Because the replication of hepatitis B virus (HBV) proceeds via an obligatory reverse transcription step in the viral capsid, cDNA is potentially vulnerable to editing by cytidine deaminases of the APOBEC3 family. To date only two edited HBV genomes, referred to as G 3 A hypermutants, have been described in vivo. Recent work suggested that HBV replication was indeed restricted by APOBEC3G but by a mechanism other than editing. The issue of restriction has been explored by using a sensitive PCR method allowing differential amplification of AT-rich DNA. G 3 A hypermutated HBV genomes were recovered from transfection experiments involving APOBEC3B, -3C, -3F, and -3G indicating that all four enzymes were able to extensively deaminate cytidine residues in minus-strand DNA. Unexpectedly, three of the four enzymes (APOBEC3B, -3F, and -3G) deaminated HBV plus-strand DNA as well. From the serum of two of four patients with high viremia, G 3 A hypermutated genomes were recovered at a frequency of Ϸ10 ؊4 , indicating that they are, albeit relatively rare, part of the natural cycle of HBV infection. These findings suggest that human APOBEC3 enzymes can impact HBV replication via cytidine deamination.hypermutation ͉ DNA editing G 3 A hypermutated retroviral genomes result from the editing of nascent DNA by APOBEC3 cytidine deaminases (1-7). This was originally demonstrated for HIV-1 and the APOBEC3G member of a cluster of seven genes (3A-3H) on human chromosome 22 (8, 9). Editing occurs on the background of a ⌬vif genome (1-5, 10). The HIV Vif protein prevents packaging of either APOBEC3F or -3G resulting in their ubiquitination and degradation by the proteasomal pathway (11)(12)(13)(14). Editing of cytidine residues in neo-synthesized minusstrand cDNA results in the formation of multiple uracil residues that are read as T during plus-strand synthesis. Albeit referred to as G 3 A hypermutants by reference to the viral plus strand, mechanistically the action is occurring on the minus strand and independently of reverse transcriptase (7). Up to 60% of G residues in a lentiviral genome can be substituted resulting in the total loss of information (15-17). As such, APOBEC3F and -3G constitute a powerful restriction mechanism to reverse transcription. Retroviruses have either to avoid replication in cells expressing APOBEC3 molecules or else evolve a mechanism that neutralizes their effect. The vif gene of human and primate lentiviruses, as well as their homologues in most of the other lentiviruses, reflect the latter solution.Hepatitis B viruses (HBVs) replicate via an obligate reverse transcription step occurring in a capsid structure close to the endoplasmic reticulum. A pair of G 3 A hypermutated genomes were identified in the blood of a chronically infected patient yet have remained unique despite a burgeoning database (18). Recent reports demonstrated that HBV replication could be strongly restricted by APOBEC3G and -3F in an experimental setting (20,21). Restriction was highlighted by a strong reduction in the proportion o...

The human APOBEC3 (A3A-A3H) locus encodes six cytidine deaminases that edit single-stranded DNA, the result being DNA peppered with uridine. Although several cytidine deaminases are clearly restriction factors for retroviruses and hepadnaviruses, it is not known if APOBEC3 enzymes have roles outside of these settings. It is shown here that both human mitochondrial and nuclear DNA are vulnerable to somatic hypermutation by A3 deaminases, with APOBEC3A standing out among them. The degree of editing is much greater in patients lacking the uracil DNA-glycolyase gene, indicating that the observed levels of editing reflect a dynamic composed of A3 editing and DNA catabolism involving uracil DNA-glycolyase. Nonetheless, hyper-and lightly mutated sequences went hand in hand, raising the hypothesis that recurrent lowlevel mutation by APOBEC3A could catalyze the transition from a healthy to a cancer genome.

DNA viruses, retroviruses and hepadnaviruses, such as hepatitis B virus (HBV), are vulnerable to genetic editing of single stranded DNA by host cell APOBEC3 (A3) cytidine deaminases. At least three A3 genes are up regulated by interferon-α in human hepatocytes while ectopic expression of activation induced deaminase (AICDA), an A3 paralog, has been noted in a variety of chronic inflammatory syndromes including hepatitis C virus infection. Yet virtually all studies of HBV editing have confined themselves to analyses of virions from culture supernatants or serum where the frequency of edited genomes is generally low (≤10−2). We decided to look at the nature and frequency of HBV editing in cirrhotic samples taken during removal of a primary hepatocellular carcinoma. Forty-one cirrhotic tissue samples (10 alcoholic, 10 HBV+, 11 HBV+HCV+ and 10 HCV+) as well as 4 normal livers were studied. Compared to normal liver, 5/7 APOBEC3 genes were significantly up regulated in the order: HCV±HBV>HBV>alcoholic cirrhosis. A3C and A3D were up regulated for all groups while the interferon inducible A3G was over expressed in virus associated cirrhosis, as was AICDA in ∼50% of these HBV/HCV samples. While AICDA can indeed edit HBV DNA ex vivo, A3G is the dominant deaminase in vivo with up to 35% of HBV genomes being edited. Despite these highly deleterious mutant spectra, a small fraction of genomes survive and contribute to loss of HBeAg antigenemia and possibly HBsAg immune escape. In conclusion, the cytokine storm associated with chronic inflammatory responses to HBV and HCV clearly up regulates a number of A3 genes with A3G clearly being a major restriction factor for HBV. Although the mutant spectrum resulting from A3 editing is highly deleterious, a very small part, notably the lightly edited genomes, might help the virus evolve and even escape immune responses.