The abundance of the short GATA1 isoform affects megakaryocyte differentiation and leukemic predisposition in mice.

Transcription factor GATA1 controls the delicate balance between proliferation, differentiation and apoptosis in both the erythroid and megakaryocytic lineages. In addition to full-length GATA1, there is an GATA1 isoform, GATA1s, that lacks the amino-terminal transactivation domain. Somatic GATA1 mutations that lead to the exclusive production of GATA1s appear to be necessary and sufficient for the development of a preleukemic condition called transient myeloproliferative disorder (TMD) in Down syndrome newborns. Subsequent clonal evolution among latent TMD blasts leads to the development of acute megakaryoblastic leukemia (AMKL). We originally established transgenic mice that express only GATA1s, which exhibit hyperproliferation of immature megakaryocytes, thus mimicking human TMD; however, these mice never developed AMKL. Here, we report that transgenic mice expressing moderate levels of GATA1s, i.e., roughly comparable levels to endogenous GATA1, were prone to develop AMKL in young adults. However, when GATA1s is expressed at levels significantly exceeding that of endogenous GATA1, the development of leukemia was restrained in a dose dependent manner. If the transgenic increase of GATA1s in progenitors remains small, GATA1s supports the terminal maturation of megakaryocyte progenitors insufficiently, and consequently the progenitors persisted, leading to an increased probability for acquisition of additional genetic modifications. In contrast, more abundant GATA1s expression compensates for this maturation block, enabling megakaryocytic progenitors to fully differentiate. This study provides evidence for the clinical observation that the abundance of GATA1s correlates well with the progression to AMKL in Down syndrome.

GATA1 Binding Kinetics on Conformation-Specific Binding Sites Elicit Differential Transcriptional Regulation.

GATA1 organizes erythroid and megakaryocytic differentiation by orchestrating the expression of multiple genes that show diversified expression profiles. Here, we demonstrate that GATA1 monovalently binds to a single GATA motif (Single-GATA) while a monomeric GATA1 and a homodimeric GATA1 bivalently bind to two GATA motifs in palindromic (Pal-GATA) and direct-repeat (Tandem-GATA) arrangements, respectively, and form higher stoichiometric complexes on respective elements. The amino-terminal zinc (N) finger of GATA1 critically contributes to high occupancy of GATA1 on Pal-GATA. GATA1 lacking the N finger-DNA association fails to trigger a rate of target gene expression comparable to that seen with the wild-type GATA1, especially when expressed at low level. This study revealed that Pal-GATA and Tandem-GATA generate transcriptional responses from GATA1 target genes distinct from the response of Single-GATA. Our results support the notion that the distinct alignments in binding motifs are part of a critical regulatory strategy that diversifies and modulates transcriptional regulation by GATA1.