Source: Plos Blogs
By Ricki Lewis, PhD
“When you hear hoof beats, think horses, not zebras.” So goes the mantra of first-year medical students. If a common disease is a horse and a rare disease a zebra, then giant axonal neuropathy (GAN), with only 50 or so recognized cases worldwide, is surely a unicorn.
Five years ago this week, 9-year-old Hannah Sames of Rexford, New York, who lives near me, received a diagnosis of GAN, a disease much like amyotrophic lateral sclerosis. And this month, thanks in part to the herculean fundraising efforts of Hannah’s Hope Fund (HHF), the cover and lead article of the Journal of Clinical Investigation reveal most of the story behind the devastating inherited disease, with repercussions that will reach far beyond the tiny GAN community.
If all goes well at the next Recombinant DNA Advisory Committee meeting at the NIH, a phase 1 clinical trial may be underway before year’s end to evaluate gene therapy for GAN. “A fire started burning deep in my core exactly 5 years ago when Hannah was diagnosed. We will not rest until we have a successful treatment for our kids. They are rare, but they are no longer neglected,” says Lori Sames, Hannah’s mom and executive director of HHF.
On March 5, 2004, when Lori and her husband Matt first glimpsed their newborn daughter’s kinky reddish fuzz, they were both delighted and puzzled. Madison, five, and Reagan, two, have stick-straight hair, as do Lori and Matt. When the birthing goop dried, Hannah’s cap of tight curls sprang to life.
For many months, the little girl seemed okay. She smiled, sat, crawled and hauled herself upright on schedule. But her footsteps were halting, hesitant. Hannah slowly grew clumsy, the strength ebbing from her legs. Lori made the usual rounds of specialists assuring her all was well, but already, filaments of protein were distending the long axons of the motor neurons running down Hannah’s legs, blocking messages to her muscles.
By Hannah’s third birthday, Lori and Matt suspected something was seriously wrong. Both of Hannah’s arches now bowed, and she tottered. More doctors gave false reassurances, hardly hiding their diagnosis of Lori as a helicopter mom. Then Lori’s sister showed cell phone video of Hannah walking to a physical therapist friend, who thought Hannah’s gait was like that of a child with muscular dystrophy. Six months of neurological tests followed, all results normal.
That’s what happens with a disease so rare that few physicians have seen it, or even heard of it. They can’t recognize a unicorn, don’t know what to test for. But finally an astute pediatric neurologist gave Matt and Lori an answer, and it didn’t come from an exome sequence or a sophisticated scan. “He took out a huge textbook and showed us a photo of a skinny little boy with kinky hair, a high forehead, and braces that went just below the knee – he looked exactly like Hannah. And he had GAN,” Lori recalls. Three days of tests at a children’s hospital in New York City confirmed the diagnosis.
Meeting with a genetic counselor was devastating. Lori recites what they learned: “Matt and I are each carriers of GAN, and we passed the disease to Hannah. Each of our two other daughters has a two in three chance of being a carrier. GAN is a rare ‘orphan genetic disorder’ for which there is no cure, no treatment, no clinical trial and no ongoing research.”
“So you are telling us this is a death sentence?” Lori recalls asking the genetic counselor.
The disease would progress slowly, the counselor said. Hannah’s legs would continue to weaken. By first grade she’d likely need a walker in addition to her ankle supports, and soon after, a wheelchair. She might lose her sight and hearing, and eventually be bedridden.
Matt and Lori walked around like zombies for a few days. And then they founded Hannah’s Hope Fund. Their basement became a war room where they used their business backgrounds to assemble the first ever research conference for GAN. As Lori taught herself molecular biology, she became convinced that gene therapy was a logical approach, but at the same time recognized the value of learning anything about GAN. They were lucky to find Jude Samulski, director of the Gene Therapy Center at the University of North Carolina at Chapel Hill, and he recommended a young investigator, Steven Gray, to lead the team. It’ll be the first gene therapy delivered to the spinal cord. A clinical trial is incredibly expensive, and HHF’s fundraising efforts are amazing – they’ve earned $1 million in just the past 8 months. They’re just one of many not-for-profits in the rare disease community who have taken the helm of funding research.
INTO THE INTERMEDIATE FILAMENTS
In parallel to the gene therapy efforts, HHF supports research into the nature of the cellular glitch behind GAN, in the labs of Robert Goldman and Puneet Opal at Northwestern University, Jean-Pierre Julien at Université Laval in Quebec, Pascale Bomont at the INSERM neurological institute in Montpelier, France, and others. The group reports on a remarkable set of experiments in May’s JCI that probe the out-of-control parts of the cell’s inner skeleton, the intermediate filaments (IFs).
GAN is the perfect disease to investigate IFs because it’s caused by a single gene and has a large, measurable effect. Other conditions may affect IFs secondarily, or reflect input from several genes or environmental exposures.
At fault in GAN is a protein called gigaxonin that normally interacts with the IFs. Most kids with GAN have abnormal forms of the protein; Hannah is highly unusual in that she lacks it entirely.
