BACH and the megakaryocyte symphony

Comment on Toki et al, page 3100

Final differentiation of an erythroid-megakaryocyte precursor cell into one or the other lineage may involve a balance between 2 related transcription factors, NF-E2, which leads to megakaryocytic differentiation, and BACH1, which leads to erythroid differentiation.

Final fate determination into the erythroid and megakaryocyte lineages involves a considerable number of overlapping hematopoietic-specific transcription factors. Understanding how these related lines undergo terminal differentiation to yield such completely different cells, each with its own rich repertoire of distinct proteins and organelles, is far from understood. Certainly such insights may not only be of interest for understanding basic hematopoiesis but may also lead to novel new clinical strategies for regulating the production of red cells and platelets.

There are 2 not mutually exclusive mechanisms by which final differentiation choice may be made by a common megakaryocyte-erythrocyte precursor: (1) the turning on of a specific gene that drives expression down a particular pathway; and (2) a stochastic system, where the relative balance of a transcription factor leads to pathway choice. A model of the former is based on the demonstration that the Ets family member Fli-1 (friend leukemia integration 1) enhances GATA-1/FOG-1 expression of many megakaryocyte-specific genes,¹  and studies altering Fli-1 expression altered megakaryocyte-specific expression.²  Is the expression of Fli-1 the deciding factor that leads to megakaryopoiesis? On the other hand, studies have shown that the ratio of GATA-1/FOG-1 expression affects megakaryocyte-erythrocyte choice so that in a GATA-1 knockdown mouse there is predominantly a deficiency in megakaryopoiesis consistent with a stochastic model.³ FIG1 

In this issue, Toki and colleagues examine a new aspect of gene regulation during megakaryopoiesis, where the relative levels of 2 related transcription factors appear to affect final fate determination. NF-E2 (nuclear factor-erythroid derived 2) or p45 is a hematopoietic-specific transcription factor that dimerizes with p18, a widely distributed Maf transcription factor.⁴  Absence of NF-E2 results in marked thrombocytopenia with increased number of immature-appearing megakaryocytes.⁵  Megakaryocytes express an NF-E2–related transcription factor, BACH1, that can also dimerize with p18.⁶  Lack of BACH1 does not affect platelet counts, but in this paper by Toki et al the authors overexpress BACH1 using a GATA-1 promoter construct previously shown to drive expression in both the megakaryocyte and erythrocyte lineages. The transgenic lines that overexpressed BACH1 had significant thrombocytopenia, though not as severe as in the NF-E2^–/– mice, although in many other ways the phenotype resembled that of the NF-E2^–/– mice. By semiquantitative reverse transcriptase–polymerase chain reaction (RT-PCR), the levels of GATA-1 and NF-E2 were roughly normal in the megakaryocytes. Chromatin immunoprecipitation (ChIP) studies using megakaryocytic cell lines suggest that BACH1 can bind to the proximal promoter of the megakaryocyte-expressed gene thromboxane A2 synthetase (TXS), and BACH1 could clearly inhibit TXS synthesis in transient expression studies. The authors propose a novel balancing act between NF-E2 and BACH1 expression during differentiation with BACH1 suppresses megakaryopoiesis without affecting erythropoiesis (see the figure).

However, while the data are consistent with this model, interpretation of overexpression studies can be problematic. The authors acknowledge this issue, though an alternative explanation that the vast excess of BACH1 binds all available p18 in the nucleus so that none was available for NF-E2 heterodimerization is also a possibility. This and other related issues remain to be examined.

Nevertheless, this paper adds to the growing appreciation of the potential complexity and subtleties of changes that occur at the junction at which final differentiation into erythrocytes and megakaryocytes occurs. It is likely that no one single factor or altered ratio of factors will underlie this process. Rather, final fate determination will likely be a closely orchestrated symphony of interactive tissue-specific and nonspecific transcription factors in concert with a primed chromatin organization. The challenge will then be how to conduct this orchestra to play the exact notes one wishes for clinical advantage. ▪