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The deaminase APOBEC3B triggers the death of cells lacking uracil DNA glycosylase.

ABSTRACT: Human cells express up to 9 active DNA cytosine deaminases with functions in adaptive and innate immunity. Many cancers manifest an APOBEC mutation signature and APOBEC3B (A3B) is likely the main enzyme responsible. Although significant numbers of APOBEC signature mutations accumulate in tumor genomes, the majority of APOBEC-catalyzed uracil lesions are probably counteracted in an error-free manner by the uracil base excision repair pathway. Here, we show that A3B-expressing cells can be selectively killed by inhibiting uracil DNA glycosylase 2 (UNG) and that this synthetic lethal phenotype requires functional mismatch repair (MMR) proteins and p53. UNG knockout human 293 and MCF10A cells elicit an A3B-dependent death. This synthetic lethal phenotype is dependent on A3B catalytic activity and reversible by UNG complementation. A3B expression in UNG-null cells causes a buildup of genomic uracil, and the ensuing lethality requires processing of uracil lesions (likely U/G mispairs) by MSH2 and MLH1 (likely noncanonical MMR). Cancer cells expressing high levels of endogenous A3B and functional p53 can also be killed by expressing an UNG inhibitor. Taken together, UNG-initiated base excision repair is a major mechanism counteracting genomic mutagenesis by A3B, and blocking UNG is a potential strategy for inducing the selective death of tumors.

SUBMITTER: Serebrenik AA

PROVIDER: S-EPMC6825264 | biostudies-literature | 2019 Oct

REPOSITORIES: biostudies-literature

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The deaminase APOBEC3B triggers the death of cells lacking uracil DNA glycosylase.

Serebrenik Artur A AA Starrett Gabriel J GJ Leenen Sterre S Jarvis Matthew C MC Shaban Nadine M NM Salamango Daniel J DJ Nilsen Hilde H Brown William L WL Harris Reuben S RS

Proceedings of the National Academy of Sciences of the United States of America 20191014 44

Human cells express up to 9 active DNA cytosine deaminases with functions in adaptive and innate immunity. Many cancers manifest an APOBEC mutation signature and APOBEC3B (A3B) is likely the main enzyme responsible. Although significant numbers of APOBEC signature mutations accumulate in tumor genomes, the majority of APOBEC-catalyzed uracil lesions are probably counteracted in an error-free manner by the uracil base excision repair pathway. Here, we show that A3B-expressing cells can be selecti ...[more]

PMID: 31611371

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Project description:UnlabelledSeveral recent studies have converged upon the innate immune DNA cytosine deaminase APOBEC3B (A3B) as a significant source of genomic uracil lesions and mutagenesis in multiple human cancers, including those of the breast, head/neck, cervix, bladder, lung, ovary, and other tissues. A3B is upregulated in these tumor types relative to normal tissues, but the mechanism is unclear. Because A3B also has antiviral activity in multiple systems and is a member of the broader innate immune response, we tested the hypothesis that human papillomavirus (HPV) infection causes A3B upregulation. We found that A3B mRNA expression and enzymatic activity were upregulated following transfection of a high-risk HPV genome and that this effect was abrogated by inactivation of E6. Transduction experiments showed that the E6 oncoprotein alone was sufficient to cause A3B upregulation, and a panel of high-risk E6 proteins triggered higher A3B levels than did a panel of low-risk or noncancer E6 proteins. Knockdown experiments in HPV-positive cell lines showed that endogenous E6 is required for A3B upregulation. Analyses of publicly available head/neck cancer data further support this relationship, as A3B levels are higher in HPV-positive cancers than in HPV-negative cancers. Taken together with the established role for high-risk E6 in functional inactivation of TP53 and published positive correlations in breast cancer between A3B upregulation and genetic inactivation of TP53, our studies suggest a model in which high-risk HPV E6, possibly through functional inactivation of TP53, causes derepression of A3B gene transcription. This would lead to a mutator phenotype that explains the observed cytosine mutation biases in HPV-positive head/neck and cervical cancers.ImportanceThe innate immune DNA cytosine deaminase APOBEC3B (A3B) accounts for a large proportion of somatic mutations in cervical and head/neck cancers, but nothing is known about the mechanism responsible for its upregulation in these tumor types. Almost all cervical carcinomas and large proportions of head/neck tumors are caused by human papillomavirus (HPV) infection. Here, we establish a mechanistic link between HPV infection and A3B upregulation. The E6 oncoprotein of high-risk, but not low-risk, HPV types triggers A3B upregulation, supporting a model in which TP53 inactivation causes a derepression of A3B gene transcription and elevated A3B enzyme levels. This virus-induced mutator phenotype provides a mechanistic explanation for A3B signature mutations observed in HPV-positive head/neck and cervical carcinomas and may also help to account for the preferential cancer predisposition caused by high-risk HPV isolates.

Project description:Two dimensional (2D) NMR and molecular dynamics simulations have been used to determine the three dimensional (3D) structure of a hairpin DNA, d-CTA-GAGGATCC-TUTT-GGATCCT (22mer; abbreviated as U2-hairpin), which has uracil at the second position from the 5' end of the tetraloop. The(1)H resonances of this hairpin have been assigned almost completely. NMR restrained molecular dynamics and energy minimization procedures have been used to describe the 3D structure of U2-hairpin. This study establishes that the stem of the hairpin adopts a right-handed B-DNA conformation, while the T(12)and T(15)nucleotides stack upon 3' and 5' ends of the stem, respectively. Further, T(14)stacks upon both T(12)and T(15). Though U(13)partially stacks upon T(14), no stacking interaction is observed between U(13)and T(12). All the individual nucleotide bases belonging to the stem and T(12)and T(15)of the loop adopt ' anti ' conformation with respect to their sugar moiety, while the U(13)and T(14)of the loop are in ' syn ' conformation. The turning phosphate in the loop is located between T(13)and T(14). This study and a concurrent NMR structural study on yet another hairpin DNA d-CTAGAGGAATAA-TTTU-GGATCCT (22mer; abbreviated as U4-hairpin), with uracil at the fourth position from the 5' end of the tetraloop throw light upon various interactions which have been reported between Escherichia coli uracil DNA glycosylase (UDG) and uracil containing DNA. The epsilon of T(12)and alpha, beta, gamma, epsilon and zeta of U(13)and gamma of T(14), which partially influence the local conformation of U(13)in U2-hairpin are all locked in ' trans ' conformation. Such stretched out backbone conformation in the vicinity of U(13)could be the reason as to why the U2-hairpin is found to be the poor substrate for its interaction with UDG compared to the other substrates in which the uracil is at first, third and fourth positions of the tetraloop from its 5' end, as reported earlier by Vinay and Varshney. This study shows that UDG actively promotes the flipping of uracil from a stacked conformation and rules out the possibility of UDG recognizing the flipped out uracil bases.

Dataset Information

The deaminase APOBEC3B triggers the death of cells lacking uracil DNA glycosylase.

Publications

The deaminase APOBEC3B triggers the death of cells lacking uracil DNA glycosylase.

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