Synthetic biology impacts several facets of the drug discovery process, and opens new perspectives toward addressing long-standing therapeutic challenges, such as infections caused by microbial biofilms, or antibiotic resistance, a crisis of global proportions. Illustrating the powerful applications in infectious diseases, Lu and Collins engineered the E. coli-specific T7 bacteriophage to express DspB, an Actinobacillus actinomycetemcomitans protein that hydrolyzes β-1,6-N-acetyl-D-glucosamine, a crucial adhesin from biofilms produced by many bacterial species.
The initial phage infection of a bacterial biofilm causes the intracellular production of progeny phage and DspB, which are both released by the lysing cells. DspB degrades the extracellular polymeric matrix of the biofilm, and the phages re-infect neighboring cells, reinitiating the cycle. This strategy, which removes the need to deliver enzymes to the sites of an infection, reduced the biofilm cell counts by 4.5 orders of magnitude.
In another study, the authors engineered a modified version of the M13 bacteriophage in which the lexA gene, a repressor of the bacterial SOS global response to DNA damage, was placed under the PLtetO inducible promoter. Combining antibiotic treatment with an antibiotic-enhancing bacteriophage increased the in vitro killing by several antibiotics and, in vivo, the survival of infected mice treated with ofloxacin.
Another landmark study reported the engineering of a yeast strain to produce artemisinin, a component of first-line combination therapies for uncomplicated malaria. The only commercial source of artemisinin is the Chinese plant Artemisia annua L. (wormwood), where the compound is present at low concentrations, making therapy expensive.
Chemically synthesizing this medication is also very costly. Ro et al. generated an artemisinic acid-producing Saccharomyces cerevisiae strain in three steps that involved manipulating its farnesyl pyrophosphate pathway to increase farnesyl pyrophosphate production and decrease its use for sterol synthesis; introducing the A. annua amorphadiene synthase gene to allow amorphadiene synthesis; and engineering an A. annua cytochrome P450 to facilitate amorphadiene oxidation into artemisinic acid. This engineered strain produced up to 100 mg/L artemisinic acid that was transported and retained on the outside of the yeast, facilitating subsequent purification.
Ajikumar et al. reported the metabolic engineering of E. coli to produce taxadiene, the first committed intermediate of the potent anticancer medication taxol, which is produced by the Pacific yew tree (Taxus brevifolia). In an approach called multivariate-modular pathway engineering, the authors combined the upstream isoprenoid pathway, native to E. coli, with the downstream, synthetic taxol pathway, and simultaneously varied gene expression within each module.
This allowed the expression of upstream and downstream pathways to be modulated by changing multiple parameters such as promoter strength or gene and plasmid copy numbers. This system yielded approximately 1 gram of taxadiene from a liter of culture, and represents a significant step toward producing taxol in a bacterial system.