Looking Into a Crystal Ball

Ronald G. Crystal, MD,
Ronald G. Crystal, MD,

Ronald G. Crystal, MD, is chair of the department of genetic medicine at Weill Cornell Medical College in New York, where he has worked for 26 years. He is one of the pioneers of gene therapy. After training with W. French Anderson, he led some of the first in vivo gene therapy studies, including the first trial for cystic fibrosis in the early 1990s. He has experienced the highs and lows of this field, emerging highly optimistic about the prospects for gene therapy. For example, he is looking forward to his own next big clinical trial for Alzheimer’s disease.

Here, Ron sits down with Kevin Davies, PhD, executive editor, The CRISPR Journal. Kevin published some of Ron’s early studies in gene therapy when Kevin was editor of Nature Genetics.


Davies: Ron, it seems as if we are in the middle of a renaissance in gene therapy. We’ve seen “cures” in a number of indications, from hereditary blindness, to sickle cell disease, to muscular dystrophy or muscular atrophy. Is that an overstatement?

Crystal: No, I think gene therapy is real now. It’s taken us 25 years or more to develop it with a lot of ups and downs. I think it is going to be a therapeutic for many different applications.


Davies: How did you first get into gene therapy?

Crystal: I am a pulmonary physician by training. I got interested in the early 1980s in a hereditary disorder called α1-antitrypsin deficiency, which is quite commonly associated with emphysema and also liver disease. It is a serum deficiency disorder, so we got the idea in the early ’80s, why not just purify plasma protein, α1-antitrypsin, and give it back? And we did.

Then, in 1986, we carried out the pivotal trial with one of the plasma fractionating companies, and it was approved by the FDA. I started thinking—this was ’87­—rather than giving it back on a weekly basis, because α1-antitrypsin has a half-life of four and a half days, why do you not give it once with a gene?

So, I had done my original postdoctoral training with French Anderson, who was one floor above me, and he had gotten interested in gene therapy. He had interacted with Eli Gilboa, who is a retrovirologist, and had gotten some retroviral vector constructs from him to transfer genes. They were working on that for T cells, which resulted in the early studies with T-cell modification.

We started working with this retrovirus vector with the α1-antitrypsin coding sequence. My first gene therapy publication in 1987 in PNAS was with a retrovirus ex vivo strategy.

Then one of my postdoctoral fellows, Paul Tolstochev, went to a company called Transgene in Strasbourg. He was the chief scientific officer there, and he called me up one day and said, “We’ve been interested in the possibility of adenovirus to transfer genes but do not have any really good ideas of what to use it for.” I said, “Oh, we have an idea.” We start thinking about in vivo gene therapy.

He introduced me to Michel Perricaudet, an adenovirologist south of Paris, and I sent over one of my postdoctoral fellows to his laboratory. We got the components, and we put together the first attempts, with β-galactosidase and then with α1-antitrypsin, and then very quickly demonstrated in vivo—by putting the adenovirus with β-galactosidase down the lung of a rat—that we could get a blue color. That was one of these eureka moments before 1990: we could really transfer genes in vivo.


Davies: 1990 is usually referred to as the launch pad of gene therapy with French Anderson’s clinical ADA deficiency trial.

Crystal: I think French’s first study was with Steve Rosenberg; they did a marking study in patients with cancer of some kind where they had a marker gene and a T cell and infused that into cancer patients.


Davies: Back in those days, the main vectors of choice were retroviruses. Was adenovirus a thing back then?

Crystal: We developed the adenovirus. After learning how to do it from Perricaudet, we were the ones who developed adenovirus for in vivo gene therapy. The first publication was in Science. We had in vivo gene therapy—α1-antitrypsin in vivo down the lung with adenovirus.

The second paper, in Cell, was with the cystic fibrosis transmembrane regulated gene. That came a little later. Ben Lewin [Cell’s editor] was gracious enough to accept it and give us a cover.


Davies: About 1995, Harold Varmus convened a panel, the gist of the conclusion being, hey guys, can we just slow down a little bit? What is your recollection of that time?

Crystal: We dosed the first patient in vivo with a virus in April 1993. There were three groups competing to be first: ourselves, Jim Wilson and Genzyme, together with Mike Welsh at Iowa. One of the interesting historic things was the NIH Recombinant Advisory Committee (RAC) meeting, where there were probably 600 people in the room. Maybe a third of them were scientists. A third were investors, and a third were the press. I think we went first; the RAC approved our proposal for CF with adenovirus. But we still had to get FDA approval. My FDA reviewer was working in a building on the NIH campus, and my group was in the clinical center. In April ’93, we were finishing up some of the details for the FDA.

Meanwhile, we had a patient waiting. One of the issues that was interesting for us, the FDA, the IRB, and the RAC, was the risk for in vivo gene therapy. One of the theoretical issues was what happens if our virus combined with some viral sequences in vivo and creates an Andromeda strain virus that would wipe out Bethesda! We built two negative pressure rooms in the clinical center specifically for the first patients. The first two patients in the trial were in those rooms for several weeks.

These patients were so gracious to participate. And we took samples of every biologic fluid we could obtain, looking for recombinant viruses. Of course, that turned out to be not an issue. Today we do it on an outpatient basis.


Davies: Let’s get back to the Varmus report.

Crystal: OK. Many of my colleagues and I met with that committee and it was interesting. I think they were correct that we were probably a little bit ahead of ourselves.

But the bottom line, and I think we have learned that from recombinant proteins and monoclonal antibodies and cell therapies, is eventually it doesn’t matter how many animals you do. You have got to do humans…These subjects who were part of these early trials were heroic. We learned from their participation. For example, with adenovirus, which is what the Gelsinger case was all about, [the issue] was innate immune responses. And the field did not really understand that risk.

I think looking back, what we did in animals did not necessarily predict what we would see in humans, and so I think that was part of it. It was an interesting period.


Davies: Gelsinger case in 1999: Was it bad luck? Was it avoidable?

Crystal: I think there were one or two patients dosed at the same level before that. So, you know, the mechanisms were never totally clear. But it was an innate immune response to the capsid. And that concept of immunity, whether innate or active immunity against the capsid of viruses, whatever the viruses are, has plagued the field. Because we’re using foreign proteins essentially in the capsids to deliver our genes. That is still something the field is struggling with.


Davies: It seems that most of the trials that are generating successful results today involve adeno-associated virus or lentivirus, are you still using adenovirus in certain scenarios?

Crystal: Yes, we are. The adenovirus is a short-term expression and it is terrific at that. It is very highly expressed, and if you use it at the right doses, the safety issues become a nonproblem. So, they are an excellent way to deliver genes short-term, for 2–3 weeks. If you want to build new structures, for example, small coronary arteries, adenovirus is ideal. The adenovirus is also very good if you want to kill things like cancer cells; there is a lot of interest in terms of still using adenoviruses for that.


*This is an edited version of an article that appears in Human Gene Therapy Clinical Development (September 2019; vol. 30, no. 3), a journal published by Mary Ann
Liebert, Inc. You can read the entire article online: DOI 10.1089/humc.2019.29047.int