“Complete Opioid Biosynthesis in Yeast,” by Galanie, et al. (2015): Discussion

In the latest issue of the journal Science, a group of synthetic biologists reported that they had engineered a strain of yeast that could synthesize the opiate molecule thebaine, a precursor to many opiate drugs. The paper, “Complete Biosynthesis of Opiates in Yeast,” was written by Stephanie Galanie, working in the laboratory of Christina Smolke at Stanford University. This report marks not only an achievement in genetic engineering, but also a potential breakthrough in biotechnology. If there was a way to synthesize thebaine and related opiates directly, without cultivating fields upon fields of poppy plants, it would be a boon to the 5.5 billion people who, according to the World Health Organization, have “low to nonexistent access to treatment for moderate or severe pain. (quoted in Galanie, et al., 2015)”

The opiates comprise a broad class of drugs that mimic endogenous opioid molecules in the brain. Opiates can relieve pain, induce euphoria, engender addiction, end life painlessly, or even block the action of other opiates. The only natural source of opiates like thebaine is the opium poppy (Papaver somniferum), from which opiates get their name. Although there are non-biological methods of synthesizing opiates, most opiates are still derived from poppies. Despite the risks and inefficiencies of agriculture, “chemical synthesis of these complex molecules is not commercially competitive.” (Galanie, et al., 2015) When the poppy straw is harvested, there are two principal extracts: morphine and thebaine. Morphine is valuable in its extracted form, while thebaine is medically valuable only after it has been processed into other potent opiates, such as hydrocodone (Oxycontin, Percoset), naltrexone (Revia), and buprenorphine (Suboxone).

Depressingly, most news outlets have approached this story from the angle of prescription drug abuse. Many commentators have even insinuated that this discovery portends a future in which most of the populace is stupefied by narcotics. If it is easy to imagine such a world, it is only because we have been exposed to countless dystopian narratives that depict a future in which cutting-edge technology has enabled addiction. But we need to avoid the fallacy of generalizing from fictional evidence. We need to weigh such hypothetical scenarios against the real, ongoing, and preventable pain of the 5.5 billion people who lack access to opiate medications. Moreover, these sensationalistic stories distract us from more pressing concerns about safety in biomedical research. For instance, there are experimental epidemiologists who are intentionally engineering “gain-of-function” mutations into the avian flu virus, transforming it into a virus that is transmissible between humans. On the one hand, virulent samples can escape the lab and cause an epidemic. But, on the other hand, if we understood the mutations that enable viruses jump between species, we’d be able to react more effectively in the event of a natural epidemic. In any case, this is research that genuinely deserves public attention and concern.

This paper by Galanie, et al., is of particular interest to me because its subject combines elements from both of the laboratories I’ve worked for. Formerly, I worked in a neuroscience lab that used opiate drugs to understand how the endogenous opioid system underlies reward-based learning. And now, my current makes use of transgenic mouse models, which have been endowed with genetic constructs that enable us to visualize aspects of their neurobiology.

Interestingly, much of the early research in synthetic biology –including the discovery of recombinant DNA– was done at my current institution, Cold Spring Harbor Laboratory. According to most historical sources, recombinant DNA was discovered by Stanley Cohen in 1972. Working at CSHL, he and his colleagues demonstrated that DNA from one organism could be stably inserted into a different organism. These discoveries paved the way for genetically modified organisms, the Human Genome Project, and transgenic mice.

In a series of famous experiments, Cohen used restriction enzymes to create a strain of E. coli that expressed DNA from two separate strains of the bacteria. Each of the two original strains contained a plasmid (a circular segment of bacterial DNA that code for nonessential functions; see Fig. 1) that conferred resistance to one of two antibiotics. Through wholly synthetic methods, Cohen engineered a strain of E. coli that possessed a fusion plasmid that made it resistant to both antibiotics. Cohen coined the term recombination to describe the incorporation of exogenous genetic material into an organism under artificial conditions.

Figure 1. Plasmids are circular loops of DNA inside a bacteria.
Figure 1. Plasmids are circular loops of DNA inside a bacteria. (Wikimedia Commons)

Stanley Cohen and his colleague, Herbert Boyer, went on to demonstrate that recombination was possible between two different species of bacteria. Some critics suggested that DNA introduced in an artificial manner was unstable. In response, Cohen and Boyer showed that recombinant genes were maintained through hundreds of replication cycles. With hindsight, the idea that the host bacteria would have a mechanism for distinguishing its original DNA from the recombinant DNA seems ridiculous. It is redolent of essentialism, the discredited idea that the every species has a unique, non-physical essence that would render its DNA incompatible with that of another species. It is now universally acknowledged in biology that there is no life essence. Moreover, the interchangeability of DNA is a logical consequence of the fact that all extant life evolved from a common ancestor that used DNA as its genetic code.

