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Published on: October 2, 2017
Chromatin reader L(3)mbt requires the Myb-MuvB/DREAM transcriptional regulatory complex for chromosomal recruitment
Daniel P Blanchard1, Daphne Georlette1, Lisa Antoszewski2
1Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720; and.
Abstract:
Lethal malignant brain tumors (lmbt) result from the loss of the conserved transcriptional repressor l(3)mbt, in Drosophila melanogaster. Similar mutations in the human homolog L3MBTL1 correlate with some cancers. The protein's C-terminal MBT repeats bind mono and dimethylated histones in vitro, which could influence recruitment of L3MBTL1 to its target sites. The L(3)mbt chromatin targeting mechanism, however, is controversial and several studies suggest insufficiency or a minor role for histone methylation in determining the site specificity for recruitment. We report that L(3)mbt colocalizes with core members of the Myb-MuvB/DREAM (MMB/DREAM) transcriptional regulatory complex genome-wide, and that L(3)mbt-mediated repression requires this complex in salivary glands and larval brains. Loss of l(3)mbt or of MMB components through mutation cause similar spurious expression of genes, including the transposon regulatory gene piwi, in terminally differentiated cells. The DNA-binding MMB core component Mip120 (Lin54) is required for L(3)mbt recruitment to chromosomes, whereas Mip130 (Lin9) (an MMB core protein) and E2f2 (an MMB transcriptional repressor) are not, but are essential for repression. Cytolocalization experiments suggest the presence of site-specific differential composition of MMB in polytene chromosomes where some loci were bound by a Myb-containing or alternatively, an E2f2 and L(3)mbt form of the complex.
Insights
Loss of the transcriptional repressor L(3)mbt causes lethal malignant brain tumors. L(3)mbt requires the Myb-MuvB/DREAM complex for repression, with Mip120 mediating its recruitment.
Area of Science:
- Developmental Biology
- Molecular Biology
- Cancer Research
Background:
- Lethal malignant brain tumors (LMBT) arise from the loss of the conserved transcriptional repressor l(3)mbt in Drosophila.
- The human homolog L3MBTL1 is implicated in certain cancers, suggesting conserved functions.
- The precise chromatin targeting mechanism of L(3)mbt remains debated, with conflicting evidence regarding the role of histone methylation.
Purpose of the Study:
- To investigate the genome-wide localization and functional requirements of the L(3)mbt transcriptional repressor.
- To elucidate the role of the Myb-MuvB/DREAM (MMB/DREAM) complex in L(3)mbt-mediated gene repression.
- To determine the specific MMB/DREAM components involved in L(3)mbt recruitment and repression.
Main Methods:
- Genome-wide colocalization studies of L(3)mbt with MMB/DREAM complex members.
- Analysis of gene expression changes in mutants lacking L(3)mbt or MMB/DREAM components.
- Cytolocalization experiments to assess differential MMB/DREAM complex composition at specific loci.
Main Results:
- L(3)mbt colocalizes genome-wide with core MMB/DREAM complex members.
- L(3)mbt-mediated repression is dependent on the MMB/DREAM complex in salivary glands and larval brains.
- Loss of l(3)mbt or MMB components leads to aberrant gene expression, including the piwi gene, in differentiated cells.
- Mip120 (Lin54) is essential for L(3)mbt recruitment, while Mip130 (Lin9) and E2f2 are required for repression.
- Evidence suggests differential MMB complex composition at polytene chromosome loci.
Conclusions:
- L(3)mbt functions as a transcriptional repressor through its association with the MMB/DREAM complex.
- The MMB/DREAM complex, particularly Mip120, is crucial for targeting L(3)mbt to its genomic sites.
- The findings reveal a novel mechanism for transcriptional repression involving L(3)mbt and MMB/DREAM, with implications for understanding cancer.
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