The Lysine Acetyltransferase Tip60 Is Required for Hematopoietic Stem Cell Maintenance

Hematopoietic stem cells (HSCs) are maintained for their two defining properties, a self-renewal and a multi-lineage differentiation ability, under various epigenetic processes. One of the epigenetic processes essential for HSC is a lysine acetylation, which was demonstrated in several previous studies utilizing knockout mice of a gene encoding a lysine acetyltransferase (KAT), such as Cbp/p300, Moz, or Mof. The lysine acetyl transferase 5, Kat5 (also called Tip60), is a member of MYST KAT family, defined by a protein containing a C₂HC-type zinc finger and an acetyl-CoA binding domain, consisting of five members: Tip60, Moz, Morf, Hbo1, and Mof. Tip60 regulates gene transcription, genome stability, cell growth and apoptosis and has been demonstrated to be essential for embryonic development.

To assess the role of Tip60 in murine HSC, we generated Tip60 conditional knockout mice in two different Cre strains (Mx1-Cre and Vav-iCre), wherein Tip60 gene was successfully deleted in HSCs. Tip60 abrogation induced embryonic and adult lethality due to hematopoietic failure. Remarkably, HSCs were rapidly depleted upon Tip60 disruption, exhibiting robust apoptosis, aberrant cell cycle progression, and accumulated DNA damages. Tip60^Δ/Δ fetal LSK cells transduced with retroviruses expressing wild-type Tip60 recovered a long-term hematopoiesis in transplantation experiments, whereas acetyl transferase defective Tip60 did not rescue the reconstitution to any detectable level, suggesting that the Tip60 acetyltransferase activity is essential for HSC function. Gene set enrichment analysis in RNA-seq demonstrated that Tip60 was significantly associated with genes involved in cell-cycle and DNA repair process, and Myc transcriptional factors target gene sets. A genome-wide chromatin profiling corroborated these findings, uncovering a strong similarity between Tip60 and c-Myc binding genomic regions. Considering similarities of HSC phenotype between our Tip60 conditional knockout mice and c-myc and N-myc double knockout mice (Laurenti, E. et al., Cell Stem Cell 2008), Tip60 maintains HSC by regulating Myc target genes.

We next evaluated alteration in histone modifications evoked by Tip60 deletion, performing ChIP-seq analysis in Tip60^f/f; Rosa26-CreERT2 and Tip60^f/f fetal c-Kit⁺ cells that were treated with 4-OHT. Intriguingly, we found that acetylated H2A.Z was enriched at the Tip60-bound active chromatin and Tip60 deletion reduced the acetylation level of H2A.Z at Myc target genes promoters, whereas neither H3K27 acetylation level, active promoter / enhancer mark, nor H3K27 tri-methylation level, repressive mark, were altered. Collectively, our results demonstrate that Tip60 - H2A.Z could be an epigenetic axis critical for active transcription of Myc target genes to maintain murine HSC.