Identification of Bioactive SNM1A Inhibitors

Two-Step Validation Approach for Tools To Study the DNA Repair Enzyme SNM1A

We report a two-step validation approach to evaluate the suitability of metal-binding groups for targeting DNA damage-repair metalloenzymes using model enzyme SNM1A. A fragment-based screening approach was first used to identify metal-binding fragments suitable for targeting the enzyme. Effective fragments were then incorporated into oligonucleotides using the copper-catalysed azide-alkyne cycloaddition reaction. These modified oligonucleotides were recognised by SNM1A at >1000-fold lower concentrations than their fragment counterparts. The exonuclease SNM1A is a key enzyme involved in the repair of interstrand crosslinks, a highly cytotoxic form of DNA damage. However, SNM1A and other enzymes of this class are poorly understood, as there is a lack of tools available to facilitate their study. Our novel approach of incorporating functional fragments into oligonucleotides is broadly applicable to generating modified oligonucleotide structures with high affinity for DNA damage-repair enzymes.

High-Throughput Exonuclease Assay Based on the Fluorescent Base Analogue 2-Aminopurine

Exonucleases are essential enzymes that remove nucleotides from free DNA ends during DNA replication, DNA repair, and telomere maintenance. Due to their essential role, they are potential targets for novel anticancer and antimicrobial drugs but have so far been little exploited. Here, we present a simple and versatile real-time exonuclease assay based on 2-aminopurine, an intrinsically fluorescent nucleotide that is quenched by neighboring bases when embedded in DNA. We show that our assay is applicable to different eukaryotic and bacterial exonucleases acting on both 3' and 5' DNA ends over a wide range of protein activities and suitable for a high-throughput inhibitor screening campaign. Using our assay, we discover a novel inhibitor of the Mycobacterium tuberculosis PHP-exonuclease that is part of the replicative DNA polymerase DnaE1. Hence, our novel assay will be a useful tool for high-throughput screening for novel exonuclease inhibitors that may interfere with DNA replication or DNA maintenance.

Dual targeting of Saccharomyces cerevisiae Pso2 to mitochondria and the nucleus, and its functional relevance in the repair of DNA interstrand crosslinks

Repair of DNA interstrand crosslinks involves a functional interplay among different DNA surveillance and repair pathways. Previous work has shown that interstrand crosslink-inducing agents cause damage to Saccharomyces cerevisiae nuclear and mitochondrial DNA, and its pso2/snm1 mutants exhibit a petite phenotype followed by loss of mitochondrial DNA integrity and copy number. Complex as it is, the cause and underlying molecular mechanisms remains elusive. Here, by combining a wide range of approaches with in vitro and in vivo analyses, we interrogated the subcellular localization and function of Pso2. We found evidence that the nuclear-encoded Pso2 contains 1 mitochondrial targeting sequence and 2 nuclear localization signals (NLS1 and NLS2), although NLS1 resides within the mitochondrial targeting sequence. Further analysis revealed that Pso2 is a dual-localized interstrand crosslink repair protein; it can be imported into both nucleus and mitochondria and that genotoxic agents enhance its abundance in the latter. While mitochondrial targeting sequence is essential for mitochondrial Pso2 import, either NLS1 or NLS2 is sufficient for its nuclear import; this implies that the 2 nuclear localization signal motifs are functionally redundant. Ablation of mitochondrial targeting sequence abrogated mitochondrial Pso2 import, and concomitantly, raised its levels in the nucleus. Strikingly, mutational disruption of both nuclear localization signal motifs blocked the nuclear Pso2 import; at the same time, they enhanced its translocation into the mitochondria, consistent with the notion that the relationship between mitochondrial targeting sequence and nuclear localization signal motifs is competitive. However, the nuclease activity of import-deficient species of Pso2 was not impaired. The potential relevance of dual targeting of Pso2 into 2 DNA-bearing organelles is discussed.

Structure and Function of SNM1 Family Nucleases

Three human nucleases, SNM1A, SNM1B/Apollo, and SNM1C/Artemis, belong to the SNM1 gene family. These nucleases are involved in various cellular functions, including homologous recombination, nonhomologous end-joining, cell cycle regulation, and telomere maintenance. These three proteins share a similar catalytic domain, which is characterized as a fused metallo-β-lactamase and a CPSF-Artemis-SNM1-PSO2 domain. SNM1A and SNM1B/Apollo are exonucleases, whereas SNM1C/Artemis is an endonuclease. This review contains a summary of recent research on SNM1's cellular and biochemical functions, as well as structural biology studies. In addition, protein structure prediction by the artificial intelligence program AlphaFold provides a different view of the proteins' non-catalytic domain features, which may be used in combination with current results from X-ray crystallography and cryo-EM to understand their mechanism more clearly.

5'-Phosphorylation Increases the Efficacy of Nucleoside Inhibitors of the DNA Repair Enzyme SNM1A

Certain cancers exhibit upregulation of DNA interstrand crosslink repair pathways, which contributes to resistance to crosslinking chemotherapy drugs and poor prognoses. Inhibition of enzymes implicated in interstrand crosslink repair is therefore a promising strategy for improving the efficacy of cancer treatment. One such target enzyme is SNM1A, a zinc co-ordinating 5'-3' exonuclease. Previous studies have demonstrated the feasibility of inhibiting SNM1A using modified nucleosides appended with zinc-binding groups. In this work, we sought to develop more effective SNM1A inhibitors by exploiting interactions with the phosphate-binding pocket adjacent to the enzyme's active site, in addition to the catalytic zinc ions. A series of nucleoside derivatives bearing phosphate moieties at the 5'-position, as well as zinc-binding groups at the 3'-position, were prepared and tested in gel-electrophoresis and real-time fluorescence assays. As well as investigating novel zinc-binding groups, we found that incorporation of a 5'-phosphate dramatically increased the potency of the inhibitors.

Synthesis and evaluation of squaramide and thiosquaramide inhibitors of the DNA repair enzyme SNM1A

SNM1A is a zinc-dependent nuclease involved in the removal of interstrand crosslink lesions from DNA. Inhibition of interstrand crosslink repair enzymes such as SNM1A is a promising strategy for improving the efficacy of crosslinking chemotherapy drugs. Initial studies have demonstrated the feasibility of developing SNM1A inhibitors, but the full potential of this enzyme as a drug target has yet to be explored. Herein, the synthesis of a family of squaramide- and thiosquaramide-bearing nucleoside derivatives and their evaluation as SNM1A inhibitors is reported. A gel electrophoresis assay was used to identify nucleoside derivatives bearing an N-hydroxysquaramide or squaric acid moiety at the 3′-position, and a thymidine derivative bearing a 5′-thiosquaramide, as candidate SNM1A inhibitors. Quantitative IC₅₀ determination showed that a thymidine derivative bearing a 5′-thiosquaramide was the most potent inhibitor, followed by a thymidine derivative bearing a 3′-squaric acid. UV–Vis titrations were carried out to evaluate the binding of the (thio)squaramides to zinc ions, allowing the order of inhibitory potency to be rationalised. The membrane permeability of the active inhibitors was investigated, with several compounds showing promise for future in vivo applications.