A recently published article in Science Advances has revealed significant insights into how oncogenic MAPK/AP1 signaling drives global epigenetic and transcriptional activation of LTR10 elements in colorectal cancer and other epithelial cancers. This research elucidates the role of LTR10 elements as enhancers that contribute to pathological transcriptional rewiring in cancer cells.
The study demonstrates that LTR10 elements, normally repressed in adult somatic tissues, show tumor-specific enhancer activity. This activity is linked to oncogenic MAPK/AP1 signaling, a pathway frequently mutated in various cancers, leading to uncontrolled tumor cell proliferation and gene expression.
Key Findings
Epigenomic Analysis of ERVs: The researchers conducted a pan-cancer epigenomic analysis, revealing multiple primate-specific endogenous retrovirus (ERV) families enriched in tumor-specific accessible chromatin across 21 solid tumor types. This implicates ERVs as a pervasive source of regulatory elements shaping gene regulation in most tumors.
LTR10 Elements as Enhancers: Focusing on LTR10 elements, the study found recurrent epigenomic signatures of enhancer activity in epithelial cancers, including colorectal cancer. Bulk and single-cell RNA sequencing of patient tumors revealed that LTR10 elements display tumor-specific transcriptional activation in approximately 30% of cases.
CRISPR-Mediated Silencing: Using CRISPR technology to silence or knock out individual LTR10 elements in HCT116 colorectal cancer cells, the researchers discovered that LTR10-derived enhancers drive AP1-dependent gene expression at multiple loci. These loci include genes with known roles in tumorigenesis and therapy resistance, such as ATG12, XRCC4, and VCAN.
Implications for MAPK Inhibition Therapy: The study shows that oncogenic AP1/MAPK signaling results in the activation of LTR10 enhancers, while MAPK inhibitors can effectively silence LTR10 regulatory activity in cancer cells. This suggests that the silencing of LTR10 ERV regulatory activity is an important mechanism underlying the therapeutic effects of MAPK inhibition.
Tandem Repeat Expansions: The researchers also discovered that LTR10 elements are frequently affected by tandem repeat expansions, which could influence their regulatory activity. These expansions or contractions in cancer cells with microsatellite instability may alter cancer-specific enhancer landscapes.
While the study provides compelling evidence of LTR10 elements contributing to cancer-specific transcriptional dysregulation, further investigation using patient-derived organoid models and immunocompetent mice is needed to validate these findings in clinical settings. Additionally, analysis of single-cell ATAC-seq datasets would be critical for identifying patient-relevant LTR10 regulatory elements and understanding their activity in the tumor microenvironment.
Despite these limitations, this research underscores the importance of LTR10 elements in MAPK/AP1-mediated transcriptional dysregulation in colorectal cancer and highlights how normally silenced transposable elements can be reactivated in cancer, leading to aberrant gene expression. The study’s insights into the epigenetic landscape of cancer-specific regulatory networks offer new avenues for potential therapeutic interventions.