Duchenne muscular dystrophy (DMD) is a debilitating genetic disease primarily affecting boys. One of more than 30 forms of muscular dystrophy, it causes those affected to become wheelchair-bound by the age of 10. Most die in their 20s or 30s from cardiac or respiratory failure.
For more than 35 years, geneticist Louis M. Kunkel ’71 has been working on DMD research at Boston Children’s Hospital. It was there, in the 1980s, that Kunkel’s lab discovered the protein called dystrophin, which is integral to muscle movement. And it is the absence of dystrophin, associated with a mutation of a gene on the X chromosome in muscle cells, which researchers now know is the cause of DMD.
“Lou is clearly the father of the dystrophin gene, and in many ways all genes that followed,” said Eric Hoffman ’82 about his former colleague’s work. In 1986, Hoffman was the first postdoctoral fellow hired by Kunkel for the lab. “His [Kunkel’s] innovative work to identify the first fragments of the gene was much like the first moon walk of human genetics.”
THE ERA OF GENOME-BASED THERAPY
The identification of dystrophin, combined with what Kunkel described as an explosion in technology and research, provides promising clues for treating and preventing DMD.
“I call this the decade of genome-based therapy,” said Kunkel, former director of the genomics program at the Harvard Stem Cell Institute.
Modern genetics research began only 151 years ago, with the work of Gregor Johann Mendel, an Augustinian friar famous for his work on the inheritance patterns of pea plants. It wasn’t until 2003 that scientists would finish sequencing, identifying, and mapping human DNA as part of The Human Genome Project. And relatively little was known about the cause of muscular dystrophy around the time when Kunkel’s lab discovered dystrophin. Although DMD was first described by Guillaume-Benjamin-Amand Duchenne in the 1860s, by the late 1980s and 1990s, scientists were still identifying neuromuscular disorders, said Kunkel.
There are at least six therapeutic approaches for DMD in various stages of clinical trials and FDA- approval. Kunkel is working to understand the genetic modulators for dystrophin deficiency—to find ways to compensate for the lack of dystrophin.
One approach he found to be promising is gene correction, a gene-splicing technique known as exon skipping—skipping over mutations during the processes of gene translation and expression so dystrophin can be produced.
“It’s not going to cure the disease…you’re not reversing the problem but halting its progression, said Kunkel. “The ideal solution would be to also reverse its damage.”
Some forms of gene therapy, like exon skipping, rely on the production of a modified dystrophin protein. The patient may not suffer from DMD but would develop a milder form of the disease, called Becker muscular dystrophy. Other cell-based therapies rely on increasing the production of a similarly functioning, substitution protein such as utrophin, which studies show can take over the function of dystrophin.
Cell therapy, said Kunkel, is probably the only approach that will reverse damage, but more research is needed. Muscles are regenerative tissue, so researchers have also been exploring the use of using progenitor cells, or stem cells, to correct for dystrophin deficiency by introducing normal cells into the muscle. The problem, Kunkel said, is that the cells stay in the place where they are introduced. Until recently, they had no way of getting the cells to produce new muscle. “We have two new lines of experiments going to address those issues,” said Kunkel. Most of this work is through the use of mouse studies.
He foresees multiple therapies being used concurrently to extend and improve a patient’s quality of life. Several academic labs and companies have been using the research facilitated by Kunkel’s findings to work on therapeutic treatments for DMD, and Kunkel continues to collaborate with international scientists and outside partners like the Broad Institute of MIT and Harvard on research.
“Five years ago, none of [these therapies] existed,” Kunkel said. “We knew what was wrong and wanted to figure out how to fix it. It wasn’t until 2010 that we thought of using therapy to fix it. So, it’s a time of hope for patients with the disorder.”
A PRACTICAL START
Kunkel, who comes from a family of scientists, was always interested in genetics. His dad was an immunologist, and his grandfather was a botanist. When he graduated from Gettysburg College, he almost followed in the footsteps of his botanist grandfather, by studying plant genetics. (He is a third- generation member of the National Academy of Sciences.)
But the botany programs didn’t offer graduate school stipends. While at Gettysburg, he had worked in biology Prof. Ralph Cavaliere’s lab and completed research in human genetics at a Cornell University lab. He decided to pursue his PhD in biology, receiving his degree from Johns Hopkins University in 1978. Gettysburg College presented Kunkel a Distinguished Alumni Award in 1989 and an honorary doctorate degree in 2000.
“I did my graduate work on identifying DNA sequences on the Y chromosome and then worked with the X chromosome as a postdoctoral fellow,” said Kunkel. A flawed gene on the X chromosome is what causes DMD. He submitted a proposal to the Muscular Dystrophy Association to map the gene involved in dystrophy. “I’ve been involved ever since,” he said.
AT A CROSSROADS
Kunkel said there’s still a significant amount of work to be done—and much at stake.
“I have gotten to know several patients [with DMD] over the years. I’ve seen boys die in the interim,” he said. But he believes this decade of research will be integral to finding a cure. “The fact is we’re at a crossroads where therapies are being developed, so it’s an exciting time.”
Hoffman agrees that despite major advances, there is a lot of work to be done.
“While Lou and I together worked to bring molecular diagnostics quickly to families…the ‘deliverable’ of a therapy to DMD patients has been much slower than we had hoped, with tortuous paths and blind alleys,” Hoffman said. “I have put much of my effort into building the infrastructures needed for robust drug development programs in DMD.”
There are some areas where Kunkel wants to step on the gas, particularly in the area of collecting genetic data, which he said will eventually become more cost-effective and the norm.
“Eventually, genetics is going to be part of the clinical record for individuals. It already exists in cancer research, and it’s going to continue happening in human genetics,” he said. “For example, how many individuals out there in the normal population have a problem producing dystrophin, yet do not exhibit DMD?”
In 2015, Kunkel and a team of researchers discovered a mutation in a Golden Retriever that protected the dog from getting the disease. The findings were published in the journal Cell, and a grant from Pfizer allows them to continue the work.
“We have a dog model that some escape disease, even with the mutation, so the supposition is that some humans have, too,” Kunkel said. “Wouldn’t it be interesting to know what gene variant allows them to escape? You can’t get that data unless you sequence millions of individuals.”
That requires time, money, and large-scale consent from patients. “But it’s going to happen [eventually],” emphasized Kunkel.
The immediate goal is to preserve the ability of kids with DMD to walk for a longer period of time and improve their overall quality of life.
“Many kids today with Duchenne go to college, get degrees and even additional degrees after college,” said Kunkel. “It’s not unusual for them to live until their 30s, so, [I say] okay, let’s see if we can boost that to the 50s, and then that’s approaching normal life expectancy. If you could enable them to be able to care better for themselves [independently], that would make a big difference. That’s my hope. And I think we’ll see some of that in the next five years.”