Two research teams have independently identified the nature of a gene mutation in C9ORF72 that has been repeatedly linked with the neurodegenerative disorders amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Their studies have identified a large hexanucleotide (GGGGCC) repeat expansion in the first intron of C9ORF7, which appears to be responsible for nearly 12% of familial FTD cases and over 22% of familial ALS samples studied as well as 3% of sporadic FTD and 4% of ALS cases.
Although the function of both the gene at C9OR72 and the effects of the hexanucleotide repeat expansion in its noncoding region have still to be determined, the researchers say evidence indicates the mutation leads to the loss of one alternatively spliced C9ORF72 transcript and to formation of nuclear RNA foci, suggesting multiple disease mechanisms.
The two teams of investigators report their findings in Neuron. The paper by Mayo Clinic Florida’s Mariely DeJesus-Hernandez, M.D., and Rosa Rademakers, M.D., and colleagues, is titled “Expanded GGGGCC hexanucleotide repeat in non-coding region of C9ORF72 causes chromosome 9p-linked frontotemporal dementia and amyotrophic lateral sclerosis”. The NIA-led collaboration, headed by Alan E. Renton, M.D., and Bryan J. Traynor, M.D., reports its findings in a paper titled “A hexanucleotide repeat expansion in C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD.”
A number of prior linkage analysis studies of ALS, FTD, and ALS-FTD have suggested that there is an important locus for the diseases on the short arm of chromosome 9. Recent genome-wide association studies by Dr. Renton’s team found that the same locus on chromosome 9p21 was associated with nearly 50% of familial ALS and nearly 25% of all ALS cases in a cohort of 405 Finnish patients. They had separately identified an ALS-FTD family from the U.K. and an apparently unrelated ALS-FTD family from the Netherlands that showed positive linkage to the chromosome 9p21 region.
Dr. Rademakers’ group, meanwhile, also found conclusive linkage to chromosome 9p in a large family (designated VSM-20) with autosomal dominant FTD/ALS. However, previous attempts to sequence all exons and exon-intron boundaries of the genes within the candidate region has failed to identify the disease-causing mutation in the family, the researchers note.
Using these cohorts of previously identified ALS, FTD, and ALS-FTD patients and independent cohorts of patients from other geographical regions, both sets of researchers carried out sequencing analyses and pinpointed the mutation to the large GC repeat expansion. According to the Mayo Clinic team's findings, whereas the GGGGCC repeat length in healthy individuals ranged from 2–23 hexanucleotide units, estimates suggested the repeat length to be 700–1,600 units in FTD/ALS patients.
Although the function of C9ORF72 and the detailed impact of the GGGGCC hexanucleotide repeat expansion on disease progression have yet to be determined, prior research has demonstrated intracellular accumulation of expanded nucleotide repeats as RNA foci in the nucleus and/or cytoplasm of affected cells, Dr. Rademakers et al. point out. This has emerged as “an important disease mechanism for the growing class noncoding repeat expansion disorders.”
The team therefore determined whether GGGGCC repeat expansions in C9ORF72 resulted in the formation of RNA foci by carrying out RNA FISH analysis of paraffin-embedded sections of postmortem frontal cortex and spinal cord tissue from patients with frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP); the team had already observed the highest frequency of C9ORF72 repeat expansions in pathologically confirmed TFLD-TDP patients.
The results showed that while multiple RNA foci were evident in the nuclei of 25% of cells in both the frontal cortex and the spinal cord samples from patients who did carry the expansion, they were found in only 1% of cells in tissue sections from noncarriers.
Importantly, the GGGGCC repeat expansion is the first genetic abnormality identified as a common cause of both FTD and ALS phenotypes, Dr. Rademakers et al. continue. “Our findings suggest multiple potential disease mechanisms associated with this repeat expansion, including a direct effect on C9ORF72 expression by affecting transcription (loss-of-function mechanism) and an RNA-mediated gain-of-function mechanism through the generation of toxic RNA foci.”
SOD1, TARDBP, FUS, OPTN and VCP, have previously been identified as causative for familial ALS, and cumulatively account for about 25% of such cases, Dr. Renton’s team points out. However, the new data indicate that the repeat expansion is more than twice as common as mutations in the SOD1 gene as a cause of familial ALS and more than three times as common as TARDBP, FUS, OPTN, and VCP mutations combined.
“Taken together with the D90A SOD1 mutation, our data show that nearly 90% of familial ALS in Finland is now explained by a simple monogenic cause,” they state. “The identification of the cause of chromosome 9p21-linked neurodegeneration allows for future screening of population-based cohorts to further unravel the overlap between ALS and FTD and to identify additional genetic and environmental factors that push an individual’s symptoms towards one end of the ALS/FTD clinical spectrum.”