Interplay between Homeobox proteins and Polycomb repressive complexes in p16INK4a regulation

Abstract

The INK4/ARF locus regulates senescence and is frequently altered in cancer. In normal cells, the INK4/ARF locus is found silenced by Polycomb repressive complexes (PRCs). Which are the mechanisms responsible for the recruitment of PRCs to INK4/ARF and their other target genes remains unclear. In a genetic screen for transcription factors regulating senescence, we identified the homeodomain-containing protein HLX1 (H2.0-like homeobox 1). Expression of HLX1 extends cellular lifespan and blunts oncogene-induced senescence. Using quantitative proteomics, we identified p16 INK4a as the key target mediating the effects of HLX1 in senescence. HLX1 represses p16 INK4a transcription by recruiting PRCs and HDAC1. This mechanism has broader implications, as HLX1 also regulates a subset of PRC targets besides p16 INK4a . Finally, sampling members of the Homeobox family, we identified multiple genes with ability to repress p16 INK4a . Among them, we found HOXA9 (Homeobox A9), a putative oncogene in leukaemia, which also recruits PRCs and HDAC1 to regulate p16 INK4a . Our results reveal an unexpected and conserved interplay between homeodomain-containing proteins and PRCs with implications in senescence, development and cancer.

Acute myeloid leukemia requires Hhex to enable PRC2-mediated epigenetic repression of Cdkn2a

“…INK4a -dependent cell cycle arrest (Martin et al 2013). Our findings identify Hhex as a crucial homeobox protein that similarly enables PRC2 function at the Cdkn2a locus to maintain the self-renewal capacity of MLL-ENL-induced AML.…”

“…INK4a and p19 ARF expression via regulation of PRC2 function Previous studies have indicated an important role for homeobox transcription factors, including Hoxa9 and Hlx1, in the recruitment of PRC2 to the Cdkn2a/b locus to facilitate transcriptional repression and the maintenenace of self-renewal potential (Martin et al 2013;Collins et al 2014). We therefore tested whether Hhex mediates repression of this locus in AML cells via PRC2-mediated epigenetic silencing using antibodies directed against the PRC2-encoded repressive mark H3K27me3 as well as the activating mark H3K4me3 for chromatin B H E T K O IGF2R NAAA SLFN5 USP18 DDX58 AMOT AGAP1 SBF2 PARP14 CDKN2A RASSF10 ADAMDEC1 TRIM34A PLXNC1 DDX60 RTP4 XAF1 IRGM2 ZBP1 BMP8A SAMD4 HOXA11 ISG15 TRIM30D HOXA11OS TGM3 CHPF TMEM198 TESC immunoprecipitation (ChIP) sequencing.…”

Deregulation of the HOXA9/MEIS1 axis in acute leukemia

“…Hoxa9 represses Ink4a expression to overcome oncogene-induced senescence during transformation by AML1-ETO in Bmi1−/− cells, as well as in Hoxa9/Meis1 transformed cells (10, 83). This repression may be the result of direct recruitment of EZH2 by Hoxa9, however recent work suggests that EZH2 may also regulate p16 in HOXA9-independent fashion in MLL -rearranged leukemias (84, 85). …”

The Molecular Balancing Act of p16INK4a in Cancer and Aging

“…Recently, H2.0-like homeobox 1 (HLX1), a homeobox (HOX) protein, was shown by Martin et al to facilitate PRC2 recruitment to the p16 INK4a promoter (Fig. 2A and (52)). While the mechanism has yet to be defined, six other homeobox-containing proteins (HOXA9, DLX3, HOXB13, HOXC13, HOXD3 and HOXD8) were similarly reported to participate in p16 INK4a silencing (52).…”