Chromosomal abnormalities are frequent events in human cancer and are especially prevalent in hematological malignancies. Here, through the characterization of chromosomal abnormalities del(20q) and t(8;21) we reveal unexpected non-canonical functions of the mammalian Hippo kinase in hematologic cancer.
Somatic heterozygous deletions on chromosome 20q are detected in several hematopoietic malignancies, including myelodysplastic syndrome (MDS), classical myeloproliferative neoplasm (MPN), MDS/MPN overlap disorders, and acute leukemias. To date, identification of genes in the del(20q) common deleted region that contribute to disease development have remained elusive. Through assessment of patient gene expression we have identified STK4 (encoding Hippo kinase MST1) as a 20q gene that is downregulated below haploinsufficient amounts in MDS and MPN. Functional modeling of hematopoietic-specific gene inactivation in mice revealed Hippo kinase loss to cause phenotypes that closely resemble those observed in patients harboring del(20q), and to cooperate with JAK2-V617F mutation in promoting adverse MPN disease progression to myelofibrosis. Mechanistic studies revealed that myelofibrotic transformation in mice was associated with cooperative effects of JAK2-V617F and Hippo kinase inactivation on IRAK1-dependent innate immune-associated proinflammatory cytokine production, including IL-1β and IL-6. In summary, we find Hippo kinase MST1 (STK4) as having a central role in the biology of del(20q)-associated hematologic malignancies.
Through biochemical and functional studies based on the characterization of t(8;21) in acute myeloid leukemia, we also define a novel Hippo-RASSF2 signaling pathway that regulates basal Rac GTPase activity in hematological cancer and can be exploited for therapy in patients. This non-canonical signaling mechanism is independent of Hippo kinase activity, and is instead mediated by a SARAH domain-dependent interaction. Using proximity-based biotin labeling, we associate RASSF2 with Rho GTPase-related complexes and identify a direct interaction with the critical Rac guanine nucleotide exchange factor (GEF), DOCK2. RASSF2 promotes DOCK2 GEF activity in vitro; and RASSF2 knockdown is sufficient to functionally abolish GTP-bound Rac and promote growth-arrest in leukemia cells. Importantly, RASSF2 expression is broadly correlated with leukemia cell sensitivity to small-molecule inhibition of DOCK2 GEF activity, revealing novel mechanistic insight and providing a functional biomarker for sensitivity to perturbation of this pathway in hematological cancer.