Recent Progress

L3MBTL2 is an integral component of the polycomb repressive complex 1.6(Fig. 1). L3MBTL2 has been indicated in transcriptional repression and chromatin compaction. It is revealed by researchers that L3MBTL2 could be modulated by SUMO2/3 at lysine residues 675 and 700. Neither the repressive ability of L3MBTL2 nor it’s binding to histone tails in vitro would be affected by SUMOylation. Through the ChIP-Seq analysis with chromatin of wild-type HEK293 cells and with chromatin of HEK293 cells stably expressing either FLAG-tagged SUMOylation-competent or SUMOylation-defective L3MBTL2 respectively, results suggest that SUMOylation of L3MBTL2 does not affect chromatin binding. Though it is worth mentioning that, a subset of L3MBTL2-target genes, in partcular those with low L3MBTL2 occupancy was de-repressed in cells expressing the FLAG-L3MBTL2 K675/700R mutant. Moreover, investigators provide evidence that SUMOylation of L3MBTL2 facilitates repression of these polycomb repressive complex 1.6-target genes. This is achieved by balancing the local H2Aub1 levels.

Fig. 1. Superposition of the crystal structures of L3MBTL1 and L3MBTL2 MBT domains. L3MBTL1 is colored in blue and L3MBTL2 is colored in red. (Y Guo et al, 2009)

Researchers also revealed that L3MBTL2 is the missing link between RNF8 and RNF168. These proteins are crucial in DNA repairment. Through further investigation, it was found that L3MBTL2 is first recruited by MDC1 and then ubiquitylated by RNF8. The ubiquitylated L3MBTL2 will then facilitate recruitment of RNF168 to the DNA site of lesion and thus promotes DNA DSB repair.

In order to investigate the role of PRC1 complexes in genomic targeting of PRC1.6, the researchers conducted ChIP-seq analysis. This demonstrated the co-localization of MGA, L3MBTL2, E2F6 and PCGF6 genome-wide. Depletion of L3MBTL2 and E2F6 excluding PCGF6 led to differential, locus-specific loss of PRC1.6 binding. This indicated that different subunits modulate PRC1.6 loading to characteristic sets of promoters. Mga, L3mbtl2 and Pcgf6 also co-localize in mouse embryonic stem cells. PRC1.6 has been associated with repression of germ cell-related genes in these cells. These findings taken together demonstrated different genomic recruitment mechanisms of this group of PRC1.6 complex. This specifies the cell type- and context-specific regulatory functions of L3MBTL2 and so on.

L3MBTL2 has been implicated by researchers in regulating chromatin architecture. In order to further define the biological functions of L3MBTL2, researcher detected the expression of L3MBTL2 in the nuclei of renal tubular epithelial cells in mice. After cisplatin treatment or unilateral ureteral obstruction, it has been revealed that ablation of L3mbtl2 in renal tubular cells resulted in increase in nuclear DNA damage, p53 activation, apoptosis, tubular injury and kidney dysfunction. Studies in vitro indicated that inhibition of L3MBTL2 promoted histone H2AX expression, p53 activation and apoptosis in mouse proximal tubular TKPTS cells. The apoptosis induced by L3mbtl2 deficiency after cisplatin treatment both in vivo and in vitro could be attenuated through inhibiting p53 activity. It is also worth mentioning that, instead of being recruited to DNA damage sites, L3MBTL2 increased nuclear chromatin density and reduced initial DNA damage load. This feature is unlike other polycomb proteins.

Researchers also discovered high level of expression of L3MBTL2 in renal tubular epithelial cells in mice. Through crossbreeding floxed L3MBTL2 mice with Ksp-Cre mice, kidney epithelial cell specific L3MBTL2 knockout mice (L3MBTL2 cKO) was generated. Under basal conditions, the L3MBTL2 cKO(cell specific L3MBTL2 knockout mice) mice were generally normal with no apparent phenotypes found in the kidney. However, when expose these mice to Cisplatin-induced acute kidney injury (AKI), their kidneys were much more injured compared to the wild-type mice. Researchers also proposed that deletion of L3MBTL2 promoted DNA damage and inhibited cell proliferation, during Cisplatin-induced AKI. In L3MBTL2 cKO kidneys, expression of Ripk3, and Mlkl was also up-regulated. This is consistent with the increased necrosis. Higher mRNA levels of TNF-α and MCP-1 in L3MBTL2 cKO kidneys compared to WT kidneys also suggest that, L3MBTL2 protects the kidney from tubular injury and inflammation in AKI.