Author Correction: L1 drives IFN in senescent cells and promotes age-associated inflammation.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

L1 drives IFN in senescent cells and promotes age-associated inflammation.

Retrotransposable elements are deleterious at many levels, and the failure of host surveillance systems for these elements can thus have negative consequences. However, the contribution of retrotransposon activity to ageing and age-associated diseases is not known. Here we show that during cellular senescence, L1 (also known as LINE-1) retrotransposable elements become transcriptionally derepressed and activate a type-I interferon (IFN-I) response. The IFN-I response is a phenotype of late senescence and contributes to the maintenance of the senescence-associated secretory phenotype. The IFN-I response is triggered by cytoplasmic L1 cDNA, and is antagonized by inhibitors of the L1 reverse transcriptase. Treatment of aged mice with the nucleoside reverse transcriptase inhibitor lamivudine downregulated IFN-I activation and age-associated inflammation (inflammaging) in several tissues. We propose that the activation of retrotransposons is an important component of sterile inflammation that is a hallmark of ageing, and that L1 reverse transcriptase is a relevant target for the treatment of age-associated disorders.

The Chromatin Landscape of Cellular Senescence.

Cellular senescence, an irreversible growth arrest triggered by a variety of stressors, plays important roles in normal physiology and tumor suppression, but accumulation of senescent cells with age contributes to the functional decline of tissues. Senescent cells undergo dramatic alterations to their chromatin landscape that affect genome accessibility and their transcriptional program. These include the loss of DNA-nuclear lamina interactions, the distension of centromeres, and changes in chromatin composition that can lead to the activation of retrotransposons. Here we discuss these findings, as well as recent advances in microscopy and genomics that have revealed the importance of the higher-order spatial organization of the genome in defining and maintaining the senescent state.

Genome-wide characterization of human L1 antisense promoter-driven transcripts.

BACKGROUND: Long INterspersed Element-1 (LINE-1 or L1) is the only autonomously active, transposable element in the human genome. L1 sequences comprise approximately 17 % of the human genome, but only the evolutionarily recent, human-specific subfamily is retrotransposition competent. The L1 promoter has a bidirectional orientation containing a sense promoter that drives the transcription of two proteins required for retrotransposition and an antisense promoter. The L1 antisense promoter can drive transcription of chimeric transcripts: 5' L1 antisense sequences spliced to the exons of neighboring genes. RESULTS: The impact of L1 antisense promoter activity on cellular transcriptomes is poorly understood. To investigate this, we analyzed GenBank ESTs for messenger RNAs that initiate in the L1 antisense promoter. We identified 988 putative L1 antisense chimeric transcripts, 911 of which have not been previously reported. These appear to be alternative genic transcripts, sense-oriented with respect to gene and initiating near, but typically downstream of, the gene transcriptional start site. In multiple cell lines, L1 antisense promoters display enrichment for YY1 transcription factor and histone modifications associated with active promoters. Global run-on sequencing data support the activity of the L1 antisense promoter. We independently detected 124 L1 antisense chimeric transcripts using long read Pacific Biosciences RNA-seq data. Furthermore, we validated four chimeric transcripts by quantitative RT-PCR and Sanger sequencing and demonstrated that they are readily detectable in many normal human tissues. CONCLUSIONS: We present a comprehensive characterization of human L1 antisense promoter-driven transcripts and provide substantial evidence that they are transcribed in a variety of human cell-types. Our findings reveal a new wide-reaching aspect of L1 biology by identifying antisense transcripts affecting as many as 4 % of all human genes.

Death by transposition - the enemy within?

Here we present and develop the hypothesis that the derepression of endogenous retrotransposable elements (RTEs) - "genomic parasites" - is an important and hitherto under-unexplored molecular aging process that can potentially occur in most tissues. We further envision that the activation and continued presence of retrotransposition contribute to age-associated tissue degeneration and pathology. Chromatin is a complex and dynamic structure that needs to be maintained in a functional state throughout our lifetime. Studies of diverse species have revealed that chromatin undergoes extensive rearrangements during aging. Cellular senescence, an important component of mammalian aging, has recently been associated with decreased heterochromatinization of normally silenced regions of the genome. These changes lead to the expression of RTEs, culminating in their transposition. RTEs are common in all kingdoms of life, and comprise close to 50% of mammalian genomes. They are tightly controlled, as their activity is highly destabilizing and mutagenic to their resident genomes.

