Amplification of hnRNP K Drives c-Myc-Dependent Malignancies and Represent a Novel Therapeutic Opportunity for Hematologic Malignancies

hnRNP K is a transcriptional and translational regulatory protein critical for maintaining hematopoietic homeostasis. We have recently shown that alterations in hnRNP K expression are sufficient to drive hematological malignancies, including lymphomas. Through reverse phase protein array (RPPA) analyses, we observed that hnRNP K is overexpressed in patients with hematological malignancies and that this overexpression is directly correlated with increased c-Myc levels and activation of putative c-Myc targets. Given the critical role of c-Myc during lymphomagenesis, these findings point to a novel mechanism by which c-Myc is overexpressed in lymphoma patients that do not carry c-Myc amplifications or translocations.

In order to examine the oncogenic potential of hnRNP K overexpression in hematological cancers and evaluate its role in driving c-Myc dependent lymphomas, we generated multiple transgenic hnRNP K mice that specifically overexpress hnRNP K in the B-cell compartment (Eµ-hnRNP K). Characterization of these Eµ-hnRNP K mice revealed that hnRNP K is significantly overexpressed in multiple transgenic lines (Fig. 1A) and resulted in a significant reduction in survival (Fig. 1B) with marked enlargement of hematopoietic tissues (spleens, lymph nodes, and thymi) and highly penetrant  lymphomas capable of invasion into distant organs (Fig. 1C).

Flow cytometry and CBC analyses of malignant peripheral blood and bone marrows revealed a significant increase in immature lymphoblasts consistent with an aggressive disease state. To examine the malignant potential of these hnRNP K overexpression dependent malignancies, we performed transplantation assays. Malignant cells from the Eµ-hnRNP K mice rapidly engrafted in recipient NSG mice and the facilitated the development of aggressive lymphomas of immature origin (Fig. 1D). Together, these results demonstrate that when overexpressed, hnRNP K behaves as a bona-fide oncogene.

To address the molecular mechanisms driving the oncogenic potential of hnRNP K overexpression, we evaluated c-Myc expression in tissues from these transgenic mice. Here, we observed a direct correlation between hnRNP K overexpression and an increase in c-Myc expression in multiple transgenic lines. To further investigate the relationship between hnRNP K overexpression and c-Myc expression, we transiently transfected HEK-293 cells with hnRNP K and observed a significant increase in c-Myc expression. Next, we performed RNA immunoprecipitation (RIP) assays using murine tissues and cell lines to determine whether hnRNP K translationally regulates c-Myc expression. These experiments revealed that hnRNP K directly interacted with the Myc transcript to facilitate its translation (Fig. 1E). Given that c-Myc overexpression is often observed in hematologic malignancies without amplification or translocation, these finding strongly suggest that hnRNP K overexpression may represent a novel mechanism to drive c-Myc expression. Thus, a more robust understanding of this cooperative mechanism may lead to more effective and personalized treatment strategies.

To this end, we are now directly evaluating our hypothesis that malignant cells that overexpress hnRNP K will be uniquely sensitive to compounds that inhibit c-Myc expression. Until recently, c-Myc was an undruggable target; however, the discovery of small molecules that inhibit the function of Bromodomain and extra terminal (BET) family members (e.g.; JQ1) have shown tremendous efficacy in disrupting c-Myc activities. Using both JQ1 and a proprietary molecule from Arvinas® (BRD4-Protac), that uses Proteolysis Targeting Chimeras (Protac), we are currently examining the therapeutic efficacy of pharmacological targeting c-Myc in hnRNP K-mediated malignancies.