C9orf72 Symposium Highlights New Ideas in Understanding and Treating ALS and FTD

The discovery of the C9orf72 gene is perhaps the most exciting discovery in ALS in the last 20 years, according to researchers hard at work to understand how the gene causes ALS and a related disorder, frontotemporal dementia (FTD). To maximize the growing understanding of the gene among the widest possible group of scientists, The ALS Association partnered with The Association for FTD to sponsor a symposium in New Orleans at the annual meeting of the Society for Neuroscience, the world’s largest assembly of neuroscientists.

“It is an exciting time for ALS and FTD research, and there are many new questions that are now going to emerge as we begin to explore this gene,” said ALS Association Chief Scientist Lucie Bruijn, Ph.D. That excitement was shared by the 170 investigators attending the symposium, many of whom are just joining the field, including established academic investigators, post-doctoral fellows, representatives from industry and clinicians.

Worldwide Importance of the C9orf72 Gene
Rosa Rademakers, Ph.D., Associate Professor of Neuroscience at the Mayo Clinic in Jacksonville, Florida, and co-discoverer of the gene, described what has been learned over the past year since the discovery was announced.

The C9orf72 gene, whose function is still unknown, normally carries a small number of copies of a six- nucleotides sequence, denoted CCCCGG. Nucleotides are the building blocks of genes. In late 2011, a team led by Dr. Rademakers, and another led by Bryan Traynor, M.D., announced that a large proportion of familial ALS cases were due to an expansion of the repeated section, leading to hundreds or thousands of copies of the six-nucleotide sequence.

Detailed analyses of populations throughout the world have since determined that the C9orf72 repeat expansion accounts for approximately 30% of all familial ALS in North America and 44% in Europe. It also explains about 5% of all sporadic (non-familial) cases worldwide, making it the most common known cause of ALS. The same mutation accounts for 40% of all FTD cases.

Furthermore, she noted, about 30% of people with the mutation have symptoms of both ALS and FTD, suggesting the two diseases may be two ends of a spectrum (see further discussion below).

The age at onset for C9-related ALS ranges from ages 27 to 83. The mutation is most commonly associated with typical ALS, but some patients also display some symptoms found in Parkinson's disease (tremor, slowed movements and rigidity) and memory impairment.

Not everyone who inherits the repeat develops ALS, it appears, which is a phenomenon known as incomplete penetrance. This may occur if the repeat shortens when cells divide during growth, but this remains speculative for now.

Examining the Pattern of Disease Spread
John Ravits, M.D., Professor of Clinical Neuroscience at the University of California at San Diego, discussed a model of disease inception and spread that takes account of important new findings about a number of neurodegenerative diseases. He noted that while ALS and FTD had long been thought of as separate diseases, the discovery of the C9orf72 gene, and the presence of TDP-43 protein accumulations in both diseases, strongly suggest they are not distinct diseases, but different manifestations, or “phenotypes,” of the same disease process. Even within ALS, he pointed out, there are different phenotypes, such as limb versus bulbar onset.

In Dr. Ravits’s model, the disease begins in a random site within the central nervous system, and that determines the first symptoms experienced by the person with ALS or FTD. In many cases, the disease then spreads to adjacent central nervous system structures, accounting for the pattern of symptom progression. For instance, if the site first affected is in the spinal cord neurons affecting the right leg, it is likely to next affect the left leg, as the neurons controlling these two are adjacent in the spinal cord. Over time, the spreading pattern becomes complex, as more regions of the nervous system are affected.

This combination of focal onset and spreading may help account for why most phenotypes can be caused by most ALS genes: over time, dissimilar initial causes are followed by similar spreading patterns, leading to similar clinical manifestations.

The mechanism of spread is still unknown, although some research suggests that misfolded protein (such as SOD1 or TDP-43) in one neuron causes that same protein to misfold in the next one. This is known to occur in other neurodegenerative diseases. Understanding the mechanism and pattern of spreading could help in the development of therapies to stop or slow the spreading process, thus limiting the progression of disease.

Is RNA Accumulation the Problem?
Clotilde Lagier-Tourenne, Ph.D., outlined her research on the mechanism by which the C9orf72 mutation causes ALS and FTD. When a gene is mutated, she said, it may cause a loss of function for the protein that is made from it. There is evidence that this occurs with the C9orf72 gene, but whether that loss is important in the disease is unknown.

There is growing evidence that the mutation causes a toxic new function to arise, not from the protein, but from an intermediate called RNA. The DNA sequence of a gene is ultimately used for making a protein but is first used to make a “working copy” from another molecule, called RNA. The CCCCGG expansion in the gene causes the RNA copy to also contain an expanded region, and scientists believe that this expanded RNA region may be the ultimate cause of the problem.

Evidence for that view comes from detection of accumulated RNA, called “foci,” in the nucleus of affected cells. These foci are believed to be very large clumps of the c9orf72 RNA, which, instead of being broken down by the cellular machinery, accumulate and become toxic. Dr. Lagier-Tourenne’s lab is developing methods for studying these RNA foci in greater detail to determine what new properties they may have and what problems they may be causing.

Clues from Myotonic Dystrophy
One idea about those problems comes from another neurologic disease, called myotonic dystrophy. This disorder is known to be due to a toxic accumulation of RNA, and recent advances have shown exactly how the RNA causes the disease. According to Maurice Swanson, Ph.D., of the University of Florida, the RNA foci in that disease trap a set of proteins that serve to control certain aspects of cell development, leading to multiple problems in different organs. This discovery has laid the foundation for developing new therapies in myotonic dystrophy.

Dr. Swanson pointed out that as ALS researchers investigate whether the C9orf72 RNA foci also trap important proteins, it will be vital to keep an open mind. “There will almost certainly be many things found to bind to RNA aggregates. Sorting out which is central will take time,” he said.

If the accumulation of RNA is indeed the problem in the disease, one therapeutic approach may be to prevent it from accumulating in the first place. Dr. Lagier-Tourenne said that initial experiments were underway to test this idea, using “antisense” molecules that bind to the RNA, and cause cellular machinery to dispose of it. Developing this strategy is a major goal of several groups.

“The discovery of the C9orf72 gene has galvanized our community of researchers to understand how this mutation causes disease and to discover new strategies for treating it. We are confident that pursuing these strategies will help us ultimately develop effective treatments for ALS based on understanding how this gene works,” said Dr. Bruijn.