CS proteins and ubiquitination: orchestrating DNA repair with transcription and cell division.

To face genotoxic stress, eukaryotic cells evolved extremely refined mechanisms. Defects in counteracting the threat imposed by DNA damage underlie the rare disease Cockayne syndrome (CS), which arises from mutations in the CSA and CSB genes. Although initially defined as DNA repair proteins, recent work shows that CSA and CSB act instead as master regulators of the integrated response to genomic stress by coordinating DNA repair with transcription and cell division. CSA and CSB exert this function through the ubiquitination of target proteins, which are effectors/regulators of these processes. This review describes how the ubiquitination of target substrates is a common denominator by which CSA and CSB participate in different aspects of cellular life and how their mutation gives rise to the complex disease CS.

Active mRNA degradation by EXD2 nuclease elicits recovery of transcription after genotoxic stress.

The transcriptional response to genotoxic stress involves gene expression arrest, followed by recovery of mRNA synthesis (RRS) after DNA repair. We find that the lack of the EXD2 nuclease impairs RRS and decreases cell survival after UV irradiation, without affecting DNA repair. Overexpression of wild-type, but not nuclease-dead EXD2, restores RRS and cell survival. We observe that UV irradiation triggers the relocation of EXD2 from mitochondria to the nucleus. There, EXD2 is recruited to chromatin where it transiently interacts with RNA Polymerase II (RNAPII) to promote the degradation of nascent mRNAs synthesized at the time of genotoxic attack. Reconstitution of the EXD2-RNAPII partnership on a transcribed DNA template in vitro shows that EXD2 primarily interacts with an elongation-blocked RNAPII and efficiently digests mRNA. Overall, our data highlight a crucial step in the transcriptional response to genotoxic attack in which EXD2 interacts with elongation-stalled RNAPII on chromatin to potentially degrade the associated nascent mRNA, allowing transcription restart after DNA repair.

XPF activates break-induced telomere synthesis.

Alternative Lengthening of Telomeres (ALT) utilizes a recombination mechanism and break-induced DNA synthesis to maintain telomere length without telomerase, but it is unclear how cells initiate ALT. TERRA, telomeric repeat-containing RNA, forms RNA:DNA hybrids (R-loops) at ALT telomeres. We show that depleting TERRA using an RNA-targeting Cas9 system reduces ALT-associated PML bodies, telomere clustering, and telomere lengthening. TERRA interactome reveals that TERRA interacts with an extensive subset of DNA repair proteins in ALT cells. One of TERRA interacting proteins, the endonuclease XPF, is highly enriched at ALT telomeres and recruited by telomeric R-loops to induce DNA damage response (DDR) independent of CSB and SLX4, and thus triggers break-induced telomere synthesis and lengthening. The attraction of BRCA1 and RAD51 at telomeres requires XPF in FANCM-deficient cells that accumulate telomeric R-loops. Our results suggest that telomeric R-loops activate DDR via XPF to promote homologous recombination and telomere replication to drive ALT.

Phosphorylation of XPD drives its mitotic role independently of its DNA repair and transcription functions.

The helicase XPD is known as a key subunit of the DNA repair/transcription factor TFIIH. However, here, we report that XPD, independently to other TFIIH subunits, can localize with the motor kinesin Eg5 to mitotic spindles and the midbodies of human cells. The XPD/Eg5 partnership is promoted upon phosphorylation of Eg5/T926 by the kinase CDK1, and conversely, it is reduced once Eg5/S1033 is phosphorylated by NEK6, a mitotic kinase that also targets XPD at T425. The phosphorylation of XPD does not affect its DNA repair and transcription functions, but it is required for Eg5 localization, checkpoint activation, and chromosome segregation in mitosis. In XPD-mutated cells derived from a patient with xeroderma pigmentosum, the phosphomimetic form XPD/T425D or even the nonphosphorylatable form Eg5/S1033A specifically restores mitotic chromosome segregation errors. These results thus highlight the phospho-dependent mitotic function of XPD and reveal how mitotic defects might contribute to XPD-related disorders.

New hallmarks of ageing: a 2022 Copenhagen ageing meeting summary.

