A tough row to hoe: when replication forks encounter DNA damage

DNA damage in cancer development: special implications in viral oncogenesis

DNA lesions arise from a combination of physiological/metabolic sources and exogenous environmental influences. When left unrepaired, these alterations accumulate in the cells and can give rise to mutations that change the function of important proteins (i.e. tumor suppressors, oncoproteins), or cause chromosomal rearrangements (i.e. gene fusions) that also result in the deregulation of key cellular molecules. Progressive acquisition of such genetic changes promotes uncontrolled cell proliferation and evasion of cell death, and hence plays a key role in carcinogenesis. Another less-studied consequence of DNA damage accumulating in the host genome is the integration of oncogenic DNA viruses such as Human papillomavirus, Merkel cell polyomavirus, and Hepatitis B virus. This critical step of viral-induced carcinogenesis is thought to be particularly facilitated by DNA breaks in both viral and host genomes. Therefore, the impact of DNA damage on carcinogenesis is magnified in the case of such oncoviruses via the additional effect of increasing integration frequency. In this review, we briefly present the various endogenous and exogenous factors that cause different types of DNA damage. Next, we discuss the contribution of these lesions in cancer development. Finally, we examine the amplified effect of DNA damage in viral-induced oncogenesis and summarize the limited data existing in the literature related to DNA damage-induced viral integration. To conclude, additional research is needed to assess the DNA damage pathways involved in the transition from viral infection to cancer. Discovering that a certain DNA damaging agent increases the likelihood of viral integration will enable the development of prophylactic and therapeutic strategies designed specifically to prevent such integration, with an ultimate goal of reducing or eliminating these viral-induced malignancies.

Palindromes in DNA-A Risk for Genome Stability and Implications in Cancer

A palindrome in DNA consists of two closely spaced or adjacent inverted repeats. Certain palindromes have important biological functions as parts of various cis-acting elements and protein binding sites. However, many palindromes are known as fragile sites in the genome, sites prone to chromosome breakage which can lead to various genetic rearrangements or even cell death. The ability of certain palindromes to initiate genetic recombination lies in their ability to form secondary structures in DNA which can cause replication stalling and double-strand breaks. Given their recombinogenic nature, it is not surprising that palindromes in the human genome are involved in genetic rearrangements in cancer cells as well as other known recurrent translocations and deletions associated with certain syndromes in humans. Here, we bring an overview of current understanding and knowledge on molecular mechanisms of palindrome recombinogenicity and discuss possible implications of DNA palindromes in carcinogenesis. Furthermore, we overview the data on known palindromic sequences in the human genome and efforts to estimate their number and distribution, as well as underlying mechanisms of genetic rearrangements specific palindromic sequences cause.

Structure of the FA core ubiquitin ligase closing the ID clamp on DNA

The Fanconi anemia (FA) pathway is essential for the repair of DNA interstrand crosslinks. Central to the pathway is the FA core complex, a ubiquitin ligase of nine subunits that monoubiquitinates the FANCI-FANCD2 (ID) DNA clamp. The 3.1 Å structure of the 1.1-MDa human FA core complex, described here, reveals an asymmetric assembly with two copies of all but the FANCC, FANCE and FANCF subunits. The asymmetry is crucial, as it prevents the binding of a second FANCC-FANCE-FANCF subcomplex that inhibits the recruitment of the UBE2T ubiquitin conjugating enzyme, and instead creates an ID binding site. A single active site then ubiquitinates FANCD2 and FANCI sequentially. We also present the 4.2-Å structures of the human core-UBE2T-ID-DNA complex in three conformations captured during monoubiquitination. They reveal the core-UBE2T complex remodeling the ID-DNA complex, closing the clamp on the DNA before ubiquitination. Monoubiquitination then prevents clamp opening after release from the core.

