{"id":2670,"date":"2019-02-15T17:43:36","date_gmt":"2019-02-15T08:43:36","guid":{"rendered":"http:\/\/163.180.4.222\/lab\/?p=2670"},"modified":"2019-02-15T17:43:36","modified_gmt":"2019-02-15T08:43:36","slug":"revealing-a-microbial-carcinogen","status":"publish","type":"post","link":"https:\/\/biochemistry.khu.ac.kr\/lab\/?p=2670","title":{"rendered":"Revealing a microbial carcinogen"},"content":{"rendered":"

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The microbiota in the human gastrointestinal system is predicted to produce hundreds of unique small molecules and secondary metabolites that may influence host health and disease (1<\/em><\/a>). Many such molecules are produced by sophisticated multienzymatic assembly lines that are encoded by bacterial biosynthetic gene clusters. One class of molecules, colibactins, are produced from the gene cluster called the polyketide synthase (pks<\/em>) island. The\u00a0pks<\/em>\u00a0island occurs in certain strains of\u00a0Escherichia coli<\/em>\u00a0and is prevalent in the microbiota of colorectal cancer (CRC) patients (2<\/em><\/a>\u20135<\/em><\/a>). However, despite more than a decade of research into the potential carcinogenic role of colibactin, little is known about its structure or mechanism of action. On page 709 of this issue, Wilson\u00a0et al.<\/em>(6<\/em><\/a>) show that colibactin alkylates DNA in cultured cells and in vivo, forming covalent modifications known as DNA adducts. These colibactin-DNA adducts are chemical evidence of DNA damage and represent a detectable signature of exposure to colibactin. Misrepaired DNA adducts may generate mutations that contribute to colorectal tumorigenesis.<\/p>\n

Colibactin was first described as an unknown product of a 54-kilobase genomic island that encodes a hybrid nonribosomal peptide synthetase\u2013polyketide synthase gene cluster, the\u00a0pks<\/em>island, in some commensal and extraintestinal pathogenic\u00a0E. coli<\/em>\u00a0strains (2<\/em><\/a>). Exposure to\u00a0pks<\/em>+<\/sup>\u00a0E. coli<\/em>\u00a0induces DNA double-strand breaks and an increased gene mutation frequency in mammalian cells in culture (3<\/em><\/a>). This raised speculation that products of\u00a0pk<\/em>s were microbial-derived genotoxins that could promote cancer. The tumorigenic potential of\u00a0pks<\/em>\u00a0products was demonstrated in a study showing that\u00a0pks<\/em>+<\/sup>\u00a0E. coli<\/em>\u00a0was abundant in colon tissue from CRC patients and promoted CRC in mouse models (4<\/em><\/a>). This tumorigenic effect was later demonstrated in several other mouse models of CRC (5<\/em><\/a>,\u00a07<\/em><\/a>,\u00a08<\/em><\/a>).<\/p>\n

Because of its instability, the structure of colibactin has been elusive (2<\/em><\/a>). Most previous work to determine the structure of colibactin has focused on identifying stable precursors using mutant strains of\u00a0E. coli<\/em>\u00a0missing the colibactin-producing peptidase ClbP, which activates colibactin precursors by removing an\u00a0N<\/em>-myristoyl-D<\/span>-asparagine \u201cprodrug group\u201d (9<\/em><\/a>). However, the precursors do not necessarily represent a final colibactin structure. Previous research suggested that colibactin alkylates DNA and forms a DNA adduct via a cyclopropane functional group, called the \u201cwarhead,\u201d which is structurally similar to other natural products that alkylate DNA (9<\/em><\/a>,\u00a010<\/em><\/a>). The importance of the cyclopropane ring was confirmed by identification of the colibactin resistance protein ClbS, which inactivates the cyclopropane ring to provide self-protection against DNA damage in the\u00a0pks<\/em>+<\/sup>\u00a0bacteria (11<\/em><\/a>). Recently, colibactin-DNA adducts with similar structures were detected in vitro using purified DNA and colibactin-producing bacteria (10<\/em><\/a>). However, there was no direct evidence or structural characterization of these colibactin-DNA adducts in a biological setting.<\/p>\n

Wilson\u00a0et al.<\/em>\u00a0used an untargeted mass spectrometry DNA adductomics approach to structurally and mechanistically define a DNA alkylation end product of colibactin exposure. They identified two stereoisomeric colibactin adducts to the DNA nucleotide adenine in cultured mammalian cells and in colonic epithelial cells of formerly germfree (sterile) mice colonized with a single\u00a0pks<\/em>+<\/sup>\u00a0E. coli<\/em>strain, providing direct evidence that these DNA adducts occur in vivo. As the authors note, the structure they uncovered does not necessarily represent the immediate colibactin-DNA adduct but is likely a degradation product of a larger colibactin adduct. This study provides important information about the structure and mechanism of action of colibactin. Furthermore, it describes a mass spectrometry method that could be used to identify other intractable compounds.<\/p>\n

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