Stag2a or Stag2b Knockout Zebrafish Are Viable, Yet Loss of Stag2a Is Associated with Abnormal Expression of Early and Late Myeloid and Lymphoid Markers

Introduction:Mutations in Stromal antigen 2 ( STAG2) are frequently found in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). In mice, loss of Stag2 is embryonic lethal. Conditional knockout of Stag2 in adult mice results in MDS-like hematopoietic anomalies. Zebrafish harbor two paralogues of STAG2 ( stag2a and stag2b). To date, a complete stag2-null zebrafish model organism has not been developed. Recently reported, a stag2b^-/- zebrafish demonstrated downregulation in primitive erythropoiesis. However, any hematopoiesis beyond 2 days post fertilization (2 dpf) was not detailed. Furthermore, a stag2a mutant zebrafish has not been reported.

Methods and Results: Using CRISPR/Cas9 genome editing, we created a stag2a mutant anda stag2b mutant zebrafish. Both mutations result in a frame shift and premature protein truncations. Unlike the murine Stag2 ^-/- mutant, zebrafish stag2a ^-/- mutants and stag2b ^-/-mutants survive to adulthood, with no obvious morphological phenotypes. Detection of hemoglobin, using O-dianisidine staining, reveals anemia at 31 hours post fertilization (31 hpf) in stag2a ^+/- mutants (10/29), stag2a ^-/-mutants (10/15), stag2b ^+/-mutants (6/11), and stag2b ^-/-mutants (5/7); wildtype siblings appear unaffected. Intriguingly, at 4 dpf, stag2a maternal-zygotic mutants show increased expression levels of early and late myeloid and lymphoid markers. ( Figure 1) Work is ongoing to analyze blood cell lineages from zebrafish whole kidney marrow, the equivalent of mammalian bone marrow, in adult stag2a and adult stag2b mutants. In addition, we are characterizing the zebrafish stag2a;stag2b double mutants, which we will present.

Conclusions: In zebrafish, stag2a and stag2b contribute to proper primitive and definitive hematopoiesis. Our organismal model of Stag2 deletion in the zebrafish will yield new insight into how loss of STAG2 may lead to MDS/AML.