KIT mutations are common in incidental gastrointestinal stromal tumors one centimeter or less in size

Abstract

Gastrointestinal stromal tumors (GISTs) are mesenchymal neoplasms of the gut wall that express the receptor tyrosine kinase KIT. Somatic mutations that result in constitutive activation of KIT kinase have been identified in a number of studies of GISTs, although the reported frequency of these mutations has varied over a wide range (20 to 92%). Several reports have suggested that KIT gene mutations are more common in malignant GISTs than in benign lesions, and it has been proposed that mutations in exon 11 of KIT are a negative prognostic factor. To maximize sensitivity for KIT mutations we have adapted denaturing high-pressure liquid chromatography as a method for screening polymerase chain reaction amplimers of exons 9, 11, 13, and 17 from GIST genomic DNA. This approach was used to assess the frequency of KIT mutations in 13 morphologically benign, incidentally discovered, GISTs identified at autopsy, endoscopy, or laparotomy for unrelated disease. Representing the smallest pathologically recognizable GISTs, these lesions ranged in size from 4 to 10 mm in diameter and were all immunohistochemically positive for KIT. Eleven of the 13 tumors had sequence-confirmed mutations in KIT, including 10 mutations in exon 11 (77%) and one mutation in exon 9 (7.7%). The remaining two tumors were wild type for exons 9, 11, and 17; one of these was also analyzed for exon 13 and was wild type in this exon as well. The mutations found in the incidental GISTs were identical to those that have been documented in larger GISTs. In addition, the overall frequency of mutations in the incidental tumors (85%) did not differ significantly from that we previously reported in a series of 72 advanced/metastatic GISTs (86%), strongly supporting the view that activating mutations in KIT are acquired very early in the development of most GISTs. The findings suggest that KIT mutations per se are of little prognostic importance in GISTs.

Figure 1.

Incidental GISTs. A: H&E-stained section of case 6 showing a 10-mm tumor centered in the muscularis propria of the stomach. The lesion was discovered during a gastroesophagectomy for a large esophageal leiomyoma. B: H&E-stained section of case 11 showing a well-circumscribed 5-mm nodule that was removed from the serosal aspect of the small bowel during surgery for endometrial adenocarcinoma. C: Close-up of case 11. Note that entrapped smooth muscle cells of the muscularis propria are morphologically similar to the tumor cells. D: Same area of case 11 as in C, immunostained for KIT. The tumor cells are strongly positive whereas the smooth muscle cells are negative.

Figure 2.

D-HPLC elution profiles of KIT exon amplimers. A: Wild-type exon 11 amplimer (case 12) run at denaturing and nondenaturing temperatures. B: Exon 11 amplimer containing a point mutation (case 6) is resolved from wild-type amplimer at the denaturing temperature. C: Comparison of the elution profiles for exon 11 amplimers representing pure wild-type sequence (case 12), pure deletion mutation (case 7), and a mixture of wild-type and deletion mutations (case 8). In the latter case there are four separate peaks that represent homotypic and heterotypic combinations of mutant and wild-type strands after re-annealing. D: The exon 9 insertion mutation discovered in case 11 has a distinctive elution pattern in comparison with the wild-type amplimer from case 12.