Mutations of the Wnt antagonist AXIN2 (Conductin) result in TCF-dependent transcription in medulloblastomas

TY - JOUR

T1 - Mutations of the Wnt antagonist AXIN2 (Conductin) result in TCF-dependent transcription in medulloblastomas

AU - Koch, Arend

AU - Hrychyk, Aksana

AU - Hartmann, Wolfgang

AU - Waha, Anke

AU - Mikeska, Thomas

AU - Waha, Andreas

AU - Schüller, Ulrich

AU - Sörensen, Nils

AU - Berthold, Frank

AU - Goodyer, Cynthia G

AU - Wiestler, Otmar D

AU - Birchmeier, Walter

AU - Behrens, Jürgen

AU - Pietsch, Torsten

N1 - (c) 2007 Wiley-Liss, Inc.

PY - 2007/7/15

Y1 - 2007/7/15

N2 - 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.

AB - 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.

KW - Adolescent

KW - Adult

KW - Axin Protein

KW - Base Sequence

KW - Blotting, Western

KW - Cell Line, Tumor

KW - Cerebellum

KW - Child

KW - Child, Preschool

KW - Cytoskeletal Proteins

KW - DNA Methylation

KW - DNA Mutational Analysis

KW - Female

KW - Gene Expression Profiling

KW - Humans

KW - Infant

KW - Male

KW - Medulloblastoma

KW - Middle Aged

KW - Mutation

KW - RNA, Messenger

KW - Reverse Transcriptase Polymerase Chain Reaction

KW - Signal Transduction

KW - TCF Transcription Factors

KW - Transcription, Genetic

KW - Wnt1 Protein

KW - Journal Article

KW - Research Support, Non-U.S. Gov't

U2 - 10.1002/ijc.22675

DO - 10.1002/ijc.22675

M3 - SCORING: Journal article

C2 - 17373666

VL - 121

SP - 284

EP - 291

JO - INT J CANCER

JF - INT J CANCER

SN - 0020-7136

IS - 2

ER -