Differential expression of KIT/PDGFRA mutant isoforms in epithelioid and mixed variants of gastrointestinal stromal tumors depends predominantly on the tumor site

Gastrointestinal stromal tumors (GISTs) form a distinctive group of mesenchymal neoplasms, showing differentiation towards the interstitial cells of Cajal. Morphologically, GISTs vary from cellular spindle cell tumors to epithelioid or mixed, epithelioid and spindle cell variants. The genotypic features underlying the morphologic differences of GISTs with vs without epithelioid components are not well defined. Acquisition of activating mutations in KIT and PDGFRA has been reported as alternative oncogenic events in the pathogenesis of GISTs. In this study, a comprehensive KIT and PDGFRA mutational analysis was performed in a group of 28 epithelioid/mixed type tumors, in order to explore whether a specific KIT/PDGFRA mutational status segregates these neoplasms from spindle cell variant GISTs. All GISTs were primary neoplasms, 16 (57.1%) originated from the stomach and 12 (42.8%) from other locations. Histomorphologically, 14 GISTs showed an epithelioid and 14 a mixed cell type pattern. Mutational analysis included KIT exons 9, 11, 13, and 17, and PDGFRA exons 12 and 18 prescreening by denaturing high-performance liquid chromatography, followed by direct sequencing. Activating mutations of KIT were found in 14 (50%) GISTs, the majority being within exon 11 (n=11; 39.2%), and the other comprised exon 9 AY 502-503 duplications (n=2; 7.2%) and exon 17 Lys --> Aln822 missense mutations (n=1; 3.6%). Most of the KIT mutant tumors (n=11; 78.6%) originated from nongastric sites. Seven (25.0%) GISTs with no detectable KIT mutations demonstrated PDGFRA mutant isoforms, carrying either D842 V mutations (n=5) or exon 18 deletions (n=2). All GISTs harboring PDGFRA mutant isoforms originated from the stomach. In seven tumors, no detectable mutations were found; all but one of nonmutant tumors initiated from the stomach and exhibited an epithelioid morphology. These findings indicate that the mutational status of epithelioid/mixed GISTs associates with the anatomical site of the tumor.

Immunohistopathological and molecular genetic features of a case in which gastrointestinal stromal tumor recurred five times

Gastrointestinal stromal tumor (GIST) is a distinct group of mesenchymal neoplasms recently shown to exhibit differentiation toward interstitial cells of Cajal. Although previous studies have shown that the clinical outcome of patients with GIST is associated with mitotic activity, the proliferation index determined by the Ki-67 labeling index, immunophenotype (CD34 and/or p53) and mutation in exon 11 of the c-kit, a definitive discrimination between benign and malignant GIST has not yet been established. We report a patient in whom malignant GIST in the abdomen recurred five times. In this case, the primary GIST and the five recurrent GIST were associated with c-kit immunoreactivity, but the mitotic index of the GIST tended to be increasingly higher with subsequent recurrences. Mutational analysis of the c-kit revealed that the primary and recurrent GIST were mutant-negative. These data indicated that 'morphologically appearing benign' tumors with lower proliferative parameters may also have the capacity of metastasis and recurrence.

[Gastrointestinal stromal tumors carrying PDGFRalpha mutations occur preferentially in the stomach and exhibit an epithelioid or mixed phenotype]

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.