Aldehyde-mediated inhibition of asparagine biosynthesis has implications for diabetes and alcoholism

Patients with alcoholism and type 2 diabetes manifest altered metabolism, including elevated aldehyde levels and unusually low asparagine levels. We show that asparagine synthetase B (ASNS), the only human asparagine-forming enzyme, is inhibited by disease-relevant reactive aldehydes, including formaldehyde and acetaldehyde. Cellular studies show non-cytotoxic amounts of reactive aldehydes induce a decrease in asparagine levels. Biochemical analyses reveal inhibition results from reaction of the aldehydes with the catalytically important N-terminal cysteine of ASNS. The combined cellular and biochemical results suggest a possible mechanism underlying the low asparagine levels in alcoholism and diabetes. The results will stimulate research on the biological consequences of the reactions of aldehydes with nucleophilic residues.

Formaldehyde regulates S-adenosylmethionine biosynthesis and one-carbon metabolism

One-carbon metabolism is an essential branch of cellular metabolism that intersects with epigenetic regulation. In this work, we show how formaldehyde (FA), a one-carbon unit derived from both endogenous sources and environmental exposure, regulates one-carbon metabolism by inhibiting the biosynthesis of S-adenosylmethionine (SAM), the major methyl donor in cells. FA reacts with privileged, hyperreactive cysteine sites in the proteome, including Cys120 in S-adenosylmethionine synthase isoform type-1 (MAT1A). FA exposure inhibited MAT1A activity and decreased SAM production with MAT-isoform specificity. A genetic mouse model of chronic FA overload showed a decrease n SAM and in methylation on selected histones and genes. Epigenetic and transcriptional regulation of Mat1a and related genes function as compensatory mechanisms for FA-dependent SAM depletion, revealing a biochemical feedback cycle between FA and SAM one-carbon units.

Efficient Synthesis of DNA Duplexes Containing Reduced Acetaldehyde Interstrand Cross-Links

DNA interstrand cross-links (ICLs) prevent DNA replication and transcription and can lead to potentially lethal events, such as cancer or bone marrow failure. ICLs are typically repaired by proteins within the Fanconi Anemia (FA) pathway, although the details of the pathway are not fully established. Methods to generate DNA containing ICLs are key to furthering the understanding of DNA cross-link repair. A major route to ICL formation in vivo involves reaction of DNA with acetaldehyde, derived from ethanol metabolism. This reaction forms a three-carbon bridged ICL involving the amino groups of adjacent guanines in opposite strands of a duplex resulting in amino and imino functionalities. A stable reduced form of the ICL has applications in understanding the recognition and repair of these types of adducts. Previous routes to creating DNA duplexes containing these adducts have involved lengthy post-DNA synthesis chemistry followed by reduction of the imine. Here, an efficient and high-yielding approach to the reduced ICL using a novel N2-((R)-4-trifluoroacetamidobutan-2-yl)-2'-deoxyguanosine phosphoramidite is described. Following standard automated DNA synthesis and deprotection, the ICL is formed overnight in over 90% yield upon incubation at room temperature with a complementary oligodeoxyribonucleotide containing 2-fluoro-2'-deoxyinosine. The cross-linked duplex displayed a melting transition 25 °C higher than control sequences. Importantly, we show using the Xenopus egg extract system that an ICL synthesized by this method is repaired by the FA pathway. The simplicity and efficiency of this methodology for preparing reduced acetaldehyde ICLs will facilitate access to these DNA architectures for future studies on cross-link repair.

