KIT mutation in a naïve succinate dehydrogenase-deficient gastric GIST

Up to 85% of gastrointestinal stromal tumors (GIST) harbor mutually exclusive mutations in the KIT or the PDGFRA gene. Among others, known as wild type GIST, succinate dehydrogenase (SDH)-deficient tumors develop due to genetic or epigenetic alterations in any of four SDH genes. Herein, we present a unique case of SDH-deficient GIST with an unusual heterogeneous SDHA and SDHB staining pattern and mutations detected in the SDHA and KIT gene. A 50-year-old patient presented with a 5 cm large gastric tumor with a multinodular/plexiform growth pattern, mixed epithelioid and spindle cell morphology, and focal pronounced nuclear atypia with hyperchromasia and high mitotic activity. Immunohistochemically, CD117 and DOG-1 were positive. SDHB and SDHA stains showed loss of expression in some of the nodules, whereas others presented with an unusually weak patchy positivity. Molecular analysis revealed a point mutation in exon 5 of the SDHA gene and a mutation in exon 11 of the KIT gene. We hypothesize that based on the allele frequency of SDHA and KIT mutations the tumor is best regarded as SDH-deficient GIST in which the SDHA mutation represents the most likely driver mutation. The identified KIT mutation raises the distinct possibility that the KIT mutation is a secondary event reflecting clonal evolution. This is the first case of a treatment naïve GIST harboring a somatic SDHA and a KIT mutation, challenging the dogma that oncogenic mutations in treatment naïve GIST are mutually exclusive.