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Drew Potenti loves her bath. Her mother, April, joins her in the tub and supports her head. Drew, gloriously weightless, moves her arms and fingers, arches her body and kicks and splashes. Alongside the tub is her ventilator, attached to her windpipe through a tracheostomy tube.
Moving so freely is a novelty for Drew, who at 18 months old is just beginning to hold up her head. When she was born in 2007, she gave only a weak cry, and was floppy and nearly motionless. Too weak to breathe well or swallow, Drew spent two weeks in Children's Hospital Boston's Neonatal Intensive Care Unit (NICU). Six weeks later, she returned to have her tracheostomy tube inserted, as well as a gastrostomy tube (G-tube) to deliver nutrients directly to her stomach and a muscle biopsy to find out why she was so weak.
Eventually, Drew was diagnosed with nemaline myopathy, a rare congenital disorder causing muscle weakness. Although fully dependent on her ventilator, she is stable and has gained a little strength. At a recent visit, Peter Kang, MD, her neurologist at Children's, found her alert and playful. But it's unclear how Drew will do in the future.
Unlike degenerative neuromuscular disorders, such as muscular dystrophy or Lou Gehrig's disease, nemaline myopathy often doesn't worsen over time. But affected children are still at risk for life-threatening complications. There is no cure, only supportive treatment.
"I'm seeing signs of improvement—Drew's a little fighter," April says. "She can move her hands and wrist, and if we put a pillow underneath her knees, she can kick her legs. She has a speaker valve on her trach tube, so she can vocalize and she makes lots of noise. I think she may eventually hold a pencil and be able to write, but I'm not sure if she'll walk. A lot of it is just waiting and seeing."
Welcoming any chance to get information, the Potentis enrolled in a research study led by Alan Beggs, PhD, in Children's Division of Genetics. Genetic testing, which Beggs's lab helped to develop, showed Drew to have a mutation in the gene for actin, a component of muscle fibers.
While normal fibers contain orderly arrays of contracting units, Drew's muscle sample was studded with clumps of protein known as nemaline rods. Beggs believes that the contractile apparatus had come apart somehow as a result of the actin mutation, causing muscle weakness. "If we can understand why nemaline rods are forming, maybe we can develop a drug to block that process and prevent muscle structure from breaking down," he says.
Beggs knows more about nemaline myopathy than almost anyone in the world. His lab is one of just a handful worldwide focused on the congenital myopathies, a group of disorders that collectively affect about 1 in 20,000 to 100,000 live births—so rare that Elizabeth DeChene, a genetic counselor in the lab, barely heard about them in her training. They are present from birth and usually seem to arise out of nowhere, with no prior family history.
About a dozen causative genes have been discovered, and Beggs has been involved in finding five of them. His lab, the main research referral center for the Americas, has more than 500 patients enrolled from all over the world. As genes are found, Beggs hopes they will reveal what's going wrong in patients' muscles and provide families with better tests and prognostic information—and ultimately treatments.
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In 1992, Beggs was completing a post-doctoral fellowship with renowned Children's geneticist Louis Kunkel, PhD. In 1986, Kunkel had achieved an early landmark of molecular genetics, identifying dystrophin as the gene and protein altered in Duchenne muscular dystrophy. Beggs was studying dystrophin and alpha-actinin, a closely protein found in nemaline rods. He cloned two human alpha-actinin genes to determine their sequence of amino acids, the first time this was done from human muscle. "We thought that a defect in alpha-actinin was causing nemaline myopathy," says Beggs. "But we sequenced the genes in many patients and never found a mutation."
Kunkel's stature in the muscular dystrophy field cast a long shadow, and Beggs, starting his own lab, saw a chance to carve his own niche. At the time, no one knew of any gene involved in nemaline myopathy—or any congenital myopathy for that matter. Beggs was also intrigued by the disease's wide range in severity. Some children only had problems keeping up in gym class; others required life-sustaining technology, too weak to cough or smile, sometimes born frozen in a fetal position.
Beggs began studying as many patients as he could find. In 1999, he and Australian collaborators established that abnormalities of actin—a protein adjacent to alpha-actinin in the muscle—are a cause of nemaline myopathy. Pooling a total of 59 patients, they identified 15 different actin mutations in varying locations on the gene. The mutations were subtle, substituting a single amino acid for another, but all caused muscle weakness.
Beggs helped map a second causative gene, alpha-tropomyosin, and, collaborating with a Finnish researcher, found mutations in a third gene, nebulin. In 2007, Beggs and Pankaj Agrawal, MD, a neonatologist who does research in the Beggs lab, identified mutations in yet another gene, cofilin-2, in a family from Bahrain.
All the mutations affect a specific structure in muscle fibers called the thin filament. Actin, the protein mutated in Drew's disease, is a main sub-component of the thin filament; it's like a string of beads supported by a scaffolding made of nebulin. Cofilin-2 helps maintain this structure. And alpha-tropomyosin helps regulate actin's interaction with a protein in the thick filament, enabling the two filaments to hook together and slide past each other, making the muscle contract. (See an interactive animation of this.)