Transcriptional landscape of repetitive elements in normal and cancer human cells.

BACKGROUND: Repetitive elements comprise at least 55% of the human genome with more recent estimates as high as two-thirds. Most of these elements are retrotransposons, DNA sequences that can insert copies of themselves into new genomic locations by a "copy and paste" mechanism. These mobile genetic elements play important roles in shaping genomes during evolution, and have been implicated in the etiology of many human diseases. Despite their abundance and diversity, few studies investigated the regulation of endogenous retrotransposons at the genome-wide scale, primarily because of the technical difficulties of uniquely mapping high-throughput sequencing reads to repetitive DNA. RESULTS: Here we develop a new computational method called RepEnrich to study genome-wide transcriptional regulation of repetitive elements. We show that many of the Long Terminal Repeat retrotransposons in humans are transcriptionally active in a cell line-specific manner. Cancer cell lines display increased RNA Polymerase II binding to retrotransposons than cell lines derived from normal tissue. Consistent with increased transcriptional activity of retrotransposons in cancer cells we found significantly higher levels of L1 retrotransposon RNA expression in prostate tumors compared to normal-matched controls. CONCLUSIONS: Our results support increased transcription of retrotransposons in transformed cells, which may explain the somatic retrotransposition events recently reported in several types of cancers.

BET inhibition targets ABC-DLBCL constitutive B-cell receptor signaling through PAX5.

B-cell receptor (BCR) signaling is essential for the diffuse large B-cell lymphoma (DLBCL) subtype that originates from activated B-cells (ABCs). ABC-DLBCL cells are sensitive to Bruton tyrosine kinase intervention. However, patients with relapsed or refractory ABC-DLBCL had overall response rates from 33% to 37% for Bruton tyrosine kinase inhibitors, suggesting the evaluation of combination-based treatment for improved efficacy. We investigated the efficacy and mechanism of the bromodomain and extraterminal motif (BET) inhibitor AZD5153 combined with the Bruton tyrosine kinase inhibitor acalabrutinib in ABC-DLBCL preclinical models. AZD5153 is a bivalent BET inhibitor that simultaneously engages the 2 bromodomains of BRD4. Adding AZD5153 to acalabrutinib demonstrated combination benefits in ABC-DLBCL cell line and patient-derived xenograft models. Differential expression analyses revealed PAX5 transcriptional activity as a novel downstream effector of this drug combination. PAX5 is a transcription factor for BCR signaling genes and may be critical for perpetually active BCR signaling in ABC-DLBCL. Our analyses further indicated significant alterations in BCR, RELB/alternative NF-κB, and toll-like receptor/interferon signaling. Validation of these results mapped a positive-feedback signaling loop regulated by PAX5. We demonstrated that AZD5153 decreased PAX5 expression, whereas acalabrutinib disruption of BCR signaling inhibited PAX5 activation. Furthermore, several interferon levels were decreased by AZD5153 and acalabrutinib in tumors. Adding interferon-beta1 (IFNß1) to cells treated with acalabrutinib partially rescued PAX5 activation. Our results demonstrate that AZD5153 enhances the efficacy of acalabrutinib through PAX5 and BCR mechanisms that are critical for ABC-DLBCL.

Homologous Recombination Repair Gene Mutations to Predict Olaparib Plus Bevacizumab Efficacy in the First-Line Ovarian Cancer PAOLA-1/ENGOT-ov25 Trial.

