MEDU-24. INSTRUMENTAL ROLE OF THE PHOTORECEPTOR PROGRAM IN GROUP 3 MEDULLOBLASTOMA THROUGH THE TRANSCRIPTION FACTOR NRL

Abstract

Medulloblastoma (MB) is the most common malignant brain tumor of childhood arising in the cerebellum. Although multimodal treatments have significantly increased the survival, 20–30% of patients remain incurable. Most of them belong to a poorly characterized MB subgroup, called group3, which expresses a retina photoreceptor-specific differentiation program. Cancer cells often express such differentiation programs unrelated to their tissue of origin. They likely result from cancer cell plasticity but are usually not thought to contribute to cancer progression. Accordingly, the functional relevance of this aberrant differentiation program to MB remains unknown. By analyzing group3 enhancers (Enh) and super-enhancers (SE), we identified key interconnected transcription factors of this subgroup, Among those, two lineage restricted transcription factors, NRL and CRX, are known to play key roles during retina development and in particular for photoreceptor specification. We identified a list of genes harboring a co-occurrence of NRL and CRX binding sites within Enh/SE, and displaying higher expression in group3. Interestingly, photoreceptors genes are enriched among this list. This analysis showed that NRL and CRX are important players in group3 MB and control the photoreceptor differentiation program. By loss-of-function experiments in cell lines and PDXs, we showed that NRL and CRX are absolutely required for MB growth in vitro and in vivo upon orthotopic grafting. We then confirmed that NRL controls the expression of photoreceptor genes in group3 MB and established that it protects MB from apoptosis. By ChIP experiments we identified one of its key target genes in this process: the anti-apoptotic BCL-XL gene. We confirmed its key role downstream of NRL by rescue experiments. Importantly, we established the potential of anti-BCL therapies in MB. In conclusion, we showed that the photoreceptor program is instrumental to MB through NRL and CRX and that the anti-BCL therapies could represent an alternative therapeutic strategy.