Megakaryocyte ontogeny: Clinical and molecular significance
2018; Elsevier BV; Volume: 61; Linguagem: Inglês
10.1016/j.exphem.2018.02.003
ISSN1873-2399
AutoresKamaleldin E. Elagib, Ashton Brock, Adam N. Goldfarb,
Tópico(s)Parvovirus B19 Infection Studies
Resumo•Fetal megakaryocytes (Mks) are phenotypically different from adult Mks.•Fetal Mks are matured as adult Mks.•Fetal Mk phenotypic features predispose to certain Mk-related diseases.•Cell-intrinsic and -extrinsic factors contribute to Mk ontogenic differences.•Fetal Mks can be established in adult mode to enhance ex vivo platelet production. Fetal megakaryocytes (Mks) differ from adult Mks in key parameters that affect their capacity for platelet production. However, despite being smaller, more proliferative, and less polyploid, fetal Mks generally mature in the same manner as adult Mks. The phenotypic features unique to fetal Mks predispose patients to several disease conditions, including infantile thrombocytopenia, infantile megakaryoblastic leukemias, and poor platelet recovery after umbilical cord blood stem cell transplantations. Ontogenic Mk differences also affect new strategies being developed to address global shortages of platelet transfusion units. These donor-independent, ex vivo production platforms are hampered by the limited proliferative capacity of adult-type Mks and the inferior platelet production by fetal-type Mks. Understanding the molecular programs that distinguish fetal versus adult megakaryopoiesis will help in improving approaches to these clinical problems. This review summarizes the phenotypic differences between fetal and adult Mks, the disease states associated with fetal megakaryopoiesis, and recent advances in the understanding of mechanisms that determine ontogenic Mk transitions. Fetal megakaryocytes (Mks) differ from adult Mks in key parameters that affect their capacity for platelet production. However, despite being smaller, more proliferative, and less polyploid, fetal Mks generally mature in the same manner as adult Mks. The phenotypic features unique to fetal Mks predispose patients to several disease conditions, including infantile thrombocytopenia, infantile megakaryoblastic leukemias, and poor platelet recovery after umbilical cord blood stem cell transplantations. Ontogenic Mk differences also affect new strategies being developed to address global shortages of platelet transfusion units. These donor-independent, ex vivo production platforms are hampered by the limited proliferative capacity of adult-type Mks and the inferior platelet production by fetal-type Mks. Understanding the molecular programs that distinguish fetal versus adult megakaryopoiesis will help in improving approaches to these clinical problems. This review summarizes the phenotypic differences between fetal and adult Mks, the disease states associated with fetal megakaryopoiesis, and recent advances in the understanding of mechanisms that determine ontogenic Mk transitions. Megakaryocytes (Mks) are specialized mammalian marrow cells responsible for platelet production. They arise from bipotent megakaryocyte-erythroid progenitors [1Weissman I.L. Anderson D.J. Gage F. Stem and progenitor cells: origins, phenotypes, lineage commitments, and transdifferentiations.Annu Rev Cell Dev Biol. 2001; 17: 387-403Crossref PubMed Scopus (750) Google Scholar]. Their differentiation includes a unique program of endomitosis that drives nuclear polyploidization and cellular enlargement. This process is accompanied by lineage consolidation involving downregulation of erythroid genes and upregulation of Mk surface markers, as well as development of cytoplasmic granules, multivesicular bodies, and demarcation membranes. Upon achieving polyploidization and enlargement, Mks undergo cytoskeletal remodeling to induce the formation of proplatelets, preplatelets, and, ultimately, platelets [2Machlus K.R. Italiano Jr, J.E. The incredible journey: from megakaryocyte development to platelet formation.J