850P - A zebrafish model of MYC-driven acute myeloid leukemia reveals that neutrophil resistance to oncogenic transformation depends on their ability to promote PP2A-mediated MYC proteasomal degradation

Background

Acute Myeloid Leukemia (AML) is the most common acute leukemia in adults. The origin can be found in chromosomal translocations as well as in accumulated mutations in genes involved in hematopoietic proliferation and differentiation. Despite being considered a heterogeneous malignancy, AML possesses many genetic aberrations, including the overexpression of MYC and the downregulation of PP2A, both essential regulators of cell proliferation, apoptosis, and differentiation, and they, directly and indirectly, regulate each other’s activity. We have generated a zebrafish model of MYC-driven AML to rapidly test the effects of different drugs in tumor development.

Methods

After the generation of a transgenic zebrafish line expressing the human MYC under the control of the neutrophils-specific promoter lysozyme (lyz:hMYC), we checked the expansion of neutrophils in whole larvae and the colonization of the adult hematopoietic organ. We investigated the effects of chemical manipulation of PP2A activity on neutrophil expansion and on MYC phosphorylation and degradation. Single-cell RNA sequencing analysis has been performed to shed light into the mechanisms of PP2A/proteasomal-mediated degradation of oncogenic MYC in AML.

Results

In vivo imaging of whole larvae revealed a robust expansion of neutrophils in zebrafish larvae which expressed MYC from 3 to 8 days post-fertilization (dpf). Surprisingly, the neutrophil number gradually returned to normal levels from 10 dpf onwards, coinciding with degradation of MYC. Pharmacological inhibition of the proteasome resulted in the stabilization of MYC protein and further expansion of transformed neutrophils. Similarly, inhibition of PP2A phenocopied the effects of proteasome inhibition. Otherwise, pharmacological activation of PP2A prevented neutrophil expansion and increased phosphorylation of S62 and T58 of MYC, suggesting a critical role PP2A in MYC proteasomal degradation.

Conclusions

Our results show that PP2A critically regulates MYC degradation in neutrophils preventing their oncogenic transformation. In addition, the zebrafish model of AML is a unique tool for chemical screening.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

University of Murcia.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.