PUBLIC RELEASE DATE:

5-Mar-2014

Contact: Shawna Williams shawna@jhmi.edu 410-955-8632 Johns Hopkins Medicine

Johns Hopkins researchers say they have found one way that a recently discovered genetic mutation might cause two nasty nervous system diseases. While the affected gene may build up toxic RNA and not make enough protein, the researchers report, the root of the problem seems to be snarls of defective genetic material created at the mutation site.

The research team, led by Jiou Wang, Ph.D., an assistant professor of biochemistry and molecular biology and neuroscience at the Johns Hopkins University School of Medicine, reports its finding March 5 on the journal Nature's website.

Two years ago, researchers linked the gene C9orf72, named for its location on the ninth human chromosome, to amyotrophic lateral sclerosis (ALS), commonly known as Lou Gehrig's disease, and to frontotemporal dementia (FTD).

In ALS, motor neurons nerve cells that carry messages from the brain to muscles degenerate and eventually die, which gradually paralyzes the patient. In FTD, neurons in the frontal and temporal lobes of the brain die. Some scientists think the same genetic and biological processes cause both disorders, but with very different symptoms, depending on where in the brain they occur.

The mutation in C9orf72 is called a hexanucleotide repeat expansion, a six-letter "word" of DNA repeated over and over, in a part of the gene that doesn't contain instructions for making any proteins. Although it's normal to have up to 20 such repeats, some people with ALS or FTD have dozens or even hundreds of them. Studies show the mutation is likely responsible for 4 to 8 percent of cases of sporadic ALS the kind that isn't necessarily hereditary and, in some groups, up to 40 percent of the kind that is.

To learn how the repeated sequence causes disease, the Johns Hopkins scientists looked at the structure of the DNA that makes up the gene and the RNA that carries its instructions. Although DNA and RNA are generally seen as long strands, they can bunch and curl to make 3-D structures.

Working with DNA and RNA they made that bore the six-letter "word" repeat, the researchers figured out that both were forming structures called G-quadruplexes. In these formations, guanines called "G" for short, one of the letters in the repeating DNA "word" link up, making stacks that stick together like tiny shelves. The RNA also forms other shapes in the repeating section hairpins and bulges. The researchers speculate that the G-quadruplexes and other structures might be getting in the way of the nucleic acids' normal functions.

Read more:
ALS-linked gene causes disease by changing genetic material's shape

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