Out of my league. I must admit that I know very little about the genetics of neurodegenerative disorders, but a recent article in the American Journal of Human Genetics caught my interest. Massive expansions of a hexanucleotide repeat in the first intron of C9orf72 are a known cause of frontotemporal lobar degeneration (FTLD) and Amyotrophic Lateral Sclerosis (ALS). With a novel method for rapid screening for these expansions, the authors investigate a large cohort of patients with neurodegenerative disorders and population controls, providing an interesting example of how seemingly clear-cut monogenic disorders acquire an unanticipated degree of complexity.
C9orf72 phenotypes. Even though Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) are both neurodegenerative disorders, they couldn’t be more different. FTD or frontotemporal lobar degeneration (FTLD) is a dementia that predominantly affects complex planning or sequencing and results in changes in social behavior or conduct. Memory, however, is usually less affected, which is in contrast to the dementia of the Alzheimer type where memory loss is the prominent feature. FTD accounts for ~20% of all dementias and usually starts in the 5th decade. Some cases have been found to overlap with ALS, a degenerative motor neuron disease, affecting the first and second motor neuron and resulting in progressive muscle weakness, atrophy and fasciculations (twitching). C9orf72 hexanucleotide expansions were found to be a major cause of FTD and ALS.
Hexanucleotide expansions. The first intron of C9orf72 includes an unstable hexanucleotide repeat (GGGGCC) that may lead to massive expansions. In unaffected individuals, typically less than 20 repeats are observed, while patients with ALS or FTD have more that 270 repeats and may even have up to ~4000 repeats. The repeats are impossible to assess using classical Sanger sequencing or Next Generation Sequencing and require other methods such as Southern Blotting to identify a specific “smear” pattern. In their recent publication, Berg and colleagues developed a screening method to estimate the repeat length, which allowed them to investigate a large cohort of patients. Hexanucleotide expansions are thought to results in so-called RNA G-quadruplexes. These stable secondary structures arise on the basis of G-rich RNA or DNA regions and might affect various cellular processes. It is, however, unknown whether the pathology seen in FTD and ALS is due to a lack of gene expression or due to the distortion of other cellular functions. Hexanucleotide expansions are also seen in other disorders, as in the case of Myotonic Dystophy with expansions within the DMPK and ZNF9 genes.
The expanding phenotype of C9orf72. The cohort of neurodegenerative diseases screened by Beck and colleagues comprised patients with FTD, ALS, but also patients with sporadic Creutzfeldt-Jakob disease, Huntington’s disease like syndromes and other neurodegenerative diseases. Interestingly, C9orf72 expansions could also be found in 1-3% in neurodegenerative disorders other than FTD and ALS, suggesting that several C9orf72-related phenotypes are not diagnosed as FTD and ALS. Also, large expansions were found in 0.15% of control individuals, suggesting that there is incomplete penetrance and that up to 90,000 individuals in the UK might carry C9orf72 expansions. As the 1958 birth cohort was screened, many of the included individuals would have already been past the age of manifestation of ALS and FTD.
A monogenic variant gets complex – lessons for EuroEPINOMICS. The overall narrative of the paper by Beck and colleagues is reminiscent of the research on microdeletions. A novel, inexpensive method allows screening of large cohorts and a genetic variant previously considered clearly monogenic turns out to have a larger phenotypic range and incomplete penetrance. In the case of C9orf72, this is particularly interesting given that there seems to be a connection between various unconnected neurodegenerative disorders. It remains to be seen whether this might also apply to pediatric neurodegenerative disorders, some of which have a certain overlap with epileptic encephalopathies. Also, hexanucleotide expansions are not detectable with standard sequencing methods, and given the frequency of these expansions in the human genome, they might present an interesting field of research for “exome-negative” patients with severe epilepsies.