αKG-mediated carnitine synthesis promotes homologous recombination via histone acetylation.

Homologous recombination (HR) deficiency enhances sensitivity to DNA damaging agents commonly used to treat cancer. In HR-proficient cancers, metabolic mechanisms driving response or resistance to DNA damaging agents remain unclear. Here we identified that depletion of alpha-ketoglutarate (αKG) sensitizes HR-proficient cells to DNA damaging agents by metabolic regulation of histone acetylation. αKG is required for the activity of αKG-dependent dioxygenases (αKGDDs), and prior work has shown that changes in αKGDD affect demethylases. Using a targeted CRISPR knockout library consisting of 64 αKGDDs, we discovered that Trimethyllysine Hydroxylase Epsilon (TMLHE), the first and rate-limiting enzyme in de novo carnitine synthesis, is necessary for proliferation of HR-proficient cells in the presence of DNA damaging agents. Unexpectedly, αKG-mediated TMLHE-dependent carnitine synthesis was required for histone acetylation, while histone methylation was affected but dispensable. The increase in histone acetylation via αKG-dependent carnitine synthesis promoted HR-mediated DNA repair through site- and substrate-specific histone acetylation. These data demonstrate for the first time that HR-proficiency is mediated through αKG directly influencing histone acetylation via carnitine synthesis and provide a metabolic avenue to induce HR-deficiency and sensitivity to DNA damaging agents.

MYC is sufficient to generate mid-life high-grade serous ovarian and uterine serous carcinomas in a p53-R270H mouse model.

Genetically engineered mouse models (GEMM) have fundamentally changed how ovarian cancer etiology, early detection, and treatment is understood. However, previous GEMMs of high-grade serous ovarian cancer (HGSOC) have had to utilize genetics rarely or never found in human HGSOC to yield ovarian cancer within the lifespan of a mouse. MYC, an oncogene, is amongst the most amplified genes in HGSOC, but it has not previously been utilized to drive HGSOC GEMMs. We coupled Myc and dominant negative mutant p53-R270H with a fallopian tube epithelium-specific promoter Ovgp1 to generate a new GEMM of HGSOC. Female mice developed lethal cancer at an average of 15.1 months. Histopathological examination of mice revealed HGSOC characteristics including nuclear p53 and nuclear MYC in clusters of cells within the fallopian tube epithelium and ovarian surface epithelium. Unexpectedly, nuclear p53 and MYC clustered cell expression was also identified in the uterine luminal epithelium, possibly from intraepithelial metastasis from the fallopian tube epithelium (FTE). Extracted tumor cells exhibited strong loss of heterozygosity at the p53 locus, leaving the mutant allele. Copy number alterations in these cancer cells were prevalent, disrupting a large fraction of genes. Transcriptome profiles most closely matched human HGSOC and serous endometrial cancer. Taken together, these results demonstrate the Myc and Trp53-R270H transgene was able to recapitulate many phenotypic hallmarks of HGSOC through the utilization of strictly human-mimetic genetic hallmarks of HGSOC. This new mouse model enables further exploration of ovarian cancer pathogenesis, particularly in the 50% of HGSOC which lack homology directed repair mutations. Histological and transcriptomic findings are consistent with the hypothesis that uterine serous cancer may originate from the fallopian tube epithelium.

Arginine methylation of BRD4 by PRMT2/4 governs transcription and DNA repair.

BRD4 functions as an epigenetic reader and plays a crucial role in regulating transcription and genome stability. Dysregulation of BRD4 is frequently observed in various human cancers. However, the molecular details of BRD4 regulation remain largely unknown. Here, we report that PRMT2- and PRMT4-mediated arginine methylation is pivotal for BRD4 functions on transcription, DNA repair, and tumor growth. Specifically, PRMT2/4 interacts with and methylates BRD4 at R179, R181, and R183. This arginine methylation selectively controls a transcriptional program by promoting BRD4 recruitment to acetylated histones/chromatin. Moreover, BRD4 arginine methylation is induced by DNA damage and thereby promotes its binding to chromatin for DNA repair. Deficiency in BRD4 arginine methylation significantly suppresses tumor growth and sensitizes cells to BET inhibitors and DNA damaging agents. Therefore, our findings reveal an arginine methylation-dependent regulatory mechanism of BRD4 and highlight targeting PRMT2/4 for better antitumor effect of BET inhibitors and DNA damaging agents.

Transcription suppression is mediated by the HDAC1-Sin3 complex in Xenopus nucleoplasmic extract.

