Unique correlation between GTF2I mutation and spindle cell morphology in thymomas (type A and AB thymomas)

Aim Recent study has revealed frequent GTF2I mutation in thymomas, with the frequency being highest in types A and AB, followed by B1, B2, B3 and thymic carcinoma. This has led to the conclusion that GTF2I mutation correlates with more indolent histology subtype and better prognosis. In our study, the GTF2I mutation was tested in thymic epithelial tumours to investigate the relation between the mutation status and histology subtype. Methods The GTF2I mutation was tested in 111 thymic epithelial tumours by Sanger sequencing. Correlations between GTF2I mutation status and clinicopathological parameters were evaluated. Results There were 16 cases of type A, including atypical type, 37 type AB, 13 B1, 23 B2, 9 B3, 6 micronodular type, 2 metaplastic type and 5 thymic carcinomas. GTF2I mutation was seen in 78.6% of type A and 83.9% of type AB, while it was not expressed in type B, metaplastic type or thymic carcinoma (p<0.001). 75% of micronodular type also showed the mutation. Both thymoma histotype and stage were significantly associated with GTF2I mutation by univariate analysis. The presence of GTF2I mutation showed a trend towards a favourable prognosis, but this is likely due to their strong association with more indolent histologic subtypes (types A and AB). Conclusions GTF2I mutation appears unique in type A and AB thymomas, including those with atypical features and micronodular type, all of which share spindle cell morphology, indicating they represent a group biologically distinct from type B thymomas.