How Deadly Pediatric Disorder Develops In Brain
UCLA scientists have discovered how a form of the rare genetic disease known as Sanfilippo syndrome develops in the young brain, causing severe mental retardation and death as early as age 14. Published this week in the early online edition of Proceedings of the National Academy of Sciences, the findings suggest that new Alzheimer's drugs may provide therapy for the currently untreatable metabolic disorder.
Four different enzyme deficiencies cause Sanfilippo syndrome, leading to the disorder's classification as type A, B, C or D. The UCLA team studied type B, the second most common form.
"We knew that Sanfilippo syndrome type B results from a mutation of the gene that produces the enzyme needed to break down sugar molecule chains in the body," said Elizabeth Neufeld, a professor of biological chemistry at the David Geffen School of Medicine at UCLA. "We studied the disease in mice bred to possess the same gene defect seen in human patients."
Neufeld's team found that mice with the defective gene produce higher amounts of two proteins called lysozyme and P-tau. They tracked the proteins to neurons in the medial entorhinal cortex — an important memory center in the brain. One of the first areas to be affected by Alzheimer’s disease, the region also has been implicated in other abnormalities in Sanfilippo syndrome.
Earlier research had linked high levels of lysozyme to the production of P-tau, a misshapen protein that helps form the strands that clump into tangles in the brain. These tangles impair neuron function and are a hallmark of Alzheimer's and other degenerative brain diseases.
"This is really exciting," said co-author Stanislav Karsten, a UCLA assistant professor of neurology and of obstetrics and gynecology. "If we can replicate our discovery of P-tau in the brains of human patients, it may be possible to treat Sanfilippo syndrome with new drugs created for Alzheimer's disease. We believe our finding will accelerate the development of an effective therapy for this heartbreaking disorder."
Many scientists have searched for P-tau in the brains of Sanfilippo syndrome patients and in animal models. The UCLA team was the first to uncover it.
"We were fortunate to find the P-tau, because it appears in only a very small part of the brain," said lead author Kazuhiro Ohmi, a UCLA assistant researcher in biological chemistry.
Sanfilippo syndrome falls under MPS, a family of disorders involving mucopolysaccharides, which are long chains of sugar molecules used to build connective tissue. After the body finishes using these sugars, it breaks them down with enzymes and disposes of them.
Children with Sanfilippo syndrome lack the enzyme to digest the molecules, and they store them instead, leading to misshapen cells and enlarged organs. While infants rarely show signs of the disorder, symptoms worsen as children grow and their cells suffer more damage.
Fewer than one in 75,000 children are born with Sanfilippo syndrome. Both parents must carry and pass on the defective gene in order for their child to be affected — a one-in-four chance. Unaffected children have a two-in-three chance of carrying the gene.
Neufeld's earlier research identified the enzyme deficiencies that cause MPS. Her findings led to tests that enabled physicians to accurately diagnose the syndromes and counsel families. She is a consultant for Zacharon Pharmaceuticals, a biotechnology firm in San Diego.