Blocking SIRT2 Flips Colorectal Tumors Into Immune Targets
Inhibiting the protein SIRT2 destabilizes a DNA repair guardian, floods tumors with killer T cells, and supercharges immunotherapy in preclinical models.
Summary
Researchers at Sun Yat-sen University discovered that the enzyme SIRT2 shields a DNA mismatch-repair protein called MLH1 from degradation. When SIRT2 is blocked—either genetically or with the drug AGK2—MLH1 breaks down, DNA damage accumulates, and the immune-alerting cGAS-STING pathway switches on. This cascade recruits CD8+ killer T cells, boosts tumor neoantigen production, and raises MHC-I surface signals so the immune system can recognize cancer cells. In mouse colorectal cancer models and patient-derived organoids, SIRT2 inhibition alone shrank tumors; combining it with anti-PD-1 checkpoint therapy drove even greater regression and produced durable immune memory, suggesting a promising strategy to convert immunotherapy-resistant colorectal cancers into treatment-responsive ones.
Detailed Summary
Colorectal cancer (CRC) is one of the leading causes of cancer death worldwide, yet most CRC tumors respond poorly to immune-checkpoint inhibitors because they harbor few mutations and maintain an immunosuppressive microenvironment. Identifying molecular switches that can flip this cold tumor environment into a hot, immune-active one is a major research priority.
Using mass spectrometry-based proteomic profiling of human CRC tissue and clinical validation, the authors found that low expression of the deacetylase enzyme SIRT2 correlated with better patient prognosis and a more immune-active tumor microenvironment. This motivated a systematic investigation into what SIRT2 actually does inside tumor cells.
The team discovered that SIRT2 physically interacts with and deacetylates the mismatch-repair protein MLH1 at three specific lysine residues (Lys402, 443, and 461). This deacetylation prevents MLH1 from being tagged for ubiquitin-mediated destruction. When SIRT2 is knocked down or pharmacologically inhibited with AGK2, MLH1 is degraded, DNA replication errors go uncorrected, DNA damage accumulates, and the innate immune sensor cGAS-STING is activated. The result is a surge in tumor neoantigens and elevated MHC-I expression—both signals that attract and arm CD8+ cytotoxic T cells.
In multiple mouse CRC models and patient-derived organoids, SIRT2 inhibition alone produced meaningful tumor regression and generated long-lasting immune memory. Crucially, combining AGK2 with anti-PD-1 therapy produced substantially greater tumor control than either agent alone, suggesting synergistic immunotherapy sensitization.
While these findings are compelling, the study is preclinical. Results from mouse models and organoids do not always translate directly to human outcomes. AGK2's pharmacokinetics, toxicity profile, and therapeutic window in humans remain to be established in clinical trials.
Key Findings
- SIRT2 deacetylates MLH1 at Lys402/443/461, protecting it from ubiquitin-mediated degradation in CRC cells.
- SIRT2 inhibition degrades MLH1, increases DNA damage, and activates the cGAS-STING innate immune pathway.
- Blocking SIRT2 boosted CD8+ T cell infiltration and cytotoxicity, causing tumor regression in mouse models and organoids.
- SIRT2 inhibition elevated tumor neoantigen load and MHC-I expression, converting cold tumors to immune-active ones.
- Combining AGK2 (SIRT2 inhibitor) with anti-PD-1 therapy drove superior tumor regression and durable immune memory in vivo.
Methodology
The study used mass spectrometry proteomics on human CRC tissue for discovery, followed by genetic knockdown and pharmacological inhibition of SIRT2 in cell lines, multiple syngeneic mouse CRC models, and patient-derived organoids. Mechanistic studies included co-immunoprecipitation, ubiquitination assays, and immune profiling of the tumor microenvironment.
Study Limitations
All data come from preclinical models; human pharmacokinetics, dosing, and safety of AGK2 are unknown. The mechanism depends heavily on MLH1 degradation, which may inadvertently increase genomic instability with long-term use. Clinical translation will require careful patient stratification based on baseline SIRT2 and MLH1 expression.
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