A cell’s inner scaffolding has three types of girders: microtubules made of the protein tubulin, microfilaments made of actin, and the intermediate filaments made of several other types of proteins. The recipes for IFs vary with cell type: keratins in hair, neurofilaments in neurons, and vimentin in fibroblasts, the connective tissue cells that make up much of our bodies. But all IFs share a basic dumbbell shape, with a head, a tail, and a long helix in the middle. The dumbbells align and aggregate into filaments.
BLOCKING CELLULAR TRASH REMOVAL
Being a nerd, when I hear “T and A” I think “tubulin and actin.” And when I hear “UPS” I don’t think of brown delivery trucks spewing package-clutching people – I think of the ubiquitin-proteasome system. The UPS is how cells round up their garbage and get rid of it.
Ubiquitin is a molecule that tags other molecules bound for destruction, like marking rotten produce at a supermarket or the tire of a parked car exceeding the time limit. Other proteins help escort the debris to proteasomes, which resemble spools that tear apart what’s dumped into them, spewing out pieces that are then further degraded.
Gigaxonin – what Hannah’s cells lack – is technically an “E3 ubiquitin ligase adaptor,” based on its DNA sequence. In Hannah’s motor neurons, hair follicles, and probably other places, the utter absence of gigaxonin means the IFs aren’t broken down and recycled. They remain extended and build up, slowly, which is why Hannah was okay for the first two years. I don’t mean this in a bad way, but Hannah is, genetically speaking, like a knockout mouse because she has two deletions of a major part of her gigaxonin genes.
Dr. Goldman and his group looked at fibroblasts from knockout mice and from three patients (called “GAN cells”; not including Hannah’s), tracking the interaction between vimentin and gigaxonin. Their discoveries confirm and extend what’s known about the disease:
• GAN affects IFs, leaving microtubules and microfilaments alone.
• GAN cells don’t overproduce intermediate filaments – the mRNA level for vimentin is normal. Rather, the filaments aren’t broken down on schedule, like garbage collectors going on strike.
• GAN cells are strikingly abnormal. Like a creeping wad of chewed gum, the glommed filaments pervade the cytoplasm, cling to the nucleus in clumps, and capture mitochondria, rendering these energy-extracting organelles swollen and misshapen. The cell’s organelles drown, swept up and suspended in the gunk of a deranged cytoskeleton.
Gigaxonin grabs onto vimentin by the helix part of the barbell. But the researchers were surprised to find that ubiquitin doesn’t enter the picture – gigaxonin must route the IFs to the proteasomes by some alternate pathway. However it happens, the researchers hypothesize, gigaxonin normally dismantles the long IFs into pieces that enzymes further chew up – like splintering a pile of logs into twigs.
GAN’s correctible! At least in a dish. Giving gigaxonin to GAN cells lowered IF levels within 72 hours, which is what gene therapy would ideally do. But kids aren’t collections of cells. If gene therapy goes off target, what could happen? If it works too well, will cells without IFs survive?
ON BEYOND ZEBRA
The research results may suggest new (or perhaps old) drug targets for GAN. Meanwhile, the slow crawl towards clinical trials continues.
In a disturbing twist, Hannah will not be among the first to receive the experimental gene therapy, because she doesn’t make gigaxonin. So if genes placed in her spinal cord enable her motor neurons to make gigaxonin, her body will see a protein it’s never encountered before — a red flag to the immune system. And so before Hannah can join the clinical trial that has not yet begun, she must have treatments to accustom her immune system to gigaxonin. That means suppressing T cells or a stem cell transplant from one of her sisters.
This week at the American Society of Gene and Cell Therapy annual meeting in Salt Lake City, researchers are brainstorming ways to modulate the immune system to accept a protein introduced with gene therapy, with Hannah the case study. “How can they safely treat her? What is the best way to suppress her immunity? And how will they determine when it’s safe to wean her?” Lori asks. She’s there, of course.
Casey’s legacy is also that what researchers learn about GAN will likely help many others. “IF aggregates form in several types of neurological disorders in addition to GAN—such as ALS and Parkinson’s disease,” says Dr. Goldman. Add to that variants of spinal muscular atrophy and Charcot-Marie-Tooth disease, Alexander disease, Lewy body dementia, and Alzheimer’s disease. ”Our results suggest new pathways for disease intervention. Finding a chemical component that can clear the aggregations and restore the normal distribution of intermediate filaments could one day lead to a therapeutic agent for many neurological disorders,” says lead author Saleemulla Mahammad, a postdoctoral researcher at Northwestern.
I hope the days when rare diseases were considered “orphans,” and ignored, are finally gone, as more and more families form not-for-profit organizations that push research far beyond what was once possible. What we learn about the unicorns and zebras can often help the horses too.
(Hannah’s story is told in chapters 10 and 11 of my book The Forever Fix: Gene Therapy And The Boy Who Saved It.