Circular RNA cia-MAF drives self-renewal and metastasis of liver tumor-initiating cells via transcription factor MAFF.

Liver tumor-initiating cells (TICs) are involved in liver tumorigenesis, metastasis, drug resistance, and relapse, but the regulatory mechanisms of liver TICs are largely unknown. Here, we have identified a functional circular RNA, termed circRNA activating MAFF (cia-MAF), that is robustly expressed in liver cancer and liver TICs. cia-MAF-KO primary cells and cia-maf-KO liver tumors harbor decreased ratios of TICs, and display impaired liver tumorigenesis, self-renewal, and metastatic capacities. In contrast, cia-MAF overexpression drives liver TIC propagation, self-renewal, and metastasis. Mechanistically, cia-MAF binds to the MAFF promoter, recruits the TIP60 complex to the MAFF promoter, and finally promotes MAFF expression. Loss of cia-MAF function attenuates the combination between the TIP60 complex and the MAFF promoter. MAFF is highly expressed in liver tumors and liver TICs, and its antisense oligo (ASO) has therapeutic potential in treating liver cancer without MAFA/MAFG gene copy number alterations (CNAs). This study reveals an additional layer for liver TIC regulation as well as circRNA function, and provides an additional target for eliminating liver TICs, especially for liver tumors without MAFA/MAFG gene CNAs.