Multiple mutations are required for cancer development, and genome sequencing has revealed that several cancers, including breast, have somatic mutation spectra dominated by C-to-T transitions1–9. Most of these mutations occur at hydrolytically disfavored10 non-methylated cytosines throughout the genome, and are sometimes clustered8. Here, we show that the DNA cytosine deaminase APOBEC3B (A3B) is a likely source of these mutations. A3B mRNA is up-regulated in the majority of primary breast tumors and breast cancer cell lines. Tumors that express high levels of A3B have twice as many mutations as those that express low levels and are more likely to have mutations in TP53. Endogenous A3B protein is predominantly nuclear and the only detectable source of DNA C-to-U editing activity in breast cancer cell line extracts. Knockdown experiments show that endogenous A3B correlates with elevated levels of genomic uracil, increased mutation frequencies, and C-to-T transitions. Furthermore, induced A3B over-expression causes cell cycle deviations, cell death, DNA fragmentation, γ-H2AX accumulation, and C-to-T mutations. Our data suggest a model in which A3B-catalyzed deamination provides a chronic source of DNA damage in breast cancers that could select TP53 inactivation and explain how some tumors evolve rapidly and manifest heterogeneity.

Background: APOBEC3A is a myeloid-specific interferon-inducible DNA C to U deaminase implicated in innate immunity. Results: APOBEC3A also elicits MeC to T editing activity in vitro with deoxy-oligonucleotides and in vivo with transfected plasmids. Conclusion: APOBEC3A accommodates both normal and larger DNA cytosine substrates. Significance: The developmental specialization and broader substrate range of APOBEC3A may be an evolutionary adaptation for physiological function in foreign DNA restriction.

A study designed to gain baseline information on strains of Escherichia coli displaying resistance to cefoxitin in Canada is described. A total of 29,323 E. coli isolates were screened at 12 participating hospital sites as part of an extended-spectrum beta-lactamase surveillance initiative. A total of 411 clinically significant, nonrepeat isolates displaying reduced susceptibilities to the NCCLS-recommended beta-lactams were submitted to a central laboratory over a 1-year period ending on 30 September 2000. Two hundred thirty-two isolates were identified as resistant to cefoxitin. All cefoxitin-resistant strains were subtyped by pulsed-field gel electrophoresis, and of these, 182 strains revealed a unique fingerprint and 1 strain was untypeable. PCR and sequence analysis of the ampC promoter region revealed 51 different promoter or attenuator variants and 14 wild-type promoters. Three promoter regions were interrupted by insertion elements, two contained IS10 elements, and one contained an IS911 variant. PCR and sequence analysis for the detection of acquired AmpC resistance (by the acquisition of ACT-1/MIR-1, CMY-2, or FOX) revealed that 25 strains contained CMY-2, including 7 of the strains found to have wild-type promoters. The considerable genetic variability in both the strain fingerprint and the promoter region suggests that AmpC-type resistance may emerge spontaneously by mutation of sensitive strains rather than by the spread of strains or plasmids in the hospital setting.Escherichia coli contains a chromosomal ampC gene which has a weak promoter as well as a transcriptional attenuator (13,14). Strains carrying the wild-type gene produce a low basal amount of AmpC and are susceptible to ampicillin. Analysis of cefoxitin-resistant strains has revealed that the ampC promoter and attenuator regions often carry mutations leading to the overproduction of the enzyme (3,4,5,7,10,14,20,21,22,29). These AmpC-overproducing strains are not only resistant to ampicillin but also usually have reduced susceptibilities to expanded-spectrum cephalosporins. Genetically, the most common mutations are those that create a strong promoter that more closely resembles the E. coli consensus promoter (12) and mutations that destabilize the attenuator.Plasmid-mediated ampC genes were first reported in 1988 (25, 31). The plasmid-mediated ampC genes are derived from inducible chromosomal genes that have become mobilized (11,24,25). The first genetically characterized plasmid-mediated AmpC was MIR-1, which was mobilized from an Enterobacter isolate (23). These plasmid-mediated genes are of special interest because their mobility allows them to emerge in one genus or species and spread to different organisms. The prevalence of plasmid-mediated AmpC-type resistance at the national level in most countries is unknown because studies have not examined the strains at the molecular level of detail required to elucidate the different mechanisms involved. A recent report from the United States, however, showed that among 752 Klebsiella spp. a...

APOBEC3G is an important innate immune molecule that causes human immunodeficiency virus type 1 (HIV-1) hypermutation, which can result in detrimental viral genome mutations. The Vif protein of wild-type HIV-1 counteracts APOBEC3G activity by targeting it for degradation and inhibiting its incorporation into viral particles. Additional APOBEC cytidine deaminases have been identified, such as APOBEC3F, which has a similar mode of action but different sequence specificity. A relationship between APOBEC3F/G and HIV disease progression has been proposed. During HIV-1 sequence analysis of the vpu/env region of 240 HIVinfected subjects from Nairobi, Kenya, 13 drastically hypermutated proviral sequences were identified. Sequences derived from plasma virus, however, lacked hypermutation, as did proviral vif. When correlates of disease progression were examined, subjects with hypermutated provirus were found to have significantly higher CD4 counts than the other subjects. Furthermore, hypermutation as estimated by elevated adenine content positively correlated with CD4 count for all 240 study subjects. The sequence context of the observed hypermutation was statistically associated with APOBEC3F/G activity. In contrast to previous studies, this study demonstrates that higher CD4 counts correlate with increased hypermutation in the absence of obvious mutations in the APOBEC inhibiting Vif protein. This strongly suggests that host factors, such as APOBEC3F/G, are playing a protective role in these patients, modulating viral hypermutation and host disease progression. These findings support the potential of targeting APOBEC3F/G for therapeutic purposes.