We now know that recombination is not a quirky thing that only happens in contrived laboratory settings. It happens frequently in nature. In technical terms, recombination is a subcategory of a broader type of genetic exchange called horizontal gene transfer. Unlike vertical gene transfer –the familiar transfer of genes from the parent to offspring– horizontal gene transfer is the exchange of genetic material in ways other than sexual or asexual reproduction. (Counterintuitively, it is also possible to have gene transfer that is neither vertical nor horizontal, but within an organism’s own genome. In bacteria and plants, there are segments of DNA called transposons [“jumping genes”] that shuffle around the genome via a cut-and-paste mechanism.) In contrast to the subtle instances of recombination in Stanley Cohen’s experiments, natural instances of horizontal gene transfer can be quite dramatic. According the endosymbiotic theory, the cellular organelles known as mitochondria and chloroplasts were once free bacteria that became endosymbionts (“symbiotic organisms that live inside”). Insofar as they reproduce autonomously and encode the means of their own replication, the plasmids inside bacteria function more like endosymbionts than native bacterial DNA. (Fig. 1) For retroviruses such as HIV, recombination is a replication strategy. These retroviruses convert themselves into DNA and insinuate themselves into the host’s genome; the ultimate intention of these viruses is to leave the genome and infect other hosts. Occasionally, however, the viral DNA will mutate to such a degree that it can no longer escape the host’s genome. If this mutated viral DNA occurs in the germ line (the sperm and ova, in humans), the virus-unto-DNA will be present in the offspring, and propagate across generations like the fusion plasmid in Stanley Cohen’s E. coli experiments. Stunning evidence for the stability of this recombinant viral DNA comes from the myriad “fossil viruses” that populate our genome. Carl Zimmer writes:

“Scientists have identified 100,000 pieces of retrovirus DNA in our genes, making up eight percent of the human genome. That’s a huge portion of our DNA when you consider that protein coding genes make up just over one percent of the genome.”

The pioneers of recombinant DNA techniques were commendable not only for their scholarship, but also for their entrepreneurship. In 1976, Cohen, Boyer, and their colleague Paul Berg founded the biotechnology company Genentech, and applied recombinant DNA techniques to a suite of biological problems, such as cancer and diabetes. Their enterprise was wildly successful, and, in 2009, the Swiss pharmaceutical giant Roche purchased Genentech for $46.8 billion.

Additionally, the founders of recombinant DNA techniques have been praised for their ethical stewardship of the technology they brought into the world. Concerned about possible biohazards, Paul Berg organized the 1975 Asilomar Conference on Recombinant DNA to develop sensible guidelines for further biotechnology research. The consensus among the conference attendees was that the risks were high, and that research should proceed cautiously. The rules agreed upon at Asilomar were adopted by the NIH in 1976. While the Asilomar Conference is generally viewed as a success, some scholars regard these guidelines as overly conservative. Commenting on a recent controversy in bioethics, Steven Pinker wrote:

Though the Asilomar recommendations have long been a source of self-congratulation among scientists, they were opposed by a number of geneticists at the time, who correctly argued that they were an overreaction which would needlessly encumber and delay important research.

Another reason why scholars like Pinker criticize the Asilomar framework is because its “excess of caution” approach is founded on an invalid premise: one cannot predict the pace of animal research by extrapolating from progress already made in bacteria. While it is relatively easy to induce recombination in bacteria, the process of engineering recombinant DNA into animals is considerably more difficult. Bacterial DNA is far more accessible, and hence easier to manipulate. By dint of being unicellular, a recombination event between two bacteria is a much more salient event: one or both of the participating bacteria come away with a new genome. For a multicellular organism, like a mouse or a dandelion, such a wholesale change in genome is impossible; it would involve transforming billions of cells at once. However, all multicellular organisms have a phase in their reproductive cycle in which they are a single cell. It is at this unicellular stage that an organism is susceptible to wholesale genetic modification. In humans, this cell is called a zygote, and is the result of the fusion of the sperm and egg. If a transgene (another term for recombinant DNA) was inserted into the zygote’s genome, that embryo would develop as though the transgene had always been present. Consider the case of a zygote with two defective copies of CFTR, the gene responsible for cystic fibrosis. A transgene that substituted the healthy version of CFTR in place of one of the defective copies would spell the difference between a healthy adult and one with cystic fibrosis.

