Dissecting the Molecular Pathways That Underlie Disease-Causing GATA1 Mutations

Abstract 3439

Missense mutations in the gene encoding hematopoietic transcription factor GATA1 cause congenital anemias and/or thrombocytopenias. All seven reported mutations give rise to amino acid substitutions within the amino-terminal zinc finger (NF), but produce a range of phenotypes. The clinical severity depends on the site and type of substitution, and different substitutions of the same residue can produce disparate phenotypes. We combined structural, biochemical, in vivo conditional rescue approaches, and genomic analyses to systematically characterize all known GATA1 mutations with the goal of determining how they alter GATA1 function to result in disease.

Introducing mutant forms of GATA1 into GATA1-null erythroid or bipotential erythromegakaryocytic cell lines essentially recapitulated patient phenotypes. The V205M, G208S, G208R, and D218Y mutations severely impaired both erythroid and megakaryocyte maturation, while the R216Q, R216W, and D218G mutations had only a mild effect on the maturation of these lineages. Global differentiation defects were reproduced at the level of individual GATA1 target genes. Moreover, the former mutants greatly impaired both the transcriptional activation and repression functions of GATA1, while the latter moderately impaired gene activation but had no effect on repression.

It had been suggested previously that GATA1 mutations could be categorized into two classes, those that impair binding of the NF to the essential GATA1 cofactor FOG1 (V205M, G208S, G208R) and those that diminish binding of the NF to DNA (R216Q and R216W). The impact of the final two mutations (D218G and D218Y) remained uncertain, as this residue is not part of any known interaction face.

Our work led to the following novel conclusions:

1. Binding studies using isothermal titration calorimetry (ITC) and chromatin immunoprecipitation (ChIP) produced concurrent results showing that the V205M, G208S, G208R, and D218Y mutations diminish the GATA1-FOG1 interaction in vitro and FOG1 recruitment to GATA1 target genes in vivo. Interestingly, in contrast to D218Y, D218G did not affect FOG1 binding in vitro or in vivo. Furthermore, G208S had a less pronounced impact on FOG1 binding than the other three mutations, thus correlating the severity of the clinical presentation with the degree of FOG1 disruption. This confirms and extends previous work linking impaired FOG1 binding to the disease phenotypes associated with this class of mutations.

2. ITC showed that R216Q and R216W disrupt DNA binding in vitro, consistent with previous in vitro studies. However, remarkably, ChIP assays revealed that neither mutation impaired in vivo GATA1 target site occupancy at any examined simple or palindromic GATA elements, suggesting that failure to bind DNA does not account for the associated clinical phenotypes.

3. Notably, the R216Q and D218G mutations selectively diminished recruitment of Tal1/SCL without affecting the interaction with FOG1 or DNA. This implicates for the first time the Tal1/SCL complex in the pathogenesis of disorders caused by GATA1 mutations. Since the Tal1/SCL complex functions mostly during GATA1 gene activation, this also explains the observation that these GATA1 mutants largely retain their ability to repress transcription. Moreover, changes in the gene expression profiles of R216Q and D218G expressing cells are highly correlated with each other but clearly distinct from the gene expression changes associated with different substitutions at the same residues (R216W or D218Y), revealing a specific subset of genes that are most sensitive to disruption of the GATA1-Tal1/SCL interaction.

4. An unexpected finding from our studies is that different substitutions of the same residue can disrupt binding to distinct cofactors (e.g. D218G impairs Tal1/SCL binding while D218Y impairs FOG1 binding), thus accounting for variable disease presentation.

In concert, our work on GATA1 mutations in their native environment reveals critical new insights not obtainable from in vitro studies. This highlights the usefulness of gene complementation studies in the relevant lineages for the dissection of transcription pathways to better understand and ultimately diagnose and treat hematologic disease.