Stanford on the mind
University teams make progress on rare CNS disease and identify a potential Parkinson’s biomarker
STANFORD, Calif.—The past few months have been fruitful ones out west, with Stanford University’s School of Medicine sharing news of encouraging discoveries in two different neurological conditions—discoveries that not only shed new light on disease pathology, but also open the door to potential treatments.
In the rare disease setting, Stanford School of Medicine scientists—along with collaborators from the University of California, San Francisco (UCSF) and the University of Cambridge—have discovered that Pelizaeus-Merzbacher disease is the result of increased sensitivity to iron in the brain. And, armed with that knowledge, they also identified a drug capable of binding to and removing iron, which boosted the survival of cells afflicted by the disease, according to a press release by Stanford’s Krista Conger.
Pelizaeus-Merzbacher is a genetic central nervous system condition that typically afflicts males, with children generally diagnosed at a very young age and presenting with developmental delays, limited muscle tone and other neuromuscular issues. According to the Genetics Home Reference (GHR), part of the NIH’s U.S. National Library of Medicine, Pelizaeus-Merzbacher is caused by mutations in the PLP1 gene, which plays a role in producing proteins that form myelin.
“This disease is one of a group of genetic disorders called leukodystrophies. Leukodystrophies are conditions that involve abnormalities of the nervous system’s white matter, which consists of nerve fibers covered by a fatty substance called myelin. Myelin insulates nerve fibers and promotes the rapid transmission of nerve impulses. In particular, Pelizaeus-Merzbacher disease involves hypomyelination, which means that the nervous system has a reduced ability to form myelin. As a result, overall neurological function is reduced,” the GHR reports.
Using skin cells from a patient with a certain PLP1 mutation, Dr. Hiroko Nobuta, a postdoctoral scholar at Stanford and lead author of the study, created induced pluripotent stem cells that were then differentiated into oligodendrocytes, a type of glial cell in the central nervous system that produces myelin. Those with the Pelizaeus-Merzbacher disease-associated mutation died before they developed into functional cells, while “cells in which the mutation had been corrected developed normally in a laboratory dish and on human brain slices,” Konger’s release noted. Even more encouraging, the corrected cells developed normally and took part in myelination when transplanted into the brains of mice.
“When Hiroko studied the cells more closely, she found that they exhibited many hallmarks of iron toxicity,” said Dr. David Rowitch, an adjunct professor of pediatrics and of neurological surgery at UCSF, a Wellcome Trust senior investigator at the University of Cambridge and a senior author of the paper. “Adding a molecule that can chelate, or bind, iron outside the cell restored the cells’ ability to become mature, functional oligodendrocytes.”
The team selected deferiprone, a small-molecule iron chelator, to combat the iron toxicity. When tested in week-old mice with a PLP1 mutation that results in a very severe form of Pelizaeus-Merzbacher—so severe that the mice usually die roughly 35 days after birth—deferiprone reduced apoptosis, encouraged the development of new myelin and led to a slight increase in survival times.
A clinical trial in Europe is the next step for this research, in hopes of seeing whether this approach is capable of slowing or stopping disease progression.
“As a researcher you hope that something you discover will eventually contribute in some way—perhaps decades later—to patient care, but this happened so much sooner than we anticipated,” commented Dr. Marius Wernig, professor of pathology at Stanford and senior author of the work. “It’s exciting to think that we could soon be testing this approach in patients.”
Moving to a much more well-known disease, another Stanford team shared news of progress in Parkinson’s disease recently. Along with researchers from Atomwise and the Mayo Clinic in Jacksonville, Fla., Stanford scientists reported on the identification of a molecular defect that seems to be ubiquitous in individuals with Parkinson’s disease or a high risk of developing the disease, but not in those without the disease or predisposition for it.
“We’ve identified a molecular marker that could allow doctors to diagnose Parkinson’s accurately, early and in a clinically practical way,” said Dr. Xinnan Wang, associate professor of neurosurgery. “This marker could be used to assess drug candidates’ capacity to counter the defect and stall the disease’s progression.”
Their work appeared online in Cell Metabolism in a paper titled “Miro1 Marks Parkinson’s Disease Subset and Miro1 Reducer Rescues Neuron Loss in Parkinson’s Models.”
Parkinson’s disease is characterized by the loss of dopaminergic neurons within the substantia nigra. It is estimated that as much as half of an individual’s dopaminergic neurons in the substantia nigra have died by the time they start presenting with symptoms.
While the cause of these neurons’ loss is not known, a prevailing theory is that it revolves around mitochondrial function. As mitochondria produce energy for cells, they also generate free radicals, which are harmful to cells in general and particularly detrimental to mitochondria. Given that “Parkinson’s is known to involve a defect in mitochondrial function … [and] Dopaminergic neurons in the substantia nigra are among the body’s hardest-working cells,” according to a Stanford news release by Bruce Goldman, the theory holds that the answer lies therein.
The Stanford discovery is also centered on mitochondria. Cells possess proteins that remove mitochondria that no longer function properly, but before they can be removed, an adaptor molecule called Miro must be removed first. As explained in the paper, “Miro1 is localized on the mitochondrial surface and mediates mitochondrial motility. Miro1 is removed from depolarized mitochondria to facilitate their clearance via mitophagy.” Wang and colleagues had previously discovered that individuals with Parkinson’s disease have a mitochondrial defect in which their cells cannot remove Miro from damaged mitochondria, which means the mitochondria themselves cannot be cleared.
The researchers worked with skin samples from 83 individuals with Parkinson’s disease, five asymptomatic relatives at high risk of developing the disease, 22 patients who had been diagnosed with other movement disorders, and 52 healthy individuals to serve as a control group. Fibroblasts were extracted and cultured, then stressed in order to damage their mitochondria and trigger the cells to remove the damaged mitochondria.
What they found was that 78 of the 83 fibroblasts cultured from Parkinson’s patients (94 percent) presented with the Miro-removal defect, as did all five of the high-risk asymptomatic relatives. However, the defect was not seen in the fibroblasts of the patients with non-Parkinson’s movement disorders or the control group. A Parkinson’s-exclusive biomarker could be an enormous boon for diagnosis and early treatment.
In hopes of finding something along treatment lines, the team screened 6,835,320 small molecules from a commercially available database in collaboration with Atomwise, whose software identified 11 molecules that would bind to Miro. Even better, the molecules were expected to bind in a way that would “facilitate its separation from mitochondria … be nontoxic, orally available and able to cross the blood-brain barrier,” according to Goldman’s piece.
The identified compounds were fed to fruit flies for one week, and of the original 11, four were found to significantly reduce Miro levels without toxicity and one was found to target Miro with the greatest specificity. That compound was administered to three strains of fruit fly that had been engineered to present with Parkinson’s-like difficulty when climbing. The compound prevented the loss of dopaminergic neurons in all three strains and preserved climbing ability in two, with no signs of toxicity. When tested on fibroblasts from a patient with sporadic Parkinson’s disease, it increased clearance of Miro after the cells were stressed.
“Our hope,” Wang said, “is that if this compound or a similar one proves nontoxic and efficacious and we can give it, like a statin drug, to people who’ve tested positive for the Miro-removal defect but don’t yet have Parkinson’s symptoms, they’ll never get it.”