C9orf72
C9orf72 is a protein which in humans is encoded by the gene C9orf72.
The human C9orf72 gene is located on the short arm of chromosome 9 open reading frame 72, from base pair 27,546,542 to base pair 27,573,863. Its cytogenetic location is at 9p21.2.
The protein is found in many regions of the brain, in the cytoplasm of neurons as well as in presynaptic terminals. Disease-causing mutations in the gene were first discovered by two independent research teams, led by Rosa Rademakers of Mayo Clinic and Bryan Traynor of the National Institutes of Health, and were first reported in October 2011. The mutations in C9orf72 are significant because it is the first pathogenic mechanism identified to be a genetic link between familial frontotemporal dementia and of amyotrophic lateral sclerosis. It is the most common mutation identified that is associated with familial FTD and/or ALS.
Gene location
Cytogenetic Location: 9p21.2Molecular Location on chromosome 9: base pairs 27,546,542 to 27,573,863
The C9orf72 gene is located on the short arm of chromosome 9 at position 21.2.
More precisely, the C9orf72 gene is located from base pair 27,546,542 to base pair 27,573,863 on chromosome 9.
Mutations
The mutation of C9ORF72 is a hexanucleotide repeat expansion of the six letter string of nucleotides GGGGCC. In a person without the mutation, there are few repeats of this hexanucleotide, typically less than 20–30, but in people with the mutation, the repeat can occur in the order of hundreds. It is known that the mutation interferes with normal expression of the protein made by C9orf72, however the function of this protein remains speculative. There are two major theories about the way that the C9ORF72 mutation causes FTD and/or ALS. One theory is that accumulation of RNA in the nucleus and cytoplasm becomes toxic, and RNA binding protein sequestration occurs. The other is that the lack of half of the C9ORF72 protein in the body causes the diseases. Additionally, RNA transcribed from the C9ORF72 gene, containing expanded GGGGCC repeats, is translated through a non-ATG initiated mechanism, which is the same mechanism as other repeat disorders. This hexanucleotide varant of a trinucleotide repeat disorder produces five different dipeptides by RAN translation, these dipeptides aggregating to contribute to overall toxicity of the mutation. The GGGGCC repeat expansion in C9orf72 is also believed to compromise nucleocytoplasmic transport through several possible mechanisms.Clinical significance
The C9ORF72 mutation is the first mutation found to be a link between familial FTD and ALS. Numerous published studies have confirmed the commonality of the C9ORF72 repeat expansion in FTD and ALS, which are both diseases without cures that have affected millions of people. Frontotemporal dementia is the second most common form of early-onset dementia after Alzheimer's disease in people under the age of 65. Amyotrophic lateral sclerosis is also devastating; it is characterized by motor neuron degeneration that eventually causes respiratory failure with a median survival of three years after onset.C9orf72 is present in approximately 40% of familial ALS and 8-10 % of sporadic ALS. It is currently the most common demonstrated mutation related to ALS - far more common than SOD1 or TDP-43.
While different mutations of various genes have been linked to different phenotypes of FTD in the past, C9orf72 specifically has been linked to behavioral variant FTD. Certain pathology in FTD caused by the C9orf72 mutation can also include:
- TDP-43 in all C9 carriers
- Ubiquitin-binding protein 62
Slow diagnosis is also common for FTD, which can often take up to a year with many patients initially misdiagnosed with another condition. Testing for a specific gene that is known to cause the diseases would help with faster diagnoses. Possibly most importantly, the identification of this hexanucleotide repeat expansion is an extremely promising avenue for possible future therapies of both familial FTD and familial ALS, once the mechanism and function of the C9ORF72 protein is better comprehended. Furthermore, present research is being done to see if there is a correlation between C9ORF72 and other neurological diseases, such as motor neuron disease and Huntington's disease.
Gene heritability
It is possible that genetic anticipation may exist for this mutation. However, only 1 in 4 families exhibited significant anticipation in this study It has been proposed that the amount of the repeat expansion increases with each successive generation, possibly causing the disease to be more severe in the next generation, showing onset up to a decade earlier with each successive generation after the carrier. The buildup of a repeat expansion with each generation is typically thought to occur because the DNA is unstable and therefore accumulates exponentially every time the gene is copied. No genetic evidence for this has yet been demonstrated for this mutation. There is also a demographic factor that should be considered in genetic predisposition, as some cohorts have found that there might be a founder effect for the C9orf72 mutation, which might have led to higher frequencies of the mutation in specific populations than others. Specifically this founder has been linked to Northern Europeans populations, namely Finland.Gene testing
Since this mutation has been found to be the most common mutation identified in familial FTD and/or ALS, it is considered one of if not the most dependable candidates for genetic testing. Patients are considered eligible if the mother or father has had FTD and/or another family member has had ALS. There are also population and location risk factors in determining eligibility. Some studies have found that the mutation has a higher frequency in certain cohorts. Athena Diagnostics announced in Spring 2012 the first clinically available testing service for detecting the hexanucleotide repeat expansion in the C9orf72 gene. Genetic counseling is recommended for the patients before a genetic test is ordered.Likely function of C9ORF72 protein
C9ORF72 is predicted to be a full-length homologue of DENN proteins. These proteins have a conserved DENN module consisting of an N-terminal longin domain, followed by the central DENN and C-terminal alpha-helical d-DENN domains. This led to DENNL72 being suggested as a new name for C9orf72.Given the molecular role of known DENN modules, the C9ORF72-like proteins were predicted to function as Guanine nucleotide exchange factors, which activate small GTPases, most likely a Rab. Studies have provided some evidence to confirm this: C9ORF72 was found to regulate endosomal trafficking and autophagy in neuronal cells and primary neurons. This suggested that certain aspects of the ALS and FTD disease pathology might result from haploinsufficiency of C9ORF72, leading to a defect in intracellular membrane traffic, which adds to neuronal damage from RNA-mediated and dipeptide toxicities by reducing function of microglia, the macrophage-like cells of the brain.
GTPase targets of a stable C9ORF72-SMCR8-WDR41 complex include the Rag GTPases that simulate mTORC1 and so regulate macro-autophagy. Also, C9ORF72 and SMCR8 regulate the function of lysosomes. Although the GTPase involved on lysosomes is not yet identified, it might feasibly be Rab7A, which along with Rab5A and Rab11A, is activated by C9ORF72-SMCR8-WDR41 functioning as a GEF.
As well as activating GTPases, the same C9ORF72-SMCR8-WDR41 complex is proposed to inactivate GTPases, i.e. as a GTPase activating protein. This activity is proposed for Rag GTPases, paralleling the Rag-GAP activity fo the FLCN-FNIP complex, which it resembles. In addition, the complex is a GAP for Rab8a and Rab11a, with cryo-EM identifying an ariginine finger conserved between FLCN and SMCR8.