We previously developed a model for desmoid tumors in the frog Xenopus tropicalis by using CRISPR/Cas9 mediated inactivation in the apc tumor suppressor gene, thereby mimicking desmoid tumor formation associated with Familial Adenomatous Polyposis. However, in order to also model spontaneous desmoid tumors, we now want to generate a complementary model where desmoid tumor formation is driven by activating point mutations in the ctnnb1 gene, using a novel modification of the CRISPR/Cas9 system. We want to use our models to validate novel genes as targets for future therapy. In addition we want to investigate whether genes that are centrally involved in collagen secretion, a typical hallmark of desmoid tumors, are associated with tumor induction or proliferation. We believe our studies in this frog model can form a nice and relevant bridge between studies in cell culture and future use of the genetic mouse model that is being established.
This project aims to further exploit a fast, semi-high throughput and cheap animal model for identifying and/or characterizing promising drug targets for treating desmoid tumors. In addition the platform allows pre-clinical assessment of novel candidate therapeutic compounds. The project builds on a recently developed genetic desmoid tumor model in the frog Xenopus tropicalis, This model, which induces mosaic loss-of-function mutation in the tumor suppressor gene apc, presents a unique and novel experimental platform that (i) allows the rapid screening and evaluation of genes that contribute to the growth of the tumor, (ii) can serve to assess the clinical relevance of novel drug targets for treating desmoid tumors and (iii) can be used as a preclinical drug screening/assessment. A new methodology was developed called CRISPR/Cas9-mediated Negative Selection Identification of genetic Dependencies (CRISPR-NSID) that allows in vivo elucidation of cancer cell vulnerabilities in genetic cancer models. The methodology hinges on the fact that for a genetic dependency there is an incapability for recovering tumors carrying biallelic frameshift mutations in this gene. We will now further expand our model by inducing the activating S45F point mutation in the ctnnb1 gene using Cas9 Base Editors. In addition we want to use the model for validating novel dependency factors coming out of an in vitro screen and to further investigate the role of the transcription factor CREB3L1 and its link to collagen synthesis and secretion in the process of desmoid tumor formation.