Structural insights into the recognition of cisplatin and AAF-dG lesion by Rad14 (XPA).

Nucleotide excision repair (NER) is responsible for the removal of a large variety of structurally diverse DNA lesions. Mutations of the involved proteins cause the xeroderma pigmentosum (XP) cancer predisposition syndrome. Although the general mechanism of the NER process is well studied, the function of the XPA protein, which is of central importance for successful NER, has remained enigmatic. It is known, that XPA binds kinked DNA structures and that it interacts also with DNA duplexes containing certain lesions, but the mechanism of interactions is unknown. Here we present two crystal structures of the DNA binding domain (DBD) of the yeast XPA homolog Rad14 bound to DNA with either a cisplatin lesion (1,2-GG) or an acetylaminofluorene adduct (AAF-dG). In the structures, we see that two Rad14 molecules bind to the duplex, which induces DNA melting of the duplex remote from the lesion. Each monomer interrogates the duplex with a ß-hairpin, which creates a 13mer duplex recognition motif additionally characterized by a sharp 70° DNA kink at the position of the lesion. Although the 1,2-GG lesion stabilizes the kink due to the covalent fixation of the crosslinked dG bases at a 90° angle, the AAF-dG fully intercalates into the duplex to stabilize the kinked structure.

Zinc finger oxidation of Fpg/Nei DNA glycosylases by 2-thioxanthine: biochemical and X-ray structural characterization.

DNA glycosylases from the Fpg/Nei structural superfamily are base excision repair enzymes involved in the removal of a wide variety of mutagen and potentially lethal oxidized purines and pyrimidines. Although involved in genome stability, the recent discovery of synthetic lethal relationships between DNA glycosylases and other pathways highlights the potential of DNA glycosylase inhibitors for future medicinal chemistry development in cancer therapy. By combining biochemical and structural approaches, the physical target of 2-thioxanthine (2TX), an uncompetitive inhibitor of Fpg, was identified. 2TX interacts with the zinc finger (ZnF) DNA binding domain of the enzyme. This explains why the zincless hNEIL1 enzyme is resistant to 2TX. Crystal structures of the enzyme bound to DNA in the presence of 2TX demonstrate that the inhibitor chemically reacts with cysteine thiolates of ZnF and induces the loss of zinc. The molecular mechanism by which 2TX inhibits Fpg may be generalized to all prokaryote and eukaryote ZnF-containing Fpg/Nei-DNA glycosylases. Cell experiments show that 2TX can operate in cellulo on the human Fpg/Nei DNA glycosylases. The atomic elucidation of the determinants for the interaction of 2TX to Fpg provides the foundation for the future design and synthesis of new inhibitors with high efficiency and selectivity.

Unexpected non-Hoogsteen-based mutagenicity mechanism of FaPy-DNA lesions.

8-Oxopurines (8-oxodG and 8-oxodA) and formamidopyrimidines (FaPydG and FaPydA) are major oxidative DNA lesions involved in cancer development and aging. Their mutagenicity is believed to result from a conformational shift of the N9-C1' glycosidic bonds from anti to syn, which allows the lesions to form noncanonical Hoogsteen-type base pairs with incoming triphosphates during DNA replication. Here we present biochemical data and what are to our knowledge the first crystal structures of carbocyclic FaPydA and FaPydG containing DNA in complex with a high-fidelity polymerase. Crystallographic snapshots show that the cFaPy lesions keep the anti geometry of the glycosidic bond during error-free and error-prone replication. The observed dG·dC→dT·dA transversion mutations are the result of base shifting and tautomerization.

Cockayne syndrome: varied requirement of transcription-coupled nucleotide excision repair for the removal of three structurally different adducts from transcribed DNA.

Hereditary defects in the transcription-coupled nucleotide excision repair (TC-NER) pathway of damaged DNA cause severe neurodegenerative disease Cockayne syndrome (CS), however the origin and chemical nature of the underlying DNA damage had remained unknown. To find out, to which degree the structural properties of DNA lesions determine the extent of transcription arrest in human CS cells, we performed quantitative host cell reactivation analyses of expression vectors containing various synthetic adducts. We found that a single 3-(deoxyguanosin-N2-yl)-2-acetylaminofluorene adduct (dG(N2)-AAF) constitutes an unsurmountable obstacle to transcription in both CS-A and CS-B cells and is removed exclusively by the CSA- and CSB-dependent pathway. In contrast, contribution of the CS proteins to the removal of two other transcription-blocking DNA lesions - N-(deoxyguanosin-8-yl)-2-acetylaminofluorene (dG(C8)-AAF) and cyclobutane thymine-thymine (TT) dimer - is only minor (TT dimer) or none (dG(C8)-AAF). The unique properties of dG(N2)-AAF identify this adduct as a prototype for a new class of DNA lesions that escape the alternative global genome repair and could be critical for the CS pathogenesis.