Protein expression systems have been developed for many host platforms with the aim of producing large amounts of protein. These can then be used in a number of applications, such as research, diagnostic and therapeutic medicines.
Eukaryotic expression systems typically contain: a strong promoter to drive transcription (the process of producing mRNA); a terminator, that contains the signals to end the mRNA strand and attach a polyadenylation sequence that aids mRNA stability; and sequences which allow for efficient translation (protein production by ribosomes from mRNA) from the mRNA.
The Hypertrans® system is a plant-based expression method developed by George Lomonossoff and Frank Sainsbury at the John Innes Centre and delivers high levels of protein expression in plants. This system was developed through the discovery that mutation, particular sequences in the promoter of the Cowpea mosaic virus (CPMV, a plant virus), resulted in the high-level expression of protein when the gene of interest replaced the native CPMV protein. The CPMV terminator sequences also appear to result in enhanced mRNA stability.
While the Hypertrans® system is based on viral sequences, it does not require viral replication for the transmission or expression. This means that the size of the gene that can be expressed in the system is not limited by the size restrictions of viral systems. In addition, since no viral replication occurs it cannot spread to other plants, meaning there are not the requirements of containment associated with viral replicating systems.
In the Hypertrans® system, the features from CPMV are combined with a gene silencing suppressor. Plants have evolved many ways to control the levels of mRNA present in a cell, thereby making it difficult to generate enough mRNA to produce large amounts of protein. This process is known as post-transcriptional gene silencing, as it effectively silences the expression of the gene. However, nature has provided mechanisms to overcome this suppression, with many plant viruses containing proteins that can prevent this silencing. In the Hypertrans® system, P19 from the tomato bushy stunt virus (David Baulcombe) is co-expressed allowing for enhanced transient expression in plants.
All of the features of the Hypertrans® system have been combined in an easy to use plasmid series (pEAQ), which allow the simple one-step cloning of your gene of interest into a vector which can be introduced into Agrobacterium and expressed in a plant.
What are the benefits of the Hypertrans® system and how does it compare to other systems?
The flexible nature of the system enables production to be scaled from small scale research quantities to pilot-scale production very quickly. The speed of the system means that it can rapidly produce large amounts of protein and so it is well suited to rapidly responding to emergencies like pandemics. And…..less time in production equates to significant patent extension, perhaps as much as 12 months.
The technology is disruptive, and to date, there are no licensed products on the market that have been produced in plant systems. To a customer that can be seen as a considerable gamble, but what is true about the system is that a feasibility study to confirm whether the protein of interest will express can be done very quickly and at a fraction of the price compared to more conventional systems. Moreover, accurate likely costs of goods can also be established very quickly because the system is directly scalable, it’s just a case of adding more plants.
There’s also the question around regulatory approval. Conventional systems often face regulatory concerns over product purity and potential adventitious agents from the use of animal-derived raw materials and infection of patients. With plants, these concerns are avoided altogether as no animal-derived products are used in any part of the manufacturing process, either upstream or downstream.
These advantages all culminate in plant expression generally being much more cost-effective and facilitating greater R&D throughput, so surely it’s worth a try?