Origin of Genome Instability and Determinants of Mutational Landscape in Cancer Cells

Genome instability is a crucial and early event associated with an increased predisposition to tumor formation. In the absence of any exogenous agent, a single human cell is subjected to about 70,000 DNA lesions each day. It has now been shown that physiological cellular processes including DNA transactions during DNA replication and transcription contribute to DNA damage and induce DNA damage responses in the cell. These processes are also influenced by the three dimensional-chromatin architecture and epigenetic regulation which are altered during the malignant transformation of cells. In this review, we have discussed recent insights about how replication stress, oncogene activation, chromatin dynamics, and the illegitimate recombination of cell-free chromatin particles deregulate cellular processes in cancer cells and contribute to their evolution. The characterization of such endogenous sources of genome instability in cancer cells can be exploited for the development of new biomarkers and more effective therapies for cancer treatment.

Pif1, RPA, and FEN1 modulate the ability of DNA polymerase δ to overcome protein barriers during DNA synthesis

Successful DNA replication requires carefully regulated mechanisms to overcome numerous obstacles that naturally occur throughout chromosomal DNA. Scattered across the genome are tightly bound proteins, such as transcription factors and nucleosomes, that are necessary for cell function, but that also have the potential to impede timely DNA replication. Using biochemically reconstituted systems, we show that two transcription factors, yeast Reb1 and Tbf1, and a tightly positioned nucleosome, are strong blocks to the strand displacement DNA synthesis activity of DNA polymerase δ. Although the block imparted by Tbf1 can be overcome by the DNA-binding activity of the single-stranded DNA-binding protein RPA, efficient DNA replication through either a Reb1 or a nucleosome block occurs only in the presence of the 5'-3' DNA helicase Pif1. The Pif1-dependent stimulation of DNA synthesis across strong protein barriers may be beneficial during break-induced replication where barriers are expected to pose a problem to efficient DNA bubble migration. However, in the context of lagging strand DNA synthesis, the efficient disruption of a nucleosome barrier by Pif1 could lead to the futile re-replication of newly synthetized DNA. In the presence of FEN1 endonuclease, the major driver of nick translation during lagging strand replication, Pif1-dependent stimulation of DNA synthesis through a nucleosome or Reb1 barrier is prevented. By cleaving the short 5' tails generated during strand displacement, FEN1 eliminates the entry point for Pif1. We propose that this activity would protect the cell from potential DNA re-replication caused by unwarranted Pif1 interference during lagging strand replication.

Resolving DNA Damage: Epigenetic Regulation of DNA Repair

Epigenetic research has rapidly evolved into a dynamic field of genome biology. Chromatin regulation has been proved to be an essential aspect for all genomic processes, including DNA repair. Chromatin structure is modified by enzymes and factors that deposit, erase, and interact with epigenetic marks such as DNA and histone modifications, as well as by complexes that remodel nucleosomes. In this review we discuss recent advances on how the chromatin state is modulated during this multi-step process of damage recognition, signaling, and repair. Moreover, we examine how chromatin is regulated when different pathways of DNA repair are utilized. Furthermore, we review additional modes of regulation of DNA repair, such as through the role of global and localized chromatin states in maintaining expression of DNA repair genes, as well as through the activity of epigenetic enzymes on non-nucleosome substrates. Finally, we discuss current and future applications of the mechanistic interplays between chromatin regulation and DNA repair in the context cancer treatment.

Characterization of a head and neck cancer-derived cell line panel confirms the distinct TP53-proficient copy number-silent subclass

INTRODUCTION

Head and neck squamous cell carcinomas (HNSCC) arise in the mucosal lining of the upper aerodigestive tract. Risk factors are exogenous carcinogen exposure, human papillomavirus (HPV) infection, and genetic predisposition such as Fanconi anemia (FA). Clinically, tumors are stratified based on stage, site and HPV-status. The majority of HPV-positive and -negative HNSCC is characterized by frequent copy number (CN) changes and an abrogated p53-pathway. A third genetically-defined HPV-negative subclass of HNSCC is emerging: tumors that lack gross chromosomal changes (CN-silent), are mostly TP53-proficient, and have a relatively favorable prognosis.