Although transgenic techniques are rarely applied to humans, transgenic animals are a staple of modern biomedical research. Mice and fruit flies with recombinant DNA have increased our understanding of development, disease, and behavior. In spite of this progress, the process of transforming an animal’s genome remains complex, expensive, time-consuming, laborious, and imprecise. As with humans, the principal impediment stems from the fact that transforming animals requires germ line manipulation. Because of this, the model organism of choice for cell biologists has traditionally been the yeast Saccharomyces cerevisiae (Fig. 2). This is the same species we use in baking and alcohol fermentation. Yeast are particularly useful in cell biology and cellular biochemistry because they combine the convenience of bacteria with the representativeness of eukaryotes. Humans, plants, and fungi are all eukaryotes, meaning that they have a nucleus, linear chromosomes, and a generally high degree of shared biochemistry. Yeast are fungi, and since fungi are eukaryotes, it is often valid to generalize from yeast to humans. Certainly it is more valid than generalizing from bacteria to humans. In other ways, yeast behave more like bacteria. Unlike most eukaryotes, yeast are single-celled. It is easier to access, isolate, and manipulate individual cells. Yeast further resemble bacteria because of their ability to undergo asexual reproduction. Although yeast cannot replicate themselves quite as rapidly as bacteria, asexual reproduction simplifies the task of maintaining stable laboratory cultures.

Figure 2. Yeast cells viewed under a microscope.
Figure 2. Yeast cells viewed under a microscope. (Wikimedia Commons)

If the ultimate goal of biomedical research is to apply our knowledge to humans, why not study the biochemistry of human cells? In general, the petri dish is an inhospitable environment for human tissue. If it is necessary to study animal cells, the next best option is insect cells, which are more viable in culture. The exception to the rule that human cells cannot thrive in culture is cancer cells. In cancer cells, the feedback mechanisms that regulate cell division have been compromised, resulting in wild proliferation irrespective of the environment. Unfortunately, the same mutations that make cancer cells so fecund in a petri dish also disqualify them as research subjects. The biochemistry of a cancer cell is too disordered and anomalous to be representative. In many cases, healthy insect cells are a better guide to human biology than human cancer cells. For a vivid illustration of just how unrepresentative cancer cells can be, compare the karyotypes (profiles of chromosomes) of a normal human cell and a HeLa cancer cell (Fig. 3). The HeLa cell has eleven additional chromosomes, and is missing five others, including both copies of Chromosome 13.

Figure 3. The karyotypes of normal human cells and HeLa cancer cells
Figure 3. The karyotypes of normal human cells and HeLa cancer cells (Berkeley Science Review)

(Note: When I refer to the article under consideration, I’ll refer to “Galanie, et al.” When I’m referring to the general research program coordinated by Christina Smolke, which pre-dates this paper, I will refer to “Smolke” or “Smolke’s research.”)

Galanie, et al., applies techniques from yeast biochemistry and genetic engineering. By the end, the thebaine-producing yeast developed by Christina Smolke and her colleagues contained genes from four plants, an animal, and a bacteria. The yeast’s native genes were also altered. In total, 21 non-native genes were engineered into the yeast. (Fig. 4)

Figure 4. Summary of the species whose genes were engineered into the yeast in Galanie, et al. (2015). (Credit: Robert F. Service)

In the past several years, synthetic biologists have recapitulated various stages of thebaine biosynthesis in yeast, but Smolke is the first to have achieved “complete biosynthesis.” This is not to say, however, that Smolke and her collaborators merely fit together pre-fabricated pieces of a puzzle. Although certain steps had been previously worked out, unifying all the steps in a single organism required expertise, ingenuity, and perseverance in the face of endless technical challenges.

