutant thymomas and position YAP1 and β-catenin IHC as accessible, cost-effective surrogates for molecular subtyping in a tumor where standard sequencing panels have limited coverage.