Alternative genome editing systems, such as zinc finger nuclease (ZFN) systems and transcription activator-like effector nuclease (TALEN) systems, are being explored (although they suffer from some of the same issues),14 and CRISPR can be augmented with other helpers including transposons.1516 Higher-precision editing is possible using base editing and prime editing, two recently developed approaches.17
Today, viral delivery is the method of choice for living organisms. Of the nearly 300 clinical trials worldwide underway with viral-vector-based gene therapies, about half use modified adeno-associated viruses (AAVs). These can target a wide range of tissues, but AAVs can be too small to deliver the needed quantity of editing payload. This necessitates using huge amounts, which can trigger dangerous immune responses; efforts are underway to re-engineer AAV to be bigger and/or evade immune response.
Nonviral delivery has become an increasingly viable alternative, thanks to rapid progress in the use of lipid, polymer, extracellular vesicles, and inorganic nanoparticle-based delivery systems.18 Physical methods, including electroporation (where an electrical pulse is used to create temporary pores in cell membranes), are promising but not yet scalable.
