Scientists studying mice have identified a possible strategy for slowing a rare, fatal childhood neurodegenerative disease known as Niemann-Pick type C, in which brain cells accumulate fat and die. The finding could also have implications for treating other neurodegenerative disease, they say.
In their study, published in the July issue of Nature Medicine, the team discovered that the synthesis of neurosteroid hormones in the brain -- a process known as neurosteroidogenesis -- is severely disrupted in mice that naturally develop the disease. They then determined that replenishing the depleted neurosteroid hormone allopregnanolone, the prime casualty of the disruption, significantly delayed the onset of some of the animals' neurological deficits, and doubled their lifespan. The treatment was particularly potent when administered early in the animals' life.
"The results were astounding," says senior author Synthia Mellon, PhD, professor of obstetrics, gynecology and reproductive sciences at University of California, San Francisco.
While the treatment did not target the cause of the disease -- a mutation in one of two genes that disrupts the transport of cholesterol within all cells of the body -- nor cure it, the therapy substantially delayed the onset of weight loss, motor coordination, mobility -- and death. It also significantly delayed the accumulation of fats in cortical cells, and the death of some neurons in the cortex and cerebellum. The study was not designed to test intellectual function, another component of the disease.
LOSS OF NEUROSTEROIDS IN NEURODEGENERATION
"The finding suggests that the progressive loss of neurosteroids contributes to neuronal degeneration in NP-C mice," says Mellon. "We didn't cure the problem. There's still a big clog in the cell, but we've circumvented it. We've treated downstream consequences of that clog."
Scientists do not know if neurosteroid synthesis is disrupted in children with the disease, so the potential efficacy of allopregnenolone in children with NP-C cannot be predicted, says Mellon. However, allopregnanolone is produced naturally in humans, and provoked no evident side effects when administered to the NP-C mice. Thus, if the neurosteroid hormone is depleted in patients, it could prove a tactic for delaying the onset or progression of some symptoms of the disease.
To determine neurosteroidogenesis activity in children with NP-C, scientists could compare brain tissue from autopsied NP-C patients with that from children who died from unrelated causes. Mellon says she and colleagues hope to be able to initiate pre-clinical studies aimed at determining safety, safe dosage and any possible side effects of allopregnenolone, with an eye to optimizing dosage and carrying out a clinical trial.
If the therapy worked, it would signify a major advance, as scientists have not made headway in repairing the culprit gene, discovered in 1997, or the protein it synthesizes. Although clinical manifestations are varied and can present at any time from intrauterine life to adulthood, patients often begin demonstrating motor and intellectual decline during late childhood, and die between the ages of eight and eighteen. The disease, which also causes enlargement of the liver, occurs in about 1 in 150,000 children.
ILLUMINATING THE ROLE OF NEUROSTEROIDS
The finding, in broader terms, provides some of the strongest evidence to date that neurosteroids may play an important role in neurological function in humans, says Mellon. If synthesis of these steroid hormones is found to be disrupted in other neurodegenerative diseases -- a question that has just begun to be investigated -- neurosteroid therapy could prove effective in stalling them, she says.
Like most other steroid hormones, neurosteroids are derived from cholesterol, a lipid that plays an important role in the formation of cell membranes. But while much is known about the role of other steroid hormones, such as testosterone, scientists are only beginning to understand the role of those that function in the brain. Neurosteroids are known to affect neuronal growth and differentiation and to modulate various moods and reactions via neurotransmitter receptors, including GABA. However, says Mellon, scientists generally have presumed that neurosteroids play a supportive, rather than an essential, role in neurological function.
The research team, whose scientific interest is illuminating the role of neurosteroids, suspected that neurosteroid synthesis would be disturbed in NP-C, due to disruption of cholesterol transport within cells, which leads to the accumulation of cholesterol and other lipids in cellular compartments. Their theory was bolstered by the fact that mice with NP-C have below normal concentrations of testosterone and underdeveloped reproductive organs, suggesting that, at least, the synthesis of sex steroid hormones in the testes is disrupted in the disease.
To test their hypothesis, the scientists compared the brains of normal, so-called "wild type," mice, with those of animals with NP-C. The results were dramatic. While neurosteroidogenesis was not affected in NP-C mice prenatally, the impact began at birth and increased over time. The quantity of pregnenolone, the first steroid produced from cholesterol, was significantly decreased. The quantity of allopregnanolone, which is converted from pregnenolone, was diminished even further.
Supporting this finding, the two enzymes that synthesize allopregnanolone, 3a-hydroxysteroid dehydrogenase and 5a-reductase, were significantly depleted in all principal brain regions (the cortex, the midbrain and the hindbrain) by birth, and were reduced to irrelevant quantities by 10 weeks.
The team then examined the impact of administering allopregnanolone to the brains of NP-C mice. The effect was equally pronounced. In a series of experiments, the drug was given progressively early in the animals' lives, at 21 to 23 days of age in the first mice examined, at 7 days of age in the last mice evaluated. The response, in terms of stalled neurological deficits, cellular damage and death, was greater the earlier the administration of the neurosteroid.
The most dramatic results reported were in mice treated with a single injection at seven days of age. They gained weight steadily until age 86 days, and lived to a mean age of 124 days. In contrast, untreated NP-C mice had a mean survival of 67 days.
In still more notable results, the researchers cited, but did not report, in their published paper that several treatments beginning at day seven in the animals' lives extended their lifespan and delayed the onset of neurological impairment even further.
"The finding suggests that allopregnanolone may be involved in a neurodevelopmental process that occurs within or before the first week of life," says Mellon.
NP-C is an autosomal recessive disease, meaning both parents must contribute a copy of the mutated gene to cause the disease, and that there is a 25 percent chance their offspring will develop it. As infants suspected of having the disease can be tested to see if they carry the mutated gene, the disease can be detected early. Thus, if allopregnanolone proves effective in humans, it could be used to stall the disease from the outset.
The obvious next challenge for the researchers, however, will be determining how brain development of the mouse compares to that of the human.
Co-authors of the study were Lisa D. Griffin, MD, PhD, formerly UCSF assistant professor of neurology, Wenhui Gong, MD, PhD, a postdoctoral fellow in the Mellon lab, and Lucie Verot, PhD, graduate student in the laboratory of Marie T. Vanier, MD, PhD, of INSERM, U189 and Laboratoire Foundation Gillet-Merieux, Lyon-Sud Medical School and Hospital, Pierre-Benite, France.
The study was funded by the National Institutes of Health, the National Niemann Pick Disease Foundation, the March of Dimes Birth Defects Foundation, the Ara Parsaeghian Medical Research Foundation and Vaincre les Maladies Lyosomales.
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