The general approach taken by Galanie, et al., consisted of working out the basic steps, and then optimizing each step. At every step, it was possible to observe the impact of their manipulation: did the yeast produce the next product in the pathway? (Fig. 5) If not, why not? If so, how would one tweak the process so that it produces more of the desired protein? The complexity of this project was so much greater than earlier achievements in yeast-based biosynthesis that innovation was crucial. The chronicle of the experiment really highlights the “engineering” part of genetic engineering. There was tinkering, trial-and-error, and troubleshooting. By analogy to other unprecedented biotechnology initiatives (The Human Genome Project, the reconstruction of the wooly mammoth genome, etc.), it is reasonable to suppose that some of the techniques that seem awkward and laborious will become optimized and standardized as the technology matures.

Figure 5. The biosynthetic pathway for thebaine production in yeast. The bolded words are genes that have been inserted into the yeast’s genome. (from Galanie, et al., Figure 1A)

The most vexing technical challenge arose from an unexpected interaction between the yeast’s native cellular milieu and one of the plant proteins that the yeast was made to express. How they solved this problem was the most fascinating part of the paper.

The enzyme salutaridine synthase (SalSyn) is native to the opium poppy, where it catalyzes one of the steps that in the opiate production pathway. In the opium poppy, SalSyn is always processed correctly, with the active sites that are responsible for catalyzing the conversion on the C-terminal facing outward into the cytosol. (Fig. 6, left side) When the SalSyn gene was imported to yeast, the protein was processed incorrectly, with the C-terminus facing inward, into the endoplasmic reticulum. Moreover, the upside-down SalSyn was glycosylated. Glycosylation is a process by which sugar-like chains are tacked onto the developing protein. Normally, the glyco-tags function as shipping labels that ensure the protein gets delivered its proper cellular destination. In the context of engineering a yeast that synthesizing opiates, however, glycosylation became a problem, because the sugar-tags blocked the SalSyn’s active sites. (Fig. 6, center)

Removing the glycosylation sites wasn’t viable, because it made SalSyn less efficient. Instead, the solution involved engineering a chimera protein. Smolke and her colleagues used a plant genome database to search for a plant protein that was sufficiently similar to SalSyn that it would catalyze the same chemical reaction, but not so similar that it would be glycosylated or inserted upside-down. They ended up inserting a protein that had a membrane component from a poppy plant, and an active component from a Goldthread flower. (Fig. 6, right side)

Figure 6. The correctly processed SalSyn protein (Left), the incorrectly processed and glycosylated SalSyn protein (Center), and the engineered SalSyn fusion protein (Right). (From Galanie, et al., Fig. 3A)

Although Smolke and her colleagues deserve praise for finding a suitable plant protein in the database and engineering a functional fusion protein, they were, ultimately, lucky that there existed a plant protein that satisfied their needs. There is currently no way to predict whether a protein like SalSyn will have an adverse interaction when expressed in an organism that doesn’t naturally manufacture it. How synthetic biologists resolve such cross-species interactions will determine the future of genetic engineering.

When engineering biomolecules, it is important to consider the handedness of those molecules. Most biological molecules are like a human hand: asymmetric, And, like a hand, every biomoledule has a particular handedness. A biomolecule and its counterpart with the opposite handedness are called stereoisomers. One of the principle reasons why biosynthesis of molecules is often preferable to standard chemical synthesis is that biosynthesis produces molecules with the optimal handedness for affecting our bodies.

According to the Curie Principle, asymmetric effects can only arise from asymmetric causes. Biosynthesis is an asymmetric process, but synthesis by standard industrial chemistry is not. In industrial chemistry, synthesizing biomolecules involves finding a similar molecule and then modifying it step by step until it matches the desired biomolecule. Unlike biological enzymes, standard laboratory chemicals catalyze reactions symmetrically, with no bias toward left-handed or right-handed variants. As a result, the final product of industrial synthesis is racemic, containing 50 percent right-handed molecules and 50 percent left-handed molecules.

Sometimes, the other-handed version of a biomolecule is merely inactive. One treatment for Parkinson’s Disease involves injecting of DOPA, the precursor to dopamine, into a patient’s brain. However, only one stereoisomer, (L)-DOPA, is biologically active. While a racemic mixture of (L)-DOPA and its counterpart, (D)-DOPA, is not medically dangerous, it would nonetheless be better to have a nonracemic solution with only (L)-DOPA.

A more benign example of how the two different stereoisomers of a compound can have different biological effects is the organic oil carvone. Carvone’s right-handed form smells like spearmint while the left-handed form smells like caraway.

