Transcriptional Activation of Pericentromeric Satellite Repeats and Disruption of Centromeric Clustering upon Proteasome Inhibition

A critical role for Pol II CTD phosphorylation in heterochromatic gene activation

How gene activation works in heterochromatin, and how the mechanism might differ from the one used in euchromatin, has been largely unexplored. Previous work has shown that in SIR-regulated heterochromatin of Saccharomyces cerevisiae, gene activation occurs in the absence of covalent histone modifications and other alterations of chromatin commonly associated with transcription.Here we demonstrate that such activation occurs in a substantial fraction of cells, consistent with frequent transcriptional bursting, and this raises the possibility that an alternative activation pathway might be used. We address one such possibility, Pol II CTD phosphorylation, and explore this idea using a natural telomere-linked gene, YFR057w, as a model. Unlike covalent histone modifications, we find that Ser2, Ser5 and Ser7 CTD phosphorylated Pol II is prevalent at the drug-induced heterochromatic gene. Particularly enriched relative to the euchromatic state is Ser2 phosphorylation. Consistent with a functional role for Ser2P, YFR057w is negligibly activated in cells deficient in the Ser2 CTD kinases Ctk1 and Bur1 even though the gene is strongly stimulated when it is placed in a euchromatic context. Collectively, our results are consistent with a critical role for Ser2 CTD phosphorylation in driving Pol II recruitment and transcription of a natural heterochromatic gene - an activity that may supplant the need for histone epigenetic modifications.

Repeat expression is linked to patient survival and exhibits single nucleotide variation in pancreatic cancer revealing LTR70:r.879A>G

Despite an overwhelming number of cancer literature reporting the links between patient survival and the expression levels of genes or mutations/single nucleotide variations (SNVs) on them, there is only limited information on repeat elements, which make at least half the human genome. Here, we analysed RNA-seq data obtained from primary pancreatic cancer tissues of 51 patients and revealed that two transposons, HERVI-int and X6A_LINE, showed an upregulation trend in the patients who lived shorter, along with 56 other potential repeats which were linked to survival. We also detected expressed single nucleotide variations (SNVs) on repeats, among which LTR70:r.879A>G stands out with the effect of its presence on this particular repeat's expression levels and a significant link to overall patient survival. Interestingly, the expression of LTR70:r.879A>G correlated with different cancer genes in comparison to its reference version highlighting the involvement of BRAF and Fumerate Hydratase with this expressed SNV. This is one of the first studies revealing possible links between repeat expression and survival in cancer and it warrants further research in this avenue.

Proteasome inhibition alters mitotic progression through the upregulation of centromeric α-Satellite RNAs

Cell cycle progression requires control of the abundance of several proteins and RNAs over space and time to properly transit from one phase to the next and to ensure faithful genomic inheritance in daughter cells. The proteasome, the main protein degradation system of the cell, facilitates the establishment of a proteome specific to each phase of the cell cycle. Its activity also strongly influences transcription. Here, we detected the upregulation of repetitive RNAs upon proteasome inhibition in human cancer cells using RNA‐seq. The effect of proteasome inhibition on centromeres was remarkable, especially on α‐Satellite RNAs. We showed that α‐Satellite RNAs fluctuate along the cell cycle and interact with members of the cohesin ring, suggesting that these transcripts may take part in the regulation of mitotic progression. Next, we forced exogenous overexpression and used gapmer oligonucleotide targeting to demonstrate that α‐Sat RNAs have regulatory roles in mitosis. Finally, we explored the transcriptional regulation of α‐Satellite DNA. Through in silico analyses, we detected the presence of CCAAT transcription factor‐binding motifs within α‐Satellite centromeric arrays. Using high‐resolution three‐dimensional immuno‐FISH and ChIP‐qPCR, we showed an association between the α‐Satellite upregulation and the recruitment of the transcription factor NFY‐A to the centromere upon MG132‐induced proteasome inhibition. Together, our results show that the proteasome controls α‐Satellite RNAs associated with the regulation of mitosis.

