Functional Difference of <scp>MYC</scp> and <scp>MYCN</scp> in Combined Hepatocellular–Cholangiocarcinoma: Regulation of Differentiation by <scp>HNF1B</scp>

MYC and MYCN oncogenes are frequently upregulated in human liver cancers, yet their functional differences remain unclear. We used a mouse model of intrahepatic cholangiocarcinoma (CCA), constructed by transposon-mediated somatic gene integration of AKT and YAP into hepatocytes, to investigate the effects of additional integration of Myc or Mycn. Both Myc and Mycn induced a poorly differentiated hepatocellular carcinoma (HCC) component, resulting in the formation of combined hepatocellular-cholangiocarcinoma (cHCC-CCA). Interestingly, the ratio of HCC to CCA components differed significantly; AKT/YAP/Mycn (AYN) tumors exhibited a lower proportion of CCA components than AKT/YAP/Myc (AYM) tumors. To explore the underlying mechanisms, we analyzed the expression levels of genes involved in liver differentiation. We found that AYN tumors, at both the mRNA and protein levels, exhibited lower expression of HNF1B, a transcription factor that is highly expressed in human CCA but not in HCC. When Hnf1b was co-introduced with AYN, the percentage of the CCA area increased significantly. Furthermore, these tumors exhibited increased expression of TEAD proteins, which interact with YAP to initiate transcription. Notably, treatment with a YAP-TEAD inhibitor suppressed AKT/YAP/Mycn/Hnf1b tumor growth. These findings indicate that Myc and Mycn play distinct roles in the phenotypic determination of primary liver tumors and suggest that their differential effects on Hnf1b expression and subsequent TEAD activation may be a key regulatory mechanism.