Endogenous Formaldehyde Is a Hematopoietic Stem Cell Genotoxin and Metabolic Carcinogen

BRCA1/FANCD2/BRG1-Driven DNA Repair Stabilizes the Differentiation State of Human Mammary Epithelial Cells

An abnormal differentiation state is common in BRCA1-deficient mammary epithelial cells, but the underlying mechanism is unclear. Here, we report a convergence between DNA repair and normal, cultured human mammary epithelial (HME) cell differentiation. Surprisingly, depleting BRCA1 or FANCD2 (Fanconi anemia [FA] proteins) or BRG1, a mSWI/SNF subunit, caused HME cells to undergo spontaneous epithelial-to-mesenchymal transition (EMT) and aberrant differentiation. This also occurred when wild-type HMEs were exposed to chemicals that generate DNA interstrand crosslinks (repaired by FA proteins), but not in response to double-strand breaks. Suppressed expression of ΔNP63 also occurred in each of these settings, an effect that links DNA damage to the aberrant differentiation outcome. Taken together with somatic breast cancer genome data, these results point to a breakdown in a BRCA/FA-mSWI/SNF-ΔNP63-mediated DNA repair and differentiation maintenance process in mammary epithelial cells that may contribute to sporadic breast cancer development.

Formaldehyde induces toxicity in mouse bone marrow and hematopoietic stem/progenitor cells and enhances benzene-induced adverse effects

FA in air for 2 weeks, mimicking occupational exposure, then measured complete blood counts, nucleated BM cell count, and myeloid progenitor colony formation. We also investigated potential mechanisms of FA toxicity, including reactive oxygen species (ROS) generation, apoptosis, and hematopoietic growth factor and receptor levels. FA exposure significantly reduced nucleated BM cells and BM-derived colony-forming unit-granulocyte-macrophage (CFU-GM) and burst-forming unit-erythroid (BFU-E); down-regulated GM-CSFRα and EPOR expression; increased ROS in nucleated BM, spleen and CFU-GM cells; and increased apoptosis in nucleated spleen and CFU-GM cells. FA and BZ each similarly altered BM mature cells and stem/progenitor counts, BM and CFU-GM ROS, and apoptosis in spleen and CFU-GM but had differential effects on other end points. Co-exposure was more potent for several end points. Thus, FA is toxic to the mouse hematopoietic system, including BM stem/progenitor cells, and it enhances BZ-induced toxic effects. Our findings suggest that FA may induce BM toxicity by affecting myeloid progenitor growth and survival through oxidative damage and reduced expression levels of GM-CSFRα and EPOR.

Proliferation of Double-Strand Break-Resistant Polyploid Cells Requires Drosophila FANCD2

Conserved DNA-damage responses (DDRs) sense genome damage and prevent mitosis of broken chromosomes. How cells lacking DDRs cope with broken chromosomes during mitosis is poorly understood. DDRs are frequently inactivated in cells with extra genomes (polyploidy), suggesting that study of polyploidy can reveal how cells with impaired DDRs/genome damage continue dividing. Here, we show that continued division and normal organ development occurs in polyploid, DDR-impaired Drosophila papillar cells. As papillar cells become polyploid, they naturally accumulate broken acentric chromosomes but do not apoptose/arrest the cell cycle. To survive mitosis with acentric chromosomes, papillar cells require Fanconi anemia proteins FANCD2 and FANCI, as well as Blm helicase, but not canonical DDR signaling. FANCD2 acts independently of previous S phases to promote alignment and segregation of acentric DNA produced by double-strand breaks, thus avoiding micronuclei and organ malformation. Because polyploidy and impaired DDRs can promote cancer, our findings provide insight into disease-relevant DNA-damage tolerance mechanisms.

Interplay between Fanconi anemia and homologous recombination pathways in genome integrity

The Fanconi anemia (FA) pathway plays a central role in the repair of DNA interstrand crosslinks (ICLs) and regulates cellular responses to replication stress. Homologous recombination (HR), the error-free pathway for double-strand break (DSB) repair, is required during physiological cell cycle progression for the repair of replication-associated DNA damage and protection of stalled replication forks. Substantial crosstalk between the two pathways has recently been unravelled, in that key HR proteins such as the RAD51 recombinase and the tumour suppressors BRCA1 and BRCA2 also play important roles in ICL repair. Consistent with this, rare patient mutations in these HR genes cause FA pathologies and have been assigned FA complementation groups. Here, we focus on the clinical and mechanistic implications of the connection between these two cancer susceptibility syndromes and on how these two molecular pathways of DNA replication and repair interact functionally to prevent genomic instability.