"Knowing this has given us a good roadmap to follow," says Beggs. "We now need to think about a therapy for each of these defects."
Beggs began looking at other congenital myopathies, and discovered two additional causative genes. Through the Internet, patients and their families around the world began to take notice and ask for testing. When April Potenti became pregnant again, Drew's future sibling was tested and found to be free of the actin mutation.
But in the research, things became murkier. Not only does each myopathy have more than one potential causative gene, some genes can cause more than one myopathy. Each gene can have many different mutations—more than 85 different actin mutations are now known—and two babies with the identical mutation can have vastly different outcomes.
"By and large, it's difficult to make predictions of how a child will do," says Beggs. "It's very hard for clinicians to keep all this information in their head, and there are exceptions to just about everything. That makes for a very frustrating situation clinically."
Adam Foye and William Ward, both in second grade, exemplify the limits of muscle biopsy in predicting how severe a disease will be. Both have forms of centronuclear myopathy, so named because the nuclei of their muscle cells, which should cluster at the cells' outer edges, are stuck in the center.
Adam was born an apparently healthy baby, but was late meeting his motor milestones. While other babies were moving around too much to be swaddled, Adam was generally still. "That was my first clue," says his mother, Sarah.
Adam's causative gene still isn't known—he's come up negative on every available test, including one for a gene Beggs identified with French collaborators after Adam was born. At night, he sleeps with a ventilating face mask to support his breathing, as well as a back brace; his spine has begun to curve because his muscles cannot fully support his growing frame, making breathing more difficult. But he can talk, walk moderate distances, ride an adaptive bike and eat soft foods.
In contrast, William's movements are extremely limited, and he has been ventilator- and G-tube-dependent since birth. He can nod "yes" and "no," and, with help, is beginning to communicate using a computer with a digital voice output. By pressing switches on his wheelchair with his head or hand, he can navigate through categories and make selections. He sees several specialists at Children's, and his care is so complex that his mother, Erin, took a job at the hospital to help make the health care experience easier for other families.
The Wards first met with Beggs soon after William was diagnosed. "He opened up this door," recalls Will's father, Mark. "We realized there was research going on, and standing before us was one of the leading researchers in the world."
Will's defective gene turned out to be myotubularin, identified about five years prior. The latest work in Beggs's lab suggests that myotubularin defects alter the flow of calcium into muscle cells, so that contraction isn't properly initiated.
When Will received his tracheostomy and G-tube, he was still in the NICU, giving Beggs the rare opportunity to collect a live muscle sample, rather than the frozen samples he relied on. The Wards were happy to consent, and the lab used William's sample to create a self-sustaining line of muscle stem cells. "We're studying the growth of these cells and will use that information as we try to develop cell therapies for his disease," Beggs says.
In April 2008, Beggs and collaborators in France reported a different advance: successful gene therapy in a mouse model of William's disease. With a single injection, they introduced normal myotubularin into the muscle to replace the defective gene, correcting the muscle pathology and significantly increasing muscle volume and contractile force.
Beggs cautions that this needs to be validated in larger animals. A collaborating lab plans to try myotubularin gene therapy in dogs, but developing a therapy for humans is likely to be much harder. "Gene therapy is in a state where we can't control what's happening," says Beggs. "If the muscle makes too much myotubularin, for example, it could be toxic."
Still, the Wards see room for hope. "In 2001, the gene was barely known," says Mark. "To know that tangible progress has been made gives us hope that it may be possible to give Will therapies to improve his muscle strength."
Genetic counselor DeChene is getting calls from more and more families whose children lack any diagnosis, who come up negative on every genetic test. If predictions are difficult when the gene is known, it's impossible to predict anything—or counsel parents wanting more children—when no gene can be found.
Beggs and colleagues are screening some patients for genes that aren't yet established as causative, just to see if anything pans out. To identify new candidate genes, they can compare patients with unaffected relatives to find a difference in their DNA—sometimes helpful when you don't know what you're looking for. To understand how the gene mutations cause weakness, they're also doing gene-expression studies, analyzing the activity of 20,000 genes at once to try to discern some pattern that's different.
"The low-hanging fruit have been picked," says Agrawal. "Now we're looking for the high-hanging. These are the patients who are falling through the cracks because science tends to focus on the big diseases."
Last fall, Children's received a $25 million grant from The Manton Foundation, one of the largest ever given to the hospital, and Beggs became head of the newly created Manton Center for Orphan Disease Research. He hopes that the congenital myopathies—and other rare "lost causes" of medicine—can now be tackled in a concerted fashion.
The road will be long. Even relatively common disorders like muscular dystrophy and cystic fibrosis, with well-established genetic causes, still have no cure. "When we started supporting Children's in 1997, there was only one doctor in the United States being funded for centronuclear myopathy," says Alison Frase of the Frase Foundation, whose son, Joshua, 14, has the disease. "If you want to know something about my son's disorder, Alan is really the one in the U.S. who is pushing this research forward. These scientists don't get a lot of recognition, and they need to." |
