The two major pathologies of non-small cell lung cancer, adenocarcinoma (ADC) and squamous cell carcinoma (SCC) are historically treated as separate diseases, even though mounting evidence shows that epigenetic reprogramming from ADC to a more SCC fate allows lung cancers to evade therapies. Our team recently developed an isogenic lung cancer model in which lineage switching from ADC to SCC was epigenetically controlled through loss of Polycomb Repressive Complex 2 (PRC2) gene repression of squamous genes. Levels of S-adenosyl methionine (SAM), the methyl donor for the complex, influence PRC2 stability and activity. Steady state metabolism showed that SAM levels were decreased significantly and cystathionine levels were increased significantly in SCC cells relative to ADC cells. Our hypothesis is that increased activity of the enzyme cystathionine beta synthase (CBS) drives epigenetic reprogramming of lung ADC to an aggressive and therapy-resistant state through reduction of SAM pools and destabilization of PRC2, leading to expression of squamous genes. First, we stained a series of human lung cancer tissues for CBS and H3K27me3 and queried TCGA data to validate that CBS is up-regulated in squamous tumors. In addition, we leveraged 3-dimensional organoid cultures and traditional 2-dimensional human non-small cell lung cancer cell lines to explore the contribution of CBS to lung cancer lineage fate. With both of these systems, we used lentiviral small hairpins targeting Cbs/CBS and a lentivirus encoding a full length Cbs/CBS cDNA to modulate gene and protein levels. Lastly, we tested differential sensitivities to the common chemotherapeutic carboplatin on matched isogenic cultures that differ only in lineage state. In human tumors, SCCs have significantly higher levels of CBS and significantly lower levels of H3K27me3 than ADCs. Knock-down of CBS decreased cystathionine and increased both homocysteine and S-adenosyl methionine levels in 2-dimensional human NSCLC cultures. We also observed a global increase in H3K27me3 mark in shCBS cultures relative to shGFP controls. In contrast, CBS over-expression decreased global levels of H3K27me3. Importantly, we observed that squamous organoids are significantly less sensitive to the common chemotherapeutic carboplatin than are ADC organoids, suggesting that lineage switching driven by increased CBS activity will drive chemotherapy resistance. We are now characterizing changes in lineage fate and carboplatin sensitivity that are caused by these genetic manipulations of CBS. Our data suggest that lung cancer lineage fate may be governed in part through expression and activity of the enzyme CBS. Because CBS is a redox-sensitive enzyme, this could be the missing link between increased oxidative stress and ultimate epigenetic reprogramming to a therapy-resistant squamous state in lung cancer. Funded by ACS IRG-85-001-25, K22 CA201036 and KY LCRP to CFB

Citation Format: Mojtaba Bakhtiari, Christine Fillmore Brainson. Determining the contribution of cystathionine beta synthase to lung cancer lineage fate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 952.