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Institute for Clinical Pathology, Department of Pathologie, University Hospital Zürich, Schmelzbergstr. 12, CH-8091, Switzerland
1 Institut for Clinical Pathology, Kantonsspital Baden, Switzerland
(Requests for offprints should be addressed to A Perren; Email: Aurel.Perren{at}usz.ch)
| Abstract |
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We examined 130 endocrine tumors of the pancreas, parathyroid gland, adrenal medulla, paraganglia, lung and gastrointestinal tract as well as follicular and c-cell-derived thyroid tumors. We found a high rate of V559E mutations in papillary thyroid carcinomas (47%), one V599E mutation in a well-differentiated gastric endocrine carcinoma (malignant carcinoid), but no activating BRAF mutations in all other endocrine tumors examined. These results point towards different pathways in tumorigenesis of endocrine tumors of various localizations and only rare involvement of the MAP kinase (MAPK) pathway in a subset of malignant neuroendocrine tumors.
| Introduction |
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| Materials and Methods |
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Fresh frozen tissue and paraffin blocks were obtained from the files of the Department of Pathology, University Hospital Zurich. A total of 130 tumors were examined including 25 PETs, 10 PTAs, 16 PCCs (all without Ret mutations), 18 PGLs, 8 B-NETs and 6 GI-NETs (carcinoids, CDs), 25 MTCs (including one multiple endocrine neoplasia type 2a (MEN2a)-associated tumor, 7 tumors with somatic Ret exon 16 mutations and 13 tumors without Ret mutation), 7 FTCs and 15 PTCs. Tumor samples and corresponding normal tissue had been snap-frozen in liquid nitrogen after surgical removal and stored at 80°C. Where no fresh tissue was available, paraffin blocks of tumor and non-neoplastic tissue were used. These tissue samples had been fixed in 4% buffered formalin and embedded in paraffin according to standard protocols.
Controls
Tumor DNA of a paraffin-embedded malignant melanoma with known BRAF V599E mutation was used as positive control; blood DNA of a healthy individual served as negative control.
Denaturing gradient gel electrophoresis (DGGE)-based mutation analysis
DNA from fresh frozen tissue was extracted using the Purgene-kit (GentraSystems, Minneapolis, MN, USA) according to the manufacturers recommendations. Where no frozen tissue was available, tumor and normal tissue were micro-dissected from 10 µm tissue sections of paraffin blocks and the DNA extracted as previously described (Perren et al. 1998).
PCR using genomic DNA as template was carried out in a 50µl mixture of 1xPCR buffer (Perkin Elmer Europe, Rotkreuz, Switzerland) containing 400 ng of template DNA, 200 µM dNTP (Roche), 1 µM of each intron-based primer (Table 1
) and 1 µl Taq Polymerase (Ampli Taq Gold, Perkin Elmer Europe). A touch-down procedure was used consisting of denaturation for 5 s at 95°C, annealing for 60 s at temperatures decreasing from 60 to 55°C during the first 11 cycles (with 0.5°C decremental steps in cycles 2 to 11), and ending with an extension step at 72°C for 60 s. Ten cycles with an annealing temperature of 55°C and 15 cycles with an annealing temperature of 45°C followed with extension times of 90 s. After a final extension for 10 min at 72°C, heteroduplex formation was induced by initial denaturation for 10 min at 98°C followed by incubations at 55°C for 30 min and 37°C for 30 min. For DGGE, 10 µl of the PCR product in 3 µl Ficoll-based loading buffer were loaded onto 10% polyacrylamide gels containing a urea-formamide gradient in 0.5xTrisacetate TAEEDTA. The amplicons were electrophoresed at 60°C and 100 V for 16 h. DNA strands were visualized using silver staining as described previously (Komminoth et al. 1994). All samples were additionally cycle sequenced using an automated sequencer (Model 373A, Applied Biosystems, Foster City, CA, USA) and the Sequencher v.3.1.1 (Gene Codes Corp., Ann Arbor, MI, USA) software.
