Abstract 2613: Histone deacetylase inhibitor induced thrombocytopenia occurs due to inhibition platelet shedding by megakaryocytes, via increased phosporylation of myosin light chain
Authors: Bishton Mark J. , Prince H Miles , Harrison Simon J. , Martin Ben P. , Emma Josefsson C. , James Chloe , Henley Katya J. , Kile Ben T. , Johnstone Ricky W. 
Published: June 2012

Abstract
Introduction: Histone deacetylase inhibitors (HDACi) are a novel group of anti-cancer drugs with marked efficacy in haematological malignancy, for which thrombocytopenia (TCP) is the predominant dose limiting toxicity, currently limiting their use in combination treatment strategies. Using the HDAC 1/2 selective HDACi romidepsin and the pan-HDACi panobinostat, we have investigated the processes which underscore this significant clinical problem. Experimental procedures and data: We have demonstrated TCP is not due to myelosuppression, but decreased platelet release from megakaryocytes (MK) based on: 1) platelet half-life studies utilizing NHS-biotin to label circulating platelets, and reticulated platelet assays using thiazole orange staining, 2) studies in Bak-/- and Bak-/-Bax-/- bone marrow reconstituted mice which excluded HDACi-induced platelet apoptosis, and 3) bone marrow (BM) sections showing increased MK number in mice treated with HDACi compared to controls. Increases in thrombopoietin (TPO) were seen in thrombocytopenic mice, and using c-Mpl-/- mice, we demonstrated that TPO is required for the MK hyperplasia and rebound thrombocyosis seen on treatment cessation. To further elucidate the pathway causing reduced platelet production, we used the human MK cell line Meg-01 and primary MK derived from fetal liver cells stimulated with TPO. Proplatelet assays of primary MK showed reductions in proplatelet extensions following HDACi exposure and a concomitant dose dependant increase in the phosphorylation of myosin light chain (MLC). The phosphorylation status of the MLC (pMLC) is regulated by the Rho GTPase family, of which Rac1/CDC42, acting via PAK1 are postulated to have opposing actions to RhoA which is known to increase pMLC and reduce proplatelet formation. Western blots of lysates from Meg-01 and primary cells showed a reduction in protein levels of all three family members following HDACi, however qRT PCR did not demonstrate HDACi to cause transcriptional repression of these proteins We were able to recapitulate alteration in pMLC levels using both pharmacological inhibitors of PAK1 and Rac1 as well as genetically, using constitutively active and dominant negative Rac1 constructs. Furthermore, by administering a murine TPO-mimetic, we were able to treat murine HDACi-induced TCP in-vivo in both non-tumor bearing mice, resulting in platelet numbers increasing to levels similar to vehicle treated controls in naïve mice. In mice with established Eμ-Myc lymphoma, platelet numbers were increased above all control groups. Conclusions: HDACi induced TCP not due to myeloablation or effects on platelet half life, but is rather due to impaired proplatelet formation, most likely via inhibition of the Rho/Rac1/CDC42 pathways. HDACi induced TCP my be overcome in a variety of settings using TPO-mimetics.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2613. doi:10.1158/1538-7445.AM2011-2613