The novel marker, DOG1, is expressed ubiquitously in GI Stromal Tumors irrespective of KIT or PDGFA mutation status.

Accurate diagnosis of gastrointestinal stromal tumors (GISTs) has gained increasing importance since imatinib mesylate, an inhibitor of the KIT tyrosine kinase, was found to reduce tumor growth. Currently, the diagnosis of GISTs relies heavily on KIT (CD117) immunoreactivity. However, KIT- negative GISTs have been reported and some of these tumors have been shown to harbor a mutation in the PDGFRA gene. We recently characterized gene expression patterns in GISTs using cDNA microarrays, and found that the gene FLJ10261, encoding a hypothetical protein, was highly expressed in GISTs, compared to other mesenchymal tumors. Synthetic peptides based on the gene sequence, which we call DOG1, were used to generate a rabbit antiserum. The immunoreactivity of this antiserum was assessed on two soft tissue tumor microarrays (TMAs). The first TMA included duplicate cores of 460 soft tissue tumors, including 22 KIT-immunoreactive GISTs. The second TMA included 127 GIST cases for which the KIT and PDGFRA mutation status was known. 136 of 139 (97.8%) of scorable GISTs on these two arrays demonstrated immunoreactivity with DOG1 antisera. The immunohistochemistry results were confirmed by in situ hybridization for DOG1, KIT and PDGFRA. Other neoplasms in the differential diagnosis of GIST, including desmoid fibromatosis (0/17) and Schwannoma (0/3), were immunonegative for DOG1. Only four non-GIST cases, including two from the abdomen, were immunoreactive for DOG1, with one of these also reacting for KIT. All 7 GIST cases with a PDGFRA mutation were DOG1 positive, while most of these failed to react for KIT. In addition, 4 of 5 scorable PDGFRA mutants were also positive by PDGFRA in situ hybridization. Two DOG1-positive, KIT-negative GISTs on the first TMA were observed to be PDGFRA positive by in situ hybridization and subsequently found to harbor mutations in exon 18 of PDGFRA. Digital images of over 4,000 immunostains and in situ hybridizations are available at the accompanying website. DOG1, a protein of unknown function, is expressed strongly on the cell surface of GISTs and is rarely expressed in other soft tissue tumors. Use of antisera against DOG1 may aid in the diagnosis of GISTs, including those that fail to express KIT antigen.