Trisomy 21 enhances human fetal erythro-megakaryocytic development

Transient myeloproliferative disorder (TMD) occurs in 10% to 20% of newborns with Down syndrome (DS) and usually resolves after birth. However, approximately 30% of TMD patients develop acute megakaryoblastic leukemia (AMKL) within 4 years, suggesting that TMD is a premalignant disorder and that both diseases originate in the fetus.^1-4  DS TMD and AMKL blasts harbor somatic mutations of GATA1, which encodes an essential hematopoietic transcription factor.^5-7  These mutations occur in exon 2 and result in exclusive expression of an amino-truncated protein, termed GATA-1 short (GATA-1s).

Presumably, GATA-1s cooperates with trisomy 21 (T21) to induce a preleukemic state that progresses to frank malignancy through additional mutations. TMD and AMKL blasts exhibit erythro-megakaryocytic features^8-10  and GATA-1 regulates the maturation, survival, and proliferation of these lineages.¹¹  In contrast, little is known about how T21 itself alters hematopoiesis, particularly in the fetus, where TMD/AMKL-initiating events occur. We analyzed fetal liver hematopoiesis in T21 and control abortuses. In vitro and in vivo transplantation studies show that T21 is associated with enhanced erythroid and megakaryocytic precursor expansion, independent of GATA1 mutations. These findings provide insight into the hematopoietic abnormalities of DS and indicate how T21 might synergize with GATA-1s to promote TMD and AMKL.

Fetal livers were obtained from pathology specimens of week 13 to 23 abortuses. Institutional Review Board–approval was obtained from The Children's Hospital of Philadelphia and The University of Pennsylvania. Informed consent was obtained in accordance with the Declaration of Helsinki. T21 was confirmed by karyotype analysis of fetal tissue. DNA sequencing, hematopoietic assays, and gene expression analysis were performed using standard methods described in Document S1 (available on the Blood website; see the Supplemental Materials link at the top of the online article). Approval from the Institutional Animal Care and Use Committee at the University of Pennsylvania was obtained for mouse studies.

DS-associated TMD and AMKL initiate in utero, as evidenced clinically^1,3  and by GATA1 mutational analysis.^7,12,13  We analyzed fetal liver specimens from DS and control abortuses at 13 to 23 weeks' gestation. Histologically, T21 fetal livers were indistinguishable from controls (not shown). We isolated fetal liver hematopoietic mononuclear cells (MNCs), amplified GATA1 exon 2 by polymerase chain reaction (PCR), and subcloned the fragments. No mutations in 31 T21 or 10 control fetal liver MNC samples were detected by direct sequencing of the PCR product and 24 independent clones from each fetal liver, consistent with the relatively low incidence of GATA1 mutations (3.8%) identified by screening 585 DS newborns.¹³  The absence of GATA1 mutations in our specimens allowed us to study the hematopoietic effects of T21 in isolation.

We analyzed T21 and control fetal liver MNCs in methylcellulose progenitor assays and liquid megakaryocyte cultures. The absolute number of colonies generated per MNC varied between different fetal livers. This was independent of gestational age and presumably due to inherent interindividual differences or variability in specimen processing before MNC isolation (Table S2). However, T21 MNCs consistently generated increased proportions of erythroid (CFU-E and BFU-E) and megakaryocyte (CFU-Mk) colonies relative to granulocyte-macrophage (CFU-GM) colonies (Figure 1A; Table S2). Moreover, the majority of T21 erythroid and megakaryocytic colonies were approximately twice the size of control colonies (Figure 1B,C). T21 fetal liver cells also gave rise to an increased proportion of large CFU-Mks (> 50 GPIIb/IIIa + cells/colony, Figure 1C). T21 megakaryocyte progenitors proliferated more rapidly in liquid culture (Figure 1D), but were indistinguishable from controls with respect to morphology, lineage marker expression (CD41a and CD42), and visual inspection for proplatelet formation (Figure 1E and not shown), indicating that cellular maturation was not affected. We performed erythropoietin (EPO) and thrombopoietin (TPO) dose response studies and found no hypersensitivity to these cytokines (not shown) or cytokine-independent colony formation. In addition, we found no difference in Epo or Tpo mRNA expression in whole fetal liver from DS samples compared with controls (not shown).

To further examine the effects of T21 on hematopoiesis, we transplanted fetal liver MNCs into sublethally irradiated NOD/SCID/IL-2Rγc^null (NOG) mice. Five to 10 weeks after transplantation, we used human-specific antibodies to assess the progeny of a range of progenitors including B cell, macrophage, granulocyte, erythroid, and megakaryocyte (Figure S1). Trisomy 21 fetal liver cells exhibited enhanced erythroid differentiation in vivo, as evidenced by increased proportions of engrafted donor cells expressing glycophorin A (Figure 1F left panel). Transplanted T21 fetal liver cells also generated increased proportions of megakaryocytes (Figure 1F right panel), although human megakaryocytic development occurred at relatively low levels, as reported by others.¹⁴  These findings are consistent with our in vitro cultures linking T21 to enhanced differentiation and proliferation of erythro-megakaryocytic lineages. TMD typically resolves spontaneously after birth, suggesting that in DS, specific properties of fetal hematopoietic progenitors and/or the fetal liver microenvironment are necessary for expansion of TMD blasts. Our transplantation experiments demonstrate that T21 hematopoietic progenitors exhibit unique abnormalities in euploid adult mice. Hence, at least some of these properties are cell-intrinsic and independent of the trisomic fetal environment or artifacts related to in vitro culture.

