Abstract A028: An isogenic H/N/KRAS-less mouse embryonic fibroblast cell line panel derived from a size sorted diploid clonal parent

Shortly after the formation of the NCI Ras Initiative, a H/N/KRAS-less mouse embryonic fibroblast (MEF) cell line panel was generated, validated, and shared internationally to aid the research community in its reinvigorated effort to apply new technical advancements to understanding the structure and function of Ras and to ultimately design inhibitors. While this panel has been beneficial to the Ras research community, an inherent variability due in part to the ploidy of the parental pool from which the panel was derived complicated comparisons between isolates and alleles. We have minimized this variation by utilizing a diploid clone derived from the DU1473 MEF; Hras-/-; Nras-/-; Kraslox/lox; RERTert/ert - parental cell line. A diploid cell population was sorted, cloned and clones were cultured continuously for up to 20 passages to identify those that retained a diploid signature. These clones were further screened for p53 mutations and mouse exome sequenced from which a final clone was selected to develop the improved cell line panel. The selected clone was treated with 4-hydroxytamoxifen (4-OHT) to activate the Cre-recombinase which excised a portion of the endogenous Kras located between two loxP sites effectively arresting cells in a senescent G1 state. Molecular verification of Kras removal was performed, and the cells were then transduced with a third generation VSV-G pseudotyped lentiviral vector which delivered different versions of the human Ras transgene as well as an antibiotic selection marker. This transduction restored the proliferative ability of the cells allowing for expansion and selection of the transduced pool. The pools were single cell cloned and subsequent clones underwent a systematic, rigorous quality control evaluation culminating with another round of exome sequencing before clones were selected to be part of the MEF 2.0 panel. Clones contained in this new panel were confirmed to be diploid, had fewer integrations than the initial panel, and performed similarly on proliferation assays as well as epidermal growth factor stimulation signaling assays. The new panel has successfully recapitulated the initial panel, but in a far more stable and reproducible cellular background. Instructions to obtain the cell line panel will be presented on the Ras Initiative website at www.cancer.gov and these cells are scheduled to become available to the Ras research community late in the spring of 2023. Citation Format: William E. Burgan, Nicole