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Chinese hamster ovary (CHO) cells are a cell line derived from the ovary of the Chinese hamster. They are often used in biological and medical research and commercially in the production of therapeutic proteins. They were introduced in the 1960s and grow as a cultured monolayer. They require the amino acid proline in their culture medium.
CHO cells are used in studies of genetics, toxicity screening, nutrition and gene expression, particularly to express recombinant proteins. Today, CHO cells are the most commonly used mammalian hosts for industrial production of recombinant protein therapeutics.
The use of the Chinese hamster in research began in 1919 where they were used in place of mice for typing pneumococci. They were subsequently found to be excellent vectors for transmission of kala-azar (also known as visceral leishmaniasis), facilitating research in epidemiology.
In 1948, the Chinese hamster was brought to the United States for breeding in research laboratories. In the following years, the Chinese hamster became noteworthy for the cell lines that were derived from its tissues. Having a very low chromosome number (2n=22) for a mammal, the Chinese hamster is an ideal model for radiation cytogenetics and tissue culture.
In 1957, Theodore T. Puck obtained a female Chinese hamster from Dr. George Yerganian's laboratory at the Boston Cancer Research Foundation and used it to derive the original Chinese hamster ovary (CHO) cell line. Since then, CHO cells have been a cell line of choice because of their rapid growth and high protein production. They have become the mammalian equivalent of E. coli in research and biotechnology today, especially when long-term, stable gene expression and high yields of proteins are required.
CHO-K1 was derived from the original cell lines; it contains a slightly lower amount of DNA than the original CHO. CHO-K1 was mutagenized to generate CHO-DXB11 (also referred to as CHO-DUKX or CHO-DUK-XB11), a cell line lacking DHFR activity (Urblaub and Chasin, 1980). These cells have a deletion of one dhfr allele and a missense mutation in the other. Subsequently, the proline-dependent CHO-pro3- strain, another derivative of the original CHO cell line, was mutagenized to yield CHO-DG44, a cell line with deletions of both dhfr alleles (Urlaub et al., 1983). These two DHFR-minus strains require glycine, hypoxanthine, and thymidine (GHT) for growth. Although not initially intended recombinant protein manufacture, DHFR-minus CHO cells were used for a number of pioneering experiments demonstrating stable transfection with an exogenous dhfr gene via selection in GHT-minus medium (Ringold et al., 1981; Kaufman and Sharp, 1982; Scahill et al., 1983). This genetic selection scheme remains one of the standard methods to establish stably transfected CHO cell lines for the production of recombinant therapeutic proteins. The multistep process begins with the molecular cloning of the gene of interest (GOI) and the dhfr gene in a single or in separate mammalian expression vectors. The plasmid DNA(s) carrying the two genes are then delivered into cells by transfection, and the cells are grown under selective conditions in GHT-minus medium. Each surviving cell will have one or more copies of the exogenous dhfr gene, usually along with the GOI, integrated in its genome (Ringold et al., 1981; Kaufman and Sharp, 1982; Scahill et al., 1983). The integrated plasmid copy number varies widely from one recombinant cell to another, but there is almost always only one integration site per cell even if multiple plasmids are transfected (Wurm 1990). The growth rate and the level of recombinant protein production of each cell line also vary widely. To obtain a few stably transfected cell lines with the desired phenotypic characteristics, it may be necessary to evaluate several hundred candidate cell lines.
The CHO and CHO-K1 cell lines can be obtained from a number of biological resource centres such as the European Collection of Cell Cultures (ECACC) which is part of the Health Protection Agency Culture Collections. CHO-K1 data, such as growth curves, timelapse videos of growth, images and subculture routine information are available from ECACC.
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