Romiplostim administration shows reduced megakaryocyte response-capacity and increased myelofibrosis in a mouse model of MYH9-RD

Abstract

Macrothrombocytopenia in MYH9-related disease (MYH9-RD) results from defects in nonmuscular myosin-IIA function. Thrombopoietin receptor agonists (eltrombopag; romiplostim) seem to improve hemostasis, but little is known about their biologic effects in MYH9-RD. We administered romiplostim to Myh9 ؊/؊ mice (100 g/kg, every 3 days, during 1 month). MKs increased to similar numbers in Myh9 ؊/؊ and wild-type (WT) mice (with an increase in immature MKs), but Myh9 ؊/؊ platelet count response was much less (2.5-fold vs 8-fold increase). A strong increase in MK nuclei emboli in the lung, in WT and Myh9 ؊/؊ mice, indicates increased transmigration of MKs from the BM. Prolonged (but not acute) treatment with romiplostim decreased expression of GPIb-IX-V complex and GPVI, but not of GPIIbIIIa, and bleeding time increased in WT mice. Microcirculation was not altered by the increased number of large platelets in any of the assessed organs, but in Myh9 ؊/؊ mice a much stronger increase in BM reticulin fibers was present after 4 weeks of romiplostim treatment vs WT mice. These data further encourage short-term use of thrombopoietic agents in patients with MYH9-RDs; however, myelofibrosis has to be considered as a potential severe adverse effect during longer treatment. Reduction of GPIbIX/GPVI expression by romiplostim requires further studies. IntroductionMYH9-related disorders (MYH9-RDs) are a group of rare diseases characterized by congenital macrothrombocytopenia that results from mutations in the MYH9 gene encoding the nonmuscle myosin IIA, the only isoform of myosin present in platelets. 1 Thrombocytopenia ranges from mild to severe and remains relatively stable in a person throughout life. Patients may experience easy bruising, epistaxis, and menorrhagia, in some rare occasions requiring blood transfusion. Other manifestations may occur later in life such as cataract, hearing loss, and nephropathy; the mechanism leading to these additional symptoms is presently unknown. 2,3 In these patients, thrombocytopenia results from defective platelet production, probably resulting from impairment in marrow MK maturation because of decreased or abnormal myosin IIA, leading to a strong decrease in their capacity to extend proplatelets. 4,5 Second-generation thrombopoietic agents, which stimulate megakaryocytopoiesis by binding to the thrombopoietin (TPO) receptor, 6 may be an option for increasing the platelet count in MYH9-RDs. Two of these TPO receptor agonists have completed phase 3 trials in primary immune thrombocytopenia (eltrombopag, a nonpeptide TPO receptor agonist, and romiplostim, a peptide TPO receptor agonist 7,8 ) and have been recently approved in several countries for treatment of certain patients with immune thrombocytopenia.Eltrombopag administration has recently been tested in patients with MYH9-RDs in a phase 2, multicenter trial, showing moderate increase in platelet counts and reduction in bleeding tendency, 9 suggesting that these new thrombopoietic agents could also represent an interesting therapeutic opti...

Immune thrombocytopenia: antiplatelet autoantibodies inhibit proplatelet formation by megakaryocytes and impair platelet production in vitro

“…Or, is the same number of MK producing more platelets? There is evidence of enhanced MK production after romiplostim treatment, 45 while other observations suggest that increases in platelet production occur without a change in MK mass. 46 Our results provide experimental evidence that TPO-R agonists may increase platelet production in ITP by boosting the number of proplatelet-bearing MK within an existing MK population.…”

Exome sequencing reveals a thrombopoietin ligand mutation in a Micronesian family with autosomal recessive aplastic anemia

“…Romiplostim and eltrombopag are FDA-approved THPO mimetics (MPL agonists) used in the treatment of patients with idiopathic thrombocytopenic purpura 48,49 and MYH9 related inherited thrombocytopenia. 50,51 More recently, eltrombopag was reported to improve hematopoiesis in patients with refractory aplastic anemia. 52 However, these patients with aplastic anemia were not genotyped for possible mutations in THPO or MPL.…”

MYH9-related inherited thrombocytopenia

“…Ромиплостим (Энплейт®, Aмджен, Нидерлан-ды), парентеральная форма миметиков эТПО, успеш-но применяется с целью повышения уровня тромбоци-тов перед плановым оперативным вмешательством у взрослых; элтромбопаг (Револейд®, Новартис, Швей-цария), пероральная форма, показал свою эффектив-ность при профилактическом его назначении перед оперативным лечением у взрослых и детей с MYH9-НТ [23][24][25].…”