Modification of histones provides a dynamic mechanism to regulate chromatin structure and access to DNA. Histone acetylation, in particular, plays a prominent role in controlling the interaction between DNA, histones, and other chromatin-associated proteins. Defects in histone acetylation patterns interfere with normal gene expression and underlie a wide range of human diseases. Here, we utilize Xenopus egg extracts to investigate how changes in histone acetylation influence transcription of a defined gene construct. We show that inhibition of histone deacetylase 1 and 2 (HDAC1/2) specifically counteracts transcription suppression by preventing chromatin compaction and deacetylation of histone residues H4K5 and H4K8. Acetylation of these sites supports binding of the chromatin reader and transcription regulator BRD4. We also identify HDAC1 as the primary driver of transcription suppression and show that this activity is mediated through the Sin3 histone deacetylase complex. These findings highlight functional differences between HDAC1 and HDAC2, which are often considered to be functionally redundant, and provide additional molecular context for their activity.

BRD4 promotes resection and homology-directed repair of DNA double-strand breaks.

Double-strand breaks (DSBs) are one of the most toxic forms of DNA damage and represent a major source of genomic instability. Members of the bromodomain and extra-terminal (BET) protein family are characterized as epigenetic readers that regulate gene expression. However, evidence suggests that BET proteins also play a more direct role in DNA repair. Here, we establish a cell-free system using Xenopus egg extracts to elucidate the gene expression-independent functions of BET proteins in DSB repair. We identify the BET protein BRD4 as a critical regulator of homologous recombination and describe its role in stimulating DNA processing through interactions with the SWI/SNF chromatin remodeling complex and resection machinery. These results establish BRD4 as a multifunctional regulator of chromatin binding that links transcriptional activity and homology-directed repair.

SWAN pathway-network identification of common aneuploidy-based oncogenic drivers.

Haploinsufficiency drives Darwinian evolution. Siblings, while alike in many aspects, differ due to monoallelic differences inherited from each parent. In cancer, solid tumors exhibit aneuploid genetics resulting in hundreds to thousands of monoallelic gene-level copy-number alterations (CNAs) in each tumor. Aneuploidy patterns are heterogeneous, posing a challenge to identify drivers in this high-noise genetic environment. Here, we developed Shifted Weighted Annotation Network (SWAN) analysis to assess biology impacted by cumulative monoallelic changes. SWAN enables an integrated pathway-network analysis of CNAs, RNA expression, and mutations via a simple web platform. SWAN is optimized to best prioritize known and novel tumor suppressors and oncogenes, thereby identifying drivers and potential druggable vulnerabilities within cancer CNAs. Protein homeostasis, phospholipid dephosphorylation, and ion transport pathways are commonly suppressed. An atlas of CNA pathways altered in each cancer type is released. These CNA network shifts highlight new, attractive targets to exploit in solid tumors.

Heterogeneous nuclear ribonucleoprotein E1 binds polycytosine DNA and monitors genome integrity.

Heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1) is a tumor suppressor protein that binds site- and structure-specifically to RNA sequences to regulate mRNA stability, facilitate alternative splicing, and suppress protein translation on several metastasis-associated mRNAs. Here, we show that hnRNP E1 binds polycytosine-rich DNA tracts present throughout the genome, including those at promoters of several oncogenes and telomeres and monitors genome integrity. It binds DNA in a site- and structure-specific manner. hnRNP E1-knockdown cells displayed increased DNA damage signals including γ-H2AX at its binding sites and also showed increased mutations. UV and hydroxyurea treatment of hnRNP E1-knockdown cells exacerbated the basal DNA damage signals with increased cell cycle arrest, activation of checkpoint proteins, and monoubiquitination of proliferating cell nuclear antigen despite no changes in deubiquitinating enzymes. DNA damage caused by genotoxin treatment localized to hnRNP E1 binding sites. Our work suggests that hnRNP E1 facilitates functions of DNA integrity proteins at polycytosine tracts and monitors DNA integrity at these sites.

BRCA1-BARD1 regulates transcription through BRD4 in Xenopus nucleoplasmic extract.

The tumor suppressor BRCA1 is considered a master regulator of genome integrity. Although widely recognized for its DNA repair functions, BRCA1 has also been implicated in various mechanisms of chromatin remodeling and transcription regulation. However, the precise role that BRCA1 plays in these processes has been difficult to establish due to the widespread consequences of its cellular dysfunction. Here, we use nucleoplasmic extract derived from the eggs of Xenopus laevis to investigate the role of BRCA1 in a cell-free transcription system. We report that BRCA1-BARD1 suppresses transcription initiation independent of DNA damage signaling and its established role in histone H2A ubiquitination. BRCA1-BARD1 acts through a histone intermediate, altering acetylation of histone H4K8 and recruitment of the chromatin reader and oncogene regulator BRD4. Together, these results establish a functional relationship between an established (BRCA1) and emerging (BRD4) regulator of genome integrity.