METHODS

A representative panel of HPV-positive, HPV-negative and FA-HNSCC-derived cell lines was genetically characterized.

RESULTS

Despite apparent differences in etiology, FA-HNSCC cell lines show comparable genetic alterations as sporadic non-FA-HNSCC-derived cell lines. Furthermore, we identified a near diploid CN-silent HPV-negative HNSCC line: VU-SCC-040. Molecular characterization uncovers the absence of TP53 mutations, a functional p53-pathway and a CASP8 mutation. TP53 gene knockout using CRISPR-Cas9 resulted in resistance to MDM2 inhibition. Whereas p53-status is often proposed as a predictive biomarker for treatment response, TP53-knockout did not change sensitivity to cisplatin, Chk1 and Wee1 inhibition. Additionally, 84 CN-silent tumors were identified in the HNSCC PanCancer cohort and shown to be enriched for female gender, HRAS and CASP8 mutations.

CONCLUSION

FA-derived HNSCC cell lines share comparable CN-profiles and mutation patterns as sporadic HPV-negative HNSCC. In contrast, a subclass of CN-silent, HPV-negative and TP53 wild-type HNSCC separates from the majority of HNSCC tumors. We show that VU-SCC-040 is a HNSCC cell model representative of this subclass.

Replication Stress and Consequential Instability of the Genome and Epigenome

Cells must faithfully duplicate their DNA in the genome to pass their genetic information to the daughter cells. To maintain genomic stability and integrity, double-strand DNA has to be replicated in a strictly regulated manner, ensuring the accuracy of its copy number, integrity and epigenetic modifications. However, DNA is constantly under the attack of DNA damage, among which oxidative DNA damage is the one that most frequently occurs, and can alter the accuracy of DNA replication, integrity and epigenetic features, resulting in DNA replication stress and subsequent genome and epigenome instability. In this review, we summarize DNA damage-induced replication stress, the formation of DNA secondary structures, peculiar epigenetic modifications and cellular responses to the stress and their impact on the instability of the genome and epigenome mainly in eukaryotic cells.

DNA damage kinase signaling: checkpoint and repair at 30 years

From bacteria to mammalian cells, damaged DNA is sensed and targeted by DNA repair pathways. In eukaryotes, kinases play a central role in coordinating the DNA damage response. DNA damage signaling kinases were identified over two decades ago and linked to the cell cycle checkpoint concept proposed by Weinert and Hartwell in 1988. Connections between the DNA damage signaling kinases and DNA repair were scant at first, and the initial perception was that the importance of these kinases for genome integrity was largely an indirect effect of their roles in checkpoints, DNA replication, and transcription. As more substrates of DNA damage signaling kinases were identified, it became clear that they directly regulate a wide range of DNA repair factors. Here, we review our current understanding of DNA damage signaling kinases, delineating the key substrates in budding yeast and humans. We trace the progress of the field in the last 30 years and discuss our current understanding of the major substrate regulatory mechanisms involved in checkpoint responses and DNA repair.

DNA G-Quadruplexes (G4s) Modulate Epigenetic (Re)Programming and Chromatin Remodeling: Transient Genomic G4s Assist in the Establishment and Maintenance of Epigenetic Marks, While Persistent G4s May Erase Epigenetic Marks

Here, the emerging data on DNA G-quadruplexes (G4s) as epigenetic modulators are reviewed and integrated. This concept has appeared and evolved substantially in recent years. First, persistent G4s (e.g., those stabilized by exogenous ligands) were linked to the loss of the histone code. More recently, transient G4s (i.e., those formed upon replication or transcription and unfolded rapidly by helicases) were implicated in CpG island methylation maintenance and de novo CpG methylation control. The most recent data indicate that there are direct interactions between G4s and chromatin remodeling factors. Finally, multiple findings support the indirect participation of G4s in chromatin reshaping via interactions with remodeling-related transcription factors (TFs) or damage responders. Here, the links between the above processes are analyzed; also, how further elucidation of these processes may stimulate the progress of epigenetic therapy is discussed, and the remaining open questions are highlighted.

