Regulation of heterochromatin formation and tumor suppression in leukemia by IKAROS, HDAC1 and EZH2 [Molt4_CT]
The IKZF1 gene encodes IKAROS – a DNA binding protein that acts as a tumor suppressor in  T-cell acute lymphoblastic leukemia (T-ALL). IKAROS can act as a transcriptional repressor via chromatin remodeling, however the mechanisms through which Ikaros exerts its tumor suppressor function via heterochromatin in T-ALL are largely unknown. We studied human and mouse T-ALL using loss-of-function and Ikzf1 re-expression approach, along with Ikzf1-wildtype primary human and mouse T-ALL and thymocytes to establish the role of  Ikaros and Ikaros-associated protein histone deacetylase 1 (HDAC1)  in global regulation of facultative heterochromatin and transcriptional repression in T-ALL.   Results identified novel Ikaros and HDAC1 functions in T-ALL: Ikaros and HDAC1 are essential for EZH2 histone methyltransferase activity, and formation of facultative heterochromatin; Recruitment of HDAC1 by Ikaros is critical for establishment of H3K27me3 and repression of active enhancers; and Ikaros-HDAC1 complexes promote formation and expansion of H3K27me3 large organized chromatin lysine domains (LOCKs) and broad genic repression domains (BGRDs) in T-ALL.  Our results establish that Ikaros' tumor suppressor function in T-ALL occurs via activation of EZH2 and HDAC1 function, global regulation of the facultative heterochromatin landscape and silencing of active enhancers that regulate oncogene expression. Overall design: The IKZF1 gene was deleted in human T-ALL cells MOLT4 using the CRISPR-Cas system to create MOLT-4 IKZF1-null cells