In an article published in the April issue of Nature Reviews Genetics, two experts at the Weill Medical College of Cornell University sum up the achievements, challenges and promise of a burgeoning field: genetic medicine.
"There have been some real success stories since the human genome was sequenced in 2001, but some disappointments, too, and real hurdles yet to be overcome," says co-author Dr. Ronald G. Crystal, the Bruce Webster Professor of Internal Medicine and Chairman of the Department of Genetic Medicine at Weill Cornell Medical College.
"However, the purpose of this paper is to highlight the nearly limitless potential of this technology, which is still in its infancy," he says.
Co-author Dr. Timothy O'Connor, Assistant Research Professor of Genetic Medicine at Weill Cornell, agrees with that assessment.
"There are, and will be, roadblocks along the way, both biological and social. But the pace of discovery suggests that all of these challenges can be met," he says.
The article, entitled "Genetic Medicines: Treatment Strategies for Hereditary Disorders," focuses on the treatment of "monogenic" disorders -- the over 1,800 inherited illnesses linked to aberrations in a single gene.
"On paper, a gene therapy 'fix' for these disorders appears simple: Introduce a 'healthy' form of the dysfunctional gene, or even a portion of that gene, to the disease site, to correct the problem," Dr. Crystal explains.
"Unfortunately, it's rarely, if ever, proved that simple."
The article enumerates a number of promising gene therapy approaches, each with its strengths -- and its Achilles' heel. They include:
Gene transfer using viral vectors. Getting a gene and its promoter to breach the outer membrane of target cells has always been a tough challenge. But harmless adenoviruses have proven useful vehicles for penetrating cells and depositing piggybacked genes.
"Trouble is, host immune responses have limited the expression of adenovirus-delivered genes," Dr. O'Connor explains.
So, researchers have turned to simpler viral vectors, such as adeno-associated viruses (AAVs). "These are capable of delivering longer gene expression, but there's been a trade-off in terms of the amount of genetic cargo they can carry and the magnitude of expression levels," Dr. O'Connor says.
Then there are the retroviruses (such as MMLV) which go one step further, permanently integrating bits of DNA into the host cell's genome.
"That's a lot more long-lasting because expression continues as the cells divide; there's not that dilution of effect," Dr. Crystal explains.
But retrovirus-delivered gene therapy has one big disadvantage here, too: Integration boosts the risk for "mutagenesis" -- cancer-linked mutations in the cell's genome.
In one of the first gene therapy trials, a majority of children with the monogenic immune disorder X-linked SCID were effectively cured by MMLV-delivered genes. Unfortunately, a minority later developed life-threatening leukemias linked to the therapy's mutagenic potential.
"We believe, however, that if you could identify spots on the chromosome where integration was safe, this risk of mutagenesis could be minimized," Dr. Crystal says.
RNA-modification therapies. Ribonucleic acid (RNA) is the intermediary player that helps turn instructions encoded in DNA into the active proteins that drive cell function. Suppressing or stimulating RNA should be another way of "fixing" genetic disorders.
One RNA-directed technology in development involves the use of "antisense oligonucleotides" (ASOs), compounds that can decrease production of an unwanted or overly expressed protein. Keeping ASOs stable within cells, without compromising their ability to target specific defects, has been a challenge, however.
Then there's RNAi, where the "I" stands for "interference." Essentially, this technology involves harnessing a natural process whereby specific molecules silence RNA activity. Here, as with other gene therapies, effective delivery across the cellular membrane has proven challenging, and the molecules' short half-lives mean effects have been transient.
"On the other hand, experiments that delivered RNAi with a viral vector have proven promising in mouse models of Huntington's disease, helping to slow progression," Dr. O'Connor says.
Other innovations include trans-splicing -- substituting a "healthy" piece of a gene in a spot normally occupied by dysfunctional DNA, rather than replacing the whole gene.
"One strategy of trans-splicing was pioneered in our lab here at Weill Cornell, and has proven promising in animal models for hemophilia and cystic fibrosis," Dr. Crystal says.
Finally, the experts noted that ribozymes -- RNA with enzymatic activity -- might also be used to fix gummed-up cellular machinery, although problems with delivery and stability have plagued this approach as well.
Embryonic stem cells. Beyond the political and ethical issues connected to this hot-button technology, therapy involving embryonic stem cells -- which can differentiate into any cell type in the body -- does have biological hurdles to overcome, as well.
"There are potential rejection issues, although most scientists believe those can be overcome," Dr. O'Connor says.
However, because of their incredible plasticity, the potential for these cells is nearly limitless. Numerous studies have already shown they can be efficiently directed to differentiate into specific cell types.
"In the future, we should be able to use embryonic stem cells to help regenerate diseased organs," Dr. Crystal says. "While current restrictions here in the U.S. limit this research, it's important that we as scientists educate the public as to the uniqueness of these cells and their therapeutic potential."
So, where does all this leave the future of genetic medicine? For every naysayer who doubts gene therapy's potential, there are scores who see today's setbacks as just speed bumps on the road to success.
"Remember, drug development is always a 10-to-15-year process, whatever the theory behind it," Dr. O'Connor observes. "And just in the last decade we've seen enormous leaps forward, such as faster high-throughput screens, hapmap technologies and other advances. It's our belief that even more astonishing advances are yet to come that will turn the dream of genetic medicine into a reality for patients at the bedside."