In vivo CRISPR–Cas9 knockout screening using quantitative PCR identifies thymosin beta‐4 X‐linked that promotes diffuse‐type gastric cancer metastasis

Abstract Gastric cancer (GC) is histologically classified into intestinal‐type gastric cancer (IGC) and diffuse‐type gastric cancer (DGC), and the latter is poorly differentiated and highly metastatic. In this study, using quantitative real‐time polymerase chain reaction, we described a complete protocol for in vivo CRISPR–Cas9‐based knockout screening of essential genes for DGC metastasis. We functionally screened 30 candidate genes using our mouse DGC models lacking Smad4, p53, and E‐cadherin. Pooled knockout mouse DGC cells were transplanted into a spleen of syngeneic immunocompetent mice to study clonal advantages in context of a complex process of liver metastasis. Tmsb4x (thymosin beta‐4 X‐linked), Hmox1, Ifitm3, Ldhb , and Itgb7 were identified as strong candidate genes that promote metastasis. In particular, Tmsb4x enhanced DGC metastasis and stomach organoid‐generated tumor growth in in vivo transplantation models. Tmsb4x promoted tumor clonogenicity and anoikis resistance. In situ hybridization analysis showed that Tmsb4x is highly expressed in E‐cadherin‐negative mouse DGC models compared with mouse IGC and intestinal cancer models. E‐cadherin deficiency also increased Tmsb4x expression in stomach organoids via Wnt signaling activation. Collectively, these results demonstrate that Tmsb4x promotes DGC metastasis. In addition, this experimental system will aid in the identification of novel target genes responsible for DGC metastasis.