Meeting the Challenge of Lysosomal Storage Diseases
December 2010- Gustavo Maegawa, flanked by a clinical genetics resident and a genetic counselor, patiently examines an energetic preschool patient, biding his time between measuring her skull and flexing her hyper-bendable wrists and elbows. When she scoots out of reach, he queries her parents, gathering clues that might help his team to pin down a diagnosis of a rare genetic disorder.
The 35-year-old physician-scientist asks, “Have you noticed if the size or shape of her head has changed?”
“How does she communicate?”
Lots of whining, except for three words in sign language: eat, more and thank you.
“Does she fall a lot?”
Too often. She always hits the same spot of her head, every time, and often ends up in the emergency room.
The question he saves until last—“What’s the major concern for you, as parents?” —elicits rapid-fire responses that have been well rehearsed with a long line of specialists sought out by this young couple who’s frustrated from getting “the runaround” ever since their seemingly healthy baby began showing alarming signs of decline: Will she progress to a normal state? Why isn’t she talking or acting like other 3-year-olds?
Hovering between blame and guilt, they inevitably broach the raw issue of accountability. They wonder aloud if the cause is something in their genetic makeup. Was it something in their genes that clashed?
Of the 18,000 to 20,000 genes that make us human, we each are carriers of about 15- to 20 or more with mutations that cause single-gene genetic disorders, Maegawa explains. These parents each apparently have variants in the same gene, and, as a result, their child has an autosomal recessive genetic disease.
As a practitioner of genetic medicine who’s actively involved in basic research, Maegawa not only empathizes with the stricken couple but uses their hounding concerns to both inform and fuel his investigations in the lab. “The best scientific and research questions come from parents of patients I encounter,” he says. “When I don’t have an answer, I feel like it’s my responsibility to try to find one.”
Born and raised in Curitiba, a capital city in the south of Brazil, Maegawa completed his intense clinical fellowship in clinical and biochemical genetics and later obtained his Ph.D. at the Hospital for Sick Children in Toronto, Canada, one of the world’s foremost pediatric health care, teaching and research centers dedicated exclusively to children. As an intern, he spent three formative months in Baltimore working with the physician-scientist he regards his mentor: the late Dr. Hugo Moser (of Lorenzo’s Oil fame), then president of the Kennedy Krieger Institute and professor of neurology and pediatrics at Johns Hopkins. Moser, who earned an international reputation with his work on adrenoleukodystrophy, earlier had worked on developing screening programs for lysosomal storage diseases, or LSDs.
There are dozens of LSDs; all are rare inherited disorders that result when the cells’ recycling centers—known as lysosomes—malfunction, causing cellular garbage to accumulate. The lysosomes’ failure to digest cellular garbage, caused by a deficiency in an enzyme required for metabolizing sugars or fats, happens as a result of a gene mutation. The most common of these rare genetic disorders, affecting one in about 50,000 individuals worldwide, is Gaucher disease; others include Pompe and Tay-Sachs diseases. Despite being individually rare, collectively the LSDs’ incidence ranges from one in 5,000 to 7,000. Many LSDs are insidious—they can go undetected for years while irreversible damage is being done to the central nervous system, bones and organs, including the brain, heart, spleen and liver.
Impressed by Moser’s model of linking bench research with clinical practice, training and community services for the developmentally disabled, Maegawa arrived at Hopkins a year ago with ambitious goals that span the clinic and lab, not least of which is discovering molecular mechanisms that ultimately will lead to drug targets and new therapies for those with LSDs.
Despite that all LSDs have a similar underlying origin, the physical symptoms and developmental delays that can arise with each disease vary according to the degree of levels of activity of the deficient enzyme, waste material stored and the types of cells damaged. Some LSDs affects bone marrow cells, for instance; and most of them, cells of the central nervous system. Even within a specific disease type, some individuals may be so subtly affected that their condition goes undetected for many decades, while others are toddlers when they drastically lose the ability to move, swallow and breathe.
LSDs happen when mutations in genes impair the manufacture and function of enzymes whose functions are to prevent accumulation of specific waste products. Some mutations impair the folding process of the enzymes they code for. Cells have quality control systems that recognize and destroy most, but not all, mis-folded enzymes before they can reach their destination in the lysosome. At least a critical threshold of the enzyme—about 10 to 15 percent of normal levels—must be present for the recycling center to operate.
Most LSD patients have deficient enzyme activity, Maegawa stresses. It’s low, not in the normal range of those who don’t have the disease, but not completely absent. The reason there’s a buildup of waste material in cells is that not quite enough of the working enzyme reaches the lysosomes.
Therefore, the issue is getting enough enzyme to the lysosome, before the cell’s quality control system destroys it. One way to do that is by using a molecule that stabilizes and chaperones faulty enzymes, essentially escorting them to their place of business.
Maegawa’s previous research involved screening an FDA-approved drug library and identifying pyrimethamine, a drug used for more than 40 years to treat malaria and toxoplasmosis, as a pharmacological chaperone for the particular enzyme (hexosaminidase A), that is deficient in GM2 gangliosidosis, (Tay-Sachs and Sandhoff diseases). In another study, he found that ambroxol, a drug used for chronic bronchitis, enhanced and stabilized mutant forms of glucocebrosidase, the enzyme deficient in Gaucher disease.
“An advantage of this approach is that small molecules can cross the blood brain barrier and reach the neuronal cells that are dramatically affected in LSDs,” Maegawa says. “The principles we learn by treating one type of LSD can be applied not only to other LSDs, but also to other neurodegenerative diseases caused by protein misfolding.”
Maegawa’s current research finds him stepping further back into the physiological malfunction and focusing not with the specific misfolded enzymes themselves, but on the machinery that regulates the complex process of enzyme folding. He is gearing up now to screen a library of 500,000 compounds for small molecules that will increase the residual enzyme activity, so that enough will make its way into the lysosomes to enable them to degrade and digest the garbage of the cells that otherwise would build up and cause damage to the brain, bones, muscles, spleen, heart and liver.
His intent is to prevent the devastating symptoms of LSDs from ever occurring by stopping these diseases in their paths.
“The strength of this strategy is its broad application, because we’re not targeting specific mutated proteins, but common molecular pathways that are implicated in many neurodegenerative diseases,” Maegawa says. “As we gain a deeper understanding of how these rare genetic diseases affect the brain, we are finding that they are excellent models for more complex diseases such as Parkinson’s and Alzheimer’s.”
Maegawa now is developing high-throughput screening assays to identify specific chemical compounds to assist folding of mutant lysosomal proteins. His plan is to screen 500,000 compounds in seven different concentrations each (that’s 3.5 million tests to measure enzyme activity) using live skin cells from his patients.
“The advantage of the live-cell high-throughput screening is that the protein of interest is evaluated in the context of an intact cell, which is crucial when studying proteins that need to cross membranes or move from one component of the cell into another,” Maegawa says.
With funding from the National Institute for Neurological Disorders and Stroke and National Tay-Sachs Disease and Allied Disorders Association (NTSAD), and collaborating with members of NIH Chemical Genomic Center, Maegawa’s close to finishing the design of the assay, after which he will move on to what he anticipates will be the most challenging part of this research: narrowing down the 5,000 or so hits he expects to get in order to focus specifically on those compounds that will make all the difference in the lives of his young patients with LSDs.
“It’s difficult to see these patients degenerating from one visit to the next,” Maegawa says, “and quite a responsibility. When you realize you’re in the forefront—you have the potential and the tools and an amazing institution behind you—you must try to meet the challenge with new concepts and try things that were not tried before.”
-- Maryalice Yakutchik