BRAF and FBXW7 (CDC4, FBW7, AGO, SEL10) Mutations in Distinct Subsets of Pancreatic Cancer

Abstract

The recognition of biologically distinct tumor subsets is fundamental to understanding tumorigenesis. This study investigated the mutational status of the serine/threonine kinase BRAF and the cyclin E regulator FBXW7 (CDC4, FBW7, AGO, SEL10) related to two distinct pancreatic carcinoma subsets: the medullary KRAS2-wild-type and the cyclin E overexpressing tumors, respectively. Among KRAS2-wild-type carcinomas, 33% (3 of 9) contained BRAF V599E mutations; one of which was identified in the pancreatic cancer cell line COLO357. Among 74 KRAS2-mutant carcinomas, no BRAF mutations were identified. Among the KRAS2/BRAF wild-type carcinomas, no mutations within pathway members MEK1, MEK2, ERK1, ERK2, RAP1B, or BAD were found. Using pancreatic cancer microarrays and immunohistochemistry, we determined that 6% (4 of 46 and 5 of 100 in two independent panels) of pancreatic adenocarcinomas overexpress cyclin E. We identified two potential mechanisms for this overexpression including the amplification/gain of CCNE1 gene copies in the Panc-1 and Su86.86 cell lines and a novel somatic homozygous mutation (H460R, in one of 11 pancreatic cancer xenografts having allelic loss) in FBXW7, which was accompanied by cyclin E overexpression by immunohistochemistry. Both BRAF and FBXW7 mutations functionally activate kinase effectors important in pancreatic cancer and extend the potential options for therapeutic targeting of kinases in the treatment of phenotypically distinct pancreatic adenocarcinoma subsets.

Deregulated cyclin E promotes p53 loss of heterozygosity and tumorigenesis in the mouse mammary gland

“…When a fraction of the Cdk2 complex is active, cyclin E is phosphorylated on residues T62 and/or T380 (Clurman et al, 1996;Won and Reed, 1996;Strohmaier et al, 2001), among other probable sites (Welcker et al, 2003), targeting it for ubiquitination by SCF hCdc4/Fbw7 and subsequent degradation by the 26S proteasome. Although phosphorylation site mutations have not been reported in tumors, mutations in hCdc4/Fbw7 have been found in breast, ovarian, pancreatic, colorectal and endometrial carcinomas (Moberg et al, 2001;Spruck et al, 2002;Calhoun et al, 2003;Rajagopalan et al, 2004) and lymphomas (Qiu et al, 2003). Interestingly, endometrial (Ekholm-Reed et al, 2004b), pancreatic (Calhoun et al, 2003) and colorectal carcinomas (Rajagopalan et al, 2004) with hCdc4/Fbw7 mutations show loss of cyclin E cell cycle regulation, which is likely to be equivalent to constitutive expression of a less-degradable allele as with the model presented here.…”

“…Although phosphorylation site mutations have not been reported in tumors, mutations in hCdc4/Fbw7 have been found in breast, ovarian, pancreatic, colorectal and endometrial carcinomas (Moberg et al, 2001;Spruck et al, 2002;Calhoun et al, 2003;Rajagopalan et al, 2004) and lymphomas (Qiu et al, 2003). Interestingly, endometrial (Ekholm-Reed et al, 2004b), pancreatic (Calhoun et al, 2003) and colorectal carcinomas (Rajagopalan et al, 2004) with hCdc4/Fbw7 mutations show loss of cyclin E cell cycle regulation, which is likely to be equivalent to constitutive expression of a less-degradable allele as with the model presented here. These tumors have also been shown to exhibit high levels of aneuploidy (Hubalek et al, 2004), consistent with the hypothesis that deregulation of cyclin E relative to the cell cycle causes genomic instability leading to carcinogenesis.…”

The V599E BRAF mutation is uncommon in biliary tract cancers

“…Our report is consistent with the emerging evidence that while BRAF mutations are common in certain cancer types, such as colorectal and thyroid cancers and melanomas, they are rare to absent in many other malignancies, such as pancreatic, gastric, renal cell, and head and neck cancers. [26][27][28][29][30] In fact, even within one given cancer type, there exist dramatic differences in frequency of mutations based on tissue origin; thus while 80% of cutaneous melanomas harbor BRAF mutations, these are essentially absent in melanomas of the uveal tract. 19,31 The rarity (or absence) or BRAF mutations is biliary cancers implies that the V599E mutation is unlikely to be a useful biomarker for early detection of these cancers in bile fluids.…”

Concepts to Target MYC in Pancreatic Cancer

“…In a subset of PDACs, FBXW7 is mutated or not expressed at all (64,65). In addition to SCF FBXW7 , other E3 ligases such as SCF SKP2 contribute to the control of MYC (59).…”