ABSTRACT
The chromosome translocations generating PAX3-FOXO1 and PAX7-FOXO1 chimeric proteins are the primary hallmarks of the paediatric fusion-positive alveolar subtype of Rhabdomyosarcoma (FP-RMS). Despite the ability of these transcription factors to remodel chromatin landscapes and promote the expression of tumour driver genes, they only inefficiently promote malignant transformation in vivo. The reason for this is unclear. To address this, we developed an in ovo model to follow the response of spinal cord progenitors to PAX-FOXO1s. Our data demonstrate that PAX-FOXO1s, but not wild-type PAX3 or PAX7, trigger the trans-differentiation of neural cells into FP-RMS-like cells with myogenic characteristics. In parallel, PAX-FOXO1s remodel the neural pseudo-stratified epithelium into a cohesive mesenchyme capable of tissue invasion. Surprisingly, expression of PAX-FOXO1s, similar to wild-type PAX3/7, reduce the levels of CDK-CYCLIN activity and increase the fraction of cells in G1. Introduction of CYCLIN D1 or MYCN overcomes this PAX-FOXO1-mediated cell cycle inhibition and promotes tumour growth. Together, our findings reveal a mechanism that can explain the apparent limited oncogenicity of PAX-FOXO1 fusion transcription factors. They are also consistent with certain clinical reports indicative of a neural origin of FP-RMS.
Subject(s)
Cell Transdifferentiation/genetics , Cell Transformation, Neoplastic/genetics , Oncogene Proteins, Fusion/metabolism , Paired Box Transcription Factors/metabolism , Rhabdomyosarcoma, Alveolar/genetics , Animals , Biopsy , Chick Embryo , Child , Cyclin D1/genetics , Datasets as Topic , Disease Models, Animal , Gene Expression Profiling , Gene Expression Regulation, Neoplastic , Humans , N-Myc Proto-Oncogene Protein/genetics , Neoplasm Invasiveness/genetics , Neural Stem Cells/pathology , Neural Tube/cytology , Oncogene Proteins, Fusion/genetics , PAX3 Transcription Factor/genetics , PAX3 Transcription Factor/metabolism , PAX7 Transcription Factor/genetics , PAX7 Transcription Factor/metabolism , Paired Box Transcription Factors/genetics , Rhabdomyosarcoma, Alveolar/pathology , S Phase/geneticsABSTRACT
Transcription factors are key orchestrators of the emergence of neuronal diversity within the developing spinal cord. As such, the two paralogous proteins Pax3 and Pax7 regulate the specification of progenitor cells within the intermediate neural tube, by defining a neat segregation between those fated to form motor circuits and those involved in the integration of sensory inputs. To attain insights into the molecular means by which they control this process, we have performed detailed phenotypic analyses of the intermediate spinal interneurons (IN), namely the dI6, V0D, V0VCG and V1 populations in compound null mutants for Pax3 and Pax7. This has revealed that the levels of Pax3/7 proteins determine both the dorso-ventral extent and the number of cells produced in each subpopulation; with increasing levels leading to the dorsalisation of their fate. Furthermore, thanks to the examination of mutants in which Pax3 transcriptional activity is skewed either towards repression or activation, we demonstrate that this cell diversification process is mainly dictated by Pax3/7 ability to repress gene expression. Consistently, we show that Pax3 and Pax7 inhibit the expression of Dbx1 and of its repressor Prdm12, fate determinants of the V0 and V1 interneurons, respectively. Notably, we provide evidence for the activity of several cis-regulatory modules of Dbx1 to be sensitive to Pax3 and Pax7 transcriptional activity levels. Altogether, our study provides insights into how the redundancy within a TF family, together with discrete dynamics of expression profiles of each member, are exploited to generate cellular diversity. Furthermore, our data supports the model whereby cell fate choices in the neural tube do not rely on binary decisions but rather on inhibition of multiple alternative fates.
