Aims: Most gastrointestinal stromal tumors (GISTs) carry gain-of-function mutations of the KIT gene encoding the receptor tyrosine kinase KIT. However, in a subset of GISTs no activating mutations are detectable in KIT. Recently, PDGFRalpha mutations have been identified as alternative oncogenic mechanism. We studied a panel of 100 GISTs for mutations in hot spot regions of KIT (exons 9, 11, 13 and 17) and PDGFRalpha (exons 12 and 18) and compared the results with pathomorphological and immunohistochemical data.
Methods: DNA from formalin-fixed and paraffin-embedded tumor tissue was extracted after microdissection from serial sections. We performed single strand conformational polymorphism analysis and direct sequencing.
Results: We found PDGFRalpha mutations in 24 of 55 GISTs with wild-type sequence in exons 9, 11, 13 and 17 of KIT. All mutations were located in exon 18 of the PDGFRca gene which encodes the tyrosine kinase domain II. None of the 45 GISTs with detectable KIT mutation carried a mutation in the PDGFRalpha gene. Interestingly, all PDGFRalpha-mutated tumors were located in the stomach whereas GISTs with exon 9 and 13 KIT mutations occurred predominantly in the small bowel. Additionally, 21 of 24 GISTs carrying PDGFRalpha mutations displayed an epithelioid or mixed phenotype. In contrast, KIT-mutated GISTs exhibited almost always a spindled histology (38 of 45 cases).
Conclusions: Our analysis provides evidence that GISTS represent distinctive entities with different genetic, biological and phenotypic features.