Redundant mechanisms driven independently by RUNX1 and GATA2 for hematopoietic development

RUNX1 is essential for the generation of hematopoietic stem cells (HSCs). Runx1-null mouse embryos lack definitive hematopoiesis and die in mid-gestation. However, although zebrafish embryos with a runxl W84X mutation have defects in early definitive hematopoiesis, some runx1(W84X/W84X) embryos can develop to fertile adults with blood cells of multilineages, raising the possibility that HSCs can emerge without RUNX1. Here, using 3 new zebrafish runx1(-/-) lines, we uncovered the compensatory mechanism for runx1-independent hematopoiesis. We show that, in the absence of a functional runx1, a cd41-green fluorescent protein (GFP)(+) population of hematopoietic precursors still emerge from the hemogenic endothelium and can colonize the hematopoietic tissues of the mutant embryos. Single-cell RNA sequencing of the cd41-GFP(+) cells identified a set of runx(-/-) -specific signature genes during hematopoiesis. Significantly, gata2b, which normally acts upstream of runxl for the generation of HSCs, was increased in the cd41-GFP(+) cells in runx1(-/-) embryos. Interestingly, genetic inactivation of both gata2b and its paralog gata2a did not affect hematopoiesis. However, knocking out runx1 and any 3 of the 4 alleles of gata2a and gata2b abolished definitive hematopoiesis. Gata2 expression was also upregulated in hematopoietic cells in Runx1(-/-) mice, suggesting the compensatory mechanism is conserved. Our findings indicate that RUNX1 and GATA2 serve redundant roles for HSC production, acting as each other's safeguard.

Automated summary

In the absence of functional runx1, a population of hematopoietic precursors still emerge, suggesting a compensatory mechanism for runx1-independent hematopoiesis. Gata2, acting upstream of runxl for HSC production, was upregulated in the absence of runx1, but genetic inactivation of both gata2b and gata2a did not affect hematopoiesis, indicating redundant roles for HSC production.