Small cell lung cancer (SCLC) is a highly metastatic and deadly form of lung cancer, with 70% SCLC patients presenting in the clinic with regional (thoracic lymph nodes) and distant (liver, brain, bone) metastatic disease. Current standard of care treatment combining platinum-based chemotherapy with immunotherapy has had marginal improvements in overall survival, with resistance occurring in most patients. As such, there is a dire need to find therapeutic vulnerabilities that can be exploited with pre-existing drugs.
SCLC is a heterogeneous disease that can be stratified into distinct molecular subtypes based on the expression of transcription factors (TFs), such as ASCL1 and NEUROD1. While intra-tumoral heterogeneity has been observed at primary tumour lesions, this heterogeneity at metastatic sites remains unknown.
To interrogate SCLC heterogeneity at metastatic sites, we performed single cell RNA-sequencing on lymph node biopsies from SCLC patients (n=12). Our analysis revealed co-expression of ASCL1+NEUROD1+ within a single cell. This was surprising as these TFs were previously thought to be mutually exclusive in patient samples. This phenomenon was validated using multiplex immunohistochemistry (mIHC), which showed, for the first time, co-localisation of TFs within a single cell. To understand whether these ASCL1+NEUROD1+ cells were a feature of metastasis, we used mIHC on a tissue microarray comprised of primary and metastatic SCLC biopsies (n=101). Interestingly, there was a significant enrichment of ASCL1+NEUROD1+ cells in metastatic biopsies compared to primary tumours.
Previous studies in genetically engineered mouse models of SCLC have indicated phenotypic plasticity of ASCL1 cells transitioning to NEUROD1 cells. Indeed, differential gene expression analysis on ASCL1+NEUROD1+ cells from patients highlighted biological pathways involved in migration and metastasis, which suggests that ASCL1+NEUROD1+ cells are a feature of metastasis.
Transplantation studies in immune-deficient mice are currently underway to investigate the enrichment of human ASCL1+NEUROD1+ cells at metastatic sites. Together, our findings suggest that metastatic microenvironments may influence the phenotypic state of SCLC cells. Therefore, understanding and inhibiting this phenotypic plasticity may be an important aspect in halting metastatic spread.