Abstract
Hyperactivation of RAS is common in human cancer, including hematological malignancies. Most oncogenic RAS proteins have been difficult to target. Identification of alternative targets that block RAS signaling is critical to develop therapies for RAS-related cancer. The biological activity of RAS proteins relies upon post-translational modifications (PTMs) that anchor RAS to cellular membranes. Protein palmitoylation regulates the membrane targeting, subcellular trafficking, and functions of proteins. We have previously examined the importance of PTMs in NRAS leukemogenesis and found for the first time that palmitoylation is essential for NRAS leukemogenesis. These studies suggest that targeting RAS palmitoylation may be an effective therapy for cancers involving RAS proteins that rely on palmitoylation for plasma membrane binding. In the previous studies, we blocked NRAS palmitoylation by mutating the palmitoylation site in NRAS. Therapeutic intervention of RAS palmitoylation requires targeting enzymes that mediate RAS palmitoylation. The reaction of protein S-palmitoylation is catalyzed by a family DHHC protein palmitoyl-acyltransferases (PATs). Thus far 24 mamalian PATs have been identified. It has been shown that DHHC9 (a 364-amino acid protein encoded by ZDHHC9, an X-linked gene), is the ortholog of yeast Ras2 PAT and constitute a mammalian PAT with specificity for H- and NRAS in vitro. Increased expression of DHHC9 has been found in various cancer. Here we investigate the role of DHHC9 in hematopoiesis and NRAS leukemogenesis in vivo. We found that ectopic expression of DHHC9 in mouse bone marrow cells hinders hematopoiesis, but is incapable of inducing hematological malignancies in mice. We also found that frequency of lineage-specific populations and hematopoietic stem cell phenotype were similar in mice with knockout alleles of ZDHHC9 as that of wild type mice, suggesting that DHHC9 is dispensable for normal hematopoiesis. Expression of oncogenic NRAS in bone marrow cells from ZDHHC9 knockout mice still induced leukemia but the mice survived longer that the wild type control mice. These results suggest that DHHC9 plays a role in the pathogenesis of NRAS-induced leukemia, but it is not the only PATs for RAS palmitoylation. We are identifying additional RAS PATs and developing cancer therapies targeting RAS palmitoylation.
