PLAGL2 Independently Drives Aberrant Erythropoiesis and Initiation of Preleukemic State

Background

The identification and understanding of early drivers in malignancy is crucial to prevent or revert preleukemic events. Del(20q) is one of the most common primary cytogenetic abnormalities found in preleukemic malignancies from myeloproliferative neoplasms to myelodysplastic syndrome (MDS). Previous studies have identified a "common retained region" within 20q11.21 that is often amplified in a subset of MDS patients. PLAGL2 is one of the 4 genes identified to be within the minimally conserved amplified region. (MacKinnon et al. 2010) Indeed, in previously published datasets of MDS hematopoietic stem and progenitor cells (HSPCs) transcriptome, PLAGL2 is significantly elevated in del(20q) patients compared to healthy controls. However, we have found that its level is also higher in HSPCs of cytogenetically normal MDS patients with low blasts. Given these findings, we sought to define the role of PLAGL2 as a potential early driver of myeloid malignancies.

Results

In healthy cord blood (CB) HSPCs, PLAGL2 overexpression enhanced proliferation ex vivo, better maintained stemness and decreased apoptosis. Colony formation assays also identified increased output of the erythroid lineage. Xenotransplanted CB CD34+ HSPCs overexpressing PLAGL2 exhibited increased engraftment competitiveness and led to splenomegaly with signs of hypercellularity after 20 weeks, features consistent with clinical observations of hematological malignancy. Grafts derived from PLAGL2 overexpressing cells reproducibly maintained a significantly larger CD34+ HSPC compartment. Intriguingly we also identified that ~50% of PLAGL2-overexpressing grafts exhibited a significant erythroid (CD71+/CD235a+) component where none was observed in the control group. This unique finding of aberrant erythropoiesis is reminiscent of clinical observations in patients with 20q11.21 amplification, where a high proportion of erythroblasts in the marrow and in some cases progression to erythroleukemia was noted. To evaluate the progression of PLAGL2-overexpressing grafts, further secondary transplantations were carried out and showed the persistence of only immature erythroid progenitors (CD71+/CD235a-) coupled with a near complete absence of lymphopoiesis in the same grafts. Together, our data strongly suggests ectopic levels of PLAGL2 can independently drive the expansion of human HSPCs and enforce features of myeloid malignancy.

To uncover the molecular mechanism underlying PLAGL2 function, we performed RNA-seq and Cut&Run in human CB CD34+ HSPCs overexpressing PLAGL2. Geneset enrichment analysis of the transcriptome and over-representation analysis of bound genes both identified signatures consistent with LSCs. We compared these findings with identically-derived omics profiles of HSPCs overexpressing PLAG1, a closely related family member that our lab has identified as a potent expander of HSCs ex vivo but not capable of promoting malignant features. We found a strong common feature in the downregulation of ribosomal components and translation machinery, then functionally validated reduced protein synthesis in PLAGL2 overexpressing HSPCs through OP-Puro assays. We have shown dampened mRNA translation to be one of the mechanisms by which PLAG1 enhances stemness and survival of HSCs, one that potentially extends to PLAGL2 as well. However, we also identified discordant signatures, notably PLAGL2's unique capacity to reduce mitochondrial translation, a pathway associated with ineffective erythropoiesis and MDS and one that we are currently exploring as a means by which PLAGL2 can enforce malignant phenotypes.

Finally, to investigate the potential of PLAGL2 as a therapeutic target in MDS, we performed shRNA knockdown in MDSL, a human MDS cell line. In vitro competitive assays with mixed wildtype cells showed steady dropout of PLAGL2 depleted cells. Currently, we are continuing to purse the dependence of primary MDS cells on PLAGL2 through in vivo xenograft models.

Conclusion

We have identified PLAGL2's potential as an early independent driver of myeloid malignancy and aberrant erythroid differentiation. An understanding of PLAGL2 and its downstream mechanisms will not only further our understanding on the development of early myeloid malignancies but also potentially provide another avenue to treat or prevent leukemia before it manifests.