Truncated BRPF1 cooperates with Smoothened to promote adult Shh medulloblastoma

Tumors are composed of proliferating cells that invade healthy tissue and grow over time. Even though it is still unclear, it is a common opinion that the cells of origin should possess a proliferative capacity (Blanpain, 2013; Visvader, 2011). Particularly for brain cancers, the transition of neural progenitors to differentiated postmitotic neurons is considered irreversible in physiological and pathological conditions. Therefore, postmitotic neurons have not been considered as suitable cell of origin for brain cancer. Here, we show that neurons reprograming may occur upon Shh activation leading to medulloblastoma (MB) formation in vivo. Human SHH medulloblastoma (MB) is a brain tumor affecting adults and infants that is thought to originate from cerebellar granule neuron progenitors. Notably, several groups have shown that Shh pathway activation (SmoM2 overexpression) in mouse granule neuron progenitors is able to induce Shh MB (Schuller et al., 2008; Z.-J. Yang et al., 2008). These progenitors are present in infants and newborn mice, but they seem to be absent in adult humans and mice (Biran, Verney, & Ferriero, 2012; Marzban et al., 2014; Z.-J. Yang et al., 2008). Furthermore, it was recently discovered that the two different forms of SHH MB are distinguished by different transcriptome/methylome levels suggesting that the adult SHH MB may originate from a different cell of origin (Cavalli et al., 2017; Kool et al., 2014). Relying on these data, we take advantage of a conditional Cre-Lox recombination system to recapitulate the human adult medulloblastoma pathogenesis in mice, demonstrating that cerebellar post-migratory mature granule neurons upon SmoM2 overexpression can dedifferentiate and give rise to SHH MB in vivo. Moreover, human adult patients present inactivating mutations of the chromatin reader BRPF1 that are associated with SMO mutations and absent in pediatric and adolescent patients. Here we found that truncated BRPF1 protein, as found in human adult patients, is able to induce medulloblastoma in adult mice upon SmoM2 activation. Notably, gene expression profiling on our samples allowed to associate “cerebellar granule progenitors-derived MB” with the human infant form while “truncated BRPF1-induced tumors” clustered with human adult SHH MB. Furthermore, as previously described by Kool et al., 2014, human adult SHH MB is characterised by the copresence of p-AKT and p-S6, compared to the human infant SHH MB that are positive for either p- AKT or p-S6 and always in a mutually exclusive way. Truncated BRPF1-induced tumors are double positive for p-AKT and p-S6, similarly to adult patients, while cerebellar granule progenitors derived MB present only p-S6. Furthermore, to define the contribution of chromatin changes in granule neurons dedifferentiation in response to Shh activation, we profiled changes in chromatin accessibility by ATAC-seq analysis on mice cerebella. SmoM2 overexpression changed the epigenetic landscape of granule neurons, enriching the number of open chromatin regions 12 associated with stem/progenitor-like genes. Moreover, the cooperation between truncated BRPF1 and SmoM2 in reshaping the chromatin arrangement of granule neurons was explored applying ATAC-seq on differentiated human cerebellar neurons derived from neuroepithelial cells. ATAC-seq analysis pointed out a synergistic mechanism between SmoM2 and truncated BRPF1 in modifying the epigenetic landscape of postmitotic neurons, increasing the chromatin accessibility of super-enhancers, associated with stemness and chromatin organization/modification genes. Our novel model of cancer development could explain the human SHH medulloblastoma onset in adult individuals where granule neuron progenitors are no more present. For these reasons, we strongly believe that our model configures as an important starting point for a new field in cancer and stem cell biology focusing on the study of mechanisms driving tumorigenesis in postmitotic cells.