title: Mutations of the Wnt antagonist AXIN2 (Conductin) result in TCF-dependent transcription in medulloblastomas.
abstract: Medulloblastomas (MBs) represent the most common malignant brain tumors in children. Most MBs develop sporadically in the cerebellum, but their incidence is highly elevated in patients with familial adenomatous polyposis coli. These patients carry germline mutations in the APC tumor suppressor gene. APC is part of a multiprotein complex involved in the Wnt signaling pathway that controls the stability of beta-catenin, the central effector in this cascade. Previous genetic studies in MBs have identified mutations in genes coding for beta-catenin and its partners, APC and AXIN1, which cause activation of Wnt signaling. The pathway is negatively controlled by the tumor suppressor AXIN2 (Conductin), a scaffold protein of this signaling complex. To investigate whether alterations in AXIN2 may also be involved in the pathogenesis of sporadic MBs, we performed a mutational screening of the AXIN2 gene in 116 MB biopsy samples and 11 MB cell lines using single-strand conformation polymorphism and sequencing analysis. One MB displayed a somatic, tumor-specific 2 bp insertion in exon 5, leading to carboxy-terminal truncation of the AXIN2 protein. This tumor biopsy showed nuclear accumulation of beta-catenin protein, indicating an activation of Wnt signaling. In 2 further MB biopsies, mutations were identified in exon 5 (Glu408Lys) and exon 8 (Ser738Phe) of the AXIN2 gene, which are due to predicted germline mutations and rare polymorphisms. mRNA expression analysis in 22 MBs revealed reduced expression of AXIN2 mRNA compared to 8 fetal cerebellar tissues. Promoter hypermethylation could be ruled out as a major cause for transcriptional silencing by bisulfite sequencing. To study the functional role of AXIN2 in MBs, wild-type AXIN2 was overexpressed in MB cell lines in which the Wnt signaling pathway was activated by Wnt-3a. In this assay, AXIN2 inhibited Wnt signaling demonstrated in luciferase reporter assays. In contrast, overexpression of mutated AXIN2 with a deleted C-terminal DIX-domain resulted in an activation of the Wnt signaling pathway. These findings indicate that mutations of AXIN2 can lead to an oncogenic activation of the Wnt pathway in MBs.
authors: Department of Neuropathology, University of Bonn Medical Center, Bonn, Germany. akoch1@uni-bonn.de
authors: Arend
authors: A
authors: Koch
authors: Aksana
authors: A
authors: Hrychyk
authors: Wolfgang
authors: W
authors: Hartmann
authors: Anke
authors: A
authors: Waha
authors: Thomas
authors: T
authors: Mikeska
authors: Andreas
authors: A
authors: Waha
authors: Ulrich
authors: U
authors: Schüller
authors: Nils
authors: N
authors: Sörensen
authors: Frank
authors: F
authors: Berthold
authors: Cynthia G
authors: CG
authors: Goodyer
authors: Otmar D
authors: OD
authors: Wiestler
authors: Walter
authors: W
authors: Birchmeier
authors: Jürgen
authors: J
authors: Behrens
authors: Torsten
authors: T
authors: Pietsch
mesh_terms: D000293:Adolescent; D000328:Adult; D060466:Axin Protein; D001483:Base Sequence; D015153:Blotting, Western; D045744:Cell Line, Tumor; D002531:Cerebellum / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D002648:Child; D002675:Child, Preschool; D003598:Cytoskeletal Proteins / Q000235:genetics* / Q000378:metabolism; D019175:DNA Methylation; D004252:DNA Mutational Analysis; D005260:Female; D020869:Gene Expression Profiling; D006801:Humans; D007223:Infant; D008297:Male; D008527:Medulloblastoma / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D008875:Middle Aged; D009154:Mutation; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology; D051820:TCF Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D051155:Wnt1 Protein / Q000235:genetics / Q000378:metabolism
chemical_list: C491761:AXIN2 protein, human; D060466:Axin Protein; D003598:Cytoskeletal Proteins; D012333:RNA, Messenger; D051820:TCF Transcription Factors; D051155:Wnt1 Protein