The 40-fold increase in childhood megakaryocyte-erythroid and B-cell leukemia in Down syndrome implicates trisomy 21 (T21) in perturbing fetal hematopoiesis. Here, we show that compared with primary disomic controls, primary T21 fetal liver (FL) hematopoietic stem cells (HSC) and megakaryocyte-erythroid progenitors are markedly increased, whereas granulocyte-macrophage progenitors are reduced. Commensurately, HSC and megakaryocyte-erythroid progenitors show higher clonogenicity, with increased megakaryocyte, megakaryocyte-erythroid, and replatable blast colonies. Biased megakaryocyte-erythroid-primed gene expression was detected as early as the HSC compartment. In lymphopoiesis, T21 FL lymphoid-primed multipotential progenitors and early lymphoid progenitor numbers are maintained, but there was a 10-fold reduction in committed PreproB-lymphoid progenitors and the functional B-cell potential of HSC and early lymphoid progenitor is severely impaired, in tandem with reduced early lymphoid gene expression. The same pattern was seen in all T21 FL samples and no samples had GATA1 mutations. Therefore, T21 itself causes multiple distinct defects in FL myelo-and lymphopoiesis.transient myeloproliferative disorder | aneuploidy | human fetus C onstitutional trisomy 21 (T21) causes Down syndrome (DS), the most common syndrome-associated chromosomal anomaly in humans (1). As well as with neurodevelopmental, cardiac, and gut anomalies (2), there is a striking increase in childhood acute leukemia in DS, even though the risk of solid tumors is much lower than with the general population (3). Intriguingly, this susceptibility to hematopoietic tumors manifests as an increased risk both of acute megakaryocyte (MK)-erythroid leukemia (known as ML-DS) by 150-fold and of acute B-lymphoblastic leukemia (B-ALL) by 33-fold (3, 4).DS leukemias display distinct characteristics that support a crucial role for T21 in their pathogenesis. Hallmarks of ML-DS are the megakaryoblastic phenotype, clinical presentation confined to the first 5 y of childhood (5, 6), an antecedent clonally linked preleukemic condition (termed transient myeloproliferative disorder, TMD) in most cases, and acquired N-terminal truncating mutations in the erythroid-MK transcription factor GATA1 (7-9). Such mutations in GATA1 are present in both ML-DS and TMD (9) but are not found in patients without DS who develop megakaryoblastic leukemia (7) and are not leukemogenic in the absence of T21 (10).Molecular, biologic, and clinical data indicate that TMD is initiated before birth (9,(11)(12)(13)(14). We previously reported that by the second trimester, the T21 fetal liver (FL) myeloid progenitor compartment is abnormal and that this occurs in the absence of GATA1 mutation (11, 12). Specifically, the MK-erythroid progenitor (MEP) population is expanded with increased cell-intrinsic MK and erythroid lineage proliferation from CD34 + cells. These data suggest that T21-mediated developmental alterations to FL myeloid progenitor development provide a cell-specific substrate for...
