Concomitant KIT/BRAF and PDGFRA/BRAF mutations are rare events in gastrointestinal stromal tumors.

AIM: The BRAF mutation is a rare pathogenetic alternative to KIT/PDGFRA mutation in GIST and causes Imatinib resistance. A recent description of KIT and BRAF mutations co-occurring in an untreated GIST has challenged the concept of their being mutually exclusive and may account for ab initio resistance to Imatinib, even in the presence of Imatinib-sensitive KIT mutations. BRAF sequencing is generally limited to KIT/PDGFRA wild-type cases. Hence, the frequency of concomitant mutations may be underestimated. METHODS: We screened for KIT (exon 9, 11 ,13 ,17), PDGFRA (exon 12,14, 18) and BRAF (exon 15) mutations a series of 407 GIST. Additionally, we evaluated the BRAF V600E mutation-specific antibody, VE1, as a surrogate for V600E mutation, on a series of 313 GIST (24 on whole sections, 288 cases on tissue array), including 6 cases molecularly ascertained to carry the BRAF V600E mutation. RESULTS: No concomitant KIT/BRAF or PDGFRA/BRAF mutations were detected. BRAF mutation was detected only in one case, wild-type for KIT/PDGFRA. All the 6 BRAF-mutant cases stained positive with the VE1 antibody. A weak VE1 expression was observed in 14/287 (4.9%) BRAF wild-type cases, as observed also in 2/6 BRAF-mutant cases. Overall in our series, sensitivity and specificity of the VE1 antobody were 100% and 95.1%, respectively. CONCLUSIONS: The concomitance of BRAF mutation with either KIT or PDGFRA mutation is rare in GIST. In these tumors, moderate/strong VE1 immunoreactivity is a valuable surrogate for molecular analysis. Instead, genotyping is warranted in the presence of weak VE1 staining.

KIT, PDGFRA, and BRAF mutational spectrum impacts on the natural history of imatinib-naive localized GIST: a population-based study.

The mutation status of KIT or PDGFRA notoriously affects the response of advanced gastrointestinal stromal tumors (GISTs) to tyrosine kinase inhibitors. Conversely, it is currently still unclear whether mutation status impinges on the prognosis of localized, untreated GISTs. Hence, at present, this variable is not included in decision making for adjuvant therapy. A series of 451 primary localized GISTs were analyzed for KIT, PDGFRA, and BRAF mutations. Univariable and multivariable analyses and a backward selection procedure were used to assess the impact of mutation status on overall survival and to identify prognostically homogenous groups. Mutation was a significant prognostic indicator of overall survival in naive, localized GISTs (P<0.001): KIT-mutated patients had a worse outcome than PDGFRA-mutated or triple-negative (KIT, PDGFRA, BRAF wild-type) cases. Multivariable Cox regression models allowed us to identify 3 molecular risk groups: group I exhibited the best outcome and included PDGFRA exon 12, BRAF, and KIT exon 13-mutated cases; group II, of intermediate clinical phenotype (HR=3.06), included triple-negative, KIT exon 17, PDGFRA exon 18 D842V, and PDGFRA exon 14-mutated cases; group III displayed the worst outcome (hazard ratio=4.52), and comprised KIT exon 9 and exon 11 and PDGFRA exon 18 mutations apart from D842V. This study highlights the prognostic impact of mutation status on the natural course of GIST and suggests that the molecular prognostic grouping may complement the conventional clinicopathologic risk stratification criteria in decision making for adjuvant therapy.