Abstract 1726: Loss of NF1 drives hormone dependent mammary carcinogenesis in a rat model with intact immune system
Authors: Wallis Deeann , Fay Christian , Bradley Kelley , Westin Erik , Bradley William , Liu Hui , Crossman David , Foote Jeremy B. , Kesterson Robert 
Published: April 2023

Abstract
Loss of NF1 plays a major role as an oncogenic driver in up to 33% of all breast cancers (BC). Loss of NF1 is also a prognostic indicator for increased cancer risk at an earlier age, poorer outcomes, and therapeutic resistance. NF1 has recently been shown to bind the estrogen receptor alpha (ER) and act as a transcriptional corepressor. This helps explain findings in ER+ BC. Changes to NF1 that abrogate ER signaling thereby lead to Ras driven tumor resistance to endocrine therapy as cells are able to grow in low levels of E2 (and tamoxifen). Hence, NF1-mutant tumors represent a distinct molecular class in need of new therapeutics targeting both Ras and ER pathways. We have generated preclinical mammalian models of NF1 loss and BC that can be used to evaluate the role of NF1/ER transcriptional signaling in BC, the role of immune cells in BC, and therapeutics. Our models include a pathogenic patient missense allele c.3827G&gt;A, p.R1276Q (knockin or KI) as well as a 14 base pair deletion c.3661_3674del, p.P1220fs*1223 (knockout or KO) model. Heterozygous (het) Nf1 female rats develop mammary gland adenocarcinoma spontaneously, but het KO rats develop multiple tumors with earlier onset. Tumors are generally Grade 2 and do not differ by genotype. By 16 weeks, 70% of KO females have developed at least one tumor whereas only 20% of KI rats and 2% of WT rats have developed tumors. This impacts survival as 76% of KI and 58% of KO females survive to one year. By 2 years, both alleles have 54% survival. The divergence in phenotype between KI and KO alleles may be due to residual function of R1276Q missense NF1 protein. A more in-depth analysis indicates that mammary tumor formation likely begins relatively early, as we find evidence of aberrant morphology and hyperplasia prior to the formation of palpable tumors. Interestingly, we find histological evidence of lung metastases and expression of breast markers GCDFP15, MGA, and CEA in the lung. Again, we see allele-specific effects in that KO rats develop lung tumors earlier than KI rats. Using single nuclei RNASeq to characterize the transcriptional profile of the mammary tumors, we find allele-specific effects beyond repression of Ras activity that drive aggressive tumor development. We identified different tumor cell populations (2 epithelial cell populations, Myeloid cells, B cells, T cells, Basal mammary cells, and WT specific cells) and identified different pathways altered due to the loss of Nf1 including Ephrin B signaling, Cyclin and Cell Cycle signaling, and Glycolysis signaling. Our overall goal is to characterize the phenotype of these rat models in terms of histopathology, Ras signaling, hormone signaling, immune components, and targeted drug response; which can then be compared with what is known regarding patients with somatic or germline inactivation of NF1 and breast cancer. Ultimately, this will provide better prognostic predictions for patients and better therapeutic options.
Citation Format: Deeann Wallis, Christian Fay, Kelley Bradley, Erik Westin, William Bradley, Hui Liu, David Crossman, Jeremy B. Foote, Robert Kesterson. Loss of NF1 drives hormone dependent mammary carcinogenesis in a rat model with intact immune system [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1726.