Cell-free transcription in Xenopus egg extract.

Soluble extracts prepared from Xenopus eggs have been used extensively to study various aspects of cellular and developmental biology. During early egg development, transcription of the zygotic genome is suppressed. As a result, traditional extracts derived from unfertilized and early stage eggs possess little or no intrinsic transcriptional activity. In this study, we show that Xenopus nucleoplasmic extract (NPE) supports robust transcription of a chromatinized plasmid substrate. Although prepared from eggs in a transcriptionally inactive state, the process of making NPE resembles some aspects of egg fertilization and early embryo development that lead to transcriptional activation. With this system, we observed that promoter-dependent recruitment of transcription factors and RNA polymerase II leads to conventional patterns of divergent transcription and pre-mRNA processing, including intron splicing and 3' cleavage and polyadenylation. We also show that histone density controls transcription factor binding and RNA polymerase II activity, validating a mechanism proposed to regulate genome activation during development. Together, these results establish a new cell-free system to study the regulation, initiation, and processing of mRNA transcripts.

Uncoupling of p97 ATPase activity has a dominant negative effect on protein extraction.

p97 is a highly abundant, homohexameric AAA+ ATPase that performs a variety of essential cellular functions. Characterized as a ubiquitin-selective chaperone, p97 recognizes proteins conjugated to K48-linked polyubiquitin chains and promotes their removal from chromatin and other molecular complexes. Changes in p97 expression or activity are associated with the development of cancer and several related neurodegenerative disorders. Although pathogenic p97 mutations cluster in and around p97's ATPase domains, mutant proteins display normal or elevated ATPase activity. Here, we show that one of the most common p97 mutations (R155C) retains ATPase activity, but is functionally defective. p97-R155C can be recruited to ubiquitinated substrates on chromatin, but is unable to promote substrate removal. As a result, p97-R155C acts as a dominant negative, blocking protein extraction by a similar mechanism to that observed when p97's ATPase activity is inhibited or inactivated. However, unlike ATPase-deficient proteins, p97-R155C consumes excess ATP, which can hinder high-energy processes. Together, our results shed new insight into how pathogenic mutations in p97 alter its cellular function, with implications for understanding the etiology and treatment of p97-associated diseases.

Chromatin Immunoprecipitation (ChIP) of Plasmid-Bound Proteins in Xenopus Egg Extracts.

Xenopus egg extracts provide a cell-free system to analyze various aspects of chromatin biology. Here we describe a modified method of chromatin immunoprecipitation (ChIP) to detect the interaction of proteins with plasmid DNA incubated in extract. The combination of ChIP and Xenopus egg extracts provides a highly versatile and tractable approach to analyze dynamic protein-DNA interactions with great spatial and temporal detail.

Identification of the influence of distal inputs on mercury loading across the mid Great Lakes region using chemical sediment chronologies.

Sediment cores from 47 inland lakes in Michigan, USA were used to assess spatial and temporal trends in loadings of mercury (Hg). Focusing/background corrected accumulation rates and inventories and peak concentrations were used to examine: 1) responses of loadings to post-1990 reductions in emissions, 2) if spatial trends are consistent with modeled Hg deposition and 3) evidence for local and distal inputs. Results showed that decreases in concentrations and anthropogenic accumulation rates of Hg were consistent with recent reductions in emissions of Hg. Most lakes exhibiting a decreasing trend were located within an area with the most emission sources. Not all lakes showed the decreasing trend with some showing increases or no change. These lakes tended to be in the northern portion of the state. In all lakes, current concentrations of Hg remain greater than long-term, historical, background concentrations. Sub-regional mean inventories and mean decadal accumulation rates exhibited a south to north gradient, consistent with previously modeled spatial trends. However, individual lake inventories and rates of accumulation compared at shorter times scales varied among lakes. Evidence for event deposition (e.g., volcanic eruptions, manufacturing) was also variable among lakes. These results suggest influence of more distal inputs of Hg, perhaps driven by well-mixed, global sources. Cause(s) of variability on shorter time scales (e.g., events) needs further work. Finally, the results reveal that understanding risks to humans and ecosystems due to exposure to Hg and developing effective abatement policy is challenging.