Aldehyde-driven transcriptional stress triggers an anorexic DNA damage response

Endogenous DNA damage can perturb transcription, triggering a multifaceted cellular response that repairs the damage, degrades RNA polymerase II and shuts down global transcription1-4. This response is absent in the human disease Cockayne syndrome, which is caused by loss of the Cockayne syndrome A (CSA) or CSB proteins5-7. However, the source of endogenous DNA damage and how this leads to the prominent degenerative features of this disease remain unknown. Here we find that endogenous formaldehyde impedes transcription, with marked physiological consequences. Mice deficient in formaldehyde clearance (Adh5-/-) and CSB (Csbm/m; Csb is also known as Ercc6) develop cachexia and neurodegeneration, and succumb to kidney failure, features that resemble human Cockayne syndrome. Using single-cell RNA sequencing, we find that formaldehyde-driven transcriptional stress stimulates the expression of the anorexiogenic peptide GDF15 by a subset of kidney proximal tubule cells. Blocking this response with an anti-GDF15 antibody alleviates cachexia in Adh5-/-Csbm/m mice. Therefore, CSB provides protection to the kidney and brain against DNA damage caused by endogenous formaldehyde, while also suppressing an anorexic endocrine signal. The activation of this signal might contribute to the cachexia observed in Cockayne syndrome as well as chemotherapy-induced anorectic weight loss. A plausible evolutionary purpose for such a response is to ensure aversion to genotoxins in food.

Two Aldehyde Clearance Systems Are Essential to Prevent Lethal Formaldehyde Accumulation in Mice and Humans

Reactive aldehydes arise as by-products of metabolism and are normally cleared by multiple families of enzymes. We find that mice lacking two aldehyde detoxifying enzymes, mitochondrial ALDH2 and cytoplasmic ADH5, have greatly shortened lifespans and develop leukemia. Hematopoiesis is disrupted profoundly, with a reduction of hematopoietic stem cells and common lymphoid progenitors causing a severely depleted acquired immune system. We show that formaldehyde is a common substrate of ALDH2 and ADH5 and establish methods to quantify elevated blood formaldehyde and formaldehyde-DNA adducts in tissues. Bone-marrow-derived progenitors actively engage DNA repair but also imprint a formaldehyde-driven mutation signature similar to aging-associated human cancer mutation signatures. Furthermore, we identify analogous genetic defects in children causing a previously uncharacterized inherited bone marrow failure and pre-leukemic syndrome. Endogenous formaldehyde clearance alone is therefore critical for hematopoiesis and in limiting mutagenesis in somatic tissues.

Bypass of Mutagenic O(6)-Carboxymethylguanine DNA Adducts by Human Y- and B-Family Polymerases

The generation of chemical alkylating agents from nitrosation of glycine and bile acid conjugates in the gastrointestinal tract is hypothesized to initiate carcinogenesis. O(6)-carboxymethylguanine (O(6)-CMG) is a product of DNA alkylation derived from nitrosated glycine. Although the tendency of the structurally related adduct O(6)-methylguanine to code for the misincoporation of TTP during DNA replication is well-established, the impact of the presence of the O(6)-CMG adduct in a DNA template on the efficiency and fidelity of translesion DNA synthesis (TLS) by human DNA polymerases (Pols) has hitherto not been described. Herein, we characterize the ability of the four human TLS Pols η, ι, κ, and ζ and the replicative Pol δ to bypass O(6)-CMG in a prevalent mutational hot-spot for colon cancer. The results indicate that Pol η replicates past O(6)-CMG, incorporating dCMP or dAMP, whereas Pol κ incorporates dCMP only, and Pol ι incorporates primarily dTMP. Additionally, the subsequent extension step was carried out with high efficiency by TLS Pols η, κ, and ζ, while Pol ι was unable to extend from a terminal mismatch. These results provide a first basis of O(6)-CMG-promoted base misincorporation by Y- and B-family polymerases potentially leading to mutational signatures associated with colon cancer.

O6-Carboxymethylguanine in DNA forms a sequence context-dependent wobble base-pair structure with thymine