Genomic instability, telomere attrition, epigenetic alterations, mitochondrial dysfunction, loss of proteostasis, deregulated nutrient-sensing, cellular senescence, stem cell exhaustion, and altered intercellular communication were the original nine hallmarks of ageing proposed by López-Otín and colleagues in 2013. The proposal of these hallmarks of ageing has been instrumental in guiding and pushing forward research on the biology of ageing. In the nearly past 10 years, our in-depth exploration on ageing research has enabled us to formulate new hallmarks of ageing which are compromised autophagy, microbiome disturbance, altered mechanical properties, splicing dysregulation, and inflammation, among other emerging ones. Amalgamation of the 'old' and 'new' hallmarks of ageing may provide a more comprehensive explanation of ageing and age-related diseases, shedding light on interventional and therapeutic studies to achieve healthy, happy, and productive lives in the elderly.

Promoters of ASCL1- and NEUROD1-dependent genes are specific targets of lurbinectedin in SCLC cells.

Small-Cell Lung Cancer (SCLC) is an aggressive neuroendocrine malignancy with a poor prognosis. Here, we focus on the neuroendocrine SCLC subtypes, SCLC-A and SCLC-N, whose transcription addiction was driven by ASCL1 and NEUROD1 transcription factors which target E-box motifs to activate up to 40% of total genes, the promoters of which are maintained in a steadily open chromatin environment according to ATAC and H3K27Ac signatures. This leverage is used by the marine agent lurbinectedin, which preferentially targets the CpG islands located downstream of the transcription start site, thus arresting elongating RNAPII and promoting its degradation. This abrogates the expression of ASCL1 and NEUROD1 and of their dependent genes, such as BCL2, INSM1, MYC, and AURKA, which are responsible for relevant SCLC tumorigenic properties such as inhibition of apoptosis and cell survival, as well as for a part of its neuroendocrine features. In summary, we show how the transcription addiction of these cells becomes their Achilles's heel, and how this is effectively exploited by lurbinectedin as a novel SCLC therapeutic endeavor.

CDK7 and MITF repress a transcription program involved in survival and drug tolerance in melanoma.

Melanoma cell phenotype switching between differentiated melanocytic and undifferentiated mesenchymal-like states drives metastasis and drug resistance. CDK7 is the serine/threonine kinase of the basal transcription factor TFIIH. We show that dedifferentiation of melanocytic-type melanoma cells into mesenchymal-like cells and acquisition of tolerance to targeted therapies is achieved through chronic inhibition of CDK7. In addition to emergence of a mesenchymal-type signature, we identify a GATA6-dependent gene expression program comprising genes such as AMIGO2 or ABCG2 involved in melanoma survival or targeted drug tolerance, respectively. Mechanistically, we show that CDK7 drives expression of the melanocyte lineage transcription factor MITF that in turn binds to an intronic region of GATA6 to repress its expression in melanocytic-type cells. We show that GATA6 expression is activated in MITF-low melanoma cells of patient-derived xenografts. Taken together, our data show how the poorly characterized repressive function of MITF in melanoma participates in a molecular cascade regulating activation of a transcriptional program involved in survival and drug resistance in melanoma.

The Long Road to Understanding RNAPII Transcription Initiation and Related Syndromes.

In eukaryotes, transcription of protein-coding genes requires the assembly at core promoters of a large preinitiation machinery containing RNA polymerase II (RNAPII) and general transcription factors (GTFs). Transcription is potentiated by regulatory elements called enhancers, which are recognized by specific DNA-binding transcription factors that recruit cofactors and convey, following chromatin remodeling, the activating cues to the preinitiation complex. This review summarizes nearly five decades of work on transcription initiation by describing the sequential recruitment of diverse molecular players including the GTFs, the Mediator complex, and DNA repair factors that support RNAPII to enable RNA synthesis. The elucidation of the transcription initiation mechanism has greatly benefited from the study of altered transcription components associated with human diseases that could be considered transcription syndromes.

The Cockayne syndrome group A and B proteins are part of a ubiquitin-proteasome degradation complex regulating cell division.

Cytokinesis is monitored by a molecular machinery that promotes the degradation of the intercellular bridge, a transient protein structure connecting the two daughter cells. Here, we found that CSA and CSB, primarily defined as DNA repair factors, are located at the midbody, a transient structure in the middle of the intercellular bridge, where they recruit CUL4 and MDM2 ubiquitin ligases and the proteasome. As a part of this molecular machinery, CSA and CSB contribute to the ubiquitination and the degradation of proteins such as PRC1, the Protein Regulator of Cytokinesis, to ensure the correct separation of the two daughter cells. Defects in CSA or CSB result in perturbation of the abscission leading to the formation of long intercellular bridges and multinucleated cells, which might explain part of the Cockayne syndrome phenotypes. Our results enlighten the role played by CSA and CSB as part of a ubiquitin/proteasome degradation process involved in transcription, DNA repair, and cell division.