Eutrophication and recovery of a Lake inferred from sedimentary diatoms originating from different habitats.

Eutrophication and recovery of Muskegon Lake resulted from a complex set of interacting factors according to diatom-inferred total phosphorus (TP), geochemical proxies, detailed modeling of land use land cover change in the watershed, and accounts of past point source management and non-native species invasions. Benthic and planktonic diatoms responded to phosphorus environments differently in this lake that receives 95% of its input from one river and has only a 23days average retention time. Planktonic diatoms reflected river conditions more than benthic diatoms, and benthos reflected lake conditions more than plankton. Inferred TP from planktonic diatoms indicated the Muskegon River was relatively nutrient rich compared to inferred TP for Muskegon Lake based on benthic diatoms before Europeans settled the watershed. Early European settlement and logging caused no changes in phosphorus condition in the Muskegon River, but modest increases in phosphorus were indicated in Muskegon Lake during the middle and late thirds of the 19th century. Extensive watershed-scale agricultural activity in the early 20th century apparently had little effect on trophic status of the lake, perhaps because it preceded high fertilizer use on farms. During the industrial and population boom in the watershed during the early half of the 20th century, river conditions changed little, but eutrophication of Muskegon Lake increased greatly. Reduction in river phosphorus by dams occurred during the first half of the 20th century. Phosphorus reduction in the lake was indicated after advanced wastewater treatment for Muskegon Township was implemented in 1973. Current diatom inferred phosphorus concentration in the lake is the same as before European settlement, however many attributes of the lake still differ because other stressors persist.

The evolving role of DNA inter-strand crosslinks in chemotherapy.

DNA crosslinking agents make up a broad class of chemotherapy agents that target rapidly dividing cancer cells by disrupting DNA synthesis. These drugs differ widely in both chemical structure and biological effect. In cells, crosslinking agents can form multiple types of DNA lesions with varying efficiencies. Inter-strand crosslinks (ICLs) are considered to be the most cytotoxic lesion, creating a covalent roadblock to replication and transcription. Despite over 50 years in the clinic, the use of crosslinking agents that specialize in the formation of ICLs remains limited, largely due to high toxicity in patients. Current ICL-based therapeutics have focused on late-stage and drug-resistant tumors, or localized treatments that limit exposure. In this article, we review the development of clinical crosslinking agents, our understanding of how cells respond to different lesions, and the potential to improve ICL-based chemotherapeutics in the future.

p97 Promotes a Conserved Mechanism of Helicase Unloading during DNA Cross-Link Repair.

Interstrand cross-links (ICLs) are extremely toxic DNA lesions that create an impassable roadblock to DNA replication. When a replication fork collides with an ICL, it triggers a damage response that promotes multiple DNA processing events required to excise the cross-link from chromatin and resolve the stalled replication fork. One of the first steps in this process involves displacement of the CMG replicative helicase (comprised of Cdc45, MCM2-7, and GINS), which obstructs the underlying cross-link. Here we report that the p97/Cdc48/VCP segregase plays a critical role in ICL repair by unloading the CMG complex from chromatin. Eviction of the stalled helicase involves K48-linked polyubiquitylation of MCM7, p97-mediated extraction of CMG, and a largely degradation-independent mechanism of MCM7 deubiquitylation. Our results show that ICL repair and replication termination both utilize a similar mechanism to displace the CMG complex from chromatin. However, unlike termination, repair-mediated helicase unloading involves the tumor suppressor protein BRCA1, which acts upstream of MCM7 ubiquitylation and p97 recruitment. Together, these findings indicate that p97 plays a conserved role in dismantling the CMG helicase complex during different cellular events, but that distinct regulatory signals ultimately control when and where unloading takes place.

DNA repair. Proteomics reveals dynamic assembly of repair complexes during bypass of DNA cross-links.

DNA interstrand cross-links (ICLs) block replication fork progression by inhibiting DNA strand separation. Repair of ICLs requires sequential incisions, translesion DNA synthesis, and homologous recombination, but the full set of factors involved in these transactions remains unknown. We devised a technique called chromatin mass spectrometry (CHROMASS) to study protein recruitment dynamics during perturbed DNA replication in Xenopus egg extracts. Using CHROMASS, we systematically monitored protein assembly and disassembly on ICL-containing chromatin. Among numerous prospective DNA repair factors, we identified SLF1 and SLF2, which form a complex with RAD18 and together define a pathway that suppresses genome instability by recruiting the SMC5/6 cohesion complex to DNA lesions. Our study provides a global analysis of an entire DNA repair pathway and reveals the mechanism of SMC5/6 relocalization to damaged DNA in vertebrate cells.

