MEDB-12. Severe developmental abnormalities and proliferative cerebellar lesions induced by combined activity of Wnt signalling and loss of SMARCA4

Abstract Almost all medulloblastomas (MB) of the WNT subgroup are characterized by hotspot mutations in CTNNB1, and mouse models have convincingly demonstrated the tumor-initiating role of these mutations as well as the tumor origin in the dorsal brain stem. Around 20 % of WNT MB additionally carry SMARCA4mutations, but the functional role of these alterations is mostly unknown. We therefore amended previously described Blbp-cre::Ctnnb1(ex3)Fl/+mice by the introduction of a floxed Smarca4 allele. In contrast to existing literature, Blbp-cre::Ctnnb1(ex3)Fl/+ mice had a maximum life span of only 17 days, even after breeding into two different genetic backgrounds (C57BL/6J and 129S2/Sv). The mice displayed a severe developmental phenotype including a thinned cerebral cortex, hydrocephalus, missing cerebellar foliation and layering as well as non-proliferative cell accumulations in brain stem and cerebellum. An additional homozygous loss of SMARCA4 even resulted in prenatal death for most mice and caused big proliferative lesions in the cerebellum at embryonal day 14.5. These lesions appear to originate from SOX2-positive progenitor cells in the cerebellar ventricular zone. In a next experiment, cells isolated from this region will be characterized in vitro and will be transplanted orthotopically to evaluate their neoplastic potential in vivo. Altogether, we hope to elucidate how a loss of SMARCA4 and mutations of Ctnnb1 cooperate during hindbrain development and tumor formation within this region.