ABSTRACT MGA, a transcription factor and member of the MYC network, is mutated or deleted in a broad spectrum of malignancies. As a critical test of a tumor suppressive role, we inactivated Mga in two mouse models of non-small cell lung cancer using a CRISPR based approach. MGA loss significantly accelerated tumor growth in both models and led to de-repression of atypical Polycomb PRC1.6, E2F and MYC-MAX targets. Similarly, MGA depletion in human lung adenocarcinoma lines augmented invasive capabilities. We further show that MGA, E2F6 and L3MBTL2 co-occupy thousands of promoters and that MGA stabilizes PRC1.6 subunits. Lastly, we report that MGA loss has also a pro-growth effect in human colon organoids. Our studies establish MGA as a bona fide tumor suppressor in vivo and suggest a tumor suppressive mechanism in adenocarcinomas resulting from widespread transcriptional attenuation of MYC and E2F targets mediated by an atypical Polycomb complex containing MGA-MAX dimers.

Loss of MGA mediated Polycomb repression promotes tumor progression and invasiveness