In TFIIH the Arch domain of XPD is mechanistically essential for transcription and DNA repair.

The XPD helicase is a central component of the general transcription factor TFIIH which plays major roles in transcription and nucleotide excision repair (NER). Here we present the high-resolution crystal structure of the Arch domain of XPD with its interaction partner MAT1, a central component of the CDK activating kinase complex. The analysis of the interface led to the identification of amino acid residues that are crucial for the MAT1-XPD interaction. More importantly, mutagenesis of the Arch domain revealed that these residues are essential for the regulation of (i) NER activity by either impairing XPD helicase activity or the interaction of XPD with XPG; (ii) the phosphorylation of the RNA polymerase II and RNA synthesis. Our results reveal how MAT1 shields these functionally important residues thereby providing insights into how XPD is regulated by MAT1 and defining the Arch domain as a major mechanistic player within the XPD scaffold.

Dysregulation of LXR responsive genes contribute to ichthyosis in trichothiodystrophy.

BACKGROUND: Trichothiodystrophy (TTD) is a rare autosomal recessive disorder characterised by brittle hairs and various systemic symptoms, including photosensitivity and ichthyosis. While photosensitivity could result from DNA repair defects, other TTD clinical features might be due to deficiencies in certain molecular processes. OBJECTIVES: The aim of this study was to understand the pathophysiological mechanism of ichthyosis in TTD, focused on the transcriptional dysregulation. METHODS: TTD mouse skin tissue and keratinocytes were pathologically and physiologically examined to identify the alteration of lipid homeostasis in TTD with ichtyosis. Gene expression of certain lipid transporter was assessed in fibroblasts derived from TTD patients and TTD mouse keratinocytes. RESULTS: Histopathology and electron microscopy revealed abnormal lipid composition in TTD mice skin. In addition to abnormal cholesterol dynamics, TTD mouse keratinocytes exhibit impaired expression of Liver X receptor (LXR) responsive genes, including Abca12, a key regulator of Harlequin ichthyosis, and Abcg1 that is involved in the cholesterol transport process in the epidermis. Strikingly, dysregulation of LXR responsive genes has been only observed in cells isolated from TTD patients who developed ichthyosis. CONCLUSIONS: Our results suggest that the altered expression of the LXR-responsive genes contribute to the pathophysiology of ichthyosis in TTD. These findings provide a new drug discovery target for TTD.

Defective transcription of ATF3 responsive genes, a marker for Cockayne Syndrome.

Cockayne syndrome (CS) is a rare genetic disorder caused by mutations (dysfunction) in CSA and CSB. CS patients exhibit mild photosensitivity and severe neurological problems. Currently, CS diagnosis is based on the inefficiency of CS cells to recover RNA synthesis upon genotoxic (UV) stress. Indeed, upon genotoxic stress, ATF3, an immediate early gene is activated to repress up to 5000 genes encompassing its responsive element for a short period of time. On the contrary in CS cells, CSA and CSB dysfunction impairs the degradation of the chromatin-bound ATF3, leading to a permanent transcriptional arrest as observed by immunofluorescence and ChIP followed by RT-PCR. We analysed ChIP-seq of Pol II and ATF3 promoter occupation analysis and RNA sequencing-based gene expression profiling in CS cells, as well as performed immunofluorescence study of ATF3 protein stability and quantitative RT-PCR screening in 64 patient cell lines. We show that the analysis of few amount (as for example CDK5RAP2, NIPBL and NRG1) of ATF3 dependent genes, could serve as prominent molecular markers to discriminate between CS and non-CS patient's cells. Such assay can significantly simplify the timing and the complexity of the CS diagnostic procedure in comparison to the currently available methods.

Correction: Variants in MED12L, encoding a subunit of the Mediator kinase module, are responsible for intellectual disability associated with transcriptional defect.

In the Acknowledgements section of the paper the authors neglected to mention that the study was supported by a grant from the National Human Genome Research Institute (NHGRI) UM1HG007301 (S.H., M.L.T.). In addition, the award of MD was associated with the authors Michelle L. Thompson and Susan Hiatt instead of PhD. The PDF and HTML versions of the Article have been modified accordingly.

