NPM1c impedes CTCF functions through cytoplasmic mislocalization in acute myeloid leukemia.

Normal cytogenetic acute myeloid leukemia (AML) frequently harbor a TCTG insertion in exon 12 of Nucleophosmin 1 (NPM1); the resulting frameshift creates a nuclear export signal (NES) and cytoplasmic localization of NPM1c. However, how NPM1c causes AML is not completely understood. NPM1 participates in multiple protein-protein interactions one of which involves the CCCTC-binding factor (CTCF). Through binding of CTCF binding sites (CBS), CTCF mediates nuclear functions including DNA looping, regulation of gene expression, and RNA splicing. We hypothesized that mislocalization of CTCF into the cytoplasm by NPM1c reduces the functional level of nuclear CTCF and so alters gene expression. We verified the interaction of CTCF with NPM1 and showed that CTCF interacts with NPM1c, with redistribution of CTCF into the cytoplasm. The interaction of CTCF and NPM1c involves the amino terminus of CTCF and the last 50 amino acids of NPM1. By interfering with the interaction of CTCF and NPM1c, CTCF becomes relocalized into the nucleus.