Though Schaffer acknowledges that the technique could be used to help pathogenic viruses evade the human immune system, potentially making them more virulent, he said that other and easier techniques already allow this frightening possibility.
AAV consists of two genes enclosed within a ball, or capsid, of proteins. The capsid proteins are what antibodies recognize, and as a result were the target of directed evolution. To provide the raw material for evolution - the genetic variation from which nature selects the best-adapted organism - the researchers created mutant viruses by introducing small variations in the genes through an error-prone polymerase chain reaction (PCR) coupled with a test tube recombination technique. After reassembling the mutant viruses inside their capsids, they introduced them to blood serum pooled from rabbits immunized against AAV, and thus containing many types of antibodies to AAV. Only the mutant viruses good at evading antibodies to AAV survived the serum.
After passing the viruses three times through increasingly more potent serum, they isolated the survivors and subjected them to another round of PCR that introduced more mutations. After passing this second generation through serum three times, they isolated viruses that could survive AAV antibodies much better than the original strain of AAV. One strain of virus was 96 times more effective than the wild AAV, and two evolved strains survived injection into mice with nearly 1,000 times the level of antibodies required to neutralize the wild virus.
By sequencing the survivor strains, the researchers discovered that the capsid proteins of the survivors differed from those of the original strain by only seven amino acid building blocks, two of which were responsible for most of the altered interacti
Source:University of California - Berkeley