Novel V600E BRAF mutations in imatinib-naive and imatinib-resistant gastrointestinal stromal tumors

Department of Pathology, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
Genes Chromosomes and Cancer (Impact Factor: 3.84). 10/2008; 47(10):853-9. DOI: 10.1002/gcc.20589
Source: PubMed

ABSTRACT BRAF and NRAS are commonly mutated in cancer and represent the most frequent genetic events in malignant melanoma. More recently, a subset of melanomas was shown to overexpress KIT and harbor KIT mutations. Although most gastrointestinal stromal tumors (GISTs) exhibit activating mutations in either KIT or PDGFRA, about 10% of the cases lack mutations in these genes. It is our hypothesis following the melanoma model that mutations in BRAF or NRAS may play a role in wild-type GIST pathogenesis. Alterations in RAS/MEK/ERK pathway may also be involved in development of imatinib resistance in GIST, particularly in tumors lacking secondary KIT or PDGFRA mutations. Imatinib-naive wild-type GISTs from 61 patients, including 15 children and 28 imatinib-resistant tumors without secondary KIT mutations were analyzed. Screening for hot spots mutations in BRAF (exons 11 and 15) and NRAS (exons 2 and 3) was performed. A BRAF exon 15 V600E was identified in 3 of 61 GIST patients, who shared similar clinical features, being 49- to 55-years-old females and having their tumors located in the small bowel. The tumors were strongly KIT immunoreactive and had a high risk of malignancy. An identical V600E BRAF mutation was also identified in one of 28 imatinib resistant GIST lacking a defined mechanism of drug resistance. In conclusion, we identified a primary BRAF V600E mutations in 7% of adult GIST patients, lacking KIT/PDGFRA mutations. The BRAF-mutated GISTs show predilection for small bowel location and high risk of malignancy. A secondary V600E BRAF mutation could represent an alternative mechanism of imatinib resistance. Kinase inhibitors targeting BRAF may be effective therapeutic options in this molecular GIST subset.

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    ABSTRACT: Gastrointestinal stromal tumour (GIST), originating from the interstitial cells of Cajal (ICCs), is characterized by frequent activating mutations of the KIT receptor tyrosine kinase. Despite the clinical success of imatinib that targets KIT, most advanced GIST patients develop resistance and eventually die of the disease. The ETS family transcription factor, ETV1, is a master regulator of the ICC lineage. Using mouse models of Kit activation and Etv1 ablation, we demonstrate that Etv1 is required for GIST initiation and proliferation in vivo, validating it as a therapeutic target. We further uncover a positive feedback circuit where MAP kinase activation downstream of KIT stabilizes the ETV1 protein and ETV1 positively regulates KIT expression. Combined targeting of ETV1 stability by imatinib and MEK162 resulted in increased growth suppression in vitro and complete tumour regression in vivo. The combination strategy to target ETV1 may provide an effective therapeutic strategy in GIST clinical management. Copyright © 2015, American Association for Cancer Research.
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