Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest cancers, characterized by high mortality rates and limited treatment options. Immunotherapy, particularly immune checkpoint blockade (ICB), has shown remarkable efficacy in treating cancers like melanoma and non-small cell lung cancer. However, many PDAC patients exhibit poor responses to ICB, mainly due to the tumor’s low immunogenicity and its immunosuppressive tumor microenvironment (TME). Inhibiting epigenetic regulators, such as SETDB1, KDM5B, and PHF8, has been shown to induce retrotransposon-mediated immune responses, thereby enhancing sensitivity to immunotherapy. While this approach shows promise in cancer treatment, its role in PDAC remains unclear. A recent study published in Gut identifies the key role of MED12 in modulating TME through the epigenetic silencing of retrotransposons, thus reshaping the immune TME in PDAC.To identify factors that regulate immune surveillance, researchers conducted an in vivo CRISPR-Cas9 epigenetic screening in PDAC tumor models using both immunocompetent and immunodeficient mice.
01 In Vivo CRISPR-Cas9 Screening Identifies MED12 as a Key Regulator of TME in Pancreatic Cancer
Using the KPC1199 PDAC cell line from C57BL/6J mice, researchers conducted an in vivo CRISPR-Cas9 screening by transducing two libraries containing 6030 single guide RNAs (sgRNAs) targeting 1036 epigenetic factors and 120 control sgRNAs. Screening in immunocompetent (C57BL/6J) and immunodeficient (NSG) mice revealed that MED12 is a key regulator of TME in PDAC. Loss of MED12 significantly increased the infiltration and cytotoxicity of immune cells, particularly CD8+ T cells, NK cells, and NK1.1+ T cells, within the tumors.
02 MED12 Loss Enhances PDAC Sensitivity to ICB Therapy
Researchers evaluated the impact of MED12 loss on ICB therapy in both subcutaneous and orthotopic KPC1199 models. The results showed that tumors with MED12 loss exhibited greater sensitivity to PD-1 blockade, marked by significant tumor shrinkage and extended survival.
03 MED12 Loss Induces Type I Interferon and NF-κB Signaling via Nucleic Acid Sensing Pathways
RNA sequencing and qPCR analysis of MED12-deficient and control KPC1199 cells revealed that MED12 loss activated Type I interferon and NF-κB signaling pathways, increased MHC class I molecule expression, and promoted antigen processing and presentation. This effect was mediated by the activation of nucleic acid sensing pathways triggered by endogenous dsRNA.
04 MED12 Suppresses Retrotransposons via H3K9me3 Marking
Using CUT&Tag and Western blot analysis, researchers assessed the effect of MED12 on retrotransposons marked by HP1A and H3K9me3. They found that MED12 interacts with HP1A to stabilize its presence on H3K9me3-modified heterochromatin, thereby suppressing retrotransposon transcription. Loss of MED12 led to downregulation of HP1A and de-repression of retrotransposons.
05 MED12 Loss Increases H3K27Ac Deposition, Enhancing Retrotransposon Activation and Immune Response
Researchers treated MED12-deficient KPC1199 cells with the BRD4 inhibitor JQ1 to assess changes in H3K27Ac levels. Results showed that MED12 loss increased H3K27Ac deposition at retrotransposons and genomic promoter regions, promoting retrotransposon transcription and immune responses. JQ1 treatment inhibited this effect, confirming the role of H3K27Ac.
06 MED12’s Role in Human PDAC Cell Lines
Researchers knocked out MED12 in several human PDAC cell lines, including PANC-1, Mia Paca2, Patu8988T, and AsPC1. They found that MED12 expression negatively correlated with immune response pathways, retrotransposon levels, and the prognosis of PDAC patients receiving ICB treatment. Patients with low MED12 expression had better responses to ICB therapy and longer survival.
This study highlights the critical role of MED12 in the epigenetic silencing of retrotransposons, reshaping the immune TME. It presents a potential therapeutic target for enhancing tumor immunogenicity and overcoming resistance to immunotherapy in PDAC.