To better localize the hematopoietic defects of DS, we analyzed multipotential progenitor compartments within the fetal liver using flow cytometry.¹⁵  The population of CD34⁺CD38⁺ progenitors was distinctly different in T21 fetal livers compared with controls (Figure 2A,B). Specifically, in T21 there was a marked increase in the proportion of megakaryocyte erythroid progenitors (MEPs, IL-3Rα⁻/CD45RA–). The proportion of common myeloid progenitors (CMPs, IL-3Rα^lo/CD45RA⁻) and granulocyte macrophage progenitors (GMPs, IL-3Rα^lo/CD45RA⁺) were commensurately reduced. Hematopoietic colony assays performed with sorted cell populations verified that these progenitors were enriched by our fractionation strategy (Figure S2). Of note, both T21 CMPs and MEPs gave rise to increased proportions of erythroid and megakaryocyte colonies relative to controls (Figure S2). Moreover, T21 erythroid colonies generated from purified fetal liver CMPs and MEPs were relatively enlarged, similar to what we observed in colony assays from unfractionated MNCs (not shown). As expected, mRNAs encoding the myeloid genes PU.1 and CEBPα were reduced in purified MEPs. Also, the erythro-megakaryocytic genes GATA1 and EpoR were barely expressed in GMPs (Figure 2C). These data further verify the enrichment for specific multilineage progenitor populations. Of note, there was a trend toward increased GATA1 and EpoR expression and decreased PU.1 and CEBPa expression within T21 CMPs, consistent with the enhanced erythro-megakaryocytic differentiation that we observed in colony assays.

Together, progenitor subfractionation indicates significant expansion of MEPs and increased tendency toward erythro-megakaryocytic development in the CMP and MEP compartments of T21 fetal liver. Trisomy 21 CMPs themselves may be inherently altered to enhance erythro-megakaryocytic production, possibly by giving rise to increased MEPs rather than GMPs. Alternatively, T21 MEPs may exhibit enhanced self-renewal. Either way, the data map the T21 hematopoietic phenotype to one or more population(s) of enriched multipotential progenitors. DS presumably alters blood development through complex mechanisms involving relatively small changes in the expression of multiple chromosome 21 genes. We examined several chromosome 21–encoded transcription factors known to influence hematopoietic fates, including ERG, RUNX1, ETS2, and SON in a hematopoietic stem cell–enriched population (CD34⁺CD38⁻) from fetal liver. While expression of these mRNAs was relatively consistent in control HSCs, there was considerable variation in T21 HSCs (Figure S3). Of note, none of these candidate chromosome 21 genes were significantly over-expressed in DS fetal liver HSCs or myeloid progenitor compartments (not shown), consistent with a model in which subtle alterations of multiple chromosome 21 genes contribute to the pathogenesis of DS phenotypes, including TMD/AMKL.

Malignant transformation is a multistep process caused by mutations that synergize to promote proliferation, survival and maturation arrest of immature progenitors. Down Syndrome–associated AMKL is an interesting example in which germline T21 cooperates with somatic GATA1 mutations to induce TMD, a preleukemic disorder characterized by clonal expansion of blasts with erythroid and megakaryocytic characteristics. Presumably additional, unknown mutations are required for progression to AMKL. Isolated mutations in GATA1 are known to block erythro-megakaryocytic maturation and promote the expansion of megakaryocytes and MEP-like precursors.^11,16-18  Here we provide the first evidence that erythroid and megakaryocytic precursors are also expanded in T21. These abnormalities occur in multilineage fetal progenitors and are at least partly cell-intrinsic. Accordingly, it is possible that T21 predisposes to AMKL by causing polyclonal expansion of a progenitor pool that is susceptible to additional transforming mutations involving GATA1 and other genes.

The authors thank the Department of Pathology at Pennsylvania Hospital for processing fetal liver specimens and are grateful to the Flow Cytometry Core at the University of Pennsylvania for advice on cell sorting and to the Stem Cell and Xenograft Core at the University of Pennsylvania for assistance with the in vivo experiments. They thank Paul Gadue for help with flow cytometry.

This work was supported by the Hope Street Kids Foundation (Alexandria, VA; S.T.C.), the American Society of Clinical Oncology (Alexandria, VA; S.T.C.), the National Institutes of Health (National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD; M.J.W.), and The Clinical & Translational Research Center (CTRC) at The Children's Hospital of Philadelphia (grant no. UL1-RR-024134 from the National Center for Research Resources, Bethesda, MD). M.J.W. is a Leukemia & Lymphoma Society (White Plains, NY) Scholar. S.T.C. is a Leukemia & Lymphoma Society Fellow.

National Institutes of Health

Contribution: S.T.C designed and conducted research, analyzed data, and wrote the paper; J.O., Y.Y., M.F, and A.K. designed and conducted research and analyzed data; J.S.B., J.K.C, A.M.G., G.D., and R.N. designed research and analyzed data; and M.J.W. designed research, analyzed data, and wrote the paper.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Mitchell J. Weiss, MD, PhD, Division of Hematology, Abramson Research Center, Room 316B, 3615 Civic Center Boulevard, Philadelphia, PA 19104; e-mail: weissmi@email.chop.edu.