The transcriptional corepressor MTG16a contains a novel nucleolar targeting sequence deranged in t (16; 21)-positive myeloid malignancies

Abstract

The MTG (Myeloid Translocation Gene) proteins are a family of novel transcriptional corepressors. We report that MTG16a, a protein isoform encoded by the MTG16 gene deranged by the t (16; 21) in myeloid malignancies, is targeted to the nucleolus. The amino acid sequence necessary for nucleolar localization was mapped to the MTG16a N-terminal region. MTG16a, like MTG8, the nuclear corepressor deranged by the t (8; 21), is capable to interact with specific histone deacetylases (HDACs) suggesting that the protein may mediate silencing of nucleolar gene transcription. In addition, MTG16a is capable to form oligomers with other MTG proteins. As a consequence of the t (16; 21) the AML1 DNA-binding domain replaces the MTG16a N-terminal region. The AML1-MTG16 fusion protein is targeted to the nucleoplasm where it is capable to oligomerize with MTG16a and interact with HDAC1 and HDAC3. The deficiency of HDAC-containing complexes at nucleolar sites and the accumulation of HDAC-containing complexes at AML1-sites may be critical in the pathogenesis of t (16; 21) myeloid malignancies.

Myeloid maturation block by AML1-MTG16 is associated with Csf1r epigenetic downregulation

“…Apparently, loss of the N-terminus of MTG16 has profound biological consequences on myeloid cell growth. We previously hypothesized a critical role for the N-terminus of MTG16 based on the observation that exogenous MTG16D2 had a different intranuclear localization than exogenous MTG16 in COS cells (Hoogeveen et al, 2002). However, we were unable to show different intranuclear localization for these two proteins in 32D/ WT1 cells.…”

“…As mentioned, we previously found that AML1-MTG16 can interact with histone deacetylases (HDACs) (Hoogeveen et al, 2002). This led us to hypothesize that AML1-MTG16 is capable of inducing epigenetic repression of endogenous AML1 target genes by tethering, via the MTG16 moiety, HDACs at AML1-target sites.…”

“…Since we previously observed that AML1-MTG16 is capable of recruiting HDACs (Hoogeveen et al, 2002), we hypothesized that Csf1r downregulation in AML1-MTG16-positive clones was associated with epigenetic histone changes. Of note, aberrant hypoacetylation of human CSF1R, highly homologous to mouse Csf1r, was shown to correlate with AML1-MTG8 downregulation in t(8;21)-positive leukemic cells, suggesting that AML1-MTG proteins exert an epigenetic constraint of CSF1R expression (Follows et al, 2003).…”

“…Both wild-type AML1 and AML1-MTG8 can bind CSF1R at AML1-binding sites (Zhang et al, 1994;Follows et al, 2003). To test whether AML1-MTG16 can bind the Csf1r promoter, we performed ChIP with an antibody directed against the MTG16 moiety (Hoogeveen et al, 2002), followed by quantitative amplification of the Csf1r FIRE region. A significant enrichment of FIRE DNA in the chromatin of AML1-MTG16-positive cells relative to the chromatin of control cells (Figure 3b) proved that AML1-MTG16 can bind the endogenous Csf1r.…”

HDAC3: taking the SMRT-N-CoRrect road to repression

“…Similarly, a common translocation in AML patients, t(8;21) results in the generation of fusion protein AML1 and ETO (eight-twenty-one or MTG8), which also recruits corepressors and HDACs by the C terminus of ETO, leading to repression of the AML1 target genes (Rowley, 1982;Erickson et al, 1992;Gelmetti et al, 1998;Lutterbach et al, 1998;Wang et al, 1998;Amann et al, 2001). Analogous effect has been described for the fusion protein MTG16a-AML1, resulting from the t(16;21) translocation, present in a number of AML patients (Gamou et al, 1998;Hoogeveen et al, 2002).…”