Abstract 4131: Plasticity of transcriptional and epigenetic cellular states in neuroblastoma is driven by core lineage transcription factors

Background: Core Regulatory Circuitries (CRCs) of lineage transcription factors associate with super-enhancers and are drivers of cellular identity. We recently showed that most neuroblastomas include two types of tumor cells with divergent gene expression profiles. Undifferentiated mesenchymal (MES) cells and committed adrenergic (ADRN) tumor cells resemble cells from developmental stages of the adrenergic lineage. Results: We identified the unique super-enhancer (SE) landscapes of MES and ADRN-type cells. SEs associated with lineage transcription factors that identified the Core Regulatory Circuitries (CRC) for each cell type. MES cells were resistant to chemotherapy. Accordingly, MES cells were strongly enriched in post-treatment samples, suggesting that MES cells could play a role in resistance and relapse development. MES and ADRN cells of isogenic origin spontaneously interconverted, showing the transcriptional plasticity of CRCs. We studied the mechanism of MES and ADRN transdifferentiation. The MES TFs PRRX1 and NOTCH were expressed as single genes in ADRN cells. Each of the genes induced a step-wise reprogramming of the ADRN transcriptome towards a MES state. The transcriptional switch was accompanied by genome-wide remodeling of the epigenome to a MES enhancer state. MES TFs repressed super-enhancers of ADRN core TFs, leading to transcriptional downregulation of the ADRN CRC. Deregulated enhancers associated with Polycomb repression. The transdifferentiation was initially reversible, but became gradually stabilized. The transgenes activated an endogenous feed-forward loop including ligands, receptors and co-factors from the NOTCH signaling route. Ultimately, this endogenous NOTCH-cascade maintained a transgene-independent MES state. Super-enhancers in stable MES cell lines associated with NOTCH receptors and co-factors, consistent with NOTCH driving MES lineage identity. Both NOTCH-induced MES cells and ADRN cells were tumorigenic in vivo. Conclusions: Our results demonstrate that single TFs from the MES CRC impose transdifferentiation via remodeling of the epigenetic and transcriptional landscape of ADRN cells, mimicking spontaneous interconversion. Plasticity of CRCs and lineage identity may have profound implications for treatment strategies in neuroblastoma. Citation Format: Johan V. Nes, Tim V. Groningen, Jan Koster, Linda Valentijn, Danny Zwijnenburg, Ellen M. Westerhout, Mohamed Hamdi, Rogier Versteeg. Plasticity of transcriptional and epi