The textbook example of the tragedy of misunderstanding the difference between a stereoisomer and its mirror counterpart is the drug thalidomide. In the early 1950s, thalidomide was produced by an industrial process that resulted in a racemic mixture with a fifty-fifty ratio of thalidomide’s two stereoisomers. Thalidomide was proven safe and effective in adult populations, and marketed as a treatment for nausea. In particular, it was widely prescribed as a remedy for morning sickness. Unfortunately, the manufacturers of thalidomide neglected to test their drug in pregnant women. One of the stereoisomers did indeed relieved morning sickness, but the other produced devastating birth defects. According to the U.S. Food and Drug Administration, “[i]n the late 1950s and early 1960s, more than 10,000 children in 46 countries were born with deformities…as a consequence of thalidomide use.”

In order to optimize each step of the biosynthetic pathway, Smolke and her team not only needed a means to detect the presence of a particular biomolecule, but also its precise quantity. For this, they used liquid chromatography mass spectroscopy (LC-MS), a technique for detecting and quantifying the presence of a specific molecule in a complex mixture. Standard LC-MS, however, was not sufficient, because that technique has no way of distinguishing between a biomolecule and its stereoisomer. Considering that one of the steps in thebaine biosynthesis is the conversion of (S)-reticuline to its stereoisomer, (R)-reticuline, the inability of standard LC-MS to distinguish the handedness of biomolecules was a significant obstacle. Therefore, Smolke relied on a more sophisticated analytical technique, chiral LC-MS, which (as the name suggests) takes the chirality of the constituent molecules into account.

In most cases in which you are trying to synthesize a biomolecule, you are trying to synthesize the naturally occurring isoform, such as (L)-DOPA. The naturally occurring stereoisomer is preferred because the other isoform is either inactive or toxic. But what if the unnatural stereoisomer was, in fact, better than the naturally occurring version? It would be as if (R)-DOPA did a better job at relieving parkinsonian symptoms than (L)-DOPA. In his outstanding book, Right Hand, Left Hand, the psychologist Chris McManus discusses an example of a stereoisomer performing better than its naturally occurring isoform. Certain animals synthesize reversed versions of ubiquitous biomolecules, for use as toxins against would-be predators. But what is a toxin to one species might be a mind-expanding agent for another. Humans have a long history of exploiting plant and animal toxins for medicinal or recreational purposes. Nicotine, for example, is a neurotoxin meant to deter insects from consuming tobacco plants. McManus highlights a poisonous frog species that synthesizes dermorphin and deltorphin, stereoisomer counterparts of the naturally occurring opiate molecules, morphine and enkephalin, respectively:

“Dermorphins and deltorphins are opioid peptides because they act on the brain in the same way as natural opiates such as morphine and heroin. In fact, weight for weight, dermorphin is a thousand times more potent than morphine, and ten thousand times more potent than the proper neurotransmitter in the brain, enkephalin. From the dermorphins and deltorphins may well come morphine substitutes that are potent pain-killers but also non-addictive and without the side effects of sedation and gastro-intestinal stasis. Of course there is also the possibility of new designer drugs to feed abuse, the juice of the poppy being replaced with a simple peptide.” (McManus 134)

If McManus’s speculation about the therapeutic value of these other-handed opiates turns out to be correct, there is no reason to suppose that synthetic biologists like Smolke couldn’t engineer this frog’s genes for dermorphin and deltorphin production into yeast. In fact, once you have a yeast that can synthesize morphine, it would only require a few more genes to synthesize dermorphin.

The experiment recounted in Galanie, et al., is known as a proof of principle. In this kind of experiment, it is necessary only to demonstrate that your method (e.g. yeast-based opiate biosynthesis) is feasible, rather than safe, moral, or economically viable. Even though proof of principle experiments require a reduced burden of proof, Galanie, et al., nevertheless defend the safety, ethics, and economic value of their research.

Safety: Smolke and her colleagues ensured the safety of their laboratory by consulting before, during, and after with the D.E.A. The lab kept meticulous records of the opiates it produced, and all samples were destroyed after testing. The lab members submitted to background checks, presumably to discover any history of drug abuse. Furthermore, the yeast were modified such that they could only grow on a particular medium, meaning that if someone were to steal a sample of the opiate-producing yeast, the yeast would die in the absence of this necessary substrate.

