How CBX proteins regulate normal and leukemic blood cells

Abstract

Hematopoietic stem cell (HSC) fate decisions are dictated by epigenetic landscapes. The Polycomb Repressive Complex 1 (PRC1) represses genes that induce differentiation, thereby maintaining HSC self-renewal. Depending on which chromobox (CBX) protein (CBX2, CBX4, CBX6, CBX7, or CBX8) is part of the PRC1 complex, HSC fate decisions differ. Here, we review how this occurs. We describe how CBX proteins dictate age-related changes in HSCs and stimulate oncogenic HSC fate decisions, either as canonical PRC1 members or by alternative interactions, including non-epigenetic regulation. CBX2, CBX7, and CBX8 enhance leukemia progression. To target, reprogram, and kill leukemic cells, we suggest and describe multiple therapeutic strategies to interfere with the epigenetic functions of oncogenic CBX proteins. Future studies should clarify to what extent the non-epigenetic function of cytoplasmic CBX proteins is important for normal, aged, and leukemic blood cells.

Keywords: CBX; HSC; PRC1; Polycomb; aging; epigenetics; hematopoiesis; leukemia; subcellular CBX localization.

Fig. 1

Epigenetic regulation of gene transcription. (A) DNA and histone modifications that repress or activate gene transcription. Only modification described in this review are shown. Ac, acetylation; me3, trimethylation; Ub, ubiquitination. (B) Canonical PRC2 and PRC1 dependent gene repression. (1) The PRC2 complex catalyzes trimethylation of H3K27, which can be (2) recognized by the CBX subunit of the PRC1 complex. (3) The subunit RING1 and PCGF can ubiquitinate H2AK119 which leads to chromatin compaction. (4) The subunits PHC and SCML interact with other PRC1 complexes resulting in chromatin looping and blocking (5) the transcription machinery whereby PRC targets are transcriptionally silenced. Figure created in Biorender.

Fig. 2

Domain annotations of CBX proteins. Each CBX protein contains a chromodomain (in red) and a PcBox (in blue). The CBX proteins use the chromodomain to interact with trimethylated marks, and the PcBox to incorporate into the PRC1 complex. CBX4 contains additional SIM motifs (purple), which are required for CBX4‐mediated SUMO E3 ligase activity. CBX8 contains a MLLT1/3 interaction site and therefore can be incorporated into the KMT2A‐r complex. The location of each feature within the human CBX protein sequence is indicated by the amino acid position above the features. Figure created in Biorender.

Fig. 3

Alternative CBX‐containing epigenetic complexes. (A) CBX4, as part of the repressive complex containing HDAC, CtBp, and PCGF4, can interact with the transcriptional activator complex containing KMT2A, LEDGF, Menin, and MOF, when CYP33 is bound to KMT2A. This half‐repressive, half‐active complex can target bivalent loci by recognizing both the repressive mark H3K27me3 and the activation mark H3K36me3 by CBX4 and LEDGF, respectively. Menin is a scaffold protein that stabilizes the binding between LEDGF and KMT2A. PCGF4 and CtBp are co‐repressors and HDAC is a deacetylase that removes the MOF‐initiated acetyl groups from H3K27, which results in gene repression. (B) CBX7 can interact with the H3K9 methyltransferase SUV39H2, SETDB1, EHMT1, and EHMT2. One hypothesis how this represses genes is that CBX7 recognizes the auto‐methylated lysines on SETDB1 and EHMT and thereby recruits the CBX7‐containing PRC1 complex to H3K9me3 covered loci to repress genes in a PRC1‐dependent way. A second hypothesis is that CBX7 targets H3K27me3, thereby recruiting SUV39H2 to the chromatin which will lead to methylation of H3K9 and deeper epigenetic silencing. (C) CBX8 can interact with the KMT2A fusion partners MLLT3 and MLLT1. One mechanism to enforce transcriptional activation of rearranged KMT2A is by binding to CBX8 and TIP60 whereby TIP60‐dependent H3K27 acetylation promotes transcriptional activation. Another mechanism to promote transcriptional activation is via binding of rearranged KMT2A to CBX8 and PAF1 which then promotes transcriptional elongation. CBX8‐KMT2A‐r binding can also compete for DOTL1‐KMT2A‐r binding, thereby blocking transcriptional activation but instead initiates PRC1‐dependent gene repression at H3K36me3 active loci. (D) As part of the non‐canonical PRC1.1 complex, containing BCL6, BCOR, KDM2B, PCGF1, RING1, and CBX8, CBX8 can recruit this complex to H3K27me3 covered loci which will initiate RING1‐dependent ubiquitination of H2A and thereby repress genes. Figure created in Biorender.