Mnt modulates Myc-driven lymphomagenesis

Kirsteen J Campbell,Peter J Hurlin,Natasha S Anstee,Cassandra J Vandenberg,Suzanne Cory

doi:10.1038/cdd.2017.131

Abstract

The transcriptional represser Mnt is a functional antagonist of the proto-oncoprotein Myc. Both Mnt and Myc utilise Max as an obligate partner for DNA binding, and Myc/Max and Mnt/Max complexes compete for occupancy at E-box DNA sequences in promoter regions. We have previously shown in transgenic mouse models that the phenotype and kinetics of onset of haemopoietic tumours varies with the level of Myc expression. We reasoned that a decrease in the level of Mnt would increase the functional level of Myc and accelerate Myc-driven tumorigenesis. We tested the impact of reduced Mnt in three models of myc transgenic mice and in p53+/− mice. To our surprise, mnt heterozygosity actually slowed Myc-driven tumorigenesis in vavP-MYC10 and Eμ-myc mice, suggesting that Mnt facilitates Myc-driven oncogenesis. To explore the underlying cause of the delay in tumour development, we enumerated Myc-driven cell populations in healthy young vavP-MYC10 and Eμ-myc mice, expecting that the reduced rate of leukaemogenesis in mnt heterozygous mice would be reflected in a reduced number of preleukaemic cells, due to increased apoptosis or reduced proliferation or both. However, no differences were apparent. Furthermore, when mnt+/+ and mnt+/− pre-B cells from healthy young Eμ-myc mice were compared in vitro, no differences were seen in their sensitivity to apoptosis or in cell size or cell cycling. Moreover, the frequencies of apoptotic, senescent and proliferating cells were comparable in vivo in mnt+/− and mnt+/+ Eμ-myc lymphomas. Thus, although mnt heterozygosity clearly slowed lymphomagenesis in vavP-MYC10 and Eμ-myc mice, the change(s) in cellular properties responsible for this effect remain to be identified.

Highlights

Myc heterodimerises with a ubiquitous bHLHZip protein, Max, and binds the E-box CACGTG to activate gene transcription.[5]
Western blot analysis verified that the level of Mnt protein was lower in mnt+/ − compared with mnt+/+ thymocytes, both on the WT and vavP-MYC17 transgenic background (Figure 1a) and in pre-B cells from WT and Eμ-myc mice (Figures 1b and c) and the level of transgenic Myc was unaltered
To study the impact of mnt heterozygosity on Myc-driven lymphomagenesis, we turned first to vavP-MYC10 mice because the level of MYC protein expressed in this line represents a critical functional threshold: When expression is increased ~ 2-fold, the mice develop predominantly T lymphomas rather than the mixture of T lymphomas and myeloid tumours found in mice heterozygous for the transgene.[43]

Summary

Introduction

Myc heterodimerises with a ubiquitous bHLHZip protein, Max, and binds the E-box CACGTG to activate gene transcription.[5] Max binds transcriptional repressors containing Myc-related bHLHZ domains, such as the Mxd proteins and Mnt.[10,22,55] Since these repressors compete with Myc for available Max and bind E boxes, they function as Myc antagonists. Their loss would be predicted to mimic Myc overexpression. More recently we developed several transgenic lines that express Myc via haemopoietic-specific regulatory elements from the vav gene.[42,43] The tumour phenotype of the vavP-MYC mice varied between lines, depending on the level of Myc achieved: high expression resulted in rapid-onset T-cell lymphomas whereas low expression resulted in late-onset myeloid tumours and intermediate levels produced both tumour types

Methods

Results

Conclusion