S-Nitrosation of Conserved Cysteines Modulates Activity and Stability of S-Nitrosoglutathione Reductase (GSNOR)

The free radical nitric oxide (NO(•)) regulates diverse physiological processes from vasodilation in humans to gas exchange in plants. S-Nitrosoglutathione (GSNO) is considered a principal nitroso reservoir due to its chemical stability. GSNO accumulation is attenuated by GSNO reductase (GSNOR), a cysteine-rich cytosolic enzyme. Regulation of protein nitrosation is not well understood since NO(•)-dependent events proceed without discernible changes in GSNOR expression. Because GSNORs contain evolutionarily conserved cysteines that could serve as nitrosation sites, we examined the effects of treating plant (Arabidopsis thaliana), mammalian (human), and yeast (Saccharomyces cerevisiae) GSNORs with nitrosating agents in vitro. Enzyme activity was sensitive to nitroso donors, whereas the reducing agent dithiothreitol (DTT) restored activity, suggesting that catalytic impairment was due to S-nitrosation. Protein nitrosation was confirmed by mass spectrometry, by which mono-, di-, and trinitrosation were observed, and these signals were sensitive to DTT. GSNOR mutants in specific non-zinc-coordinating cysteines were less sensitive to catalytic inhibition by nitroso donors and exhibited reduced nitrosation signals by mass spectrometry. Nitrosation also coincided with decreased tryptophan fluorescence, increased thermal aggregation propensity, and increased polydispersity-properties reflected by differential solvent accessibility of amino acids important for dimerization and the shape of the substrate and coenzyme binding pockets as assessed by hydrogen-deuterium exchange mass spectrometry. Collectively, these data suggest a mechanism for NO(•) signal transduction in which GSNOR nitrosation and inhibition transiently permit GSNO accumulation.

Measurement of Endogenous versus Exogenous Formaldehyde-Induced DNA-Protein Crosslinks in Animal Tissues by Stable Isotope Labeling and Ultrasensitive Mass Spectrometry

DNA-protein crosslinks (DPC) arise from a wide range of endogenous and exogenous chemicals, such as chemotherapeutic drugs and formaldehyde. Importantly, recent identification of aldehydes as endogenous genotoxins in Fanconi anemia has provided new insight into disease causation. Because of their bulky nature, DPCs pose severe threats to genome stability, but previous methods to measure formaldehyde-induced DPCs were incapable of discriminating between endogenous and exogenous sources of chemical. In this study, we developed methods that provide accurate and distinct measurements of both exogenous and endogenous DPCs in a structurally specific manner. We exposed experimental animals to stable isotope-labeled formaldehyde ([(13)CD2]-formaldehyde) by inhalation and performed ultrasensitive mass spectrometry to measure endogenous (unlabeled) and exogenous ((13)CD2-labeled) DPCs. We found that exogenous DPCs readily accumulated in nasal respiratory tissues but were absent in tissues distant to the site of contact. This observation, together with the finding that endogenous formaldehyde-induced DPCs were present in all tissues examined, suggests that endogenous DPCs may be responsible for increased risks of bone marrow toxicity and leukemia. Furthermore, the slow rate of DPC repair provided evidence for the persistence of DPCs. In conclusion, our method for measuring endogenous and exogenous DPCs presents a new perspective for the potential health risks inflicted by endogenous formaldehyde and may inform improved disease prevention and treatment strategies. Cancer Res; 76(9); 2652-61. ©2016 AACR.

Studies on the Glutathione-Dependent Formaldehyde-Activating Enzyme from Paracoccus denitrificans

Formaldehyde is a toxin and carcinogen that is both an environmental pollutant and an endogenous metabolite. Formaldehyde metabolism, which is probably essential for all aerobic cells, likely proceeds via multiple mechanisms, including via a glutathione-dependent pathway that is widely conserved in bacteria, plants and animals. However, it is unclear whether the first step in the glutathione-dependent pathway (i.e. formation of S-hydroxymethylglutathione (HMG)) is enzyme-catalysed. We report studies on glutathione-dependent formaldehyde-activating enzyme (GFA) from Paracoccus denitrificans, which has been proposed to catalyse HMG formation from glutathione and formaldehyde on the basis of studies using NMR exchange spectroscopy (EXSY). Although we were able to replicate the EXSY results, time course experiments unexpectedly imply that GFA does not catalyse HMG formation under standard conditions. However, GFA was observed to bind glutathione using NMR and mass spectrometry. Overall, the results reveal that GFA binds glutathione but does not directly catalyse HMG formation under standard conditions. Thus, it is possible that GFA acts as a glutathione carrier that acts to co-localise glutathione and formaldehyde in a cellular context.

What is the DNA repair defect underlying Fanconi anemia?

Fanconi anemia (FA) is a rare human genetic disease characterized by bone marrow failure, cancer predisposition, and genomic instability. It has been known for many years that FA patient-derived cells are exquisitely sensitive to DNA interstrand cross-linking agents such as cisplatin and mitomycin C. On this basis, it was widely assumed that failure to repair endogenous interstrand cross-links (ICLs) causes FA, although the endogenous mutagen that generates these lesions remained elusive. Recent genetic evidence now suggests that endogenous aldehydes are the driving force behind FA. Importantly, aldehydes cause a variety of DNA lesions, including ICLs and DNA protein cross-links (DPCs), re-kindling the debate about which DNA lesions cause FA. In this review, we discuss new developments in our understanding of DPC and ICL repair, and how these findings bear on the question of which DNA lesion underlies FA.