Developmental hematopoiesis: ontogeny, genetic programming and conservation. Exp Hematol. 2014;42:669-683. [PMID: 24950425 DOI: 10.1016/j.exphem.2014.06.001]

Creating cellular diversity through transcription factor competition

The development of blood cells has long served as a model system to study the generation of diverse mature cells from multipotent progenitors. The article by Org et al (2015) reveals how transcription factor competition on primed DNA templates may contribute to embryonic blood cell specification during the early stages of mesoderm development. The study not only provides new insights into the functionality of the key haematopoietic transcription factor Scl/Tal1, but also provides a potentially widely applicable framework for transcription factor-mediated cell fate specification.

Imprinted genes in myeloid lineage commitment in normal and malignant hematopoiesis

Genomic imprinting is characterized by the parent-of-origin monoallelic expression of several diploid genes because of epigenetic regulation. Imprinted genes (IGs) are key factors in development, supporting the ability of a genotype to produce phenotypes in response to environmental stimuli. IGs are highly expressed during prenatal stages but are downregulated after birth. They also affect aspects of life other than growth such as cognition, behavior, adaption to novel environments, social dominance and memory consolidation. Deregulated genomic imprinting leads to developmental disorders and is associated with solid and blood cancer as well. Several data have been published highlighting the involvement of IGs in as early as the very small embryonic-like stem cells stage and further during myeloid lineage commitment in normal and malignant hematopoiesis. Therefore, we have assembled the current knowledge on the topic, based mainly on recent findings, trying not to focus on a particular cluster but rather to have a global view of several different IGs in hematopoiesis.

Identification of Cdca7 as a novel Notch transcriptional target involved in hematopoietic stem cell emergence

Guiu et al. use ChIP-on-chip analysis for the Notch partner RBPj, using embryonic tissue from the aorta-gonad-mesonephros region to identify potential novel Notch target genes involved in HSC emergence. They show that c-MYC–responsive gene Cdca7 is expressed in different HSC and progenitor subpopulations and that CDCA7 is important for maintaining the undifferentiated phenotype. Cdca7 acts downstream of Notch in HSCs in zebrafish, mouse, and human, indicating a highly conserved Notch/RBPj/Cdca7 axis in hematopoietic development.

Hematopoietic stem cell (HSC) specification occurs in the embryonic aorta and requires Notch activation; however, most of the Notch-regulated elements controlling de novo HSC generation are still unknown. Here, we identify putative direct Notch targets in the aorta-gonad-mesonephros (AGM) embryonic tissue by chromatin precipitation using antibodies against the Notch partner RBPj. By ChIP-on-chip analysis of the precipitated DNA, we identified 701 promoter regions that were candidates to be regulated by Notch in the AGM. One of the most enriched regions corresponded to the Cdca7 gene, which was subsequently confirmed to recruit the RBPj factor but also Notch1 in AGM cells. We found that during embryonic hematopoietic development, expression of Cdca7 is restricted to the hematopoietic clusters of the aorta, and it is strongly up-regulated in the hemogenic population during human embryonic stem cell hematopoietic differentiation in a Notch-dependent manner. Down-regulation of Cdca7 mRNA in cultured AGM cells significantly induces hematopoietic differentiation and loss of the progenitor population. Finally, using loss-of-function experiments in zebrafish, we demonstrate that CDCA7 contributes to HSC emergence in vivo during embryonic development. Thus, our study identifies Cdca7 as an evolutionary conserved Notch target involved in HSC emergence.