The rare YAP1 subtype of Small Cell Lung Cancer revisited in a biobank of 39 Circulating Tumour Cell Patient Derived eXplant models (CDX): A brief report

Introduction Recent consensus defines four SCLC subtypes based on transcription factor (TF) expression; ASCL1, NEUROD1, POU2F3 and YAP1 respectively. The rare YAP1 subtype associates with ‘neuroendocrine (NE) low’ cells amongst SCLC cell lines and patient samples. We evaluated YAP1 in 39 phenotypically diverse Circulating tumour cell patient Derived eXplant models (CDX) and revisit YAP1 in terms of prevalence, cell phenotype, inter- and intra-tumour heterogeneity. Methods YAP1 transcript and protein expression were assessed by RNA sequencing (RNAseq) and immunohistochemistry or multiplexed immunofluorescence of NE and non-NE CDX subpopulations. Physically separated NE and non-NE CDX ex vivo culture lysates were western blotted for YAP1, NE marker synaptophysin (SYP) and AXL. Results RNAseq normalised for the 4 subtype TFs identified YAP1 expression in 14/39 CDX. Ten CDX expressed YAP1 protein and 8 had strong YAP1 expression confined to rare non-NE cell clusters. This was confirmed in ex vivo CDX cultures where adherent non-NE cells lacking SYP expression expressed YAP1. However, in 2 CDX, weaker cellular YAP1 expression was observed, widely dispersed in SYP positive NE cells. Conclusions YAP1 was predominantly expressed in non-NE cell clusters in SCLC CDX but 2/39 CDX expressed YAP1 in NE cells. CDX22P with relatively high YAP1 expression is an ASCL1 NE subtype with a low NE score and an outlier within this subtype within our CDX biobank. These descriptive data reveal subtly different YAP1 expression profiles paving the way for functional studies comparing YAP1 signalling in non-NE and low NE cell contexts for potential personalised therapy approaches.