Cell Biol. 2013; 201: 785-796Crossref PubMed Scopus (398) Google Scholar]. During mammalian embryogenesis, primitive Mks with limited polyploidization capacity appear early in the yolk sac. The first definitive Mks arise from hematopoietic progenitors in the fetal liver (FL) and are then produced in the bone marrow (BM). In humans, there are clear phenotypic differences between fetal/neonatal (collectively referred to as fetal) and adult Mks and these differences have major health implications. The smaller size of fetal Mks has been well established for several decades (Table 1). Using immunohistochemical staining and light microscope morphometry on healthy human tissues, Allen Graeve and de Alarcon estimated diameters of 14.0–15.2 µm for fetal Mks at 12–21 weeks gestation compared with 18.4–20.6 µm for adult Mks [3Allen Graeve J.L. de Alarcon P.A. Megakaryocytopoiesis in the human fetus.Arch Dis Child. 1989; 64: 481-484Crossref PubMed Scopus (43) Google Scholar]. Subsequent studies confirmed the diameters of human fetal Mks at 3 months gestation to range from 12.4–14.8 µm depending on whether marrow or liver was analyzed. Although diameters reached 16.1 µm by 7 months gestation, fetal Mks remained consistently smaller than those produced in human adult BM (21.9 µm in diameter) [4Ma D.C. Sun Y.H. Chang K.Z. Zuo W. Developmental change of megakaryocyte maturation and DNA ploidy in human fetus.Eur J Haematol. 1996; 57: 121-127Crossref PubMed Scopus (37) Google Scholar]. A unimodal Gaussian distribution of Mk size persists in neonates and infants until approximately 24 months. At this age, the size distribution becomes bimodal, with subpopulations of smaller and larger Mks, and by age 4, most Mks have transitioned into a larger size range characteristic of adulthood [5Fuchs D.A. McGinn S.G. Cantu C.L. Klein R.R. Sola-Visner M.C. Rimsza L.M. Developmental differences in megakaryocyte size in infants and children.Am J Clin Pathol. 2012; 138: 140-145Crossref PubMed Scopus (21) Google Scholar]. Although most studies have focused on healthy individuals, the size difference between fetal and adult Mks has also been observed in thrombocytopenic subjects [6Sola-Visner M.C. Christensen R.D. Hutson A.D. Rimsza L.M. Megakaryocyte size and concentration in the bone marrow of thrombocytopenic and nonthrombocytopenic neonates.Pediatr Res. 2007; 61: 479-484Crossref PubMed Scopus (57) Google Scholar].Table 1Phenotypic characteristics of fetal and adult MksParameterFetal/Neonatal MksAdult MksReferencesSizeSmallerLarger3Allen Graeve J.L. de Alarcon P.A. Megakaryocytopoiesis in the human fetus.Arch Dis Child. 1989; 64: 481-484Crossref PubMed Scopus (43) Google Scholar, 4Ma D.C. Sun Y.H. Chang K.Z. Zuo W. Developmental change of megakaryocyte maturation and DNA ploidy in human fetus.Eur J Haematol. 1996; 57: 121-127Crossref PubMed Scopus (37) Google Scholar, 5Fuchs D.A. McGinn S.G. Cantu C.L. Klein R.R. Sola-Visner M.C. Rimsza L.M. Developmental differences in megakaryocyte size in infants and children.Am J Clin Pathol. 2012; 138: 140-145Crossref PubMed Scopus (21) Google Scholar, 6Sola-Visner M.C. Christensen R.D. Hutson A.D. Rimsza L.M. Megakaryocyte size and concentration in the bone marrow of thrombocytopenic and nonthrombocytopenic neonates.Pediatr Res. 2007; 61: 479-484Crossref PubMed Scopus (57) Google ScholarPolyploidizationLess polyploidMore polyploid4Ma D.C. Sun Y.H. Chang K.Z. Zuo W. Developmental change of megakaryocyte maturation and DNA ploidy in human fetus.Eur J Haematol. 1996; 57: 121-127Crossref PubMed Scopus (37) Google Scholar, 7de Alarcon P.A. Graeve J.L. Analysis of megakaryocyte ploidy in fetal bone marrow biopsies using a new adaptation of the feulgen technique to measure DNA content and estimate megakaryocyte ploidy from biopsy specimens.Pediatr Res. 1996; 39: 166-170Crossref PubMed Scopus (44) Google Scholar, 8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google ScholarProliferationHyperproliferative in ex vivo cultureLess proliferative in ex vivo culture8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google Scholar, 9Liu Z.J. Italiano Jr, J. Ferrer-Marin F. et al.Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes.Blood. 2011; 117: 4106-4117Crossref PubMed Scopus (84) Google ScholarMaturationExpress Mk maturation markersExpress Mk maturation markers8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google Scholar, 9Liu Z.J. Italiano Jr, J. Ferrer-Marin F. et al.Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes.Blood. 2011; 117: 4106-4117Crossref PubMed Scopus (84) Google ScholarProplatelet FormationForm fewer proplateletsForm more proplatelets8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google Scholar, 10Ignatz M. Sola-Visner M. Rimsza L.M. et al.Umbilical cord blood produces small megakaryocytes after transplantation.Biol Blood Marrow Transplant. 2007; 13: 145-150Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar Open table in a new tab Concurrent with smaller size, fetal Mks also have a lower ploidy that increases with ontogenic stage [7de Alarcon P.A. Graeve J.L. Analysis of megakaryocyte ploidy in fetal bone marrow biopsies using a new adaptation of the feulgen technique to measure DNA content and estimate megakaryocyte ploidy from biopsy specimens.Pediatr Res. 1996; 39: 166-170Crossref PubMed Scopus (44) Google Scholar]. Using in situ DNA staining of human FL tissue sections, investigators found the percentage of Mks with 8N ploidy to increase from 16% at 3 months gestation to 33% at 6 months. Mks ≥ 16N were seen only after 7–8 months gestation. In BM, only 24% of late fetal Mks had ≥ 64N ploidy compared with 68% of adult BM Mks [4Ma D.C. Sun Y.H. Chang K.Z. Zuo W. Developmental change of megakaryocyte maturation and DNA ploidy in human fetus.Eur J Haematol. 1996; 57: 121-127Crossref PubMed Scopus (37) Google Scholar]. Similar findings have been reported with ex vivo culture-derived Mks from cord blood (CB) versus adult peripheral blood (PB) progenitors [8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google Scholar]. Specifically, ~ 80% of CB-derived Mks had 2N ploidy and 2.6% were 8N, whereas 40% of the PB-derived Mks had ≥ 8N ploidy [8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google Scholar]. Fetal and adult Mks also differ in mitotic rates, with multiple experiments showing increased proliferation of fetal Mks. In Mk cultures conducted under standardized conditions, Liu et al. described a 70-fold expansion of CB CD34+ cells compared with 5-fold in PB CD34+ cells [9Liu Z.J. Italiano Jr, J. Ferrer-Marin F. et al.Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes.Blood. 2011; 117: 4106-4117Crossref PubMed Scopus (84) Google Scholar]. In another similarly performed study, CB progenitors expanded 60-fold and PB cells underwent only a 10-fold amplification [8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google Scholar]. The ontogenic differences in progenitor expandability inversely reflect their capacity for polyploidization, suggesting that the diminished proliferation of adult progenitors may result from enhanced transition to endomitosis [9Liu Z.J. Italiano Jr, J. Ferrer-Marin F. et al.Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes.Blood. 2011; 117: 4106-4117Crossref PubMed Scopus (84) Google Scholar]. Although fetal Mks undergo incomplete enlargement and polyploidization, they fully upregulate most lineage-specific factors, including the membrane receptors and granule components necessary for platelet formation and function. Therefore, fetal and adult Mks express similar levels of membrane proteins integrin alpha-IIb (CD41), integrin beta-3 (CD61), and glycoprotein Ibα (CD42b) [11Kato A. Kawamata N. Tamayose K. et al.Ancient ubiquitous protein 1 binds to the conserved membrane-proximal sequence of the cytoplasmic tail of the integrin alpha subunits that plays a crucial role in the inside-out signaling of alpha IIbbeta 3.J Biol Chem. 2002; 277: 28934-28941Crossref PubMed Scopus (36) Google Scholar]. Importantly, the CD42 complex represents a marker of late-stage Mk differentiation [12Ruggeri Z.M. De Marco L. Gatti L. Bader R. Montgomery R.R. Platelets have more than one binding site for von Willebrand factor.J Clin Invest. 