Pro-Platelet Formation Is Impaired in Megakaryocytes of MYH9 -Related Disease Patients

Introduction

The MYH9 gene encodes myosin-IIA, a nonmuscle myosin heavy chain that facilitates shape changes in diverse cell types. The mutation of MYH9 may lead to the MYH9 -related disease (MYH9 -RD), an autosomal dominant disorder with macrothrombocytopenia and leukocyte inclusion bodies at birth and the risk of developing nephropathy, deafness and cataracts during infancy or adult life. Currently, at least 44 different MYH9 mutations in all 40 exons have been reported. Mutated MYH9 protein expression is reduced in megakaryocytes and is clumped in a few aggregates together with the wild-type protein in neutrophils, where they form the inclusion bodies. In a mouse model, myosin-IIA protein inhibits pro-platelet formation and is regulated by Rho-associated kinase (ROCK). However, how the mutated myosin-IIA influences the pro-platelet formation (PPF) of megakaryocytes in patients has not been explored. The aim of this study is to determine the function and regulation of mutated myosin-IIA in MYH9 -RD patients' megakaryocytes.

Methods

One kindred with inherited macrothrombocytopenia was identified and 5 healthy donors were enrolled as controls. Peripheral blood smear review, slit lamp examination, pure-tone audiometry, renal ultrasonography and examination of blood (urea, creatinine, electrolytes) and urine were performed for each family member to diagnose and evaluate the disease. Bidirectional polymerase chain reaction (PCR) amplification and DNA sequence analysis were performed on all 40 MYH9 exons for each patient. Platelets from peripheral blood and megakaryocytes from bone marrow were fractionated for electron microscopy. Bone marrow tissues from patients were stained with fluorescein-labeled antibodies against CD41 and CD144 to observe the distribution of mature megakaryocytes in the vascular niche by confocal fluorescence microscopy. CD34+ cells were isolated from bone marrow samples by cell sorting for in vitro culture with TPO. On day 8, megakaryocytes were enriched using an albumin density gradient and seeded onto 96-well plates in serum-free medium with or without Y27632, the ROCK inhibitor. At the same time every day for 5 days, the percentage of proplatelet-displaying megakaryocytes was quantified by inverted microscopy.

Results

Five patients in the kindred were diagnosed with MYH9 -RD based upon peripheral blood smear review, one of whom had severe renal failure. None of the patients suffered from deafness or cataracts. A pathogenic heterozygous sequence variant in exon 39 (c.5521G>A, p.E1841K) was detected, which substitutes glutamate at codon 1841 to lysine. This mutation is one of the 44 reported pathogenic mutations. Electron microscopic results showed that platelets from patients had increased diameters and irregular shapes while megakaryocytes from patients had smaller diameters and a disorganized demarcation membrane system (DMS). Immunofluorescence results of bone marrow tissues showed that proplatelet-displaying mature megakaryocytes in MYH9 -RD patients located distally to the vascular niche, compared with the control group. CD34+ cells from bone marrow differentiated into megakaryocytes after in vitro culture with TPO. However, the PPF of megakaryocytes from MYH9 -RD patients appeared much earlier than that from controls. At the same time point, PPF megakaryocytes in MYH9 -RD patients were significantly increased compared to those of the control group. When cultured with Y27632, the PPF appearance time and proplatelet-displaying megakaryocyte percentage in the control group were advanced, while the MYH9 -RD group was not influenced.

Conclusion

The mutation in exon 39 of the MYH9 gene may lead to morphological abnormalities of megakaryocytes and platelets. The regulation of myosin-IIA function by ROCK in patients may be decreased when the MYH9 gene is mutated. As a result, the timing of PPF of megakaryocytes with mutated myosin-IIA in the bone marrow niche of MYH9 -RD patients may be advanced, and their mis-location away from the vascular niche may lead to reduced platelet production and increased platelet destruction.