Ethics: According to the World Health Organization, there are 5.5 billion people who have “low to nonexistent access to treatment for moderate or severe pain.” It is scandalous that anyone has to endure needless suffering, much less a majority of human beings. Most pet-owners in the United States would be furious if their veterinarian didn’t have enough pain medication when their pet needed surgery. We should feel just as much compassion for the billions of people whose children and elderly parents are suffering because pain-killing drugs are unavailable or too expensive. Since poppy farming is not meeting the global demand, it is a moral imperative to find alternative sources of opiates. If synthetic biologists like Smolke are able to scale up opiate synthesis in yeast, it could potentially relieve the suffering of billions of people.

Economic Viability: Having a yeast that is capable of thebaine biosynthesis is significant because it is only genes necessary to convert thebaine into hydrocodone (the main ingredient in the second most widely prescribed drug in the United States, Oxycontin). A yeast that could synthesize hydrocodone directly would supplant not only agricultural production of thebaine, but also the industrial chemistry that converts raw thebaine into hydrocodone. In fact, the authors did indeed generate a yeast that produced hydrocodone, though they emphasize that they didn’t optimize it to be efficient. This relatively slight modiification will vastly increase the economic viability of yeast-based opiate biosynthesis.

Just as Stanley Cohen, Herbert Boyer, and Paul Berg founded Genentech to profit from recombinant DNA, Christina Smolke has recently founded her own company, Antheia, which seeks to push yeast opioid synthesis into commercial viability. It is reasonable to expect that many investors will be eager to fund Smolke’s start-up, along with many researchers eager to lend their talents to her upcoming projects. Even so, significant challenges lie ahead. In order for “yeast-based production of opioids to be a feasible alternative to poppy farming,” it will require “over a 100,000-fold improvement” in efficiency (Galanie, et al., 2015)

Is this degree of improvement achievable? One cause for optimism is the comparable improvement achieved in arteminisin-producing yeast strains. Arteminisin is an antimalarial drug that –like thebaine– was previously produced only by plant sources. And, like thebaine, the first yeast that synthesized arteminisin was orders of magnitude less productive than agricultural sources. But, within a few years, “researchers boosted the output of the artemisinin-making yeast by a similar amount.” (Service, 2015) Today, yeast-based production of arteminisin accounts for one-third of global production. It is important to note that the arteminisin pathway involves only 3-6 genes. By contrast, thebaine biosynthesis requires 21 genes. This difference suggests that progress in yeast-based thebaine biosynthesis might proceed at a slower pace than that of arteminisin.

In his fascinating book, Superintelligence, the philosopher Nick Bostrom provides a theoretical framework for predicting the rate of improvement for any given technology. Although Bostrom applies this model to machine intelligence, its explanatory range is more general. For any technology, the rate of improvement is proportional to optimization power and inversely proportional to system recalcitrance. Figure 7 depicts this relation in a mathematical ratio. Optimization power refers to the effort being applied to a problem. We can suppose that Smolke and her new company will be working very hard on this problem, though the overall progression of the technology will be limited by the fact that only one group has the proprietary privileges and technical know-how to synthesize opiates from yeast. The Human Genome Project, by contrast, was a multinational collaboration among at least twenty scientific institutions. System recalcitrance refers to how easy it is to make the system more productive through additional effort. We might describe a system with low recalcitrance as having “low-hanging fruit.” Conversely, when a system is highly recalcitrant, further investment would result in “diminishing returns.” If the analogy to artemisinin-producing yeast is apposite, then it is reasonable to expect substantial progress in yeast-based opiate synthesis within several years.

Screen Shot 2015-08-22 at 8.41.52 PM
Figure 7. Nick Bostom’s framework for predicting the development of a particular technology.

After finishing Smolke’s paper, I wondered whether subsequent optimization necessarily needs to come from human ingenuity. Why couldn’t they enhance opiate output through selective breeding? Humanity has a long history of radically reshaping plants and animals through selective breeding. Although yeast are valued as model organisms for their asexual reproduction, they are fully capable of reproducing sexually. It would therefore be possible to breed generations of yeast, and select for high thebaine output.

Perhaps the trajectory of the opiate-producing yeast would resemble the corn plants that were bred for high oil content. In a dramatic illustration of the power of artificial selection to increase output of a particular gene product, the corn’s oil yield quadrupled in fewer than eighty generations (Fig. 8).

