The Muggle Dittany: Plant Synthetic Biology-Based Therapeutics
Introduction
For a witch or a wizard reading the title, they must be now on their nostalgic tour to the woods where Hermione Granger poured the essence of dittany on Ron’s wounds after he splinched. But for the confused Muggles (non-magical people), we owe a mini explanation- In the Harry Potter series, dittany was the all-heal medicine used on wounds.
Many years ago, the existence of medicine as powerful as the imaginary dittany would be laughed off in conversations witnessed by little pharmacies on the nooks and corners of London when the books were first released; but now, decades later, the narrative has undergone quite the change!
Owing to the fast improvement in Synthetic Biology, plant SynBio has ended up as a go-to for improving next-gen medicines. This article summarizes contemporary development towards robust and predictable engineering of plants to benefit the manufacturing of recent and new-gen medicines.
What is artificial biology in this context?
Plant Synthetic Biology Concepts and Techniques
The engineering concept of the design-build-test–learn (DBTL) cycle reinforces the application of synthetic biology to organismic engineering. Synthetic biology speeds up the rate at which we move around the DBTL cycle. Using this foundation, recently, a plant standard has been developed based on Type IIS restriction endonuclease assembly.
The eventual goal of Synthetic Biology is to possess biological blueprints to understand and modify an organism. For plants, these goals are still very distant. However, progress has surely been made with the help of techniques like —
(a) Transgenic techniques that have been used in early plant synthetic biology approaches
(b) The development of precise gene-editing technologies can precisely manipulate a DNA sequence and gene expression levels
( c) Gene stacking and whole pathway engineering are methods that allow a relatively easy and rapid way to rearrange genes in a metabolic pathway and to test out different combinations.
This can be essential to alleviate metabolic bottlenecks or identify potential co-factors during plant research. All of these techniques play an important role in making medicines using plant-based synthetic biology.
Application in Pharmaceutical Production
The concept of plant-based pharmaceutical production (pharming) has been worked on for 30 years and recently has led to innovative results. Originally, this was based on the concept of production of edible vaccines in crop plants. These would be cheap to produce as they avoid the expensive costs associated with manufacturing or supply chain issues such as refrigeration.
However, it rapidly became clear that this goal was not achievable. The challenges of regulating dose could not be overcome. As a result, with a modified approach, there is now acceptance that while plant-produced vaccines and pharmaceuticals have many benefits over other expression systems, some processing of the plant material will be required before the vaccination of the patient.
In the field of therapeutics, plants have been chosen for the production of monoclonal antibodies (mAbs), as opposed to other industrial production systems such as bacteria, yeast, insect, or mammalian cells due to a combination of reasons based on cost, safety, and the potential of plant systems to produce mAbs with the correct post-translational modifications.
For example, mammalian production systems, which dominate the market, are expensive with very less prospect of scaling up, whereas bacteria are unable to produce most of the correct post-translational glycosylation required for the mAb activity.
Yeast, insect cells, and plants, since they are eukaryotes, can perform complex post-translational glycosylations. However, in all these systems, the exact nature of the glycosylation differs depending on the species, and all show significant differences to mammalian systems. As a result, the concept of glycoengineering has been developed, in which the expression system is modified to produce mammalian-like glycosylated mAbs. Plants have proved to be particularly amenable to glycoengineering, and this has been accelerated by synthetic biology tools.
Examples of Plant Therapeutics
Perhaps the most well-known example of a plant-produced therapeutic mAb is ZMapp™, a promising treatment for the Ebola virus. This therapeutic consists of three mouse/human chimeric mAbs that target the virions' surface glycoprotein and are produced in a strain of Nicotiana benthamiana (tobacco) called ΔXF. ΔXF plants have reduced xylosyltransferase (XT) and fucosyltransferase (FT) activity and produce authentic human protein glycosylation.
Additional plant-produced therapeutics are currently in clinical trials or have been approved. For example, Elelyso™ (taliglucerase-α), a treatment for Gaucher’s disease, is produced in carrot cell culture and was approved by the FDA in 2012.
Plants also hold promise for the commercial production of pharmaceuticals or pharmaceutical precursors.
One example is the anti-malarial drug artemisinin, the main ingredient in the ACT therapies (first-line malaria therapy in endemic countries) used globally. Artemisinin is found at relatively low abundance in the Artemisia annua plant, which means production can’t meet the increasing demands. Scientists have now learnt that the precursor, artemisinic acid, can be converted chemically in a low-cost process to artemisinin, and therefore it has become an important biotechnological target. The initial strategy used synthetic biology to engineer artemisinic acid production in yeast by overexpressing 14 genes and conditionally repressing two more. While this approach was highly successful, producing yields over 25 g/L, the costs of large-scale yeast fermentation have made this challenging to produce commercially.
Where on from here?
As the precision with which plants can be engineered increases, so will the range of applications. The discovery of novel plant compounds with immune system modulating activities has already become an increasingly important area of research, and SynBio aids to the progress in this field by fueling new possibilities. The identification, characterization and SynBio based mimicking of plant compounds that augment new or existing vaccines have been and will be of significant interest to immunologists all around the globe.
Finally, it remains to be seen how the public will view synthetic biology and gene-editing in light of previous concerns over GMOs and medicines as products of synthetic biology.
