Background: Immunotherapy resistance in STK11-mutant lung cancer

A preclinical study published in PNAS reports that removing CRTC2 or genetically blocking CRTC2–CREB binding restores response to anti-PD-1 treatment in STK11-deficient tumors. The key evidence came from a mouse colorectal cancer model. In human lung cancer cell xenograft models, suppressing CRTC2 expression stalled tumor growth; patient data showed an association between related gene-expression patterns and response to existing treatment.

The paper states that loss-of-function STK11 mutations occur in about 15–20% of patients with non-small cell lung cancer (NSCLC) and are associated with low PD-L1 expression, fewer intratumoral T cells, and resistance to PD-1/PD-L1 inhibitors. LKB1, the protein encoded by STK11, regulates cell growth and metabolism, but the pathways linking its loss to tumor immune evasion remain incompletely understood.

The researchers removed LKB1 in the MC38 colorectal cancer model, which normally responds to anti-PD-1 treatment, to compare the development and reversal of resistance. Some available mouse lung cancer models do not respond to anti-PD-1 even when STK11 is intact, making it difficult to isolate the resistance mechanism.

Key findings: CRTC2’s role demonstrated by genetic manipulation

LKB1-deficient MC38 tumors did not respond to anti-PD-1. Adding a CRTC2 gene deletion restored treatment response. The MC38 efficacy experiments used eight mice per group; anti-PD-1 antibodies were administered once weekly for 2–4 weeks to assess tumor growth. CRTC2-deficient tumors also showed a reduction in granulocytic myeloid-derived suppressor cells (gMDSCs) and changes in the composition of immune-cell populations, including CD4+ T cells.

The mechanism centers on the kinases SIK and MARK, which act downstream of LKB1, and the transcriptional coactivator CRTC2. These enzymes phosphorylate CRTC2, keeping it in the cytoplasm. The explanation is that without LKB1, CRTC2 moves into the nucleus, binds the transcription factor CREB, and regulates gene expression. To test this interaction, the researchers reintroduced either normal CRTC2 or the F40A mutant, which cannot bind CREB, into CRTC2-deficient cells. Normal CRTC2 restored resistance to anti-PD-1, whereas the F40A mutant did not. This is distinct from testing the efficacy of a specific CRTC2 inhibitor.

Effects on lung cancer growth itself were also observed. After human STK11-mutant NSCLC cell lines A549 and H2122 were implanted into immunodeficient mice, CRTC2 expression was reduced with short hairpin RNA (shRNA) in established tumors, and tumor growth stalled. This experiment supports a role for CRTC2 in tumor maintenance, but does not demonstrate the efficacy of combining CRTC2 suppression with immunotherapy in lung cancer.

Significance and outlook: Drug-target potential and the limits of clinical evidence

Using expression data from human tumors and mouse models, the researchers derived an STK11/CRTC2 gene-expression signature comprising 291 genes. They then found an association between this signature and response to pembrolizumab plus chemotherapy in data from 57 patients with nonsquamous NSCLC in the control arm of the CANOPY-1 clinical trial. This was neither a study administering a CRTC2-targeted treatment to patients nor an analysis validating a predictive test for clinical use.

The findings provide a rationale for discovering drugs that disrupt CRTC2–CREB binding. However, restoration of immunotherapy response needs to be tested in additional lung cancer models, and the effects of CRTC2 inhibition on the function of T cells and other immune cells also need study. The authors’ expectations regarding safety remain a hypothesis inferred from prior studies of CRTC2-deficient mice. The provided paper does not establish low toxicity of a drug used in combination or confirm the initiation of a human clinical trial.

The research models and experimental results are available in the full PNAS paper.