Variants in MED12L, encoding a subunit of the mediator kinase module, are responsible for intellectual disability associated with transcriptional defect.

PURPOSE: Mediator is a multiprotein complex that allows the transfer of genetic information from DNA binding proteins to the RNA polymerase II during transcription initiation. MED12L is a subunit of the kinase module, which is one of the four subcomplexes of the mediator complex. Other subunits of the kinase module have been already implicated in intellectual disability, namely MED12, MED13L, MED13, and CDK19. METHODS: We describe an international cohort of seven affected individuals harboring variants involving MED12L identified by array CGH, exome or genome sequencing. RESULTS: All affected individuals presented with intellectual disability and/or developmental delay, including speech impairment. Other features included autism spectrum disorder, aggressive behavior, corpus callosum abnormality, and mild facial morphological features. Three individuals had a MED12L deletion or duplication. The other four individuals harbored single-nucleotide variants (one nonsense, one frameshift, and two splicing variants). Functional analysis confirmed a moderate and significant alteration of RNA synthesis in two individuals. CONCLUSION: Overall data suggest that MED12L haploinsufficiency is responsible for intellectual disability and transcriptional defect. Our findings confirm that the integrity of this kinase module is a critical factor for neurological development.

TFIIE orchestrates the recruitment of the TFIIH kinase module at promoter before release during transcription.

In eukaryotes, the general transcription factors TFIIE and TFIIH assemble at the transcription start site with RNA Polymerase II. However, the mechanism by which these transcription factors incorporate the preinitiation complex and coordinate their action during RNA polymerase II transcription remains elusive. Here we show that the TFIIEα and TFIIEß subunits anchor the TFIIH kinase module (CAK) within the preinitiation complex. In addition, we show that while RNA polymerase II phosphorylation and DNA opening occur, CAK and TFIIEα are released from the promoter. This dissociation is impeded by either ATP-γS or CDK7 inhibitor THZ1, but still occurs when XPB activity is abrogated. Finally, we show that the Core-TFIIH and TFIIEß are subsequently removed, while elongation factors such as DSIF are recruited. Remarkably, these early transcriptional events are affected by TFIIE and TFIIH mutations associated with the developmental disorder, trichothiodystrophy.

Functional interplay between TFIIH and KAT2A regulates higher-order chromatin structure and class II gene expression.

The TFIIH subunit XPB is involved in combined Xeroderma Pigmentosum and Cockayne syndrome (XP-B/CS). Our analyses reveal that XPB interacts functionally with KAT2A, a histone acetyltransferase (HAT) that belongs to the hSAGA and hATAC complexes. XPB interacts with KAT2A-containing complexes on chromatin and an XP-B/CS mutation specifically elicits KAT2A-mediated large-scale chromatin decondensation. In XP-B/CS cells, the abnormal recruitment of TFIIH and KAT2A to chromatin causes inappropriate acetylation of histone H3K9, leading to aberrant formation of transcription initiation complexes on the promoters of several hundred genes and their subsequent overexpression. Significantly, this cascade of events is similarly sensitive to KAT2A HAT inhibition or to the rescue with wild-type XPB. In agreement, the XP-B/CS mutation increases KAT2A HAT activity in vitro. Our results unveil a tight connection between TFIIH and KAT2A that controls higher-order chromatin structure and gene expression and provide new insights into transcriptional misregulation in a cancer-prone DNA repair-deficient disorder.

C1D is not directly involved in the repair of UV-damaged DNA but protects cells from oxidative stress by regulating gene expressions in human cell lines.

A small nuclear protein, C1D, has roles in various cellular processes, transcription regulation, genome stability surveillance, DNA repair and RNA processing, all of which are required to maintain the host life cycles. In the previous report, C1D directly interacts with XPB, a component of the nucleotide excision repair complex, and C1D knockdown reduced cell survival of 27-1 cells, CHO derivative cells, after UV irradiation. To find out the role of C1D in UV-damaged cells, we used human cell lines with siRNA or shRNA to knockdown C1D. C1D knockdown reduced cell survival rates of LU99 and 786-O after UV irradiation, although C1D knockdown did not affect the efficiency of the nucleotide excision repair. Immunostaining data support that C1D is not directly involved in the DNA repair process in UV-damaged cells. However, H2O2 treatment reduced cell viability in LU99 and 786-O cells. We also found that C1D knockdown upregulated DDIT3 expression in LU99 cells and downregulated APEX1 in 786-O cells, suggesting that C1D functions as a co-repressor/activator. The data accounts for the reduction of cell survival rates upon UV irradiation.

