Abstract 2977: Myc and Miz1 in medulloblastoma
Authors: Vo BaoHan T. , Wolf Elmar , Kawauchi Daisuke , Rehg Jerold , Finkelstein David , Murphy Brian , Eilers Martin , Roussel Martine F. 
Published: May 2015

Abstract
Medulloblastoma (MB) is the most common malignant pediatric brain tumor, which arises in the cerebellum. Human MBs are classified into four subgroups: Wingless (WNT), Sonic Hedgehog (SHH), Group 3 (G3) and Group 4 (G4). In 2012, we developed a mouse model of the most aggressive and least curable G3 MB, by overexpressing MYC but not MYCN, in Trp53-null cerebellar granule neuron progenitors (GNPs), which were implanted into the cortices of recipient CD-1 nu/nu mice. Remarkably, enforced expression of MYCN in Trp53-null GNPs induced a SHH MB. This was unexpected since MYC and MYCN bind to the same E-box DNA binding sequences. We hypothesized that the difference between MYC- and MYCN-induced MBs might be due to their interaction with different partners in GNPs. Human G3 MBs are distinguished by MYC overexpression. The MYC proto-oncogene encodes a bHLH/leucine-zipper transcription factor that forms heterodimers with MAX to drive transcription. However, MYC can repress transcription when the MYC/MAX heterodimer is recruited to core promoter sequences by MIZ1, a member of the POZ-domain/zinc-finger transcription factor. MIZ1, ubiquitously expressed during embryonic development, can either be a transcription activator or repressor depending on its binding partners.
To investigate whether the MYC/MIZ1 complex contributed to G3 MBs, we used a mutant of MYC which no longer binds to MIZ1 in which a Valine is substituted for an Aspartic acid amino acid at position 394 (MYCV394D). We enforced the expression of MYC or MYCV394D in GNPs purified from seven-day-old Trp53-null mouse pups using retroviral transduction, and implanted transduced GNPs into the cerebral cortices of recipient mice. MYCV394D-induced tumors developed later than MYC-induced G3 MBs. Histopathological analysis revealed that MYC-induced tumors exhibited a large cell/anaplastic (LCA) phenotype and were classified as G3 MBs; however, MYCV394D-induced tumors were described as unclassified primitive neuroectodermal tumors. In vitro, MYCV394D-induced tumors formed tumorspheres that were highly apoptotic and less proliferative compared to MYC tumorspheres. Global gene expression of MYCV394D tumors was distinct from murine G3 and SHH subgroup MBs. Particularly, genes needed for neuronal differentiation and which are strongly repressed by MYC in G3 MBs, were no longer repressed by the MYCV394D mutant, indicating that the MYC/MIZ1 complex was responsible for their repression.
To analyze whether the MYC/MIZ1 complex was present on chromatin in G3 MBs, we performed ChIP-Seq for MYC and MIZ1. In contrast to Trp53-null GNPs, where MIZ1 only binds 140 promoters, 11.549 promoters were occupied by MIZ1 in G3 MBs, 9.353 of which were co-occupied by MYC (81%). Because as many of the joined binding sites neither contained a canonical E-box sequence nor a bona fide MIZ1 binding motif, we speculate that cooperative binding of oncogenic MYC with MIZ1 is necessary for binding.
Our data suggest that the MYC-MIZ1 interaction is critical for the development of G3 MBs.
Citation Format: BaoHan T. Vo, Elmar Wolf, Daisuke Kawauchi, Jerold Rehg, David Finkelstein, Brian Murphy, Martin Eilers, Martine F. Roussel. Myc and Miz1 in medulloblastoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2977. doi:10.1158/1538-7445.AM2014-2977