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Gene discovery points to Iron build-up as a major neuronal toxin

 
  June, 22 2006 20:04
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Discovery of the PLA2G6 gene, whose mutated forms trigger several genetic disorders categorized as neuroaxonal dystrophies, could shed light on the nerve cell degeneration that leads to Parkinson's and Alzheimer's diseases, both known to be associated with brain iron accumulation.

In a study published online June 18 in the journal Nature Genetics, Susan J. Hayflick, M.D., professor of molecular and medical genetics, pediatrics and neurology in the OHSU School of Medicine and an international team of geneticists describe PLA2G6's discovery using DNA from families with infantile neuroaxonal dystrophy, or INAD, and a related disorder known as neurodegeneration with brain iron accumulation, or NBIA.

In INAD, also known as Seitelberger disease, symptoms start by age 2 and worsen over time, and include loss of cranial and facial control and the ability to sit, crawl or walk, as well as deteriorating vision and speech, according to the National Institute of Neurological Disorders and Stroke, a branch of the National Institutes of Health. Children with the disease die between ages 5 and 10.

NBIA, sometimes called Hallervorden-Spatz syndrome, manifests itself between the teen years and adulthood. Symptoms include involuntary muscle contractions, rigidity and spasms in the limbs, face and torso, as well as confusion, disorientation, seizures, stupor and dementia. Rapid deterioration, punctuated by stable periods, lasts one to two months, with the rate of progression correlating with the patient's age – the later the onset, the better the patient fares.

There is no cure nor standard treatment for either disease, which are inherited in a recessive fashion. Incidence is 1 in 500,000 to 1 million.

PLA2G6's discovery means a clinical test can be developed to help families determine their chances of passing the disorders to their children.

The disorders are caused by a build-up of iron in the basal ganglia. The iron accumulation causes the axons that transmit electrical impulses from the nerve cell body to its terminal to swell, interrupting the signal transduction to downstream neurons.

PLA2G6 is thought to encode an enzyme that breaks down lipids involved in the reconstruction of a cell's membrane following damage by light and other toxins. When the gene is mutated, lipid metabolism is altered and iron builds up, triggering disease.

"I studied our entire INAD patient population for mutations in this gene and found over 44 different changes in the gene which would lead to disease," said study co-author Shawn Westaway, Ph.D., research assistant professor of molecular and medical genetics at OHSU.

Working with scientists at the University of Birmingham School of Medicine, United Kingdom, Hayflick and Westaway collected DNA from 30 to 40 families affected by the diseases and narrowed the search for the suspect gene to a 100-gene block of DNA on chromosome 22, the second smallest chromosome in humans that contains 500 to 800 genes. The team then looked for genes in the region whose function was suggestive of the symptoms and parts of the body affected by the diseases, and the search was further narrowed to 75 genes.

After scouring the 75 genes, "we finally found mutations in PLA2G6 in a large kindred with multiple generations of affected individuals, and in three other smaller families," Westaway said.

The chromosomes containing the mutations are then compared to almost 200 control chromosomes not affected by the disease. "The severity of the mutation is usually a very good clue that the gene has been found," she said. "That evidence is confirmed by continuing to find different, but severe, mutations in the same gene in new patients diagnosed with INAD, which we have done."

PLA2G6 is among 18 lipid-metabolizing genes in a protein family known as phospholipase A2 (PLA2), and INAD is the first inherited disorder associated with mutations in one of these genes. Its discovery "unequivocally" links PLA2 defects to neurodegeneration, researchers say, which is significant because similar lipid metabolism changes are seen in neurodegeneration associated with ischemia from stroke, spinal cord trauma, head injury and Alzheimer's disease, making this metabolic pathway a potential drug target.

In addition, iron is known to accumulate with age in brain regions attacked by Alzheimer's and Parkinson's diseases. "This is a common end effect of many neurodegenerative disorders," Hayflick said.


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