N-Nitrosation of glycine and its derivatives generates potent alkylating agents that can lead to the formation ofO6-carboxymethylguanine (O6-CMG) in DNA.O6-CMG has been identified in DNA derived from human colon tissue and its occurrence has been linked to diets high in red and processed meats, implying an association with the induction of colorectal cancer. By analogy toO6-methylguanine,O6-CMG is expected to be mutagenic, inducing G-to-A mutations that may be the molecular basis of increased cancer risk. Previously, the crystal structure of the DNA dodecamer d(CGCG[O6-CMG]ATTCGCG) has been reported, in whichO6-CMG forms a Watson–Crick-type pair with thymine similar to the canonical A:T pair. In order to further investigate the versatility ofO6-CMG in base-pair formation, the structure of the DNA dodecamer d(CGC[O6-CMG]AATTTGCG) containingO6-CMG at a different position has been determined by X-ray crystallography using four crystal forms obtained under conditions containing different solvent ions (Sr2+, Ba2+, Mg2+, K+or Na+) with and without Hoechst 33258. The most striking finding is that the pairing modes ofO6-CMG with T are quite different from those previously reported. In the present dodecamer, the T bases are displaced (wobbled) into the major groove to form a hydrogen bond between the thymine N3N—H and the carboxyl group ofO6-CMG. In addition, a water molecule is bridged through two hydrogen bonds between the thymine O2atom and the 2-amino group ofO6-CMG to stabilize the pairing. These interaction modes commonly occur in the four crystal forms, regardless of the differences in crystallization conditions. The previous and the present results show thatO6-CMG can form a base pair with T in two alternative modes: the Watson–Crick type and a high-wobble type, the nature of which may depend on the DNA-sequence context.

Structures of DNA duplexes containing O6-carboxymethylguanine, a lesion associated with gastrointestinal cancer, reveal a mechanism for inducing pyrimidine transition mutations

N-nitrosation of glycine and its derivatives generates potent alkylating agents that can lead to the formation of O(6)-carboxymethylguanine (O(6)-CMG) in DNA. O(6)-CMG has been identified in DNA derived from human colon tissue, and its occurrence has been linked to diets high in red and processed meats. By analogy to O(6)-methylguanine, O(6)-CMG is expected to be highly mutagenic, inducing G to A mutations during DNA replication that can increase the risk of gastrointestinal and other cancers. Two crystal structures of DNA dodecamers d(CGCG[O(6)-CMG]ATTCGCG) and d(CGC[O(6)-CMG]AATTCGCG) in complex with Hoechst33258 reveal that each can form a self-complementary duplex to retain the B-form conformation. Electron density maps clearly show that O(6)-CMG forms a Watson-Crick-type pair with thymine similar to the canonical A:T pair, and it forms a reversed wobble pair with cytosine. In situ structural modeling suggests that a DNA polymerase can accept the Watson-Crick-type pair of O(6)-CMG with thymine, but might also accept the reversed wobble pair of O(6)-CMG with cytosine. Thus, O(6)-CMG would permit the mis-incorporation of dTTP during DNA replication. Alternatively, the triphosphate that would be formed by carboxymethylation of the nucleotide triphosphate pool d[O(6)-CMG]TP might compete with dATP incorporation opposite thymine in a DNA template.

The nitrosated bile acid DNA lesion O6-carboxymethylguanine is a substrate for the human DNA repair protein O6-methylguanine-DNA methyltransferase

The consumption of red meat is a risk factor in human colorectal cancer (CRC). One hypothesis is that red meat facilitates the nitrosation of bile acid conjugates and amino acids, which rapidly convert to DNA-damaging carcinogens. Indeed, the toxic and mutagenic DNA adduct O⁶-carboxymethylguanine (O⁶-CMG) is frequently present in human DNA, increases in abundance in people with high levels of dietary red meat and may therefore be a causative factor in CRC. Previous reports suggested that O⁶-CMG is not a substrate for the human version of the DNA damage reversal protein O⁶-methylguanine-DNA methyltransferase (MGMT), which protects against the genotoxic effects of other O⁶-alkylguanine lesions by removing alkyl groups from the O⁶-position. We now show that synthetic oligodeoxyribonucleotides containing the known MGMT substrate O⁶-methylguanine (O⁶-MeG) or O⁶-CMG effectively inactivate MGMT in vitro (IC₅₀ 0.93 and 1.8 nM, respectively). Inactivation involves the removal of the O⁶-alkyl group and its transfer to the active-site cysteine residue of MGMT. O⁶-CMG is therefore an MGMT substrate, and hence MGMT is likely to be a protective factor in CRC under conditions where O⁶-CMG is a potential causative agent.