PURPOSE: The PAOLA-1/ENGOT-ov25 trial of maintenance olaparib plus bevacizumab for newly diagnosed advanced high-grade ovarian cancer demonstrated a significant progression-free survival (PFS) benefit over placebo plus bevacizumab, particularly in patients with homologous recombination deficiency (HRD)-positive tumors. We explored whether mutations in non-BRCA1 or BRCA2 homologous recombination repair (non-BRCA HRRm) genes predicted benefit from olaparib plus bevacizumab in PAOLA-1. METHODS: Eight hundred and six patients were randomly assigned (2:1). Tumors were analyzed using the Myriad MyChoice HRD Plus assay to assess non-BRCA HRRm and HRD status; HRD was based on a genomic instability score (GIS) of ≥ 42. In this exploratory analysis, PFS was assessed in patients harboring deleterious mutations using six non-BRCA HRR gene panels, three devised for this analysis and three previously published. RESULTS: The non-BRCA HRRm prevalence ranged from 30 of 806 (3.7%) to 79 of 806 (9.8%) depending on the gene panel used, whereas 152 of 806 (18.9%) had non-BRCA1 or BRCA2 mutation HRD-positive tumors. The majority of tumors harboring non-BRCA HRRm had a low median GIS; however, a GIS of > 42 was observed for tumors with mutations in five HRR genes (BLM, BRIP1, RAD51C, PALB2, and RAD51D). Rates of gene-specific biallelic loss were variable (0% to 100%) in non-BRCA HRRm tumors relative to BRCA1-mutated (99%) or BRCA2-mutated (86%) tumors. Across all gene panels tested, hazard ratios for PFS (95% CI) ranged from 0.92 (0.51 to 1.73) to 1.83 (0.76 to 5.43). CONCLUSION: Acknowledging limitations of small subgroup sizes, non-BRCA HRRm gene panels were not predictive of PFS benefit with maintenance olaparib plus bevacizumab versus placebo plus bevacizumab in PAOLA-1, irrespective of the gene panel tested. Current gene panels exploring HRRm should not be considered a substitute for HRD determined by BRCA mutation status and genomic instability testing in first-line high-grade ovarian cancer.

The landscape of therapeutic vulnerabilities in EGFR inhibitor osimertinib drug tolerant persister cells.

Third-generation EGFR tyrosine kinase inhibitors (EGFR-TKIs), including osimertinib, an irreversible EGFR-TKI, are important treatments for non-small cell lung cancer with EGFR-TKI sensitizing or EGFR T790M resistance mutations. While patients treated with osimertinib show clinical benefit, disease progression and drug resistance are common. Emergence of de novo acquired resistance from a drug tolerant persister (DTP) cell population is one mechanism proposed to explain progression on osimertinib and other targeted cancer therapies. Here we profiled osimertinib DTPs using RNA-seq and ATAC-seq to characterize the features of these cells and performed drug screens to identify therapeutic vulnerabilities. We identified several vulnerabilities in osimertinib DTPs that were common across models, including sensitivity to MEK, AURKB, BRD4, and TEAD inhibition. We linked several of these vulnerabilities to gene regulatory changes, for example, TEAD vulnerability was consistent with evidence of Hippo pathway turning off in osimertinib DTPs. Last, we used genetic approaches using siRNA knockdown or CRISPR knockout to validate AURKB, BRD4, and TEAD as the direct targets responsible for the vulnerabilities observed in the drug screen.

AZD4573 Is a Highly Selective CDK9 Inhibitor That Suppresses MCL-1 and Induces Apoptosis in Hematologic Cancer Cells.

PURPOSE: Cyclin-dependent kinase 9 (CDK9) is a transcriptional regulator and potential therapeutic target for many cancers. Multiple nonselective CDK9 inhibitors have progressed clinically but were limited by a narrow therapeutic window. This work describes a novel, potent, and highly selective CDK9 inhibitor, AZD4573. EXPERIMENTAL DESIGN: The antitumor activity of AZD4573 was determined across broad cancer cell line panels in vitro as well as cell line- and patient-derived xenograft models in vivo. Multiple approaches, including integrated transcriptomic and proteomic analyses, loss-of-function pathway interrogation, and pharmacologic comparisons, were employed to further understand the major mechanism driving AZD4573 activity and to establish an exposure/effect relationship. RESULTS: AZD4573 is a highly selective and potent CDK9 inhibitor. It demonstrated rapid induction of apoptosis and subsequent cell death broadly across hematologic cancer models in vitro, and MCL-1 depletion in a dose- and time-dependent manner was identified as a major mechanism through which AZD4573 induces cell death in tumor cells. This pharmacodynamic (PD) response was also observed in vivo, which led to regressions in both subcutaneous tumor xenografts and disseminated models at tolerated doses both as monotherapy or in combination with venetoclax. This understanding of the mechanism, exposure, and antitumor activity of AZD4573 facilitated development of a robust pharmacokinetic/PD/efficacy model used to inform the clinical trial design. CONCLUSIONS: Selective targeting of CDK9 enables the indirect inhibition of MCL-1, providing a therapeutic option for MCL-1-dependent diseases. Accordingly, AZD4573 is currently being evaluated in a phase I clinical trial for patients with hematologic malignancies (clinicaltrials.gov identifier: NCT03263637).See related commentary by Alcon et al., p. 761.

