A subset of gastrointestinal stromal tumors previously regarded as wild-type tumors carries somatic activating mutations in KIT exon 8 (p.D419del)

Abstract

About 10-15% of gastrointestinal stromal tumors (GISTs) carry wild-type sequences in all hot spots of KIT and platelet-derived growth factor receptor alpha (PDGFRA) (wt-GISTs). These tumors are currently defined by having no mutations in exons 9, 11, 13, and 17 of the KIT gene and exons 12, 14, and 18 of the PDGFRA gene. Until now, the analysis of further exons is not recommended. However, we have previously published a report on a KIT exon 8 germline mutation, which was associated with familial GIST and mastocytosis. We therefore investigated whether KIT exon 8 mutations might also occur in sporadic GIST. We screened a cohort of 145 wt-GISTs from a total of 1351 cases from our registry for somatic mutations in KIT exon 8. Two primary GISTs with an identical exon 8 mutation (p.D419del) were detected, representing 1.4% of all the cases analyzed. Based on all GISTs from our registry, the overall frequency of KIT exon 8 mutations was 0.15%. The first tumor originating in the small bowel of a 53-year-old male patient had mostly a biphasic spindled-epithelioid pattern with a high proliferative activity (14 mitoses/50 HPF) combined with a second low proliferative spindle cell pattern (4/50 HPF). The patient developed multiple peritoneal metastases 29 months later. The second case represented a jejunal GIST in a 67-year old woman who is relapse-free under adjuvant imatinib treatment. We conclude that about 1-2% of GISTs being classified as 'wild type' so far might, in fact, carry KIT mutations in exon 8. Moreover, this mutational subtype was shown to be activating and imatinib sensitive in vitro. We therefore propose that screening for KIT exon 8 mutations should become a routine in the diagnostic work-up of GIST and that patients with an exon 8 mutation and a significant risk for tumor progression should be treated with imatinib.

Figure 1

Mutational status of 1351 GIST cases in the Bonn–Cologne GIST registry (n=1351), KIT mutations were found in 72.99% and PDGFRA mutations in 16.43% of cases. The remaining 10.58% were classified as wt-GISTs. The overall frequency of KIT exon 8 mutations is 0.15%.

Figure 2

Histopathological and molecular features of a GIST with KIT exon 8 mutation. In case 1, epithelioid tumor cells with eosinophilic to clear cytoplasm were aggregated in an alveolar fashion ((a), × 200, H&E). A second tumor cell component showed a spindeled cell appearance and was situated in close proximity to the epithelioid areas ((b,c), × 200, H&E). Abundant necrosis was noted ((d), × 200, H&E). Metastatic tissue revealed epithelioid tumor cells ((e,f), × 200, H&E). Strong expression of the KIT receptor ((g), × 200, KIT). The p.D419del mutation occurred heterozygously in the primary tumor ((h), top) and homozygously in the metastasis ((h), bottom). FISH analysis showed polysomy by presenting multiple orange KIT signals as well as multiple green CEP4 signals in both the primary tumor and metastasis (i).

Figure 3

Histopathological and molecular features of a GIST with KIT exon 8 mutation. In case 2, a spindled cell morphology was noted ((a), × 200, H&E). Immunohistochemistry revealed strong positivity for DOG1 (b, × 200) and KIT (c). Sanger sequencing showed a heterozygous deletion in KIT exon 8 (p.D419del) (d).