APOBEC3 proteins are potent restriction factors against retroviral infection in primates. This restriction is accompanied by hypermutations in the retroviral genome that are attributable to the cytidine deaminase activity of the APOBEC3 proteins. Studies of nucleotide sequence diversity among endogenous gammaretroviruses suggest that the evolution of endogenous retroelements could have been shaped by the mutagenic cytidine deaminase activity of APOBEC3. In mice, however, APOBEC3 appears to restrict exogenous murine retroviruses in the absence of detectable levels of deamination. AKV is an endogenous retrovirus that is involved in causing a high incidence of thymic lymphoma in AKR mice. A comparative analysis of several mouse strains revealed a relatively low level of APOBEC3 expression in AKR mice. Here we show that endogenous mouse APOBEC3 restricts AKV infection and that this restriction likely reflects polymorphisms affecting APOBEC3 abundance rather than differences in the APOBEC3 isoforms expressed. We also observe that restriction of AKV by APOBEC3 is accompanied by G3A hypermutations in the viral genome. Our findings demonstrate that APOBEC3 acts as a restriction factor in rodents affecting the strain tropism of AKV, and they provide good support for the proposal that APOBEC3-mediated hypermutation contributed to the evolution of endogenous rodent retroviral genomes.

Deamination of cytosine (C), 5-methylcytosine (mC) and 5-hydroxymethylcytosine (hmC) occurs spontaneously in mammalian DNA with several hundred deaminations occurring in each cell every day. The resulting potentially mutagenic mispairs of uracil (U), thymine (T) or 5-hydroxymethyluracil (hmU) with guanine (G) are substrates for repair by various DNA glycosylases. Here,weshowthat targetedinactivation of the mouse Smug1 DNA glycosylase gene is sufficient to ablate nearly all hmU-DNA excision activity as judged by assay of tissue extracts from knockout mice as well as by the resistance of their embryo fibroblasts to 5-hydroxymethyldeoxyuridine toxicity. Inactivation of Smug1 when combined with inactivation of the Ung uracil-DNA glycosylase gene leads to a loss of nearly all detectable uracil excision activity. Thus, SMUG1 is the dominant glycosylase responsible for hmU-excision in mice as well as the major UNG-backup for U-excision. Both Smug1-knockout and Smug1/Ung-double knockout mice breed normally and remain apparently healthy beyond 1 year of age. However, combined deficiency in SMUG1 and UNG exacerbates the cancer predisposition of Msh2−/− mice suggesting that when both base excision and mismatch repair pathways are defective, the mutagenic effects of spontaneous cytosine deamination are sufficient to increase cancer incidence but do not preclude mouse development.

Excision of uracil introduced into the immunoglobulin loci by AID is central to antibody diversification. While predominantly carried out by the UNG uracil‐DNA glycosylase as reflected by deficiency in immunoglobulin class switching in Ung−/− mice, the deficiency is incomplete, as evidenced by the emergence of switched IgG in the serum of Ung−/− mice. Lack of switching in mice deficient in both UNG and MSH2 suggested that mismatch repair initiated a backup pathway. We now show that most of the residual class switching in Ung−/− mice depends upon the endogenous SMUG1 uracil‐DNA glycosylase, with in vitro switching to IgG1 as well as serum IgG3, IgG2b, and IgA greatly diminished in Ung−/−Smug1−/− mice, and that Smug1 partially compensates for Ung deficiency over time. Nonetheless, using a highly MSH2‐dependent mechanism, Ung−/−Smug1−/− mice can still produce detectable levels of switched isotypes, especially IgG1. While not affecting the pattern of base substitutions, SMUG1 deficiency in an Ung−/− background further reduces somatic hypermutation at A:T base pairs. Our data reveal an essential requirement for uracil excision in class switching and in facilitating noncanonical mismatch repair for the A:T phase of hypermutation presumably by creating nicks near the U:G lesion recognized by MSH2.

Brugada syndrome (BrS) is associated with ventricular tachycardia originating particularly in the right ventricle (RV). We explore electrophysiological features predisposing to such arrhythmic tendency and their possible RV localization in a heterozygotic Scn5a+/− murine model. Nav1.5 mRNA and protein expression were lower in Scn5a+/− than wild-type (WT), with a further reduction in the RV compared with the left ventricle (LV). RVs showed higher expression levels of Kv4.2, Kv4.3 and KChIP2 in both Scn5a+/− and WT. Action potential upstroke velocity and maximum Na+ current (INa) density were correspondingly decreased in Scn5a+/−, with a further reduction in the RV. The voltage dependence of inactivation was shifted to more negative values in Scn5a+/−. These findings are predictive of a localized depolarization abnormality leading to slowed conduction. Persistent Na+ current (IpNa) density was decreased in a similar pattern to INa. RV transient outward current (Ito) density was greater than LV in both WT and Scn5a+/−, and had larger time constants of inactivation. These findings were also consistent with the observation that AP durations were smallest in the RV of Scn5a+/−, fulfilling predictions of an increased heterogeneity of repolarization as an additional possible electrophysiological mechanism for arrhythmogenesis in BrS.

from explanted aortas (p<0.05). Explanted aortas from Nox2-Tg had significantly higher levels of secreted pro-inflammatory cytokine, cyclophilin A (Cypa) at both baseline and after 5 days of in vivo AngII treatment compared to WT littermates. Compared to primary WT EC and VSMC, Nox2-Tg primary EC, but not primary VSMC, had increased ROS production which was accompanied by increased Cypa secretion and ERK1/2 activation. Furthermore, conditioned media from Nox2-Tg EC induced a greater ERK1/2 phosphorylation compared to the media of WT controls. In conclusion, we demonstrate for the first time that a specific increase in endothelial ROS through the over-expression of Nox2 is sufficient to induce aortic dissection in response to AngII stimulation. Endothelial secreted Cypa could be the signalling mechanism by which increased endothelial ROS regulates the inflammatory response and the susceptibility to aortic dissection.