Defining ortholog-specific UHRF1 inhibition by STELLA for cancer therapy

Abstract

UHRF1 maintains DNA methylation by recruiting DNA methyltransferases to chromatin. In mouse, these dynamics are potently antagonized by a natural UHRF1 inhibitory protein STELLA, while the comparable effects of its human ortholog are insufficiently characterized, especially in cancer cells. Herein, we demonstrate that human STELLA (hSTELLA) is inadequate, while mouse STELLA (mSTELLA) is fully proficient in inhibiting the abnormal DNA methylation and oncogenic functions of UHRF1 in human cancer cells. Structural studies reveal a region of low sequence homology between these STELLA orthologs that allows mSTELLA but not hSTELLA to bind tightly and cooperatively to the essential histone-binding, linked tandem Tudor domain and plant homeodomain (TTD-PHD) of UHRF1, thus mediating ortholog-specific UHRF1 inhibition. For translating these findings to cancer therapy, we use a lipid nanoparticle (LNP)-mediated mRNA delivery approach in which the short mSTELLA, but not hSTELLA regions are required to reverse cancer-specific DNA hypermethylation and impair colorectal cancer tumorigenicity. There is growing evidence indicating the different roles of natural UHRF1 inhibitory protein STELLA in mouse-derived cells from its human ortholog. Here, the authors report the differences of hSTELLA versus mSTELLA in inhibiting the maintenance or de novo DNA methylation functions of UHRF1 in human cancer cells, and lipid nanoparticle-delivered mSTELLA reversing cancer-specific DNA hypermethylation thereby impairing colorectal cancer tumorigenicity.