AZD4320, A Dual Inhibitor of Bcl-2 and Bcl-xL, Induces Tumor Regression in Hematologic Cancer Models without Dose-limiting Thrombocytopenia.

PURPOSE: Targeting Bcl-2 family members upregulated in multiple cancers has emerged as an important area of cancer therapeutics. While venetoclax, a Bcl-2-selective inhibitor, has had success in the clinic, another family member, Bcl-xL, has also emerged as an important target and as a mechanism of resistance. Therefore, we developed a dual Bcl-2/Bcl-xL inhibitor that broadens the therapeutic activity while minimizing Bcl-xL-mediated thrombocytopenia. EXPERIMENTAL DESIGN: We used structure-based chemistry to design a small-molecule inhibitor of Bcl-2 and Bcl-xL and assessed the activity against in vitro cell lines, patient samples, and in vivo models. We applied pharmacokinetic/pharmacodynamic (PK/PD) modeling to integrate our understanding of on-target activity of the dual inhibitor in tumors and platelets across dose levels and over time. RESULTS: We discovered AZD4320, which has nanomolar affinity for Bcl-2 and Bcl-xL, and mechanistically drives cell death through the mitochondrial apoptotic pathway. AZD4320 demonstrates activity in both Bcl-2- and Bcl-xL-dependent hematologic cancer cell lines and enhanced activity in acute myeloid leukemia (AML) patient samples compared with the Bcl-2-selective agent venetoclax. A single intravenous bolus dose of AZD4320 induces tumor regression with transient thrombocytopenia, which recovers in less than a week, suggesting a clinical weekly schedule would enable targeting of Bcl-2/Bcl-xL-dependent tumors without incurring dose-limiting thrombocytopenia. AZD4320 demonstrates monotherapy activity in patient-derived AML and venetoclax-resistant xenograft models. CONCLUSIONS: AZD4320 is a potent molecule with manageable thrombocytopenia risk to explore the utility of a dual Bcl-2/Bcl-xL inhibitor across a broad range of tumor types with dysregulation of Bcl-2 prosurvival proteins.

Reorganization of chromosome architecture in replicative cellular senescence.

Replicative cellular senescence is a fundamental biological process characterized by an irreversible arrest of proliferation. Senescent cells accumulate a variety of epigenetic changes, but the three-dimensional (3D) organization of their chromatin is not known. We applied a combination of whole-genome chromosome conformation capture (Hi-C), fluorescence in situ hybridization, and in silico modeling methods to characterize the 3D architecture of interphase chromosomes in proliferating, quiescent, and senescent cells. Although the overall organization of the chromatin into active (A) and repressive (B) compartments and topologically associated domains (TADs) is conserved between the three conditions, a subset of TADs switches between compartments. On a global level, the Hi-C interaction matrices of senescent cells are characterized by a relative loss of long-range and gain of short-range interactions within chromosomes. Direct measurements of distances between genetic loci, chromosome volumes, and chromatin accessibility suggest that the Hi-C interaction changes are caused by a significant reduction of the volumes occupied by individual chromosome arms. In contrast, centromeres oppose this overall compaction trend and increase in volume. The structural model arising from our study provides a unique high-resolution view of the complex chromosomal architecture in senescent cells.

