hnRNP K Overexpression Synergizes with Mutant NPM1 to Drive Acute Myeloid Leukemia Progression

NPM1 mutations are one of the most common alterations observed in acute myeloid leukemia (AML). When coupled with wild type FLT3 status in cytogenetically normal (CN) patients, NPM1 mutations confer favorable prognoses compared with other alterations. However, a subset of CN NPM1^Mut :FLT3^Wt patients with AML have dismal outcomes, suggesting that uncharacterized alterations influence the outcomes in these patients. To address this, we performed reverse phase protein array (RPPA) analysis on CD34+ bone marrow cells isolated from 43 de novo CN NPM1^Mut :FLT3^Wt AML patient as well as healthy donor controls. Through these analyses, we observed that overexpression of heterogeneous nuclear ribonucleoprotein K (hnRNP K) associated with extremely poor outcomes within this a priori favorable prognostic group, as almost 90% of patients with increased hnRNP K expression died within 12 months of diagnosis while nearly 40% of individuals with normal hnRNP K expression survived seven years (Figure 1A).

hnRNP K is a multifunctional RNA and DNA binding protein whose expression is often altered in cancers. To directly examine the functional relationship between hnRNP K overexpression and mutant NPM1 in hematologic malignancies, we generated tissue-specific transgenic mouse models with the ability to overexpress hnRNP K (hnRNP K^Tg) in the presence or absence of mutant Npm1 (Npm1^Tg). By crossing these mice with Vav-Cre expressing mice, we specifically activated hnRNP K overexpression and mutant NPM1 expression in the hematological compartment.

Using Lin-CD117+ hematopoietic stem cells (HSCs) from hnRNP K^Tg, Npm1^Tg, and hnRNP K^Tg;Npm1^Tg mice, we observed significant changes in differentiation and proliferation potential in colony formation assays. Overexpression of hnRNP K alone significantly increased the number of colonies compared to wild type and Npm1^Tg HSCs while expression of mutant Npm1^Tg resulted in increased numbers of cells compared to wild type and hnRNP K^Tg HSCs. Importantly, the combination of hnRNP K overexpression and mutant Npm1 resulted in a cumulative increase in both the number of colonies and number of cells, indicating that hnRNP K and mutant NPM1 cooperate to dictate differentiation and proliferation potential in HSCs (Figure 1B). Next, we examined the in vivo impact of hnRNP K overexpression and mutant Npm1 expression by analyzing the bone marrows of Npm ^Tg, hnRNP K^Tg, and Npm1^Tg;hnRNP K^Tg mice. Within the first six months of life, these mice rapidly developed significant myeloid hyperplasias as determined by flow cytometry and pathologic analyses (Figure 1C).

Together, our findings reveal that mutant Npm1 and hnRNP K overexpression result in similar myeloid phenotypes. However, these genetic alterations are also cooperative, suggesting both increased hnRNP K expression and mutant NPM1 synergize to impact hematopoietic phenotypes and drive AML progression through similar pathways but potentially via unique molecular processes. Currently, we are investigating the direct interaction and global relationship between hnRNP K and mutant Npm1 in regulating tumor suppressor and oncogenic programs (e.g.; p53- and c-Myc pathways).

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