**Distinct Tumor Microenvironments Are a Defining Feature of Strain-Specific CRISPR**/**Cas9-Induced MPNSTs**

**Amanda Scherer 1,2, Victoria R. Stephens 1,3, Gavin R. McGivney 2,4, Wade R. Gutierrez 2,4,5, Emily A. Laverty 1,2, Vickie Knepper-Adrian 1,2 and Rebecca D. Dodd 1,2,\***


Received: 5 May 2020; Accepted: 21 May 2020; Published: 23 May 2020

**Abstract:** The tumor microenvironment plays important roles in cancer biology, but genetic backgrounds of mouse models can complicate interpretation of tumor phenotypes. A deeper understanding of strain-dependent influences on the tumor microenvironment of genetically-identical tumors is critical to exploring genotype–phenotype relationships, but these interactions can be di fficult to identify using traditional Cre/loxP approaches. Here, we use somatic CRISPR/Cas9 tumorigenesis approaches to determine the impact of mouse background on the biology of genetically-identical malignant peripheral nerve sheath tumors (MPNSTs) in four commonly-used inbred strains. To our knowledge, this is the first study to systematically evaluate the impact of host strain on CRISPR/Cas9-generated mouse models. Our data identify multiple strain-dependent phenotypes, including changes in tumor onset and the immune microenvironment. While BALB/c mice develop MPNSTs earlier than other strains, similar tumor onset is observed in C57BL/6, 129X1 and 129/SvJae mice. Indel pattern analysis demonstrates that indel frequency, type and size are similar across all genetic backgrounds. Gene expression and IHC analysis identify multiple strain-dependent di fferences in CD4+ T cell infiltration and myeloid cell populations, including M2 macrophages and mast cells. These data highlight important strain-specific phenotypes of genomically-matched MPNSTs that have implications for the design of future studies using similar *in vivo* gene editing approaches.

**Keywords:** CRISPR/Cas9; MPNST; mouse models; sarcoma; tumor microenvironment
