The highly sought-after plasma protein, human serum albumin (HSA), can now be produced at high yield and purity in rice, according to a report published today (October 31) in Proceedings of the National Academy of Sciences. Using the rice-derived protein in place of its blood-derived counterpart will not only ease demand but also eliminate the risk of spreading diseases.
“The authors have demonstrated large-scale purification and functional equivalence to human HSA,” said Richard Twyman of Pharma-Planta, an EU consortium aimed at producing pharmaceutical proteins in plants, and the University of Warwick, UK, who was not involved in the study. “They have potentially developed a very good alternative to provide HSA in the clinic.”
HSA is used for a variety of clinical applications such as the treatment of blood loss, serious burns, and abdominal fluid retention caused by cirrhosis (scarring) of the liver. In addition, HSA has served as a vehicle for vaccine and drug delivery, and as a cell culture supplement in the production of vaccines and pharmaceuticals.
“There is a high demand for plasma HSA, but it is in really short supply,” explained lead researcher Daichang Yang of Wuhan University, China. Currently, the only supply of HSA is that extracted from human blood. Besides the limited availability of blood donors, “using plasma HSA also has a high risk for spreading diseases, such as AIDS and hepatitis,” Yang said. “We considered using a plant-based production to satisfy the market demand and reduce the risk.”
Yang and his colleagues are not the first to attempt genetically engineering human HSA production in other species. Yeast, bacteria, potatoes, and tobacco plants have all been tried, but each has met with feasibility issues. “The problem with microbial HSA has been that tiny traces of microbial contaminants… are either toxic or induce immune reactions in humans, which means they require extensive purification and this adds tremendously to the costs,” explained Twyman. HSA produced in potatoes or tobacco does not carry such contamination risks, but neither of these sources has provided sufficient yield to be cost-effective.
To bump up yield, Yang and his colleagues turned to rice. Specifically, they targeted the part of rice we eat, the endosperm inside the seed—a natural nutrient storage organ, and thus an excellent site for the accumulation and long-term stable storage of recombinant proteins. By driving expression of the HSA gene in the endosperm, the team managed to obtain 2.74 grams of pure HSA protein per kilogram of rice seed, more than 25 times the 0.1 grams of HSA collected from one kilogram of tobacco leaves.
In addition to obtaining a high yield, the rice-derived HSA shared the same molecular weight, crystal structure, molecular binding sites, and other biochemical characteristics as that of blood-derived HSA. These similarities translated to functional equivalence in tests for ligand binding, promotion of cell growth in culture, and the treatment of fluid retention in a rat model of liver cirrhosis.
The rice-derived HSA also displayed an equivalent immunogenicity to that of plasma HSA. That is, animals injected with either rice HSA or plasma HSA developed similar immune reactions.
“The chemical, biophysical and immunological characterization of the highly purified protein from rice… shows it to be identical in every respect to human serum albumin isolated from blood plasma,” said Diter von Wettstein of Washington State University, who was not involved in the study.
Despite these similarities, before rice HSA can be used in humans it will need to go through extensive clinical trials. Furthermore, to generate sufficient quantities of HSA to meet global demand—an estimated 500 tons a year—production will need to be scaled up to open-field farming, which will bring its own share of red tape. So, while the prospects for rice HSA are exciting, Twyman says, “we are not going to see blood-derived HSA replaced tomorrow.”
Y. He et al., “Large-scale production of functional human serum albumin from transgenic rice seeds,” Proceedings of the National Academy of Sciences, doi/10.1073/pnas.1109736108, 2011.