The Erg-onomics of myeloproliferation in Down syndrome

In this issue of Blood, Ng and colleagues engineer a series of unique mice that provide genetic evidence for increased gene dosage of the ETS transcription factor, Erg, as a required determinant of the myeloproliferative features observed in the most studied mouse model of DS.

Newborns and very young children with Down syndrome (DS) exhibit a spectrum of uncommon myeloid abnormalities including an unusual transient myeloproliferative disorder (TMD) that is observed in less than 10% of DS infants at birth. TMD is characterized by circulating myeloblasts in the peripheral blood and immature megakaryoblasts infiltrating the fetal liver, bone marrow, and sometimes other tissues. Remarkably, although clonal, in most infants it spontaneously disappears. This condition is preleukemic for the 20% to 30% of cases that go on within 1 to 4 years to develop a Down syndrome–associated acute megakaryocytic leukemia (DS-AMKL). Recent observations describe acquired exon 2 mutations in the X-linked GATA1 gene in virtually all DS-associated TMD and AMKL samples analyzed.¹  Identification of the same GATA1 mutations in the cases where DS-AMKL follows TMD in the same patient, but the lack of AMKL progression in most TMD persons, along with important recent observations from a number of laboratories, indicates that the development of DS-AMKL is a multistep and multifactorial process. Current models suggest that first, a fetal liver hematopoietic cell trisomic for chromosome 21 may exhibit aberrant hematopoietic progenitor functions. Second, a specific exon 2 GATA1 mutation is acquired, with resultant interaction between chromosome 21 gene product(s) and mutated GATA-1. This provides a selective advantage for these cells, allowing expansion and other behaviors. Finally, acquisition of additional genetic and/or epigenetic events are required for full progression to AMKL. As such, the DS-associated myeloid (and lymphoid) disorders serve as fascinating model(s) of leukemic progression.^2,3 

Several mouse models are available to study aspects of DS pathogenesis. Most studied is the Ts65Dn mouse, partially trisomic for a 15.6-Mb region that contains 104 genes conserved on the human chromosome 21 (Hsa21).²  Careful analysis of blood development in these mice identified persistent macrocytosis similar to that seen in some DS persons, and in “older” mice the development of a myeloproliferative disease (MPD) characterized by a significant thrombocytosis, megakaryocytic hyperplasia, dysplastic megakaryocytes, and myelofibrosis.⁴  In a genetic cross with a mouse strain mutant in one of leading chromosome 21 candidate megakaryocytic disease genes, trisomy of Aml1/Runx1, was ruled out as required for the megakaryocytic hyperplasia and myelofibrosis in this model.⁴  Using a similar approach, now Ng et al have identified Erg as one of the key trisomy genes that does contribute to the myeloproliferative phenotype associated with this DS mouse model.⁵ 

ERG is contained in the Down syndrome critical region (DSCR) and is expressed in human acute megakaryoblastic leukemia cell lines and primary patient samples of TMD and DS-AMKL.⁶  ERG is a member of the ETS family of transcription factors, and is ecessary for hematopoietic stem cell function and normal platelet development.⁷  Although the ERG locus is involved in rare chromosomal translocations in acute myelogenous leukemia (AML), ERG has been found to be overexpressed in normal karyotype AML cases and in cases with complex cytogenetics including those containing acquired trisomy 21. To determine whether trisomy of functional Erg drives the development of the myeloproliferative phenotype observed in Ts65Dn mice, Ng et al⁵  crossed trisomic Ts65Dn mice to mice carrying the Erg loss-of-function Erg^mld2 mutation,⁷  generating mice disomic for functional Erg, but trisomic for all the other genes within the DSCR of Ts65Dn. The Ts65Dn trisomic mice developed progressive thrombocytosis, megakaryocytosis, megakaryocytic dysplasia, and extramedullary hematopoiesis. Reticulin fibrosis was identified in 3 of 9 bone marrows, but not in wild-type mice or Ts65Ds mice disomic for Erg. Similarly, in the mice engineered to have only 2 functional Erg alleles, amelioration of MPD features and correction of thrombocytosis, megakaryocytosis, and extramedullary hematopoiesis to wild-type levels was observed.

Although these results clearly demonstrate a role for Erg in the myeloproliferative disease seen in trisomic Ts65Dn mice, intriguingly, Ts1CjeDS mice, a DS model that has Erg trisomy but contains 23 fewer murine homologues of the human chromosome 21 DSCR, do not develop a myeloproliferative disorder.⁸  The Ts1CjeDS mice do have macrocytosis and decreased red cell number like the Ts65Dn mice. Whether the different phenotypes are the result of the 23 genes trisomic in Ts65Dn and disomic in Ts1CjeDS, or strain-specific genetic background differences remains to be determined. A series of micro-RNAs outside of the DSCR but potentially linked to the normal regulation of megakaryopoiesis have been hypothesized to perhaps participate.^2,3  Neither the Ts65Dn nor Ts1CjeDS strain develops TMD or DS-AMKL. Interestingly, a recently published high-density genomic array analysis of human partial trisomy 21 DS cases including 3 cases of DS-TMD/AMKL has narrowed the common TMD/AMKL amplification region to an 8.3-Mb segment containing RUNX1, ETS2, and ERG.⁹  All 3 TMD/AMKL persons in this study also shared an unexpected deletion of the short arm of the human chromosome 21, far away from the DSCR, which may also contribute to the phenotype.

With these complexities aside, how might Erg overexpression contribute to abnormal myelopoiesis when coupled with an exon 2 Gata1 mutation in the same cell? Previous work had demonstrated that overexpression of ERG, ETS2, or the related ETS factor FLI-1 in Gata1 mutant or GATA1s knock-in fetal liver progenitors leads to aberrant megakaryopoiesis, immortalization of fetal liver progenitors, and activation of the JAK/STAT pathway.¹⁰  Much work remains, but the results presented by Ng et al illustrate that precise modeling of candidate gene copy number with careful hematologic analysis for an uncommon Down syndrome–associated myeloid disorder may be an early step toward ultimately providing significant insights into our general understanding of much more common myeloproliferative disorders and leukemic transformation.