1983; 72: 1-12Crossref PubMed Scopus (286) Google Scholar]. CB progenitor-derived Mks also demonstrate abundant expression of the platelet proteins von Willebrand factor and P-selectin [9Liu Z.J. Italiano Jr, J. Ferrer-Marin F. et al.Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes.Blood. 2011; 117: 4106-4117Crossref PubMed Scopus (84) Google Scholar]. CB Mks were also observed to have mature ultrastructural characteristics such as an enlarged cytoplasm, abundant granules, and a well-developed demarcation membrane system. Similar observations have been made with murine neonatal Mks [9Liu Z.J. Italiano Jr, J. Ferrer-Marin F. et al.Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes.Blood. 2011; 117: 4106-4117Crossref PubMed Scopus (84) Google Scholar]. The most important phenotypic difference between fetal and adult Mks concerns their platelet-producing efficiency. Despite their mature marker expression, CB-derived Mks produce 3-fold fewer proplatelets and platelets on a per-cell basis compared with PB-derived Mks [8Mattia G. Vulcano F. Milazzo L. et al.Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release.Blood. 2002; 99: 888-897Crossref PubMed Scopus (195) Google Scholar]. This difference likely results from the fetal polyploidization deficit, which limits enlargement and ultimately restricts the allocation of cell mass toward platelet formation. Whether fetal Mks have additional proplatelet formation defects independent of their size remains to be determined. The distinct phenotypic features of fetal Mks may have a variety of clinical consequences (Table 2). Considered below are four problems in which this phenotype has been implicated as a major contributing factor. The first of these problems, neonatal thrombocytopenia, occurs in ~5% of all neonates, 22–35% of neonatal intensive care unit admissions, and ~73% of low-birth-weight infants < 1,000 g [13Ferrer-Marin F. Stanworth S. Josephson C. Sola-Visner M. Distinct differences in platelet production and function between neonates and adults: implications for platelet transfusion practice.Transfusion. 2013; 53 (quiz 2813): 2814-2821Crossref PubMed Scopus (52) Google Scholar]. The propensity for thrombocytopenia is inversely proportional to gestational age and results from cell-intrinsic defects in Mk morphogenesis (i.e., enlargement and polyploidization) [13Ferrer-Marin F. Stanworth S. Josephson C. Sola-Visner M. Distinct differences in platelet production and function between neonates and adults: implications for platelet transfusion practice.Transfusion. 2013; 53 (quiz 2813): 2814-2821Crossref PubMed Scopus (52) Google Scholar]. One of the most common inciting features consists of sepsis, most likely due to an increased demand placed on platelet production. In a recent study in The Netherlands, sepsis was identified in 7% of all hospitalized neonates and severe thrombocytopenia (< 50,000 platelets/µL) occurred in 20% of septic patients [14Ree I.M.C. Fustolo-Gunnink S.F. Bekker V. Fijnvandraat K.J. Steggerda S.J. Lopriore E. Thrombocytopenia in neonatal sepsis: Incidence, severity and risk factors.PLoS ONE. 2017; 12 (e0185581)Crossref PubMed Scopus (34) Google Scholar]. The presence of thrombocytopenia in this cohort increased risk of mortality almost 4-fold. Management of neonatal thrombocytopenia remains controversial, with platelet transfusions frequently provided to prevent intraventricular hemorrhage (IVH). A recent clinical trial confirmed that neonatal thrombocytopenia predisposes to IVH but found no correlation between the degree of risk and the degree of thrombocytopenia and could identify no benefit associated with platelet transfusion [21Sparger K.A. Assmann S.F. Granger S. et al.Platelet transfusion practices among very-low-birth-weight infants.JAMA Pediatr. 