A research team from the departments of biochemistry and pathology at the University of Pennsylvania’s School of Dental Medicine has found a successful technique using genetic engineering to coax lettuce and tobacco plants to produce foreign proteins in their leaves.
The applications include producing the human clotting factor to make hemophilia treatments, polio viruses to make vaccines, or a wormwood plant to synthesize malaria drugs.
Prof. Henry Daniell, who led the team, has found that while his plant-based drug production platform could efficiently express bacterial genes as well as short human genes, it wasn’t so easy to express viral genes and longer human genes.
Daniell and his colleagues hypothesized that this could be linked to the differences between plants, animals, bacteria and viruses in how they use the DNA code to produce proteins.
“Plant chloroplasts are bacteria-like, or prokaryotic, and humans are eukaryotic,” Daniell said in a Penn news piece. “So that’s the challenge: How can we make a chloroplast recognize a human gene and transform it like its own to make a protein?”
In an article titled “Codon Optimization to Enhance Expression Yields Insights into Chloroplast Translation,” and published in the journal Plant Physiology, Daniell explains how proteins are made up of building blocks called amino acids which are also produced according to three-letter strings of DNA, called codons. There are 64 codons but only 20 amino acids, since multiple codons encode the same amino acid. Daniell’s team found that different organisms actually have different preferences for which codon they use to produce a given amino acid.
Their paper focused on these species-specific preferences. The researchers assessed the genomes of 133 plant species to see which codons were used most frequently to code for particular amino acids. Using results from this analysis, the team designed software that could convert any given DNA sequence into the sequence that would be preferred by either lettuce or tobacco plants.
The team then evaluated whether this “codon optimization” process could, consequently, increase levels of protein expression, using a head-to-head comparison of the optimized gene – the output of the software – versus the native gene in two different proteins, one for hemophilia treatment and one for a polio vaccine.
“These two advances — improving the expression levels of protein and quantifying an exact dose — were key questions the [U.S. Food and Drug Administration] has had about our work,” Daniell said. “Now that we’ve addressed these issues, we’re closer than ever to getting these therapies to the clinic.”
Their findings reveal that codon optimization has a significant impact: The process led to higher expression levels of hemophilia clotting factor five to six times that of the native protein, and to higher levels of the poliovirus protein 26 times that of its native sequence.
The team made the software freely available for other researchers to use, so that more advances can be made in the field.
Below is a video of Daniell explaining his research at Penn: