Smarca2 genetic ablation is phenotypically benign in a safety assessment of tamoxifen-inducible conditional knockout rats

SMARCA2 and SMARCA4 are the ATPases of the SWI/SNF chromatin remodeling complex, which play a significant role in regulating transcriptional activity and DNA repair in cells. SMARCA2 has become an appealing synthetic-lethal, therapeutic target in oncology, as mutational loss of SMARCA4 in many cancers leads to a functional dependency on residual SMARCA2 activity. Thus, for therapeutic development, an important step is understanding any potential safety target-associated liabilities of SMARCA2 inhibition. To best mimic a SMARCA2 therapeutic, a tamoxifen-inducible (TAMi) conditional knockout (cKO) rat was developed using CRISPR technology to understand the safety profile of Smarca2 genetic ablation in a model system that avoids potential juvenile and developmental phenotypes. As the rat is the prototypical rodent species utilized in toxicology studies, a comprehensive toxicological and pathological assessment was conducted in both heterozygote and homozygous knockout rats at timepoints up to 28 days, alongside relevant corresponding controls. To our knowledge, this represents the first TAMi cKO rat model utilized for safety assessment evaluations. No significant target-associated phenotypes were observed when Smarca2 was ablated in mature (11- to 15-week-old) rats; however subsequent induction of SMARCA4 was evident that could indicate potential compensatory activity. Similar to mouse models, rat CreERT2-transgene and TAMi toxicities were characterized to avoid confounding study interpretation. In summary, a lack of significant safety findings in Smarca2 cKO rats highlights the potential for therapeutics targeting selective SMARCA2 ATPase activity; such therapies are predicted to be tolerated in patients without eliciting significant on-target toxicities.