Segonds-Pichon, A. | Search bioRxiv

bioRxiv2021

GLP is critical for oogenesis exhibiting a G9A-independent role in transcriptional repression

GLP (EHMT1) is a multifunctional protein, best known for its role as an H3K9me1 and H3K9me2 methyltransferase through its reportedly obligatory dimerization with G9A (EHMT2). Here, we investigate the role of GLP in the oocyte in comparison to G9A using oocyte-specific conditional knockout mouse models (G9a cKO, Glp cKO, G9a-Glp cDKO). Loss of GLP in Glp cKO and G9a-Glp cDKO oocytes re-capitulated meiotic defects observed in the G9a cKO; however, there was a significant impairment in oocyte maturation and developmental competence in Glp cKO and G9a-Glp cDKO oocytes beyond that observed in the G9a cKO. Consequently, loss of GLP in oogenesis results upon fertilisation in mid-gestation embryonic lethality. To assess the molecular functions of GLP and G9A, we applied a multi-omics approach, supported by immunofluorescence, to identify changes in epigenomic, transcriptomic and proteomic signatures in cKO oocytes. H3K9me2 was equally depleted in all cKO oocytes, whereas H3K9me1 was decreased only upon loss of GLP. The transcriptome, DNA methylome and proteome were markedly more affected in G9a-Glp cDKO than G9a cKO oocytes, with transcriptional de-repression associated with increased protein abundance and gains in genic DNA methylation in G9a-Glp cDKO oocytes. Together, our findings suggest that GLP contributes to transcriptional repression in the oocyte, independent of G9A, and is critical for oogenesis and oocyte developmental competence.

developmental biology↗

bioRxiv2021

DNA replication during acute MEK inhibition drives acquisition of resistance through amplification of the BRAF oncogene

Mutations and gene amplifications that confer drug resistance emerge frequently during chemotherapy, but their mechanism and timing is poorly understood. Here, we investigate BRAFV600E amplification events that underlie resistance to the MEK inhibitor selumetinib (AZD6244/ARRY-142886) in COLO205 cells, a well-characterised model for reproducible emergence of drug resistance, and show that de novo amplification of BRAF is the primary path to resistance irrespective of pre-existing amplifications. Selumetinib causes long-term G1 arrest accompanied by reduced expression of DNA replication and repair genes, but cells stochastically re-enter the cell cycle during treatment despite continued repression of pERK1/2. Most DNA replication and repair genes are re-expressed as cells enter S and G2, however, mRNAs encoding a subset of factors important for error-free replication and chromosome segregation including TIPIN, PLK2 and PLK3 remain at low abundance. This suggests that DNA replication in drug is more error prone and provides an explanation for the DNA damage observed under long-term RAF-MEK-ERK1/2 pathway inhibition. To test the hypothesis that DNA replication in drug promotes de novo BRAF amplification, we exploited the combination of palbociclib and selumetinib. Combined treatment with selumetinib and a dose of palbociclib sufficient to reinforce G1 arrest in selumetinib-sensitive cells, but not to impair proliferation of resistant cells, delays the emergence of resistant colonies, meaning that escape from G1 arrest is critical in the formation of resistant clones. Our findings demonstrate that acquisition of MEK inhibitor resistance often occurs through de novo gene amplification and can be suppressed by impeding cell cycle entry in drug.

molecular biology↗