Gastric mesenchymal tumor with PTCH1::GLI2 fusion: a distinct subset of GLI1/GLI2-altered tumors?

Gastric mesenchymal tumors with GLI2 gene fusions are extremely rare, and only two cases, both with PTCH1::GLI2, have been reported. Here, we describe a third case involving a 47-year-old man. The tumor, measuring 4.5 cm in size, was distributed throughout the gastric wall transmurally with an infiltrative growth pattern. It had two spindle cell components, one arranged in tight bundles and the other in a loose reticular to vague fascicular pattern, and an epithelioid component lacking glandular formation. Immunohistochemically, CAM5.2 showed focal positivity in all components, SMA was positive in both spindle cell components, whereas desmin was positive only in the former. No recurrence was observed over 5 years. RNA sequencing identified a PTCH1::GLI2 fusion with GLI1, GLI2, and PTCH1 upregulation. The breakpoint was identical to those in the previously reported cases. Taken together, these findings suggest that PTCH1::GLI2 fusion may define a distinctive subtype of gastric mesenchymal tumors within the GLI1/GLI2-altered spectrum.