2016; 170: 687-694Crossref PubMed Scopus (36) Google Scholar]. Against such a tenuous benefit must be weighed the risks of platelet transfusion, which include bacterial infection, transfusion-mediated lung injury, alloimmunization, and financial cost. Thrombopoietin (Tpo) receptor agonists have gained widespread clinical use in enhancing platelet production in adults. However, compelling in vitro data predict that these agents will lack efficacy in neonates because Tpo stimulation paradoxically exacerbates defects in infantile Mk morphogenesis [22Pastos K.M. Slayton W.B. Rimsza L.M. Young L. Sola-Visner M.C. Differential effects of recombinant thrombopoietin and bone marrow stromal-conditioned media on neonatal versus adult megakaryocytes.Blood. 2006; 108: 3360-3362Crossref PubMed Scopus (46) Google Scholar].Table 2Clinical significance of fetal Mks and challenges of ex vivo platelet formationClinical and Therapeutic Impact of Mk OntogenyReferencesNeonatal thrombocytopenia13Ferrer-Marin F. Stanworth S. Josephson C. Sola-Visner M. Distinct differences in platelet production and function between neonates and adults: implications for platelet transfusion practice.Transfusion. 2013; 53 (quiz 2813): 2814-2821Crossref PubMed Scopus (52) Google Scholar, 14Ree I.M.C. Fustolo-Gunnink S.F. Bekker V. Fijnvandraat K.J. Steggerda S.J. Lopriore E. Thrombocytopenia in neonatal sepsis: Incidence, severity and risk factors.PLoS ONE. 2017; 12 (e0185581)Crossref PubMed Scopus (34) Google Scholar Premature neonates Neonates with sepsisDelayed platelet recovery after CB-HSC transplantation10Ignatz M. Sola-Visner M. Rimsza L.M. et al.Umbilical cord blood produces small megakaryocytes after transplantation.Biol Blood Marrow Transplant. 2007; 13: 145-150Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 15Solh M. Brunstein C. Morgan S. Weisdorf D. Platelet and red blood cell utilization and transfusion independence in umbilical cord blood and allogeneic peripheral blood hematopoietic cell transplants.Biol Blood Marrow Transplant. 2011; 17: 710-716Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar Need for more frequent platelet transfusions after CB-SC transplantationsMegakaryoblastic neoplasia16Arber D.A. Brunning R.D. Orazi A. Porwit A. Peterson L. Thiele J. Acute myeloid leukaemia with recurrent genetic abnormalities.in: Swerdlow S.H. Camp E. Harris N.L. World Health Organization classification of tumours of haematopoietic and lymphoid tissues. IARC Press, Lyon, France2017: 130-171Google Scholar, 17Nikolaev S.I. Santoni F. Vannier A. et al.Exome sequencing identifies putative drivers of progression of transient myeloproliferative disorder to AMKL in infants with Down syndrome.Blood. 2013; 122: 554-561Crossref PubMed Scopus (58) Google Scholar, 18de Rooij J.D. Branstetter C. Ma J. et al.Pediatric non-Down syndrome acute megakaryoblastic leukemia is characterized by distinct genomic subsets with varying outcomes.Nat Genet. 2017; 49: 451-456Crossref PubMed Scopus (83) Google Scholar Down syndrome transient myeloproliferative disorder Acute megakaryoblastic leukemia with the RBM15-MKL1 fusionChallenges of ex vivo platelet production as a source for platelets19Gollomp K. Lambert M.P. Poncz M. Current status of blood "pharming": megakaryoctye transfusions as a source of platelets.Curr Opin Hematol. 2017; 24: 565-571Crossref PubMed Scopus (9) Google Scholar, 20Thon J.N. Mazutis L. Wu S. et al.Platelet bioreactor-on-a-chip.Blood. 2014; 124: 1857-1867Crossref PubMed Scopus (152) Google Scholar Poor proliferative capacity of adult Mks Infantile nature of CB-derived and iPSC-derived Mks Inefficient platelet formation ex vivo Open table in a new tab The second problem consists of delayed platelet recovery in umbilical CB hematopoietic stem cell (CB-HSC) transplantation recipients [10Ignatz M. Sola-Visner M. Rimsza L.M. et al.Umbilical cord blood produces small megakaryocytes after transplantation.Biol Blood Marrow Transplant. 