Hopkins scientists have reported a novel method of killing specific populations of cells that could have important applications to cancer and virus therapies.
The technique involves specialized strands of DNA that block cell replication, according to their report published in the Journal of Antimicrobial Chemotherapy.
The strands in question are called "anti-gene padlocks," and their name is quite instructive.
By physically and irreversibly wrapping themselves around a target gene, the padlocks effectively cut that gene off from the machinery of DNA replication necessary to maintain a cell's status quo.
The padlock technology is the newest in a series of breakthrough therapies all aimed at using diseased cells' genetic profiles to target them for elimination.
Antisense RNA and small interfering RNA (siRNA), for example, have proven effective in killing cancer cells, but both of those approaches are post-transcriptional (that is, they function by interfering with messenger RNA which carries a transcribed "copy" of the genetic code). Anti-gene padlocks, on the other hand, function by interacting directly with the DNA helix, blocking transcription and replication before they can occur.
Led by James Eshleman, a researcher at the School of Medicine, the team custom-built a short segment of DNA - the padlock - to be complementary to the DNA in a gene in the bacterium E. coli. (In genetic parlance, two strands of DNA are complementary if they're able to bind to each other.)
They then used an electric current to open the bacterium's cell membrane and injected the padlock into its nucleus. Once bound to its target, a specialized protein called a DNA ligase grabbed the two loose ends of the soon-to-be padlock and linked them together.
That ligating process is the genetic equivalent of permanently tying a shoelace.
With a padlock firmly surrounding a target gene, gene-copying cellular machinery can't do its job.
Without the ability to copy its genome, a cell can no longer divide, let alone fix any problems with its own genome. In the long-term, this could mean a reduction in the number of virus-infected cells or the size of a cancerous tumor.
Nonetheless, only 30 to 40 percent of the target-possessing E. coli was killed. The researchers attributed the modest success rate to the fact that the padlocks in this particular experiment were made from standard DNA, which they knew to be susceptible to normal degradation by enzymes called exonucleases.
As the team's goal was simply to prove the viability of the anti-gene padlock technology, they weren't too concerned and suggest that modifying the DNA backbone in certain ways will protect the padlocks from being broken down. What's more, the gene they targeted in E. coli wasn't essential to the bacterium's survival.
Choosing a more crucial gene will likely increase the rate of elimination of cells bearing the target gene.
This is the crux of the therapeutic potential of the padlock technology: By selecting a gene that is both unique to a diseased cell and essential to its survival, the padlocks will be able to specifically home in on sick cells while avoiding damaging healthy ones.
The potential applications are remarkably numerous; cancer and viral infection are two obvious possibilities, but the team also suggests the intriguing prospect of applying the technology to fighting drug-resistant infectious disease, including methicillin-resistant Staphylococcus aureus (MRSA).
The fact that the number of MRSA infections treated in U.S. hospitals doubled between 1999 and 2005 suggests no shortage of target genes ripe for a padlock.
