Researchers at Stanford University have identified enzymes that could make it easier to produce the anticancer drug etoposide (Science 2015, DOI: 10.1126/science.aac7202). Etoposide is currently synthesized from podophyllotoxin, a natural product first isolated from the Himalayan mayapple plant, Podophyllum hexandrum.
“Podophyllum is not an easy plant to cultivate and grow,” says Elizabeth S. Sattely, who led the study. “It’s a challenge to sustainably grow the plant to get enough compound.”
Such difficulties could be avoided by producing an etoposide precursor in a different, easier-to-handle organism. Sattely and grad student Warren Lau have now identified the enzymes in the mayapple plant that could enable such a move.
The pathway comprises 10 genes, four of which were already known. Sattely and Lau found the other six genes by searching the RNA produced by wounded mayapple plants, which make extra deoxypodophyllotoxin, a proposed podophyllotoxin precursor, in response to being wounded. The team used bacterial plasmids to transfer the pathway into Nicotiana benthamiana, a wild relative of tobacco plants that’s easy to grow.
The pathway the researchers assembled has an unexpected bonus. It contains two enzymes that convert deoxypodophyllotoxin into the etoposide aglycone, which differs from etoposide only in that it lacks a disaccharide group. The aglycone also requires fewer steps to convert into etoposide than does podophyllotoxin.
Currently, the engineered tobacco plants produce only nanogram amounts of the etoposide precursor. Although she thinks the task will be challenging, Sattely plans to engineer the new pathway into yeast, a prolific microorganism, to improve the yield.
The work is interesting and timely, but there’s a long way to go before significant amounts of the precursor could be obtained from tobacco, says Norman G. Lewis, an expert on plant metabolic engineering at Washington State University. “The amounts presently described are tiny, and there will be significant technical hurdles to overcome to get this into a commercially viable system in either tobacco or yeast,” he says. “The problems they will face on this, however, are common to everyone” in metabolic engineering and synthetic biology.