Metastasis-repressive CircRNA MASCOT Regulates Distribution of the Splicing Factor hnRNP M

Alternative splicing (AS) diversifies the transcriptome complexity and is usually hijacked by cancer cells. Here, we identified heterogeneous nuclear ribonucleoprotein M (hnRNP M) as an indispensable contributor to EMT-related AS reprogramming, whose sub-nuclear distribution of hnRNP M is altered in metastatic tumors. Upon TGFβ stimulation, hnRNP M trends to disassociate with a circRNA called MASCOT (hnRNP M-ASsociated Circular RNA Of TGFβ) that demonstrated an impressive inhibitory effect on EMT process and tumor metastasis. Mechanistically, the anti-metastatic effect of MASCOT is attributed to not only its competition with U2 RNA splicing factors to recruit hnRNP M in the nucleoplasm, but also the anchorage of hnRNP M with nucleolin into the nucleolus to cause a temporospatial separation of hnRNP M from spliceosome. Moreover, the correlation between MASCOT expression and hnRNP M sub-nuclear localization was validated in clinical specimens. Our study deciphers the regulatory codes controlling AS networks and highlights MASCOT as a promising therapeutic to treat metastasis. Guo et al. revealed a dynamic sub-nuclear distribution of hnRNP M mediated by the circRNA called MASCOT which functions as a scaffold to fasten hnRNP M with nucleolin to prevent hnRNP M from being recruited by spliceosomes and thus suppresses the EMT-related alternative splicing events and tumor metastasis.