A novel primary KIT exon 9 single nucleotide substitution c.1427G>T (p.Ser476Ile) in a gastrointestinal stromal tumor

Abstract

Dear Sir: Gastrointestinal stromal tumors (GISTs) are mesenchymal tumors that originate from the interstitial cells of Cajal. These neoplasms are characterized by the expression of the receptor tyrosine kinase KIT (CD117, c-Kit), and mutations in the ckit proto-oncogene resulting in constitutive activation of the receptor are associated with the tumorigenesis of most GISTs [1]. For unresectable and metastatic tumors, the multitargeted tyrosine kinase inhibitor imatinib mesylate is considered the first choice of treatment. We report the case of a 44-year-old male who initially presented with a GIST of the stomach measuring 16.0 cm in August 2007 at his local hospital. The tumor was incompletely resected by partial gastrectomy, and adjuvant imatinib at 400 mg/d was started in September 2007. Following 15 months of imatinib, the patient developed hepatic and peritoneal metastases (January 2009). He then asked for a recommendation for further treatment at our sarcoma center. We recommended a dose escalation of imatinib to 800 mg/d. Computed tomography showed disease stabilization at first follow-up in March 2009 but a progression of the disease 3 months later. The patient’s treatment regimen was changed to 37.5 mg/d of sunitinib in a continuous daily schedule, which resulted in disease stabilization for 6 months and, subsequently, progressive hepatic disease. The histopathological analysis revealed a mixed type GIST (Fig. 1a, b) with a mitotic index of ten mitoses/50 high-power fields (1HPF=0.23 mm). Immunohistochemical staining of the GIST showed a positive reaction with antibodies against c-KIT, CD34, and DOG1. The assessment of tumor behavior, which was performed following the NIH consensus guidelines [2] or the classification proposed by Miettinen and Lasota [3], revealed a “high-risk” for malignancy. We performed a sequence analysis of KIT exons 9, 11, 13, and 17 as well as PDGFRα exons 12 and 18 in the primary tumor (central tumor section) and adjacent nonneoplastic formalin-fixed and paraffin-embedded tissue. The analysis was carried out by PCR amplification (Primus 25 Thermocycler, MWG, Ebersberg, Germany) followed by direct sequencing of both the forward and reverse strands with an ABI 3130 Genetic Analyzer (Applied Biosystems, Foster City, CA, USA). We used two independent PCR reactions for sequence analysis, which was additionally confirmed by a second test in a different institution (GenOPath GbR Bonn, Germany). As a result of the genetic analysis of the tumor tissue, we identified a novel primary KIT exon 9 single nucleotide substitution c.1427G>T (p.Ser476Ile) that was not detectable in the adjacent normal tissue (Fig. 2). The electropherogram of the tumor tissue showed almost no wild-type component, so that a homozygous mutation status is very likely. It was shown that the loss of the wild-type allele may be present at F. Grabellus (*) : S.-Y. Sheu :K. W. Schmid :K. Worm Institute of Pathology and Neuropathology, University Hospital Essen, University of Duisburg-Essen, Hufelandstrasse 55, 45122 Essen, Germany e-mail: florian.grabellus@uk-essen.de