Klin Padiatr 2011; 223 - A3
DOI: 10.1055/s-0031-1292584

FAM131B-BRAF Fusion Gene Resulting From 7q34 Deletion Leads to MAPK Pathway Activation in Pilocytic Astrocytoma

H Cin 1, C Meyer 2, R Herr 3, WG Janzarik 4, 5, S Lambert 6, DTW Jones 1, K Jacob 7, A Benner 8, H Witt 1, 9, M Remke 1, 9, S Bender 1, 9, F Falkenstein 10, TN Van Anh 5, H Olbrich 5, 11, A von Deimling 12, 13, A Pekrun 14, AE Kulozik 9, A Gnekow 10, W Scheurlen 15, O Witt 9, 16, H Omran 11, N Jabado 7, 17, VP Collins 6, T Brummer 3, R Marschalek 2, P Lichter 1, A Korshunov 12, 13, SM Pfister 1, 9
  • 1Division of Molecular Genetics, German Cancer Research Center, Heidelberg, Germany
  • 2Institute of Pharmaceutical Biology, Diagnostic Center of Acute Leukemia (DCAL), Goethe-University, Frankfurt, Germany
  • 3Centre for Biological Systems Analysis (ZBSA), Centre for Biological Signalling Studies BIOSS and Faculty of Biology, Albert-Ludwigs-University Freiburg, Germany
  • 4Department of Neurology, University Hospital Freiburg, Germany
  • 5Department of Pediatric Neurology and Muscle Disorders, University Hospital Freiburg, Germany
  • 6Division of Molecular Histopathology, Department of Pathology, University of Cambridge, UK
  • 7Departments of Human Genetics, McGill University Health Center, Montreal, Canada
  • 8Division of Biostatistics, German Cancer Research Center (DKFZ), Heidelberg, Germany
  • 9Department of Pediatric Oncology, Hematology and Immunology, University Hospital Heidelberg, Germany
  • 10Department of Pediatrics, Klinikum Augsburg, Augsburg, Germany
  • 11Clinic and Polyclinic for Pediatrics, Department of General Pediatrics, University Hospital Muenster, Germany
  • 12Department of Neuropathology, University Hospital Heidelberg, Germany
  • 13Clinical Cooperation Unit Neuropathology, German Cancer Research Center, Heidelberg, Germany
  • 14Klinikum Bremen Mitte gGmbH, Prof-Hess-Kinderklinik, Klinikum Bremen-Mitte, Bremen, Germany
  • 15Cnopf'sche Kinderklinik, Nürnberg Children's Hospital, Nürnberg, Germany
  • 16Clinical Cooperation Unit Pediatric Oncology, German Cancer Research Center, Heidelberg Germany
  • 17Departments of Pediatrics, Montreal Children's Hospital, McGill University Health Center, Montreal, Canada

Pilocytic astrocytoma (PA) constitutes the most common pediatric brain tumor. Despite the benign clinical behavior reported for PA, patients with tumors arising in inaccessible anatomical locations, show a poorer prognosis and tumor recurrence remains a great clinical challenge. However, there has been a paucity of genetic markers regarding PA tumorigenesis. Recently, we and others have shown that tandem duplication at 7q34, resulting in KIAA1549-BRAF fusion genes and constitutive activation of the MAPK signaling pathway, is a hallmark genetic lesion in PA development. Alternative mechanisms of MAPK activation include BRAF and KRAS point mutations, RAF1 fusions, and Neurofibromatosis-associated NF1 mutations.

In order to define the spectrum of underlying genetic driver alterations leading to aberrant MAPK activation, we screened 125 PA from all anatomic sites of the brain for mutations in KRAS, NRAS, PTPN11, BRAF and RAF1 and performed multiplex and long-distance inverse (LDI) PCR to identify BRAF and RAF1 fusion genes. To define the mechanism giving rise to the formation of novel BRAF fusion genes and understand their phenotype, we used microarray-based comparative genomic hybridization (aCGH) and performed in vitro experiments for functional assessment.

We characterized fusions targeting RAF kinases in 72% (90/125) of cases. Sequencing confirmed all previously reported variants of KIAA1549-BRAF and yielded novel variants of this fusion gene and of SRGAP3-RAF1. Mutations in KRAS and BRAF, as well as RAF1 gene fusions were mostly mutually exclusive with BRAF rearrangements. Most importantly, we identified FAM131B as an alternative fusion partner of BRAF, resulting from an interstitial deletion of ˜2.5 Mb at 7q34, in 3 cases. Investigating the functional characteristics of FAM131B-BRAF, we demonstrated constitutive MEK phosphorylation and activation of MAPK downstream effectors. Furthermore, we showed that FAM131B-BRAF is able to transform NIH3T3 cells.

Taken together, our findings further stress the role of RAF kinase fusions as a central oncogenic mechanism in the development of PA, and strengthen their potential role as both a tumor-specific marker for molecular diagnosis and an ideally suited target for molecular therapeutic intervention.