Directed molecular evolution by somatic hypermutation

After rearrangement of immunoglobulin gene segments, the immune system evolves the antibody repertoire by mutating the immunoglobulin variable region at a high rate. While this somatic hypermutation was thought to occur only at the variable region, recent studies suggest that hypermutation can occur at locations throughout the genome. Building upon this notion, we sought to exploit this mechanism as a mutagenesis tool. We created a substrate based on GFP that could be screened using flow cytometry and showed that retroviral infection can deliver the transgene to genomic locations that support hypermutation. Infected cells generated various GFP mutants with increased fluorescence intensity and analysis revealed mutations not only at the chromophore, but also an unexpected mutation at a distant residue. Our results demonstrate in principle that immunoglobulin somatic hypermutation can be a potent means of mutagenesis. With appropriate selection conditions it may be utilized to evolve gene products with desired properties.