Pharmaceuticals and Genetically Modified Algae

Fig.1 - Scanning E.M. Image of Chlamydomonas

A tiny green alga is showing promise in the production of genetically modified (GM) pharmaceuticals such as monoclonal antibodies, interleukins and nerve growth hormones.

Chlamydomonas reinhardtii (figure 1) is currently the most favoured single celled alga for the expression of human genes. There are a number of reasons for this. Firstly, its genome has been sequenced and it has been cultured successfully in laboratories for many years. The alga can in fact be grown on a simple medium of salts in the presence of light, where it can photosynthesise to produce glucose for itself. It can also be grown on an acetate medium in the absence of light, and can be economically cultured on a small or large scale (up to 500 000 litres).

Another reason for its popularity in scientific work is that the DNA of its chloroplasts, mitochondria and nucleus can be easily modified genetically. In other words, human genes (sections of DNA) can be introduced into these structures using genetic engineering techniques.

One such technique is microinjection, in which a microscopic needle inserts the desired gene into the nucleus or other cellular organelles. Another method, electroporation (figure 2), allows the introduction of foreign DNA into membranes by exposing them to a high electric potential difference. This acts to increase their permeability.

Fig.2 - Electroporator with Cuvette Loaded

Problems Associated With Protein Expression

There are difficulties posed by GM algal technology, however. In terms of nuclear expression of transgenes, for instance, allowance has to be made for ‘codon bias’; the disproportionately high GC (guanine - cytosine) level that occurs in the Chlamydomonas genome. This results in synthesised human genes which have been modified to adapt to this bias.

As Professor Joe Cummins, 2005, points out, the human immune system may reject pharmaceuticals derived from such DNA and 
therefore ‘synthetic human DNA in the alga should not be deemed equivalent until it has been tested for untoward effects on humans and the environmental biota.’ 
Proteins expressed by Chlamydomonas may in some cases undergo post translational modification to more effectively emulate the behaviour of the original human protein, unlike other GM organisms such as yeasts. Indeed, yeasts and bacteria are often incapable of fully expressing monoclonal antibodies (immunoglobulin proteins) or more complex proteins.In addition, processes not as yet fully explained within the Chlamydomonas nucleus tend to silence foreign gene expression. Several genes thought to be responsible for this phenomenon have, however, been identified and are being investigated.

With respect to expression of transgenes by the chloroplast, codon bias (this time in terms of high AT levels) also presents a problem, as does the low level of translation of foreign genes within this organelle. Recent studies, however, have shown that the Chlamydomonas chloroplast is capable of expressing complex monoclonal antibodies such as the anti- Herpes simplex antibody, HSV8lsc. Given the low cost of cultivating Chlamydomonas reinhardtii, the potential thus exists to produce this antibody on a large scale in the near future.

Environmental and Health Concerns

Proposals for such extensive cultivation in Hawaii in 2005, however, met with a vociferous backlash from scientists and community members alike. A permit granted to Mera Pharmaceuticals to establish the wide scale production of GM antibodies in large outdoor bioreactors in Kona, was contested on the basis of the risk of horizontal gene transfer through the soil and waterways. It was argued that the outdoor cultivation of Chlamydomonas could result in the alga transferring its transgenes to related soil and aquatic algae and bacteria.

Indeed, the very success of transgene expression by modified chloroplasts is itself a risk because the high level of GM proteins (including antibiotic resistant marker proteins) and foreign DNA produced increases the chances of horizontal contamination of related species. Ho and Cummins (2005), stress that the similarity between chloroplast and bacterial genomes further adds to this risk - at least 87 easily transformable bacterial soil species may be vulnerable if GM Chlamydomonas cultures are not effectively contained.

Once the above issues are explored the future of GM pharmaceuticals from these microalgae looks promising.

References

Cummins, J., 2005, Human Gene Products in the Micro Alga Chlamydomonas reinhardtii, i-sis.org, accessed 10/3/2010

Cummins, J. and Ho, M.,2005, ‘GM Pharmaceuticals From Common Algae’ ibiblio.org, accessed 9/3/2010

Franklin, S. and Mayfield, S., 2004, Prospects For Molecular Farming in the Green Alga Chlamydomonas reinhardtii, Current Opinion in Plant Biology, sciencedirect.com, accessed 9/3/2010