Figure 8. The average oil content of corn selectively bred for high oil yield across 80 generations. (From Boyd and Silk, How Humans Evolved: Second Edition. [New York: W.W. Norton and Company, Inc., 2000] p. 74; retrieved from PBS)
Figure 8. The average oil content of corn selectively bred for high oil yield across 80 generations.
(From Boyd and Silk, How Humans Evolved: Second Edition. [New York: W.W. Norton and Company, Inc., 2000] p. 74; retrieved from PBS)
However, the corn example might be inapposite because selective breeding requires genetic variation. The reason those corn plants could increase in oil yield over many years was not because the plants were evolving new genes, but because selection process was concentrating all the genes that increased oil yield while winnowing down those that didn’t share those genes. At some point, the corn plants will stop increasing in oil content. When this happens, it will not necessarily because the oil content has begun to compromise the corn’s ability to survive, but because every corn plant will be invariant with respect to their oil-related genes. Since the corn’s oil content was the only selection criterion, it is akin to breeding identical clones. Smolke’s yeast are also, for all intents and purposes, identical clones. Unlike a natural population, there is no simply diversity for evolution to winnow down.

The ultimate economic impact of yeast-based opiate biosynthesis notwithstanding, this paper by Galanie, et al., shows the practical value of basic research. This recent achievement was made possible by decades of exploratory research into the hidden mechanisms of bacteria, flowers, and yeast –research, which, at the time may have been difficult to justify in terms of economic utility. Nevertheless, that corpus of information is now being applied to the biomedical sciences. Investment in scientific research benefits society.

Life’s diversity is frequently extolled, and biodiversity is a key goal of conservationism. In terms of sheer economic value, however, diversity is difficult to justify. As a result, many choose to frame it as an aesthetic good. It was, in Darwin’s words, “endless forms most beautiful and most wonderful.” But life is more than life forms. It is also a vast repository of information, strategies for wringing meaningful work out of insensate chemical. It is this informational diversity that is worth preserving. This information is intelligible because it is wrought in DNA, the universal genetic code. Modern advances in synthetic biology attest to our progress in mastering this amazing –and largely untapped– natural resource.

The power to engineer organisms is the ultimate validation of our knowledge about the workings of cellular mechanisms. To have assembled all the knowledge about cell biology into a textbook is impressive, but to apply that knowledge in order to reconfigure and optimize another creature is deeply gratifying. The ability to engineer an organism is an outside criterion of verification, the biological equivalent of an airplane designer observing that her plane flies properly instead of falling from the sky. Perhaps, like the corn that was bred for high oil content, there is a limit to how much an evolved life-form can be re-shaped. However much we might praise the utility of any given model organism, there is a point after which further efficiency cuts into their viability. There is currently a program in synthetic biology to develop single-celled “template organisms” that are loaded with gene clusters that enable the artificial cells to do a particular job, and not expend any resources on extraneous functions. Even if synthetic biology doesn’t produce template organisms, it is likely that synthetic biologists working with single-celled organisms like yeast will converge on this solution. Along with the insertion of novel genes, Smolke also silenced the activity of native yeast genes. This suggests that further optimizations might come from eliminating any genes that interfere directly or indirectly with thebaine biosynthesis. Once that is done, the genes that code for anything besides basic functions and thebaine biosynthesis will be erased, since these genes waste metabolic resources that might otherwise be dedicated to manufacturing thebaine. The ultimate product of all this culling will be an organism that resembles the notional template organism more so than a wild-type yeast.

The research chronicled in Galanie, et al., is a technological accomplishment. Moreover, the byproducts of optimizing this particular technique will catalyze subsequent advances. It’s moral implications are no less significant; the promise of meeting demand for opiate medication in the developing world is worth our investment in this research program.

References:

Galanie, S., Thodey, K., Trenchard, I. J., Interrante, F., & Smolke, C. D. (2015). Complete biosynthesis of opioids in yeast, (August), 1–11.

Service, R.F. (2015. Modified yeast produce opiates from sugar. Science, 14 August 2015: 349 (6249), 677. [DOI:10.1126/science.349.6249.677]

Advertisements

One thought on ““Complete Opioid Biosynthesis in Yeast,” by Galanie, et al. (2015): Discussion

  1. A well-reasoned, fiercely empathetic call for access to pain relief for the millions of people whose suffering goes unrecognized by most of us lucky enough to have never had to witness the suffering of late stage cancer victims without affordable morphine. That we treat our own pets better than people invisible to us is sad. That we might reject a technology that would enable a billion-fold reduction in suffering is even sadder.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s