Mapping H4K20me3 onto the chromatin landscape of senescent cells indicates a function in control of cell senescence and tumor suppression through preservation of genetic and epigenetic stability.

BACKGROUND: Histone modification H4K20me3 and its methyltransferase SUV420H2 have been implicated in suppression of tumorigenesis. The underlying mechanism is unclear, although H4K20me3 abundance increases during cellular senescence, a stable proliferation arrest and tumor suppressor process, triggered by diverse molecular cues, including activated oncogenes. Here, we investigate the function of H4K20me3 in senescence and tumor suppression. RESULTS: Using immunofluorescence and ChIP-seq we determine the distribution of H4K20me3 in proliferating and senescent human cells. Altered H4K20me3 in senescence is coupled to H4K16ac and DNA methylation changes in senescence. In senescent cells, H4K20me3 is especially enriched at DNA sequences contained within specialized domains of senescence-associated heterochromatin foci (SAHF), as well as specific families of non-genic and genic repeats. Altered H4K20me3 does not correlate strongly with changes in gene expression between proliferating and senescent cells; however, in senescent cells, but not proliferating cells, H4K20me3 enrichment at gene bodies correlates inversely with gene expression, reflecting de novo accumulation of H4K20me3 at repressed genes in senescent cells, including at genes also repressed in proliferating cells. Although elevated SUV420H2 upregulates H4K20me3, this does not accelerate senescence of primary human cells. However, elevated SUV420H2/H4K20me3 reinforces oncogene-induced senescence-associated proliferation arrest and slows tumorigenesis in vivo. CONCLUSIONS: These results corroborate a role for chromatin in underpinning the senescence phenotype but do not support a major role for H4K20me3 in initiation of senescence. Rather, we speculate that H4K20me3 plays a role in heterochromatinization and stabilization of the epigenome and genome of pre-malignant, oncogene-expressing senescent cells, thereby suppressing epigenetic and genetic instability and contributing to long-term senescence-mediated tumor suppression.

Transposable elements become active and mobile in the genomes of aging mammalian somatic tissues.

Transposable elements (TEs) were discovered by Barbara McClintock in maize and have since been found to be ubiquitous in all living organisms. Transposition is mutagenic and organisms have evolved mechanisms to repress the activity of their endogenous TEs. Transposition in somatic cells is very low, but recent evidence suggests that it may be derepressed in some cases, such as cancer development. We have found that during normal aging several families of retrotransposable elements (RTEs) start being transcribed in mouse tissues. In advanced age the expression culminates in active transposition. These processes are counteracted by calorie restriction (CR), an intervention that slows down aging. Retrotransposition is also activated in age-associated, naturally occurring cancers in the mouse. We suggest that somatic retrotransposition is a hitherto unappreciated aging process. Mobilization of RTEs is likely to be an important contributor to the progressive dysfunction of aging cells.

Genomes of replicatively senescent cells undergo global epigenetic changes leading to gene silencing and activation of transposable elements.

Replicative cellular senescence is an important tumor suppression mechanism and also contributes to aging. Progression of both cancer and aging include significant epigenetic components, but the chromatin changes that take place during cellular senescence are not known. We used formaldehyde assisted isolation of regulatory elements (FAIRE) to map genome-wide chromatin conformations. In contrast to growing cells, whose genomes are rich with features of both open and closed chromatin, FAIRE profiles of senescent cells are significantly smoothened. This is due to FAIRE signal loss in promoters and enhancers of active genes, and FAIRE signal gain in heterochromatic gene-poor regions. Chromatin of major retrotransposon classes, Alu, SVA and L1, becomes relatively more open in senescent cells, affecting most strongly the evolutionarily recent elements, and leads to an increase in their transcription and ultimately transposition. Constitutive heterochromatin in centromeric and peri-centromeric regions also becomes relatively more open, and the transcription of satellite sequences increases. The peripheral heterochromatic compartment (PHC) becomes less prominent, and centromere structure becomes notably enlarged. These epigenetic changes progress slowly after the onset of senescence, with some, such as mobilization of retrotransposable elements becoming prominent only at late times. Many of these changes have also been noted in cancer cells.