Synergistic lethality between BRCA1 and H3K9me2 loss reflects satellite derepression.

has two histone H3 Lys9 methyltransferases, MET-2 (SETDB1 homolog) and SET-25 (G9a/SUV39H1 related). In worms, we found simple repeat sequences primarily marked by H3K9me2, while transposable elements and silent tissue-specific genes bear H3K9me3. RNA sequencing (RNA-seq) in histone methyltransferase (HMT) mutants shows that MET-2-mediated H3K9me2 is necessary for satellite repeat repression, while SET-25 silences a subset of transposable elements and tissue-specific genes through H3K9me3. A genome-wide synthetic lethality screen showed that RNA processing, nuclear RNA degradation, the BRCA1/BARD1 complex, and factors mediating replication stress survival are necessary for germline viability in worms lacking MET-2 but not SET-25. Unlike mutants, -null worms accumulated satellite repeat transcripts, which form RNA:DNA hybrids on repetitive sequences, additively with the loss of BRCA1 or BARD1. BRCA1/BARD1-mediated H2A ubiquitination and MET-2 deposited H3K9me2 on satellite repeats are partially interdependent, suggesting both that the loss of silencing generates BRCA-recruiting DNA damage and that BRCA1 recruitment by damage helps silence repeats. The artificial induction of MSAT1 transcripts can itself trigger damage-induced germline lethality in a wild-type background, arguing that the synthetic sterility upon BRCA1/BARD1 and H3K9me2 loss is directly linked to the DNA damage provoked by unscheduled satellite repeat transcription

Yeast telomerase and the SUN domain protein Mps3 anchor telomeres and repress subtelomeric recombination.

Telomeres form the ends of linear chromosomes and protect these ends from being recognized as DNA double-strand breaks. Telomeric sequences are maintained in most cells by telomerase, a reverse transcriptase that adds TG-rich repeats to chromosome ends. In budding yeast, telomeres are organized in clusters at the nuclear periphery by interactions that depend on components of silent chromatin and the telomerase-binding factor yeast Ku (yKu). In this study, we examined whether the subnuclear localization of telomeres affects end maintenance. A telomere anchoring pathway involving the catalytic yeast telomerase subunits Est2, Est1, and Tlc1 is shown to be necessary for the perinuclear anchoring activity of Yku80 during S phase. Additionally, we identify the conserved Sad1-UNC-84 (SUN) domain protein Mps3 as the principal membrane anchor for this pathway. Impaired interference with Mps3 anchoring through overexpression of the Mps3 N terminus in a tel1 deletion background led to a senescence phenotype and to deleterious levels of subtelomeric Y' recombination. This suggests that telomere binding to the nuclear envelope helps protect telomeric repeats from recombination. Our results provide an example of a specialized structure that requires proper spatiotemporal localization to fulfill its biological role, and identifies a novel pathway of telomere protection

Perinuclear Anchoring of H3K9-Methylated Chromatin Stabilizes Induced Cell Fate in C. elegans Embryos.

Interphase chromatin is organized in distinct nuclear sub-compartments, reflecting its degree of compaction and transcriptional status. In Caenorhabditis elegans embryos, H3K9 methylation is necessary to silence and to anchor repeat-rich heterochromatin at the nuclear periphery. In a screen for perinuclear anchors of heterochromatin, we identified a previously uncharacterized C. elegans chromodomain protein, CEC-4. CEC-4 binds preferentially mono-, di-, or tri-methylated H3K9 and localizes at the nuclear envelope independently of H3K9 methylation and nuclear lamin. CEC-4 is necessary for endogenous heterochromatin anchoring, but not for transcriptional repression, in contrast to other known H3K9 methyl-binders in worms, which mediate gene repression but not perinuclear anchoring. When we ectopically induce a muscle differentiation program in embryos, cec-4 mutants fail to commit fully to muscle cell fate. This suggests that perinuclear sequestration of chromatin during development helps restrict cell differentiation programs by stabilizing commitment to a specific cell fate. PAPERCLIP

Promoter- and RNA polymerase II-dependent hsp-16 gene association with nuclear pores in Caenorhabditis elegans.

Some inducible yeast genes relocate to nuclear pores upon activation, but the general relevance of this phenomenon has remained largely unexplored. Here we show that the bidirectional hsp-16.2/41 promoter interacts with the nuclear pore complex upon activation by heat shock in the nematode Caenorhabditis elegans. Direct pore association was confirmed by both super-resolution microscopy and chromatin immunoprecipitation. The hsp-16.2 promoter was sufficient to mediate perinuclear positioning under basal level conditions of expression, both in integrated transgenes carrying from 1 to 74 copies of the promoter and in a single-copy genomic insertion. Perinuclear localization of the uninduced gene depended on promoter elements essential for induction and required the heat-shock transcription factor HSF-1, RNA polymerase II, and ENY-2, a factor that binds both SAGA and the THO/TREX mRNA export complex. After induction, colocalization with nuclear pores increased significantly at the promoter and along the coding sequence, dependent on the same promoter-associated factors, including active RNA polymerase II, and correlated with nascent transcripts

Histone H3K9 methylation is dispensable for Caenorhabditis elegans development but suppresses RNA:DNA hybrid-associated repeat instability

Genome Instability and Cance