Methylation maintains HSC division fate.

Hematopoietic stem cells (HSCs) are responsible for maintaining a sufficient pool of self-renewing cells that can continuously differentiate into lineage-specific hematopoietic cells throughout the lifetime of an individual (1). The choice to self-renew or differentiate is most likely achieved through coordination of extrinsic and intrinsic cell fate signals that drive symmetrical or asymmetrical cell divisions. Several examples of extrinsic signals have been described that regulate HSC self-renewal or differentiation, including Notch signaling in the bone marrow niche (2). Intrinsic mechanisms that regulate these alternative modes of HSC division are still relatively unknown; however, alterations in epigenetic inheritance have been hypothesized as a driving factor. DNA methylation is a heritable epigenetic mark that is highly symmetrical, where 99% of methylated CpG dinucleotides are found to be methylated on both DNA strands (3). DNA methylation patterns are reestablished upon cell division during S phase by the action of DNA methyltransferase 1 (DNMT1). DNMT1 recognizes hemimethylated DNA and remethylates the newly synthesized daughter strand, facilitating epigenetic inheritance in the genome during cell division. In PNAS, a study by Zhao et al. (4) describes how disruption in DNA methylation maintenance has the potential to alter HSC division modes and deregulate lineage-specific gene expression spontaneously, ultimately blocking the self-renewal capacity of HSCs, which leads to their rapid depletion. The molecular mechanism by which DNA methylation regulates transcription and lineage specification is known to be dependent on the complement of factors that can recognize, and are recruited at, sites of differential methylation. Gene promoters often harbor regions of high CpG density known as CpG islands (CGIs) that are typically hypomethylated. Hypomethylation in promoter CGIs may serve as a recruitment signal for transcriptional activators or CXXC-domain– containing proteins that establish a transcriptionally active chromatin (5). Alternatively, methylated CpGs could prevent binding of transcriptional regulators or attract methyl-CpG–binding proteins that specifically recognize methylation marks and recruit histone-modifying enzymes and chromatin remodeling factors to silence chromatin (6). Whether changes in DNA methylation patterns are the cause or consequence of self-renewal or differentiation cues in hematopoietic cells has been explored using genetic ablation of regulators of DNA methylation in mice. Deletion or hypomorphic loss of function in Dnmt1 impairs the self-renewal capacity of HSCs, drastically reducing HSC numbers, suggesting that a critical threshold of DNAmethylation is required to maintain homeostasis within the HSC pool (7, 8). On the contrary, deficiency in the de novo methyltransferasesDnmt3a andDnmt3b promotes increased HSC self-renewal, yet blocks differentiation of all mature hematopoietic lineages (9). Similarly, loss of function in members of the Ten-Eleven-Translocation (TET) family of DNA demethylases promotes increased HSC and progenitor cell self-renewal, but also lineage-specific differentiation bias (10). In the study by Zhao et al. (4), ubiquitin-like with PHD and ring finger domain 1 (UHRF1), a regulator of DNA methylation maintenance, has now been shown to regulate self-renewal and differentiation fate epigenetically in HSCs. Previous studies found that systemic deletion of Uhrf1 is embryonically lethal and Uhrf1deficient embryonic stem cells exhibit a dramatic loss in DNA methylation (11, 12). UHRF1 (also known as ICBP90 in human and Np95 in mouse) specifically recognizes hemimethylated DNA via a SET and RINGassociated (SRA) domain and is an essential cofactor of maintenance methylation by recruiting DNMT1 to replication forks during S phase (11–13). The majority of defects observed in Uhrf1-deficient HSCs in the study by Zhao et al. (4) phenocopy those defects seen withDnmt1 deficiency. Pan-hematopoietic deletion of Uhrf1 is embryonically lethal due to fetal liver (FL)-HSC depletion, and inducible deletion in adult HSCs also causes a rapid decline in survival and loss of total HSC numbers. In addition, Uhrf1-deficient HSCs lose their capacity for self-renewal in competitive bone marrow transplantation assays.