The mixed lineage leukemia fusion partner AF9 binds specific isoforms of the BCL-6 corepressor

Abstract

The mixed lineage leukemia (MLL) gene at chromosome band 11q23 is commonly involved in reciprocal translocations that are detected in acute leukemias. Evidence suggests that the resulting MLL fusion genes contribute to leukemogenesis. AF9 is a common MLL fusion partner in acute myeloid leukemia. The AF9 protein functions as a transcriptional activator in artificial reporter gene assays and a structurally related protein in yeast, ANC1/TFG3, is a component of the SWI/SNF complex. Apart from these observations, little is known about the biologic function of AF9 in mammals. We have found that a recently described transcriptional repressor, BCL-6 corepressor (BCoR), interacts with the carboxy-terminus of AF9. The interaction of AF9 with BCoR has been confirmed by independent in vitro and in vivo proteinbinding studies. The BCoR gene is expressed as several alternatively spliced transcripts. AF9 only binds BCoR isoforms that contain a unique 34 aa sequence located in the mid-portion of the protein. In artificial reporter gene assays, a BCoR isoform that binds AF9 efficiently suppresses AF9 transcriptional activity, while a nonbinding isoform does not. These results indicate that different isoforms of BCoR have unique biologic properties and that cell function may be partly determined by the different isoforms that are present within the cell.

Characterization of Bcor expression in mouse development

“…RNAi knock-down of Bcor in zebrafish results in eye, skeletal and nervous system abnormalities consistent with those found in MCOPS2 patients (Ng et al, 2004). BCOR has also been shown to interact with and repress transcriptional activation of the MLL fusion protein AF9 (Srinivasan et al, 2003), a known regulator of Hox gene expression (Collins et al, 2002) and skeletal development, further implicating BCOR as a key developmental regulator. The pleiotropic effects induced by the loss of functional BCOR in humans and zebrafish (Ng et al, 2004) clearly illustrate the essential role of BCOR during embryogenesis and emphasizes the importance of determining the spatial and temporal expression of BCOR during development.…”

“…Although alternative splicing in the coding region has been reported ( Fig. 1F and (Srinivasan et al, 2003)) this can only affect transcript size by up to 156 bp. In silico analysis of spliced expressed sequence tags (EST) databases supported use of alternative promoters and polyadenylation sites at the mouse Bcor genomic locus.…”

“…1F) (Srinivasan et al, 2003). Only isoforms a and b contain sequences required for the interaction with the transcriptional regulator AF9 (Ng et al, 2004;Srinivasan et al, 2003).…”

“…1F) (Srinivasan et al, 2003). Only isoforms a and b contain sequences required for the interaction with the transcriptional regulator AF9 (Ng et al, 2004;Srinivasan et al, 2003). To determine which isoforms are differentially expressed in the embryo and adult mouse tissues, we conducted RT-PCR using primers that span exon 4 to exon 10 on samples used in Figure 1B and 1D.…”

Af9/Mllt3 interferes with Tbr1 expression through epigenetic modification of histone H3K79 during development of the cerebral cortex

“…Af9 is also expressed in neurons dispersed over all cortical layers and diverges in this respect from other SVZ markers, such as Svet1 and Cux2 (6). On the molecular level, AF9 mediates transcriptional activation and was classified as a proto-oncogene (9). The AF9 protein interacts with many different factors and has been implicated in different cellular processes (9)(10)(11).…”