Gene delivery

Gene delivery is the process of introducing foreign genetic material, such as DNA or RNA, into host cells. Genetic material must reach the nucleus of the host cell to induce gene expression. Successful gene delivery requires the foreign genetic material to remain stable within the host cell and can either integrate into the genome or replicate independently of it. This requires foreign DNA to be synthesized as part of a vector, which is designed to enter the desired host cell and deliver the transgene to that cell's genome. Vectors utilized as the method for gene delivery can be divided into two categories, recombinant viruses and synthetic vectors (viral and non-viral). Gene delivery is the process of introducing foreign genetic material, such as DNA or RNA, into host cells. Genetic material must reach the nucleus of the host cell to induce gene expression. Successful gene delivery requires the foreign genetic material to remain stable within the host cell and can either integrate into the genome or replicate independently of it. This requires foreign DNA to be synthesized as part of a vector, which is designed to enter the desired host cell and deliver the transgene to that cell's genome. Vectors utilized as the method for gene delivery can be divided into two categories, recombinant viruses and synthetic vectors (viral and non-viral). In complex multicellular eukaryotes (more specifically Weissmanists), if the transgene is incorporated into the host's germline cells, the resulting host cell can pass the transgene to its progeny. If the transgene is incorporated into somatic cells, the transgene will stay with the somatic cell line, and thus its host organism. Gene delivery is a necessary step in gene therapy for the introduction or silencing of a gene to promote a therapeutic outcome in patients and also has applications in the genetic modification of crops. There are many different methods of gene delivery for various types of cells and tissues. Viral based vectors emerged in the 1980s as a tool for transgene expression. In 1983, Siegel described the use of viral vectors in plant transgene expression although viral manipulation via cDNA cloning was not yet available. The first virus to be used as a vaccine vector was the vaccinia virus in 1984 as a way to protect chimpanzees against hepatitis B. Non-viral gene delivery was first reported on in 1943 by Avery et al. who showed cellular phenotype change via exogenous DNA exposure. There are a variety of methods available to deliver genes to host cells. When genes are delivered to bacteria or plants the process is called transformation and when it is used to deliver genes to animals it is called transfection. This is because transformation has a different meaning in relation to animals, indicating progression to a cancerous state. For some bacteria no external methods are need to introduce genes as they are naturally able to take up foreign DNA. Most cells require some sort of intervention to make the cell membrane permeable to DNA and allow the DNA to be stably inserted into the hosts genome. Chemical based methods of gene delivery can uses natural or synthetic compounds to form particles that facilitate the transfer of genes into cells. These synthetic vectors have the ability to electrostatically bind DNA or RNA and compact the genetic information to accommodate larger genetic transfers. Chemical vectors usually enter cells by endocytosis and can protect genetic material from degradation. One of the simplest method involves altering the environment of the cell and then stressing it by giving it a heat shock. Typically the cells are incubated in a solution containing divalent cations (often calcium chloride) under cold conditions, before being exposed to a heat pulse. Calcium chloride partially disrupts the cell membrane, which allows the recombinant DNA to enter the host cell. It is suggested that exposing the cells to divalent cations in cold condition may change or weaken the cell surface structure, making it more permeable to DNA. The heat-pulse is thought to create a thermal imbalance across the cell membrane, which forces the DNA to enter the cells through either cell pores or the damaged cell wall. Another simple methods involves using calcium phosphate to bind the DNA and then exposing it to cultured cells. The solution, along with the DNA, is encaspulated by the cells and a small amount of DNA can be integrated into the genome. Liposomes and polymers can be used as vectors to deliver DNA into cells. Positively charged liposomes bind with the negatively charged DNA, while polymers can be designed that interact with DNA. They form lipoplexes and polyplexes respectively, which are then up-taken by the cells. The two systems can also be combined. Polymer-based non-viral vectors uses polymers to interact with DNA and form polyplexes.