The Cold-Shock Protein Ybx1 Is Required for Development and Maintenance of Acute Myeloid Leukemia (AML) in Vitro and In Vivo

The family of cold shock proteins (CSPs) is highly conserved and consists of 8 members, including Ybx1-3, Csde1 and Lin28. Ybx1 is a multifunctional DNA/RNA binding protein that modulates gene transcription and translation during inflammation and malignant transformation. Recently, our group identified Ybx1 as a mediator of Jak2 signaling in MPN that protects Jak2-mutated cells from Jak-inhibitor induced apoptosis.

In a recently published genome wide CRISPR-Cas9 dropout screen in AML cell lines, depletion of Ybx1 resulted in the highest dropout indices compared to other CSP members, with strongest dependencies in cell lines harboring MLL-rearrangements. Protein expression of Ybx1 in healthy individuals (n=10), primary MDS (n=54) and AML (n=58) bone marrow (BM) biopsies, revealed high protein expression in the majority of AML and MDS cases. Consistently, gene expression data revealed high mRNA expression of Ybx1 in AML samples compared to normal controls.

Genetic inactivation of Ybx1 in human AML cell lines by RNAi resulted in reduced proliferative capacity. Therefore, we sought to investigate the requirement for Ybx1 in malignant transformation. We used BM cells from a previously published conventional knockout (ko) mouse model (Lu et al., 2005) in which homozygous deletion is embryonically lethal due to brain malformation. We sorted Lineage-Sca1+Kit+ (LSK-) cells from the BM of heterozygous (Ybx1+/-) and wildtype (Ybx1+/+) mice. Cells were retrovirally infected with either MLL-AF9 (MA9) or HoxA9 and Meis1a (HA9M1) to assess for disease development by serial plating in methylcellulose. Haploinsufficiency for Ybx1 in MA9- or HA9M1 transformed cells limited re-plating capacity to 2-4 rounds. When we injected 2,5x 10⁴ MA9-infected LSK cells into sublethally irradiated recipient mice, recipients of MA9-Ybx1+/- cells (n=8) and MA9-Ybx1+/+ (n=10) showed development of AML. However, recipients of MA9-Ybx1+/- cells had a significant delay in AML development (median survival 67.5 days for Ybx1+/+ versus 101.5 days for Ybx1+/- animals, p=0.0078**). This effect appeared even more pronounced when 1x 10⁶ whole BM cells were transplanted into sublethally irradiated secondary recipients. Besides a significant delay in AML development (median survival 37.5 days for recipients of MA9-Ybx1+/+ versus 79 days for MA9-Ybx1+/- BM, p=0.0042**), disease penetrance was reduced by 40%, indicating that haploinsufficiency for Ybx1 impairs development of MA9 driven AML. In contrast, immunophenotypic abundance of stem- and progenitor cells in Ybx1+/+ versus Ybx1+/- animals revealed comparable numbers in all relevant subpopulations. Serial competitive transplantation of Ybx1+/+ and Ybx1+/- BM into primary and secondary recipient animals showed no competitive disadvantage or lack of self-renewal capacity of Ybx1+/- cells.

To address the question whether Ybx1 may also be essential for maintenance of AML, we used RNAi to deplete Ybx1 in already established MA9 driven AML. LSK cells from BL/6 mice transformed with MA9 were injected into primary recipient mice. After AML onset, MA9-LSK cells were sorted and infected with either one of 3 shRNAs against Ybx1 or non-targeting (NT-) control. Lentiviral knockdown of 40% reduced colony formation by more than 50% but did not limit the re-plating capacity in vitro. When injected into sub-lethally irradiated recipient mice, lentiviral knockdown (kd) of Ybx1 resulted in a significant delay in AML development (median survival 39.5 days for NT-control versus 53 days for Ybx1 kd, p=0.0446*).

To validate our findings, we used a newly generated conditional ko mouse model for Ybx1, in which exon 3 coding for the cold-shock domain is deleted by activation of an Mx1-Cre-recombinase following pIpC administration. Preliminary results provide first evidence that genetic deletion of Ybx1 after onset of MA9 driven leukemia resulted in improved survival of primary recipient (median survival 73 versus 83 days) and a reduced penetrance in secondary recipient mice.

Taken together our results may provide first evidence for a functional role of Ybx1 in MLL-AF9 driven AML. As Ybx1 seems to be dispensable for normal hematopoietic cells, these findings may offer a potential therapeutic index. Experiments to assess for the requirement for Ybx1 in maintenance of murine and human AML as well as analysis on proteomic and transcriptional changes following Ybx1 deletion are currently under way.