Nap1L4a Cooperates with Scl/Klf1 to Recruit H2A.Z in Mediating Interactions Among Cis-Regulatory Elements and Transcription Required for Primitive Erythropoiesis in Zebrafish.

The chromatin remodeler nucleosome assembly protein 1-like 4 (Nap1L4) is highly expressed in megakaryocyte-erythroid progenitors (MEPs) and erythroid cells. Mutations, deletions, and aberrant expressions of Nap1L4 are observed in diseases such as acute myeloid leukemia (AML). However, the roles of Nap1l4a in erythropoiesis and related diseases, as well as the underlying mechanisms, remain unknown. Here, it is demonstrated that zebrafish nap1l4a homozygous mutants (nap1l4a) are more sensitive to hypoxia stress during the early embryonic stage and exhibit impaired primitive erythropoiesis. Mechanistically, zebrafish Nap1l4a interacts with the erythropoietic transcription factors (TFs) Scl and Klf1, and recruits the histone variant H2A.Z. This interaction remodels the cis-regulatory element (CRE) landscape and promotes nascent RNA transcription of erythropoietic genes. Meanwhile, Nap1l4a deficiency impairs chromatin accessibility at the epigenetic regulators kdm6b and kmt2c. This results in expanded H3K27me3 and diminished H3K4me1 in erythrocytes, leading to altered histone landscapes at erythropoiesis TF loci and reduced TF expression. Moreover, Nap1l4a regulates primitive erythropoiesis by transcriptionally and epigenetically modulating the canonical WNT/β-Catenin pathway. Together, the findings reveal a lineage-selective transcription, with histone epigenomics-dependent role for nap1l4a in vertebrate primitive erythropoiesis. These findings highlight potential mechanisms underlying human blood disorders and hypoxia responses associated with Nap1l4a deficiency.