Abstract 826: Global DNA methylation levels regulate PD-L1 expression in melanoma

The programmed death-ligand 1 (PD-L1) receptor is an important immune checkpoint and is often upregulated in cancer cells to allow immune evasion. In melanoma, the patients with PD-L1 expression and absense of tumour infiltrating lymphocytes (TILs) (i.e. “constitutive PD-L1 or PD-L1CON”) show worse response rates and prognosis than patients with PD-L1 expression and the presence of TILs (i.e. “inducible PD-L1 or PD-L1IND”). However, how PD-L1 expression is regulated in melanoma cells remains elusive. Understanding the mechanisms of how PD-L1 is regulated is important for predicting responses for anti-PD-L1 treatment and for developing new combinatorial therapies. We hypothesised that epigenetic state regulates constitutive and inducible PD-L1 expression in melanoma. To address the hypothesis, we have generated whole-genome scale DNA methylomes (using reduced representation bisulfite sequencing) and transcriptomes (RNA-Seq) for patient derived melanoma cell lines (in PD-L1IND and PD-L1CON groups). We discovered extensive global hypomethylation in the constitutive lines, particularly pronounced in intergenic repeat regions and gene bodies. A high proportion of hypomethylated regions exhibited dichotomous methylation patterns indicating a common regulatory mechanism between the PD-L1IND and PD-L1CON lines. RNA-Sequening data indicated that the hypomethylated state of the PD-L1CON cells was correlated with higher upregulation of the differentially expressed genes at a global-scale and the upregulated genes were associated with cancer hallmark properties. The upregulated genes exhibited signatures of viral mimicry and cytosolic sensing of dsRNA similar to what has been observed after DNA methyltransferase inhibitor (DNMTi) treatment in cancers. DNMTi treatment increased PD-L1 transcription in the PD-L1IND cell lines. PD-L1CON samples showed greater resistance to DNMTi compared to PD-L1 negative samples. This was observed at protein-coding genes, transposable elements and long non-coding RNAs. Moreover, genes involved in the innate immune pathway was strongly increased upon DNMTi mediated hypomethylation in the PD-L1IND cell lines (FDR-adjusted P-value = 0.02), however there was no expression change in the PD-L1 positive cells (Adjusted P-value = 1.0). Furthermore, we found that the IRF1 transcription factor, which plays a critical role in the innate immune pathway and also binds to the PD-L1 promoter, is significantly upregulated upon DNMTi in the PD-L1 negative cell lines (Adjusted P-value = 0.01) but not in the PD-L1 positive cells (Adjusted P-value = 0.9). These results show that global hypomethylation levels regulate PD-L1 expression in melanoma. We believe these results are the first to show that DNA methylation levels play a role in regulating PD-L1 on melanoma and suggest they may have important implications for combined treatments targeting methylation (DNMTi) and PD1/PD-L1 (anti-PD1 antibodies). Citation Format: Aniruddha Chatterjee, Antonio Ahn, Euan J. Rodger, Peter A. Stockwell, Matthew Parry, Jyoti Motwani, Stuart J. Gallagher, Elena Shklovskaya, Jessamy Tiffen, Peter Hersey, Michael R. Eccles. Global DNA methylation levels regulate PD-L1 expression in melanoma [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 826.