The discovery of the JAK2V617F mutation in most patients with Ph-negative myeloproliferative neoplasms has led to the development of JAK2 kinase inhibitors. However, JAK2 inhibitor therapy has shown limited efficacy and dose-limiting hematopoietic toxicities in clinical trials. In the present study, we describe the effects of vorinostat, a small-molecule inhibitor of histone deacetylase, against cells expressing JAK2V617F and in an animal model of polycythemia vera (PV). We found that vorinostat markedly inhibited proliferation and induced apoptosis IntroductionMyeloproliferative neoplasms (MPNs) are a group of clonal hematopoietic malignancies that include chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). 1,2 These diseases are characterized by excessive proliferation of myeloid/erythroid lineage cells. A somatic point mutation (V617F) in the JAK2 tyrosine kinase has been found in most patients with PV and in 50%-60% patients with ET and PMF. [3][4][5][6] JAK2V617F is a constitutively active tyrosine kinase that can transform factor-dependent hematopoietic cell lines into cytokine-independent cells. 3,4 Expression of the JAK2V617F mutant activates multiple downstream signaling pathways, such as Stat, Erk, and PI3K/Akt pathways. 3,7,8 Current therapies for MPNs include phlebotomy and myelosuppressive therapy (eg, hydroxyurea and anagrelide) for PV and ET and transfusions and supportive care for PMF. However, these empiric treatments are unlikely to cure or offer remission to patients with MPNs, so there is a clear need for new therapies for MPNs. The discovery of the JAK2V617F mutation in PV, ET, and PMF has led to the development of inhibitors of JAK2. Several JAK2 inhibitors are undergoing clinical trials. Although JAK2 inhibitors are effective in reducing splenomegaly and improving constitutional symptoms, significant hematopoietic toxicities, including anemia and thrombocytopenia, are observed in the majority of patients after this treatment, 9,10 which is consistent with the known function of JAK2 in normal hematopoiesis. 11,12 Ruxolitinib, a JAK1/JAK2 inhibitor, has been approved for the treatment of myelofibrosis. However, a recent report on long-term outcomes with Ruxolitinib treatment found improvement in constitutional symptoms, but no significant benefit in survival for myelofibrosis patients. 13 In addition, there is an increased rate of discontinuation of Ruxolitinib therapy because of severe hematopoietic toxicities or lack of response. 13 It is also possible that drug resistance may emerge in some patients treated with JAK2 inhibitors, similar to what is observed with the ABL inhibitor imatinib in CML patients. 14 Therefore, identifying additional new therapies targeting JAK2V617F or pathways downstream of JAK2V617F would be beneficial for the treatment of patients with MPNs.Acetylation is an important posttranslational modification that serves as a key modulator of chromatin structure and gene transcription, and provides a me...

Valproic acid has been previously associated with hematologic toxicity, including a reversible myelodysplasia-like syndrome without chromosomal abnormalities. We now report three cases of acute leukemia with features of secondary leukemia associated with valproic acid therapy: two cases of acute myelogenous leukemia with multilineage dysplasia, one with trisomy 8 and one with monosomy 7, and one case of secondary acute lymphoblastic leukemia with del (7) (q22q34), del (9) (q21.11q22), del (11) (q12q23). One patient had a previous myelodysplastic syndrome while on valproic acid. Valproic acid has been previously shown to be a histone deacetylase inhibitor. Inhibition of histone deacetylase causes a relaxation of chromatin structure and thus increases susceptibility to DNA damage and sensitizes cells to radiation. We propose that valproic acid therapy may lead to secondary leukemia by increasing DNA damage through chronic inhibition of histone deacetylase. Am.

Valproic acid (VA) is the most commonly used antiepileptic drug. It has a wide spectrum of hematologic toxicity including thrombocytopenia, anemia, leukopenia, macrocytosis and the presence of the Pelger-Huet abnormality. Serious but reversible hematologic toxicity resembling a myelodysplastic syndrome (MDS) has also been reported. In these cases, cytogenetics, when tested, was normal and progression to acute leukemia was not reported. We now report three cases of acute leukemia associated with VA therapy, accompanied by cytogenetic abnormalities and features suggestive of secondary leukemia. Three patients were recognized as having acute leukemia associated with VA at a single institution over an eight-year period. All three cases had characteristics of secondary acute leukemia. Two had Acute Myelogenous Leukemia (AML) with multilineage dysplasia (one patient with monosomy 7 and one with trisomy 8) and one had Acute Lymphocytic Leukemia (ALL)(partial deletions of chromosome 7, 9, and 11, with a breakpoint at 11q23). One patient with AML had a recognized preceeding myelodysplastic syndrome with normal cytogenetics that acquired trisomy 8 at progression. All three patients had no other risk factors for secondary leukemia. Two patients had been treated with VA for Lenox-Gaustat syndrome for > 20 years and 2 years, and one was treated for idiopathic epilepsy for > 15 years. The leukemogenic mechanism for secondary leukemia is thought to be DNA damage from radiation, chemotherapy drugs, organic solvents or inability to repair such damage from genetic defects in DNA repair enzymes. Chromatin is a dynamic structure and changes in it regulate multiple processes. It is modified through post-translational modifications of the DNA binding histone proteins, including acetylation of lysinses on the tails of histone proteins. Histone acetylation is associated with gene expression and deacetylation is associated with inhibition of gene expression. DNA in chromatin that has a relaxed conformation due to increased histone acetylation is more susceptible to DNA damage by radiation. VA was recently shown to be an inhibitor of histone deacetylase (HDAC). It causes hypereacetylation of histones in cultured cells and relieves HDAC induced transcriptional repression. These findings suggest a novel mechanism of leukemogenesis by VA. We hypothesize that chronic HDAC inhibition by VA results in hyperacetylation of chromatin and relaxation in chromatin structure. This, in turn, may cause increased sensitivity to double-stranded DNA breaks caused by low-level exposure to radiation, exogneous chemicals, or perhaps by cellular oxidant stress. Alternatively, chronic inhibition of HDAC may decrease the ability to repair DNA damage. The resultant DNA damage may subsequently lead to leukemogenesis through several genetic pathways. Interestingly, VA is being studied as a potential treatment for leukemia and MDS and it has been shown to induce expression of silenced tumor suppressor genes, and cause differentiation and/or apoptosis of leukemia cell lines in vitro. The fact that VA may have therapeutic effects in some cases of leukemia is, however, not incompatible with it also being potentially leukemogenic. Further study is needed to better define the incidence and mechanism of VA associated acute leukemia.