AAV2 X increases AAV6 rep/cap-driven rAAV production

We have recently identified a new gene, involved in DNA replication, at the far 3′ end of the adeno-associated virus type 2 (AAV2) genome. The AAV type 6 (AAV6) genome has a disrupted X open reading frame (ORF) whose two halves, when combined, have full-length homology and comparable size to AAV2 X. Hypothesizing that AAV6 X is inactive, we assessed if AAV2 X augments recombinant (r)AAV2 DNA replication and virion production, but with rep and cap trans-functions of AAV6. Using AAV2 X expressing HEK293 cell lines we show AAV2 X significantly boosts rAAV DNA replication/virion production, driven by AAV6 rep/cap as it does the AAV2 rep/cap system. Protein BLAST search for homology between AAV2 X and various AAV Rep78 proteins suggests that X might be AAV8 Rep78-derived and have some of its activities. These data suggest that AAV2 X, and the corresponding X genes of other AAV types/clades, warrant further study.