In May 2022, a clinical trial is quietly underway in London, when a British teenager named Elisa becomes the first patient to receive an experimental treatment generated using “base-editing”, an emerging form of DNA engineering. Alyssa, 13, had exhausted all conventional treatments for blood cancer, and was enrolled to receive immune cells from a healthy volunteer that had been programmed to hunt down and kill leukemia cells. Within a month, the disease was undetectable.
Whether this highly refined form of cell therapy pioneered by our team at Great Ormond Street Hospital in London has permanently cleared the cancer, and whether subsequent patients will also respond, only time will tell. Nonetheless, the breathless pace of research development around gene therapy, always accompanied by impressive technological improvements, provides room for optimism, with good reason.
It’s been almost 20 years since the Human Genome Project, an international research program involving 20 universities in six countries, completed a decades-long quest to sequence the DNA code that makes up our genes and chromosomes. Some 3 billion letter pairs – or bases – were found to be organized into 20,000 or more genes, and which ultimately protect and control the secrets of life in each nucleated cell.
The code, encrypted in bundles, or genes, rests on just four DNA bases and instructs the molecular machinery found inside cells and orders the production of proteins. If DNA code is the script, then proteins are the actors who play their roles to determine how cells assemble, grow, function, interact, sleep, wake up, replicate and die. Huh.
At the turn of the millennium, there was still a great deal of uncertainty as to how quickly these vast data sets could be used to improve human health. Variation in the DNA code naturally arises between individuals, and sometimes certain changes – commonly known as mutations – lead to diseases. Some dominant mutations – for example, those for the neurodegenerative disorder Huntington’s disease – act alone. Other mutations – as in the case of the inherited blood disorder beta thalassemia or the lung disease cystic fibrosis – are problematic only in combination.
The question being asked at that time was how can mutations be permanently fixed or their effects reversed? At that time some of the earliest reports of successful “genetic therapy” were emerging.
The holy grail of gene therapy remains the quest to mutate or replace DNA ‘on site’ in a single, efficient and reliable therapy.
For example, in France in 2001, pediatric immunologists Alain Fischer and Maria Cavazano used a modified virus, with its own genes deleted, to inject a therapeutic gene into bone marrow collected from infants born without a functioning immune system. to be carried into the marrow cells. Reinjecting these “repair” stem cells back into the infants helped in life-saving recovery, restored immunity and cleared up the infection.
Later, it became clear that randomly pushing extra copies of genes into chromosomes can disrupt how cells proliferate and can lead to cancer under certain circumstances. This prompted researchers to modify and upgrade virus delivery systems. As a result, there are dozens of experimental treatments being tested today to add extra copies of the gene to cells. Few treatments have reached the market authorization stage, including beta thalassemia and some forms of hemophilia, which would otherwise require life-long blood product support. Overall, the numbers treated to date are relatively small, and long-term monitoring will give us a better picture of their effectiveness and safety. For now, the exorbitant price tags will raise eyebrows and restrict access, but development continues at a rapid pace, and better, safer and more effective strategies are inevitable.
The holy grail of gene therapy remains the quest to mutate or alter DNA “on site” in a single, efficient and reliable therapy. That aspiration was given a boost in 2012, when an enzyme system called Crispr/Cas9, first discovered in bacteria, was reprogrammed to properly cleave, or cut, human DNA. It was not the first such platform to be developed. In fact, our team deployed existing molecular scissors tools for editing “T cells” – a type of white blood cells that are a part of the immune system – against leukemia. However, Crispr is a highly adaptable, inexpensive, and easy-to-use technology. It is indeed a breakthrough technology that has earned its developers, Emmanuel Charpentier and Jennifer Doudna, the 2020 Nobel Prize in Chemistry.
Simultaneous advances in sequencing technologies and computing power now enable entire genomes to be sequenced within hours. A new wave of therapeutic strategies is already in development. Several clinical trials using Crispr/Cas9 are underway, mostly to disrupt genes, including in our hospital where, earlier this year, we published how CRISPR can more efficiently kill T cells. Engineer can.
This week, at the American Society of Hematology’s annual conference in New Orleans, we described how a new generation of molecular tools — called base-editors — can be used to treat other types of “off-the-shelf” T cells. is being applied to generate Aggressive leukemia.
Base-editors were invented at the Massachusetts Institute of Technology as recently as 2016, and can chemically alter single letters of the DNA code. The technology draws on CRISPR’s guidance system to reach very precise locations on chromosomes deep inside the center of cells. Instead of cutting DNA, molecular machines deploy local chemistry to replace letters within reach of their enzyme arms. The cells Elisa received were the most complex ever generated, and she recovered within a month of treatment.
This is all very encouraging, but of course this and other clinical trials will need to treat more patients and follow them for longer periods of time.
Nonetheless, a pretty remarkable technological leap is underway, and it looks like there will be more iterations and clever refinements to come. Solutions are being investigated to address delivery to other cell types and to limit potential immune responses. New questions and dilemmas will certainly arise, not least as the revolution unfolds around regulatory oversight, cost, and accessibility of these cutting-edge technologies.
For now, at least one relieved family will be celebrating the holidays together, and others are looking forward to the new year.
Published: December 23, 2022, 6:00 pm