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| Results |
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One gastric, well-differentiated endocrine carcinoma (malignant carcinoid) with liver metastasis showed a somatic BRAF V599E mutation, no alterations were detected in the remaining 5 gastrointestinal CDs.
No alterations indicative for mutations of BRAF exon 11 and 15 were identified in any of the 25 PETs, 10 PTAs, 16 PCCs, 18 PGLs, 25 MTCs, 7 FTCs and 8 bronchial CDs tested. The results are summarized in Table 2
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| Discussion |
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BRAF is a member of the MAPK pathway inducing a mitogenic response upon stimulation of tyrosine kinase receptors (Liebmann 2001). In colon carcinomas, BRAF mutations were only found in tumors without K-RAS mutations, indicating an equivalent tumorigenic effect (Davies et al. 2002, Rajagopalan et al. 2002). A similar exclusive correlation was observed in PTCs for BRAF mutations and Ret/PTC translocations. Kimura et al.(2003) and Soares et al.(2003) showed no overlap between tumors with Ret/PTC translocation, BRAF or RAS mutations in 78 and 50 PTCs respectively, indicating that in PTCs the two alterations lead to very similar downstream effects. Our series included one PTC with an inactivating PTEN mutation and concomitant loss of PTEN protein expression (Dahia et al. 1997, Gimm et al. 2000), but no BRAF exon 15 mutation. As BRAF is known to be downstream of G protein-coupled receptor (GPCR) signaling (Liebmann 2001), the inactivating mutation of the tumor suppressor PTEN could lead to a similar downstream effect.
The work presented is the first study examining BRAF in endocrine tumors other than PTCs and FTCs. We detected one somatic BRAF V599E mutation in a metastasizing endocrine tumor of the stomach (malignant carcinoids). This is evidence for a rare involvement of the MAPK pathway in these tumors. The fact that a malignant endocrine tumor bears this mutation might be indicative for an event of tumor progression. Interestingly, we previously described a similar phenomenon of a PET harboring a PTEN mutation (Perren et al. 2000) which also turned out to be of a malignant phenotype.
The absence of BRAF exon 11 and 15 mutations in all other examined types of endocrine tumors including the neural-crest-derived MTC, PGL and PCC seems interesting. In addition to MEN2-associated familial MTC, PCC and PTA, a subset of sporadic forms of the first two tumors are also known to harbor activating Ret tyrosine kinase receptor mutations (Komminoth et al. 1995, van der Harst et al. 1998). However, we did not detect any BRAF mutation in sporadic MTCs, PCCs and PGLs. The Ret mutation status of the MTCs and PCCs of this study was known and 7 MTC samples with somatic Ret M918T mutation as well as one MEN2a (C634Y)-associated MTC were included (Table 2
). Somatic or germline Ret mutations were absent in all 13 informative PCC samples. Therefore BRAF mutations are absent in these tumors irrespective of Ret mutation status. These data are indirect evidence that signaling of the constitutively active forms of mutated Ret differs from Ret/PTC translocation induced signaling. In neural-crest-derived tumors, pathways other than the MAPK downstream pathway of Ret seem to be of importance. On the other hand, FTCs are more likely to harbor RAS mutations than PTCs; however, we and others could not detect any BRAF V599E mutation in FTCs.
In summary our study confirms a high rate of BRAF V559E mutations in PTCs. We detected a single V599E mutation in a metastasizing gastric endocrine carcinoma which might point towards a role of the MAPK pathway in malignant transformation of these tumors. BRAF mutations were absent in all neural-crest-derived tumors including MTCs, PCCs and PGLs. As a subset of these tumors is known to contain activating Ret mutations, our results indicate that pathways other than the MAPK pathway are downstream targets of Ret mutations in these tumors.
| Funding |
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| References |
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