16P - Investigation of c-MYC role in DNA-PK-mediated activation of STING pathway in SCLC

Background

Small cell lung cancer (SCLC) is characterised by TP53 and RB1 loss and amplification of MYC in approximately 30% of patients. Our group and others have shown that DNA damaging therapies (chemotherapy, radiotherapy, or DDR inhibitors, DDRi) synergize with immune checkpoint inhibition in SCLC in vivo models by activating a Stimulator of Interferon Genes (STING)-mediated innate immune response. Also, the transcription factor MYC is known to be positively correlated with the expression of CXCL10 and CCL5, the two downstream chemokines of STING pathway, in SCLC clinical datasets. Based on these findings, we hypothesised that the expression of MYC family members defines distinct molecular states that may be associated with unique responses to treatment with DDRi.

Methods

We explored the landscape of DDR pathways, STING, and immune-related cytokines expression both at mRNA and protein expression levels among a panel of n=9 SCLC cell lines classified as MYChigh or MYClow. We then tested the two SCLC cell line subsets for micronucleus (MN) formation and innate immune activation in response to cisplatin 0.5 μM and/or DNA-PK inhibitor (DNA-PKi) 2 μM.

Results

STING and PD-L1 mRNA expression were higher in MYChigh cells as compared to MYClow SCLC cells (p=0.02). Also, in cisplatin-treated SCLC panel, we found that DDR pathway promoted STING upregulation preferentially in MYChigh subset. In parallel, in vitro treatment with DNA-PKi further increased STING expression in MYChigh cells while reduced or had no effect on STING in MYClow SCLC cells. MN assay showed increased MN formation, particularly in DNA-PKi treated MYChigh cells, resulting in cytosolic DNA release and STING pathway activation. Furthermore, DNA damage accumulation by DNA-PKi promoted tumor recognition and NK cell activity by inducing tumor cell display of NKG2D ligands MICA/B and ULBP1/2/3. Overall, MYChigh cells were more responsive to innate immune cell activity possibly due to higher baseline levels of replication stress.

Conclusions

Our study report that c-MYC expression levels in SCLC cells differentially impact DNA-PK inhibition-mediated activation of STING pathway and presents STING as a downstream of targetable DDR pathway that could offer therapeutic benefit in a subset of MYChigh SCLC patients.

Editorial acknowledgement

Clinical trial identification

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.