In this issue of Blood, Elagib and colleagues describe a novel connection in megakaryopoiesis between the zinc finger transcription factor GATA-1 and the major mediator of transcriptional elongation, P-TEFb.

The precise patterns of gene expression in cells and organisms are regulated primarily at the level of transcription through recruitment of RNA polymerase II (RNAP II). Although transcriptional initiation has received the most attention, it is clear that for certain genes, elongation of nascent transcripts is the critical step. Positive transcription elongation factor b (P-TEFb), the major effector of transcriptional elongation, is composed of a cyclin-dependent kinase, Cdk9, and its associated cyclins (cyclins T1 and T2; reviewed in Price,1  Peterlin and Price,2  and Price3 ). P-TEFb is subject to complex regulation by both positively acting factors, including the bromodomain protein Brd4 and a number of recruiting proteins, and negative factors that include the HEXIM1 and HEXIM2 proteins and 7SK snRNA. The principal targets of this kinase are serine residues in the C-terminal domain of RNAP II.

To exert its actions, P-TEFb must first be recruited to sites where transcription has been initiated. In addition to the general chromatin remodeling protein Brd4, specific activators, including NF-kB, c-Myc, MyoD, and steroid hormone receptors, have been shown to bind P-TEFb. Now, the important hematopoietic transcription factor GATA-1 can be added to this list. In an article in this issue of Blood, Elagib et al describe a novel interaction between GATA-1 and P-TEFb in megakaryopoiesis. Using the K562 human leukemia cell line, they show that cyclin T1 coprecipitates with GATA-1 in cells induced to undergo megakaryocytic differentiation. While the interaction of these proteins may be direct, this was not unequivocally established. However, the authors did show that RNAP II processivity in a well characterized GATA-1 target gene, integrin αIIb, increases with megakaryocytic differentiation.

This newly described intersection between GATA-1 and P-TEFb both reflects megakaryocytic differentiation and appears capable of modulating it. Using a loss-of-function approach involving Cdk9 knockdown and a gain-of-function approach using HEXIM1 knockdown, P-TEFb function correlated with differentiated gene expression. Interestingly, Cdk9 knockdown was associated with ectopic expression of certain erythroid markers and even hemoglobinization of these cells, confirming previous observations of enhanced erythroid differentiation with P-TEFb inhibition and suggesting that the level of P-TEFb activity regulates lineage choice in the bipotent megakaryocytic-erythroid progenitor. Impressively, these findings in K562 cells were confirmed in primary human hematopoietic cells by 2 approaches — flavopiridol treatment and Cdk9 knockdown.

Finally, the interactions between GATA-1 and P-TEFb in abnormal megakaryopoiesis were explored using mice with an induced mutation in an upstream regulatory region in the Gata-1 locus (GATA-1Lo) that severely reduces transcription of the gene in the megakaryocytic lineage. Flavopiridol treatment of mice homozygous or hemizygous for this mutant Gata-1 allele, but not wild-type controls, caused both impairment of megakaryocytic differentiation and the reversible accumulation of megakaryoblasts. Interestingly, these clinical findings resemble those observed in the transient myeloproliferative disorder associated with Down syndrome. These individuals have been found to carry mutations in GATA-1 that produce an amino-terminally truncated form of the protein; however, aberrant transcriptional elongation in GATA-1–regulated genes has not been described. Thus, the findings of Elagib et al could provide new insight into the pathogenesis and, conceivably, the treatment of this human myeloproliferative disorder.

Conflict-of-interest disclosure: The author declares no competing financial interests. ■

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