P04.07.B CHARACTERIZATION OF ARHGAP36/MYC-INDUCED GROUP 3 MEDULLOBLASTOMA

Abstract

Abstract BACKGROUND Medulloblastoma (MB) is one of the most common pediatric brain cancers of the central nervous system. It is stratified into 4 molecular subtypes (WNT, SHH, group 3, and group 4), which differ from one to the other for various features, such as cellular morphology, age of onset, set of mutations, and percentage of survival. A common point between the four MB subgroups is the absence of treatments for this disease. The current gold standard treatment has deleterious side effects for the patients, which spans from growth defects to low hormone levels and problems with learning. Analyzing transcriptomic data of group 3 MB patients we found ARHGAP36 as a possible gene involved in tumor formation or propagation. Rho GTPase-Activating Protein 36 (ARHGAP36) is a gene that plays a role in regulating PKA activity. Specifically, the ARHGAP36 protein can inhibit PKA function through two distinct mechanisms. Firstly, it acts as a pseudosubstrate for the catalytic subunit of the PKA protein. Secondly, it can drive protein degradation via a lysosome-mediated process. MATERIAL AND METHODS We took advantage of the Piggybac/transposon technology to induce a strong and stable overexpression of our genes of interest to generate two different models, one in-vivo and one in-vitro with induced pluripotent stem cells-derived human cerebellar organoids. We also exploited the cre/LoxP system in transgenic mice to overexpress ARHGAP36 under the control of specific promoters. The analysis was conducted using standard techniques such as RT-qPCR and Western Blot, together with immunofluorescence of the cerebellar section and live imaging of the modified cerebellar organoids. The gene expression profile of the tumors obtained in mice was analyzed by RNA sequencing and compared to known group 3 MB mouse models. RESULTS We analyzed RNA-sequencing data from medulloblastoma patients and found that high expression of ARHGAP36 correlated with poor prognosis in patients. Through PKA inhibition, ARHAP36 induces ectopic activation of SHH signaling and cell proliferation leading to abnormal development of the cerebellum in mouse embryos. Notably, despite its inductive activity on the SHH pathway, ARHGAP36 expression is higher in group 3 and group 4 MB, rather than in the SHH subgroup. In this study, we demonstrate that intracranial transfection of P0 mouse pups with plasmids overexpressing MYC and ARHGAP36 can generate group 3 medulloblastoma in-vivo. We studied and then confirmed the involvement of the binding with PKA as a tumorigenic mechanism in the MYC/ARHGAP36 co-overexpression using several mutated forms of the gene. We studied the obtained tumors for the presence of group 3 MB immunophenotypical markers, as well as their gene expression profile by RNA-seq. CONCLUSION These findings can become of key interest as possible druggable targets to efficiently treat group 3 medulloblastoma patients in the future.