Loss of DOG-1 expression associated with shift from spindled to epithelioid morphology in gastric gastrointestinal stromal tumors with KIT and platelet-derived growth factor receptor α mutations.

Most gastric gastrointestinal stromal tumors (GISTs) display spindle cell morphology and coexpress CD117 (KIT), DOG-1, and CD34. Secondary loss of DOG-1 has not been reported. We present two gastric GISTs which showed loss of DOG-1 in the epithelioid component but retained its expression in the minor spindle cell component. Patients were a 67-year-old man and an 80-year-old woman with 4.8-cm and 3.5-cm gastric GISTs harboring mutations in KIT exon 11 (c.1729_1758dup30; p.P577_R586dup) and platelet-derived growth factor receptor α (PDGFRA) exon 18 (c.2527_2538del12; p.I843_D846del), respectively. Both were predominantly epithelioid with a minor microscopic spindle cell component (3-12 mm). The spindle cell component was CD117(+)CD34(+)DOG-1(+) in both cases. The epithelioid component in case 1 was CD117(+)CD34(+)DOG-1(-). In case 2, the epithelioid component strongly expressed PDGFRA (dot-like) but lost CD117, CD34, and DOG-1. These cases confirm the immunophenotypic heterogeneity as secondary events in GIST. Loss of DOG-1 in KIT-negative PDGFRA mutants should not preclude diagnosis.