Convenient and efficient syntheses of oligodeoxyribonucleotides containing O(6)-(carboxymethyl)guanine and O(6)-(4-oxo-4-(3-pyridyl)butyl)guanine

O(6)-(carboxymethyl)guanine (O(6)-CMG) and O(6)-(4-oxo-4-(3-pyridyl)butyl)guanine (O(6)-pobG) are toxic lesions formed in DNA following exposure to alkylating agents. O(6)-CMG results from exposure to nitrosated glycine or nitrosated bile acid conjugates and may be associated with diets rich in red meat. O(6)-pobG lesions are derived from alkylating agents found in tobacco smoke. Efficient syntheses of oligodeoxyribonucleotides (ODNs) containing O(6)-CMG and O(6)-pobG are described that involve nucleophilic displacement by the appropriate alcohol on a common synthetic ODN containing the reactive base 2-amino-6-methylsulfonylpurine. ODNs containing O(6)-pobG and O (6)-CMG were found to be good substrates for the S. pombe alkyltransferase-like protein Atl1.

Atl1 regulates choice between global genome and transcription-coupled repair of O(6)-alkylguanines

Nucleotide excision repair (NER) has long been known to remove DNA lesions induced by chemical carcinogens, and the molecular mechanism has been partially elucidated. Here we demonstrate that in Schizosaccharomyces pombe a DNA recognition protein, alkyltransferase-like 1 (Atl1), can play a pivotal role in selecting a specific NER pathway, depending on the nature of the DNA modification. The relative ease of dissociation of Atl1 from DNA containing small O(6)-alkylguanines allows accurate completion of global genome repair (GGR), whereas strong Atl1 binding to bulky O(6)-alkylguanines blocks GGR, stalls the transcription machinery, and diverts the damage to transcription-coupled repair. Our findings redraw the initial stages of the NER process in those organisms that express an alkyltransferase-like gene and raise the question of whether or not O(6)-alkylguanine lesions that are poor substrates for the alkyltransferase proteins in higher eukaryotes might, by analogy, signal such lesions for repair by NER.

Insights into the DNA stabilizing contributions of a bicyclic cytosine analogue: crystal structures of DNA duplexes containing 7,8-dihydropyrido [2,3-d]pyrimidin-2-one

The incorporation of the bicyclic cytosine analogue 7,8-dihydropyrido[2,3-d]pyrimidin-2-one (X) into DNA duplexes results in a significant enhancement of their stability (3–4 K per modification). To establish the effects of X on the local hydrogen-bonding and base stacking interactions and the overall DNA conformation, and to obtain insights into the correlation between the structure and stability of X-containing DNA duplexes, the crystal structures of [d(CGCGAATT-X-GCG)]₂ and [d(CGCGAAT-X-CGCG)]₂ have been determined at 1.9–2.9 Å resolutions. In all of the structures, the analogue X base pairs with the purine bases on the opposite strands through Watson–Crick and/or wobble type hydrogen bonds. The additional ring of the X base is stacked on the thymine bases at the 5′-side and overall exhibits greatly enhanced stacking interactions suggesting that this is a major contribution to duplex stabilization.

Crystal structures of DNA duplexes stabilized by bicyclic-C residues

Chemical modification of nucleic acids is being studied extensively as an approach for the development of nucleic acid-based therapies. We found that a nucleotide carrying 7,8-dihydropyrido[2,3-d]pyrimidin-2-one (bicyclic-C or X), which is a cytosine derivative with a propene attached at the N4 and C5 atoms, increases the stability of DNA duplexes. To establish the conformational effects of X on DNA and to obtain insight into the correlation between the structure and stability of X-containing DNA duplexes, the crystal structures of [d(CGCGAATT-X-GCG)](2) and [d(CGCGAAT-X-CGCG)](2) have been determined at 2.9 A resolutions. In both duplexes, the bicyclic-C bases form pairs with the counter bases through hydrogen bonds, and stabilize the duplex formation in part by stacking interactions between X and the subsequent thymine base of the same strand.