DNA Damage and Associated DNA Repair Defects in Disease and Premature Aging

Genetic information is constantly being attacked by intrinsic and extrinsic damaging agents, such as reactive oxygen species, atmospheric radiation, environmental chemicals, and chemotherapeutics. If DNA modifications persist, they can adversely affect the polymerization of DNA or RNA, leading to replication fork collapse or transcription arrest, or can serve as mutagenic templates during nucleic acid synthesis reactions. To combat the deleterious consequences of DNA damage, organisms have developed complex repair networks that remove chemical modifications or aberrant base arrangements and restore the genome to its original state. Not surprisingly, inherited or sporadic defects in DNA repair mechanisms can give rise to cellular outcomes that underlie disease and aging, such as transformation, apoptosis, and senescence. In the review here, we discuss several genetic disorders linked to DNA repair defects, attempting to draw correlations between the nature of the accumulating DNA damage and the pathological endpoints, namely cancer, neurological disease, and premature aging.

Germline genome protection: implications for gamete quality and germ cell tumorigenesis

BACKGROUND

Germ cells have a unique and critical role as the conduit for hereditary information and therefore employ multiple strategies to protect genomic integrity and avoid mutations. Unlike somatic cells, which often respond to DNA damage by arresting the cell cycle and conducting DNA repair, germ cells as well as long-lived pluripotent stem cells typically avoid the use of error-prone repair mechanisms and favor apoptosis, reducing the risk of genetic alterations. Testicular germ cell tumors, the most common cancers of young men, arise from pre-natal germ cells.

OBJECTIVES

To summarize the current understanding of DNA damage response mechanisms in pre-meiotic germ cells and to discuss how they impact both the origins of testicular germ cell tumors and their remarkable responsiveness to genotoxic chemotherapy.

MATERIALS AND METHODS

We conducted a review of literature gathered from PubMed regarding the DNA damage response properties of testicular germ cell tumors and the germ cells from which they arise, as well as the influence of these mechanisms on therapeutic responses by testicular germ cell tumors.

RESULTS AND DISCUSSION

This review provides a comprehensive evaluation of how the developmental origins of male germ cells and their inherent germ cell-like DNA damage response directly impact the development and therapeutic sensitivity of testicular germ cell tumors.

CONCLUSIONS

The DNA damage response of germ cells directly impacts the development and therapeutic sensitivity of testicular germ cell tumors. Recent advances in the study of primordial germ cells, post-natal mitotically dividing germ cells, and pluripotent stem cells will allow for new investigations into the initiation, progression, and treatment of testicular germ cell tumors.

Intertwined: SAMHD1 cellular functions, restriction, and viral evasion strategies

SAMHD1 was initially described for its ability to efficiently restrict HIV-1 replication in myeloid cells and resting CD4⁺ T cells. However, a growing body of evidence suggests that SAMHD1-mediated restriction is by far not limited to lentiviruses, but seems to be a general concept that applies to most retroviruses and at least a number of DNA viruses. SAMHD1 anti-viral activity was long believed to be solely due to its ability to deplete cellular dNTPs by enzymatic degradation. However, since its discovery, several new functions have been attributed to SAMHD1. It has been demonstrated to bind nucleic acids, to modulate innate immunity, as well as to participate in the DNA damage response and resolution of stalled replication forks. Consequently, it is likely that SAMHD1-mediated anti-viral activity is not or not exclusively mediated through its dNTPase activity. Therefore, in this review, we summarize current knowledge on SAMHD1 cellular functions and systematically discuss how these functions could contribute to the restriction of a broad range of viruses besides retroviruses: herpesviruses, poxviruses and hepatitis B virus. Furthermore, we aim to highlight different ways how viruses counteract SAMHD1-mediated restriction to bypass the SAMHD1-mediated block to viral infection.