Cockayne's Syndrome A and B Proteins Regulate Transcription Arrest after Genotoxic Stress by Promoting ATF3 Degradation.

Cockayne syndrome (CS) is caused by mutations in CSA and CSB. The CSA and CSB proteins have been linked to both promoting transcription-coupled repair and restoring transcription following DNA damage. We show that UV stress arrests transcription of approximately 70% of genes in CSA- or CSB-deficient cells due to the constitutive presence of ATF3 at CRE/ATF sites. We found that CSB, CSA/DDB1/CUL4A, and MDM2 were essential for ATF3 ubiquitination and degradation by the proteasome. ATF3 removal was concomitant with the recruitment of RNA polymerase II and the restart of transcription. Preventing ATF3 ubiquitination by mutating target lysines prevented recovery of transcription and increased cell death following UV treatment. Our data suggest that the coordinate action of CSA and CSB, as part of the ubiquitin/proteasome machinery, regulates the recruitment timing of DNA-binding factors and provide explanations about the mechanism of transcription arrest following genotoxic stress.

Comparative performance analysis of human iPSC-derived and primary neural progenitor cells (NPC) grown as neurospheres in vitro.

Developmental neurotoxicity (DNT) testing performed in rats is resource-intensive (costs, time, animals) and bears the issue of species extrapolation. Thus, reliable alternative human-based approaches are needed for predicting neurodevelopmental toxicity. Human induced pluripotent stem cells (hiPSCs) represent a basis for an alternative method possibly being part of an alternative DNT testing strategy. Here, we compared two hiPSC neural induction protocols resulting in 3D neurospheres: one using noggin and one cultivating cells in neural induction medium (NIM protocol). Performance of Nestin+/SOX2+ hiPSC-derived neural progenitor cells (NPCs) was compared to primary human NPCs. Generally, primary hNPCs first differentiate into Nestin+ and/or GFAP+ radial glia-like cells, while the hiPSC-derived NPCs (hiPSC-NPC) first differentiate into ßIII-Tubulin+ neurons suggesting an earlier developmental stage of hiPSC-NPC. In the 'Neurosphere Assay', NIM generated hiPSC-NPC produced neurons with higher performance than with the noggin protocol. After long-term differentiation, hiPSC-NPC form neuronal networks, which become electrically active on microelectrode arrays after 85days. Finally, methylmercury chloride inhibits hiPSC-NPC and hNPC migration with similar potencies. hiPSC-NPCs-derived neurospheres seem to be useful for DNT evaluation representing early neural development in vitro. More system characterization by compound testing is needed to gain higher confidence in this method.

MED12-related XLID disorders are dose-dependent of immediate early genes (IEGs) expression.

Mediator occupies a key role in protein coding genes expression in mediating the contacts between gene specific factors and the basal transcription machinery but little is known regarding the role of each Mediator subunits. Mutations in MED12 are linked with a broad spectrum of genetic disorders with X-linked intellectual disability that are difficult to range as Lujan, Opitz-Kaveggia or Ohdo syndromes. Here, we investigated several MED12 patients mutations (p.R206Q, p.N898D, p.R961W, p.N1007S, p.R1148H, p.S1165P and p.R1295H) and show that each MED12 mutations cause specific expression patterns of JUN, FOS and EGR1 immediate early genes (IEGs), reflected by the presence or absence of MED12 containing complex at their respective promoters. Moreover, the effect of MED12 mutations has cell-type specificity on IEG expression. As a consequence, the expression of late responsive genes such as the matrix metalloproteinase-3 and the RE1 silencing transcription factor implicated respectively in neural plasticity and the specific expression of neuronal genes is disturbed as documented for MED12/p.R1295H mutation. In such case, JUN and FOS failed to be properly recruited at their AP1-binding site. Our results suggest that the differences between MED12-related phenotypes are essentially the result of distinct IEGs expression patterns, the later ones depending on the accurate formation of the transcription initiation complex. This might challenge clinicians to rethink the traditional syndromes boundaries and to include genetic criterion in patients' diagnostic.