Human APOBEC3G prevents emergence of infectious endogenous retrovirus in mice

Endogenous retroviruses (ERV) are found throughout vertebrate genomes and failure to silence their activation can have deleterious consequences on the host. Introduction of mutations that subsequently prevent transcription of ERV loci is therefore an indispensable cell-intrinsic defense mechanism that maintains the integrity of the host genome. Abundant in vitro and in silico evidence have revealed that APOBEC3 cytidine-deaminases, including human APOBEC3G (hA3G) can potently restrict retrotransposition; yet in vivo data demonstrating such activity is lacking, particularly since no replication competent human ERV has been identified. In mice deficient for Toll-like receptor 7 (TLR7), transcribed ERV loci can recombine and generate infectious ERV. In this study, we show that mice deficient in the only copy of Apobec3 in the genome did not have spontaneous reactivation of ERVs, nor elevated ERV reactivation when crossed to Tlr7-/- mice. In contrast, expression of a human APOBEC3G transgene abrogated emergence of infectious ERV in the Tlr7-/- background. No ERV RNA was detected in the plasma of hA3G+Apobec3-/-Tlr7-/- mice, and infectious ERV virions could not be amplified through co-culture with permissive cells. These data reveal that hA3G can potently restrict active ERV in vivo, and suggest that the expansion of the APOBEC3 locus in primates has helped restrict ERV reactivation in the human genome.\n\nImportanceAlthough APOBEC3 proteins are known to be important antiviral restriction factors in both mice and humans, their roles in the restriction of endogenous retroviruses (ERV) have been limited to in vitro studies. Here, we report that human APOBEC3G expressed as a transgene in mice prevents the emergence of infectious ERV from endogenous loci. This study reveals that APOBEC3G can powerfully restrict active retrotransposons in vivo and demonstrates how ectopic expression of human factors in transgenic mouse models can be used to investigate host mechanisms that inhibit retrotransposons and reinforce genomic integrity.