Chromatin reader L(3)mbt requires the Myb–MuvB/DREAM transcriptional regulatory complex for chromosomal recruitment

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 sitespecific 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.epigenetics | bar coding | tumorigenesis

Dynamic Control of X Chromosome Conformation and Repression by a Histone H4K20 Demethylase

“…Malignant brain tumor repeat (MBT) proteins associate with H4K20me1 nucleosomes and repress gene expression (Blanchard et al, 2014; Trojer et al., 2007). Hence, we tested whether the MBT repeat proteins of C. elegans (Harrison et al., 2007) contribute to DC.…”

“…In flies and mammals, malignant brain tumor repeat (MBT) proteins associate with nucleosomes enriched in H4K20me1 or H4K20me2, compact the chromatin fiber, and help repress transcription (Blanchard et al, 2014; Trojer et al, 2007). Knockout of worm MBT proteins failed to disrupt DC, implying that if H4K20me1 binding proteins modulate X-chromosome conformation and gene repression, they will define a new class of H4K20me1-specific histone “readers”.…”

Oncogenic transformation of Drosophila somatic cells induces a functional piRNA pathway

“…This may be attributable to the alteration of the SWH pathway, as Piwi and Ago3 become expressed in hyperactive Yki wing disc cells. This suggests that transcription factors controlled by Yki can directly or indirectly contribute to the regulation of germline gene expression programs (Polesello and Tapon 2007;Blanchard et al 2014;Zhang et al 2015). Among the reactivated germline genes are also components of secondary biogenesis pathways, including Aub and Ago3.…”

“…In the nucleus, Yki normally cooperates with other transcription factors to promote the expression of genes that enhance growth (e.g., cyclin E) and survival (e.g., DIAP-1 and bantam). A causal role for the altered expression of SWH pathway targets in l(3) mbt mutant tumors is supported by studies in which mutation of bantam or yki or enforced expression of Ex suppressed tumorigenesis (Richter et al 2011;Blanchard et al 2014;Zhang et al 2015).…”

“…This could be due to the fact that germline transcription factors, which are targets of l(3)mbt, also become activated (Richter et al 2011;Meier et al 2012;Blanchard et al 2014;Zhang et al 2015). These include LINT, which has been implicated in the regulation of Piwi expression, as well as vasa (vas), bag of marbles (bam), and benign gonial cell neoplasm (bgcn), all of which play important roles in the regulation of germ cell division.…”