2007; 13: 145-150Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar]. For transplantation, CB-HSCs offer several advantages over adult HSCs and may represent the only curative option for hard-to-match patients with lethal diseases [23Gluckman E. History of cord blood transplantation.Bone Marrow Transplant. 2009; 44: 621-626Crossref PubMed Scopus (118) Google Scholar]. A major drawback of CB-HSC transplantation has been inferior platelet recovery. In a study of adult leukemia/lymphoma patients, CB-HSC recipients experienced a 3-fold delay in the time to platelet independence compared with recipients of adult PB-HSCs [15Solh M. Brunstein C. Morgan S. Weisdorf D. Platelet and red blood cell utilization and transfusion independence in umbilical cord blood and allogeneic peripheral blood hematopoietic cell transplants.Biol Blood Marrow Transplant. 2011; 17: 710-716Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar]. This delay translated into a 2-fold increase in the number of platelet transfusions required. In pediatric transplantation recipients, CB-HSCs were associated with a 2.3-fold delay in platelet recovery and marrow morphometry documented equivalent Mk numbers but decreased Mk size in CB-HSC recipients compared with adult HSC recipients [10Ignatz M. Sola-Visner M. Rimsza L.M. et al.Umbilical cord blood produces small megakaryocytes after transplantation.Biol Blood Marrow Transplant. 2007; 13: 145-150Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar]. In fact, the differences in Mk size correlated directly with the differences in platelet recovery. The third clinical problem concerns the leukemic propensity of fetal Mk progenitors. Two distinct Mk neoplasms occur almost exclusively in neonates: (1) Down syndrome-associated transient myeloproliferative disorder (DS-TMD) and (2) acute megakaryoblastic leukemia with the RBM15-MKL1 gene fusion (AMKL R-M) [16Arber D.A. Brunning R.D. Orazi A. Porwit A. Peterson L. Thiele J. Acute myeloid leukaemia with recurrent genetic abnormalities.in: Swerdlow S.H. Camp E. Harris N.L. World Health Organization classification of tumours of haematopoietic and lymphoid tissues. IARC Press, Lyon, France2017: 130-171Google Scholar]. Epidemiologic profiles suggest that fetal status may constitute an oncogenic "hit" for these entities. Such a concept is supported in DS-TMD by the spontaneous disease regression as neonates age over several months [16Arber D.A. Brunning R.D. Orazi A. Porwit A. Peterson L. Thiele J. Acute myeloid leukaemia with recurrent genetic abnormalities.in: Swerdlow S.H. Camp E. Harris N.L. World Health Organization classification of tumours of haematopoietic and lymphoid tissues. IARC Press, Lyon, France2017: 130-171Google Scholar]. Recent whole-exome sequencing studies provide further support. One such study has shown that the majority of DS-TMD cases carry no secondary genetic abnormalities beyond the hallmark GATA1s mutations coupled with trisomy 21 [17Nikolaev S.I. Santoni F. Vannier A. et al.Exome sequencing identifies putative drivers of progression of transient myeloproliferative disorder to AMKL in infants with Down syndrome.Blood. 2013; 122: 554-561Crossref PubMed Scopus (58) Google Scholar]. A subsequent study demonstrated that DS-AMKL arises from a TMD clone that acquires additional mutations in multiple genes, including cohesin components, CTCF, epigenetic regulators such as EZH2 and KANSL1, and members of signaling pathways such as the JAK family and RAS pathways [24Yoshida K. Toki T. Okuno Y. et al.The landscape of somatic mutations in Down syndrome-related myeloid disorders.Nat Genet. 2013; 45: 1293-1299Crossref PubMed Scopus (233) Google Scholar]. Similarly, AMKL R-M displays a strikingly sparse mutational landscape compared with other classes of non-Down syndrome AMKL [18de Rooij J.D. Branstetter C. Ma J. et al.Pediatric non-Down syndrome acute megakaryoblastic leukemia is characterized by distinct genomic subsets with varying outcomes.Nat Genet. 