A mutation in a gene located on chromosome 9 and known as C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS), but exactly how it causes this disease has been unclear.
Now, Aaron Haeusler at Johns Hopkins University, and colleagues, have made major strides in understanding the complicated steps that occur between this gene mutation and the loss of motor neurons — the hallmark of ALS.
MDA funded Jiou Wang, assistant professor of biochemistry and molecular biology at Johns Hopkins, on this study.
The new findings, published March 13, 2014, in the journal Nature, are likely to provide information that will help physicians and scientists counteract the chain of events that starts with an expansion in the C9ORF72 gene and ends in paralysis of voluntary muscles.
Abnormally shaped DNA, RNA result from gene expansion
Haeusler and colleagues based their study in part on findings announced in 2011 that showed that an abnormal expansion in the DNA of the C9ORF72 gene appeared to be responsible for about 30 percent of familial ALS cases and perhaps 4 percent of sporadic (nonfamilial) ALS cases in North America.
One somewhat surprising aspect of the 2011 findings was that the C9ORF72 gene does not appear to be the code for a functional protein. The gene’s DNA is, however, converted to RNA, a step that normally occurs between DNA and protein production in cells. (Recent evidence has shown there may be some protein pieces made from the C9ORF72 gene as well, although their significance is uncertain.)
It became clear that the expanded DNA was causing a problem because of its effects on the DNA itself or the RNA made from it but not from the action or inaction of any C9ORF72 protein.
The mutation in the C9ORF72 gene is an abnormal expansion involving a sequence of six nucleotides — DNA building blocks — that is repeated far more times than is normal. The nucleotides are guanine (G) and cytosine (C), and their repeated sequence is GGGGCC. The usual number of GGGCC repeated sequences (“repeats”) is between two and 25, but people with ALS can have far more than that — with some showing thousands of these repeats.
In experiments conducted in cells with and without C9ORF72 expansions, the researchers on the new study found that the expanded GGGGCC DNA repeats become expanded RNA GGGGCC repeats, and that these expansions results in abnormal and harmful shapes made out of DNA, RNA, or both.
Abnormal RNA traps cellular proteins
The abnormally shaped bits of genetic material that result from the repeat expansions can wreak havoc with a cell’s internal machinery, the investigators found.
They impair the normal DNA-to-RNA conversion process, causing a lack of full-length C9ORF72 RNA (which may be a problem), and also causing small, abnormal bits of C9ORF72 RNA to accumulate in the cell, which is definitely a problem.
Important cellular proteins can become ensnared by these bits of abnormal RNA and prevented from carrying out their usual roles. (This phenomenon is known to be the cause of types 1 and 2 myotonic muscular dystrophy, a disorder in which abnormal gene expansions form a toxic web in which important proteins are ensnared.)
The researchers identified one of the trapped proteins as nucleolin, and they suspect that significant cellular abnormalities result from this protein’s inability to carry out its usual functions.
The findings may also have implications for other disorders. “There are over 30 repeat expansion diseases, including many neuromuscular disorders [Friedreich’s ataxia, myotonic muscular dystrophy, oculopharyngeal muscular dystrophy and spinal-bulbar muscular atrophy are in this group], for which effective treatments are lacking,” Wang said in correspondence with MDA. “This study points to the structures of the nucleotide repeats as a potential cause of one or more of these diseases.”
A leap forward
J. Paul Taylor, chair of Cell & Molecular Biology at St. Jude Children’s Research Hospital in Memphis, Tenn., authored an accompanying editorial about the new C9ORF72 paper. Taylor has received MDA research support. In his March 13 Nature News & Views piece, he says, “Haeusler and colleagues have taken a leap forward” in understanding C9ORF72-related disease and that their findings “will propel efforts to determine the role of individual RNA-binding proteins in disease.”