Composition of bacterial and archaeal communities during landfill refuse decomposition processes.

Little is known about the archaeal and the bacterial diversities in a landfill during different phases of decomposition. In this study, the archaeal and the bacterial diversities of Laogang landfill (Shanghai, China) at two different decomposition phases (i.e., initial methanogenic phase (IMP) and stable methanogenic phase (SMP)), were culture-independently examined using PCR-based 454 pyrosequencing. A total of 47,753 sequences of 16S rRNA genes were retrieved from 69,954 reads and analyzed to evaluate the diversities of the archaeal and bacterial communities. The most predominant types of archaea were hydrogenotrophic Methanomicrobiales, and of bacteria were Proteobacteria, Firmicutes, and Bacteroidetes. As might be expected, their abundances varied at decomposition phases. Archaea Methanomicrobiales accounts for 97.6% of total archaeal population abundance in IMP and about 57.6% in SMP. The abundance of archaeal genus Halobacteriale was 0.1% in IMP and was 20.3% in the SMP. The abundance of Firmicutes was 21.3% in IMP and was 4.3% in SMP. The abundance of Bacteroidetes represented 11.5% of total bacterial in IMP and was dominant (49.4%) in SMP. Both the IMP and SMP had unique cellulolytic bacteria compositions. IMP consisted of members of Bacillus, Fibrobacter, and Eubacterium, while SMP harbored groups of Microbacterium. Both phases had Clostridium with different abundance, 4-5 folds higher in SMP.

BRCA1 promotes unloading of the CMG helicase from a stalled DNA replication fork.

The tumor suppressor protein BRCA1 promotes homologous recombination (HR), a high-fidelity mechanism to repair DNA double-strand breaks (DSBs) that arise during normal replication and in response to DNA-damaging agents. Recent genetic experiments indicate that BRCA1 also performs an HR-independent function during the repair of DNA interstrand crosslinks (ICLs). Here we show that BRCA1 is required to unload the CMG helicase complex from chromatin after replication forks collide with an ICL. Eviction of the stalled helicase allows leading strands to be extended toward the ICL, followed by endonucleolytic processing of the crosslink, lesion bypass, and DSB repair. Our results identify BRCA1-dependent helicase unloading as a critical, early event in ICL repair.

Inferring sources for mercury to inland lakes using sediment chronologies of polycyclic aromatic hydrocarbons.

Sediment chronologies from inland lakes suggest the influence of local to sub-regional scale sources for mercury (Hg). However, apportionment of sources for Hg using sediment chronologies is difficult due to the mixing of sources and pathways. Mercury and polycyclic aromatic hydrocarbons (PAH) often share common sources and pathways into the environment. Thus, chronologies of PAHs in dated cores of sediments might be a useful tool to infer sources of Hg. Sediment cores from seven inland lakes of Michigan were collected for measurement of PAHs and Hg and dated by use of (210)Pb. PAH concentrations and ratios of kinetic and thermodynamic PAH compounds were used to infer sources of Hg. Ratios indicate the existence of modern combustion sources to each lake and historic combustion sources to lakes near cement kilns and an iron foundry. Coal combustion sources were identified for two lakes near urban centers. Whereas a petroleum combustion source was identified for a lake that has a coal fired power plant along its shoreline. These results have implications for the cycling of Hg on local to regional scales.

XPF-ERCC1 acts in Unhooking DNA interstrand crosslinks in cooperation with FANCD2 and FANCP/SLX4.

DNA interstrand crosslinks (ICLs), highly toxic lesions that covalently link the Watson and Crick strands of the double helix, are repaired by a complex, replication-coupled pathway in higher eukaryotes. The earliest DNA processing event in ICL repair is the incision of parental DNA on either side of the ICL ("unhooking"), which allows lesion bypass. Incisions depend critically on the Fanconi anemia pathway, whose activation involves ubiquitylation of the FANCD2 protein. Using Xenopus egg extracts, which support replication-coupled ICL repair, we show that the 3' flap endonuclease XPF-ERCC1 cooperates with SLX4/FANCP to carry out the unhooking incisions. Efficient recruitment of XPF-ERCC1 and SLX4 to the ICL depends on FANCD2 and its ubiquitylation. These data help define the molecular mechanism by which the Fanconi anemia pathway promotes a key event in replication-coupled ICL repair.