Chromatin Remodeling By GATA1s Alters Megakaryocytic Gene Expression

The GATA1 transcription factor is crucial for the regulation of gene expression during the differentiation of hematopoietic progenitors to erythrocytic and megakaryocytic lineages. Mutations in GATA1 are pervasively found in myeloid leukemia of Down syndrome (ML-DS), a subtype of acute myeloid leukemia that is common in children with Down syndrome. These mutations typically occur in the N-terminal region of the GATA1 gene and lead to the production of a truncated isoform known as GATA1s, which retains the DNA-binding capacity. Expression of the truncated form GATA1s disrupts normal hematopoietic differentiation and leads to the over-production of megakaryocytic progenitors in the fetal liver. In Down syndrome children, this results in a non-lethal blood disorder known as transient abnormal myelopoiesis (TAM), which in many cases progresses to myeloid leukemia.

Our study investigates the impact of the GATA1s isoform on megakaryopoiesis and its underlying molecular mechanisms. We use mouse hematopoietic progenitors and CRISPR-edited human HSPCs and perform transcriptomic and epigenomic analyses to understand how GATA1s differentially controls megakaryocytic gene expression. Our findings indicate that the expression of GATA1s promotes a premature increase of megakaryocyte-specific transcription already in hematopoietic stem cells. The upregulation of megakaryocyte markers is observed in HSCs from both fetal liver and adult bone marrow, though with varying effects across subpopulations. Moreover, the response to megakaryocyte-inducing acute stressors, such as interferon, differs between cells expressing GATA1 and those expressing GATA1s. Despite this early bias, GATA1s-expressing cells show incomplete differentiation into megakaryocytes in culture. ChIP-seq of normal and mutant GATA1 shows that despite both isoforms bind to a majority of shared binding sites, a fraction of these sites is isoform-specific. This is linked to changes in chromatin accessibility, which present different dynamics of gain and loss along megakaryocyte differentiation and which are associated to the number of GATA1 motifs on DNA. Furthermore, Hi-C analysis of global chromatin interactions shows changes in long-range genomic interactions associated to the deregulation of Mk-specific transcripts.

In summary, our work illustrates how GATA1s disrupts normal megakaryopoiesis by inducing early lineage bias and impairing differentiation through remodeling of the chromatin landscape and gene expression. These findings enhance our understanding of GATA1s-related disorders, such as myeloid leukemia of Down syndrome, and may guide future therapeutic strategies.