Signature changes in the expressions of protein-coding genes, lncRNAs, and repeat elements in early and late cellular senescence

Replicative cellular senescence is the main cause of aging. It is important to note that early senescence is linked to tissue regeneration, whereas late senescence is known to trigger a chronically inflammatory phenotype. Despite the presence of various genome-wide studies, there is a lack of information on distinguishing early and late senescent phenotypes at the transcriptome level. Particularly, the changes in the noncoding RNA portion of the aging cell have not been fully elucidated. By utilising RNA sequencing data of fibroblasts, hereby, we are not only reporting changes in gene expression profiles and relevant biological processes in the early and late senescent phenotypes but also presenting significant differences in the expressions of many unravelled long noncoding RNAs (lncRNAs) and transcripts arisen from repetitive DNA. Our results indicate that, in addition to previously reported L1 elements, various LTR and DNA transposons, as well as members of the classical satellites including HSAT5 and α-satellites (ALR/Alpha), are expressed at higher levels in late senescence. Moreover, we revealed finer links between the expression levels of repeats with the genes located near them and known to be involved in cell cycle and senescence. Noncoding elements reported here provide a new perspective to be explored in further experimental studies.

The Genomic Landscape of Centromeres in Cancers

Centromere genomics remain poorly characterized in cancer, due to technologic limitations in sequencing and bioinformatics methodologies that make high-resolution delineation of centromeric loci difficult to achieve. We here leverage a highly specific and targeted rapid PCR methodology to quantitatively assess the genomic landscape of centromeres in cancer cell lines and primary tissue. PCR-based profiling of centromeres revealed widespread heterogeneity of centromeric and pericentromeric sequences in cancer cells and tissues as compared to healthy counterparts. Quantitative reductions in centromeric core and pericentromeric markers (α-satellite units and HERV-K copies) were observed in neoplastic samples as compared to healthy counterparts. Subsequent phylogenetic analysis of a pericentromeric endogenous retrovirus amplified by PCR revealed possible gene conversion events occurring at numerous pericentromeric loci in the setting of malignancy. Our findings collectively represent a more comprehensive evaluation of centromere genetics in the setting of malignancy, providing valuable insight into the evolution and reshuffling of centromeric sequences in cancer development and progression.

Expression dynamics of repetitive DNA in early human embryonic development

BACKGROUND

The last decade witnessed a number of genome-wide studies on human pre-implantation, which mostly focused on genes and provided only limited information on repeats, excluding the satellites. Considering the fact that repeats constitute a large portion of our genome with reported links to human physiology and disease, a thorough understanding of their spatiotemporal regulation during human embryogenesis will give invaluable clues on chromatin dynamics across time and space. Therefore, we performed a detailed expression analysis of all repetitive DNA elements including the satellites across stages of human pre-implantation and embryonic stem cells.

RESULTS

We uncovered stage-specific expressions of more than a thousand repeat elements whose expressions fluctuated with a mild global decrease at the blastocyst stage. Most satellites were highly expressed at the 4-cell level and expressions of ACRO1 and D20S16 specifically peaked at this point. Whereas all members of the SVA elements were highly upregulated at 8-cell and morula stages, other transposons and small RNA repeats exhibited a high level of variation among their specific subtypes. Our repeat enrichment analysis in gene promoters coupled with expression correlations highlighted potential links between repeat expressions and nearby genes, emphasising mostly 8-cell and morula specific genes together with SVA_D, LTR5_Hs and LTR70 transposons. The DNA methylation analysis further complemented the understanding on the mechanistic aspects of the repeatome's regulation per se and revealed critical stages where DNA methylation levels are negatively correlating with repeat expression.

CONCLUSIONS

Taken together, our study shows that specific expression patterns are not exclusive to genes and long non-coding RNAs but the repeatome also exhibits an intriguingly dynamic pattern at the global scale. Repeats identified in this study; particularly satellites, which were historically associated with heterochromatin, and those with potential links to nearby gene expression provide valuable insights into the understanding of key events in genomic regulation and warrant further research in epigenetics, genomics and developmental biology.