Dedifferentiation in Gastrointestinal Stromal Tumor to an Anaplastic KIT Negative Phenotype – a Diagnostic Pitfall. Morphologic and Molecular Characterization of 8 Cases Occurring either de-novo or after Imatinib Therapy

摘要

Most GIST>s can be recognized by their monotonous cytologic features and overexpression of KIT oncoprotein. Altered morphology and loss of CD117 reactivity has been described previously after chronic imatinib treatment; however, this phenomenon has not been reported in imatinib-naïve tumors. Eight patients with abrupt transition from a classic CD117-positive spindle cell GIST to an anaplastic CD117-negative tumor were investigated for underlying molecular mechanisms of tumor progression. Pathologic and molecular analysis was performed on each of the two components. Genomic DNA PCR for KIT, PDGFRA, BRAF and KRAS hot spot-mutations and FISH for detecting KIT gene copy number alterations were performed. TP53 mutational analysis was performed in 5 cases. There were 7 males and 1 female, with an age range of 23–65 years. Five of the primary tumors were located in the stomach, while one case each originated in small bowel, colon and rectum. In 3 patients, the dedifferentiated component occurred in the setting of imatinib-resistance, while the remaining 5 occurred de novo. The dedifferentiated component had an anaplastic appearance, including one angiosarcomatous phenotype, with high mitotic activity and necrosis, and showed complete loss of CD117 (8/8) and CD34 (5/8) expression, and de novo expression of either cytokeratin (4/8) or desmin (1/8). There was no difference in the KIT genotype between the two components. However, two imatinib-resistant tumors showed co-existence of KIT exon 11 and exon 13 mutations. FISH showed loss of one KIT gene in 3 cases and low level amplification of KIT in 2 other cases in the CD117-negative component, compared to the CD117-positive area. TP53 mutation was identified in 1/5 cases tested, being present in both components. In summary, dedifferentiation in GIST may occur either de novo or after chronic imatinib exposure and can represent a diagnostic pitfall. This phenomenon is not related to additional KIT mutations, but might be secondary to genetic instability, either represented by loss of heterozygosity or low level of KIT amplification.