2017; 49: 451-456Crossref PubMed Scopus (83) Google Scholar]. Murine models also highlight the importance of ontogenic stage, with knockins for both GATA1s and RBM15-MKL1 displaying Mk abnormalities that are largely restricted to the fetal liver period [25Li Z. Godinho F.J. Klusmann J.H. Garriga-Canut M. Yu C. Orkin S.H. Developmental stage-selective effect of somatically mutated leukemogenic transcription factor GATA1.Nat Genet. 2005; 37: 613-619Crossref PubMed Scopus (230) Google Scholar, 26Mercher T. Raffel G.D. Moore S.A. et al.The OTT-MAL fusion oncogene activates RBPJ-mediated transcription and induces acute megakaryoblastic leukemia in a knockin mouse model.J Clin Invest. 2009; 119: 852-864PubMed Google Scholar]. The fourth problem relates to recent initiatives to develop donor-independent sources of platelets to treat patients with thrombocytopenia. The need for such sources is emerging in developed countries due to steadily rising platelet demands coupled with restricted donor supplies [19Gollomp K. Lambert M.P. Poncz M. Current status of blood "pharming": megakaryoctye transfusions as a source of platelets.Curr Opin Hematol. 2017; 24: 565-571Crossref PubMed Scopus (9) Google Scholar]. Recent refinements in ex vivo culture of Mks derived from a variety of sources have enhanced feasibility of producing bioactive platelets or Mks for transfusion [19Gollomp K. Lambert M.P. Poncz M. Current status of blood "pharming": megakaryoctye transfusions as a source of platelets.Curr Opin Hematol. 2017; 24: 565-571Crossref PubMed Scopus (9) Google Scholar, 20Thon J.N. Mazutis L. Wu S. et al.Platelet bioreactor-on-a-chip.Blood. 2014; 124: 1857-1867Crossref PubMed Scopus (152) Google Scholar]. A major limiting factor in this process is the poor proliferative capacity of adult type Mks in culture despite efficient platelet biogenesis. In contrast, CB Mks proliferate extensively in culture but show limited platelet production. New technology has permitted Mk generation from induced pluripotent stem cells (iPSCs), raising the possibility of personalized platelet cultivation [27Takayama N. Nishimura S. Nakamura S. et al.Transient activation of c-MYC expression is critical for efficient platelet generation from human induced pluripotent stem cells.J Exp Med. 2010; 207: 2817-2830Crossref PubMed Scopus (240) Google Scholar]. However, the infantile nature of iPSC-derived Mks greatly restricts the efficiency of platelet production in this system [28Bluteau O. Langlois T. Rivera-Munoz P. et al.Developmental changes in human megakaryopoiesis.J Thromb Haemost. 2013; 11: 1730-1741Crossref PubMed Scopus (51) Google Scholar]. Therefore, efficient scale-up will require a biphasic system in which Mk expansion is accomplished in fetal mode, followed by induction of an adult program to maximize platelet production (Fig. 1). Multiple signaling and transcriptional programs control megakaryopoiesis. The phenotypic differences between fetal and adult Mks likely arise from ontogenic differences in these programs. Studies from the past three decades have revealed several molecular differences between fetal and adult stage megakaryopoiesis (Table 3). Most of these differences are cell intrinsic, but the microenvironment may also contribute to Mk ontogenic transitions. In addition, recent work suggests that developmental origin may also distinguish fetal from adult Mk progenitors [29Notta F. Zandi S. Takayama N. et al.Distinct routes of lineage development reshape the human blood hierarchy across ontogeny.Science. 2016; 351: aab2116Crossref PubMed Scopus (428) Google Scholar]. In particular, adult Mk progenitors appear to originate from the HSC compartment, whereas fetal Mk progenitors also arise from committed progenitors downstream of HSCs. These differences in cell of origin could also contribute to the distinct phenotypic features of fetal and adult Mks.Table 3Molecular and signaling diffe
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