Intracellular Trafficking of DNA for Gene Therapy

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Under almost all conditions using any method, the levels of gene transfer to any cell or tissue are low because many barriers exist for the efficient delivery of genes to cells. The primary goal of our laboratory is to identify and overcome the intracellular barriers to promote effective gene delivery and therapy. Exogenous viral or non- viral DNA must cross the plasma membrane, travel through the cytoplasm and the cytoskeletal networks, cross the nuclear envelope, localize to specific regions within the nucleus, and be transcribed in order for gene therapy to be successful. We have shown that once in the cytoplasm, plasmids carrying DNA nuclear targeting sequences (DTS) that are required for nuclear import in non-dividing cells rapidly associate with transcription factors that mediate movement along microtubules and across the nuclear envelope. NF-kB is one such factor that binds to several ubiquitously active DTSs and is required for DNA nuclear import, but in the cytoplasm it is maintained in a sequestered state, unable to bind DNA. The question then is how is NF-kB activated to bind to plasmids and mediate their cytoskeletal movement and nuclear import? In the case of NF-kB, a major pathway for its activation is through a set of cytoplasmic dsDNA sensors, such as cGAS-STING, that are part of the innate immune system and drive inflammatory responses. When dsDNA binds to cGAS, signaling cascades are initiated that result in activation of key pro-inflammatory transcription factors (including NF-kB) and ultimately production of pro-inflammatory cytokines. Thus, a major focus in the gene therapy space has been to block activation of these sensors to reduce inflammation. However, we have observed that when cGAS is silenced, cytoplasmically injected plasmids fail to traffic to the nucleus. We hypothesize that limited activation of one or more of these sensors is actually needed for low level activation of key transcription factors in order to facilitate DNA nuclear import in non-dividing cells. If we can find ways to limit sensor activation, but not abolish it, this will allow for enhanced gene delivery with limited accompanying inflammation. We have also spent considerable effort detailing the distribution of plasmids inside the nucleus and have found that the subnuclear mislocalization of plasmids can affect their transcriptional activity. We have found that plasmids localize to discrete transcriptional domains within the nucleus based on the type of promoter (Pol I, Pol II, or Pol III) they carry and that when two different promoter types are placed on one plasmid, not only is the intranuclear distribution of the DNA different that either promoter type alone, but transgene expression is significantly reduced. We will dissect the pathways used for DNA movement within the nucleus and exploit them to improve transgene expression based on the subnuclear localization of the transfected DNA. Our specific aims are to (1) determine whether cytosolic dsDNA sensors are required for DNA nuclear import; (2) evaluate whether residence time of DNA in the cytoplasm affects sensor activation and transfection efficiency; and (3) characterize how subnuclear organization affects exogenous DNA expression.