Our findings confirm that the newly discovered C9ORF72 mutation is an important cause of familial FTD. In our series of cases, selected for the presence of the most common molecular subtype of FTLD (FTLD-TDP), the C9ORF72 mutation was found to be present in all autosomal dominant families in which multiple members were affected by a combination of FTD and ALS. In addition, all autosomal dominant families in our series with a pure FTD phenotype and FTLD-TDP pathology were explained by mutations in either C9ORF72 or GRN, with the two genetic abnormalities being of similar frequency in this group. We did not find mutations in any sporadic cases or those with only a weak family history; however, the number of such cases in our series was very small. The entry criteria of our study also prohibited us from determining the frequency of the C9ORF72 mutation in families or sporadic cases with clinically pure ALS. Nonetheless, our findings suggest that C9ORF72 is the most common genetic cause of familial FTLD-TDP and likely accounts for the majority of families with FTD/ALS.
As our series was centred in a dementia research clinic, there was probably a selection bias towards families with FTD. Despite this, over half of the subjects had ALS, providing us a broad view of the spectrum of clinical phenotypes associated with the C9ORF72 mutation. A striking finding was the degree of clinical heterogeneity, particularly at the time of initial presentation. The age of onset had a particularly broad range (40 years), while the disease duration also varied considerably (15 years). The initial diagnoses included behavioural variant FTD (37%), PNFA (13%) and ALS (30%). Significant clinical variation was also common within individual families. However, despite the heterogeneity in their initial presentations, a notable feature of our subjects was the degree to which the phenotypes converged as their disease progressed. Whereas all but one subject had a single diagnosis at the time of initial presentation, by the end of the disease course, 23% fulfilled diagnostic criteria for both FTD and ALS, 27% of those with FTD had features of both behavioural variant FTD and PNFA and 13% had a combination of ALS with both PNFA and behavioural variant FTD. In fact, the degree of overlap was even greater when milder symptoms were taken into account; 67% of those with ALS had at least some abnormalities of behaviour and/or language and 50% of those with FTD had some motor dysfunction. Although there was some variation in the pattern of motor neuron dysfunction, it was generally typical of classical ALS with mixed upper and lower motor neuron findings. Extrapyramidal dysfunction and memory problems were common but usually mild with late onset.
The spectrum of clinical features in our patients from families with the C9ORF72
mutation was similar to what has been reported previously in a number of chromosome 9p-linked families, all of which demonstrated a combination of FTD and ALS with marked clinical variability among family members (Momeni et al., 2006
; Morita et al., 2006
; Vance et al., 2006
; Valdmanis et al., 2007
; Luty et al., 2008
; Le Ber et al., 2009
; Gijselinck et al., 2010
; Boxer et al., 2011
; Pearson et al., 2011
). However, we have shown that the C9ORF72
mutation may also be responsible for a significant proportion of families with a pure FTD phenotype. Distinguishing families with autosomal dominant FTD caused by the C9ORF72
mutation from those with either GRN
mutations may prove difficult on clinical grounds since these three groups show similar demographic features, frequencies of different FTD phenotypes and intrafamilial heterogeneity (Kelly et al., 2009
In all cases, neuropathological assessment found TDP-43 immunoreactive inclusions in a broad range of neuroanatomical regions. Cases with FTD all had typical findings of FTLD-TDP while those with ALS all showed degeneration of both upper and lower motor neurons with TDP-43 immunoreactive neuronal cytoplasmic inclusions. Similar to the clinical phenotypes, there was significant overlap in the pathology between the major diagnostic groups; both cases with a final diagnosis of pure ALS had some TDP-43 immunoreactive pathology in the extramotor neocortex and hippocampus, while 10/11 (91%) cases with pure FTD had at least rare TDP-43 immunoreactive neuronal cytoplasmic inclusions in lower motor neurons ().
The cortical pattern of TDP-43 immunoreactive pathology in our subjects was always consistent with FTLD-TDP type B, which is the form that is most commonly found in patients with both FTD and ALS (Mackenzie et al., 2011
). However, slightly more than a third of cases in our series also had pathological changes of FTLD-TDP type A. The presence of this dual pattern of FTLD-TDP was strongly associated with advanced age and longer disease duration. Interestingly, in Alzheimer's disease and many other common neurodegenerative conditions, TDP-43 immunoreactive pathology is found in 25–50% of cases as an age-related secondary pathological change (Arai et al., 2009
). It usually has features of FTLD-TDP type A and is associated with the less common allele of the rs5848 polymorphism in the GRN
gene (Dickson et al., 2010
). This suggests that FTLD-TDP type B might be the primary pathology associated with the C9ORF72
mutation and that additional TDP-43 immunoreactive pathology develops in a genetically susceptible subset of patients with advancing age.
In three subjects, we also found sufficient numbers of neocortical senile plaques and neurofibrillary tangles to warrant a pathological diagnosis of Alzheimer's disease. Although this could simply represent the coincidental occurrence of a common age-related disorder in some of our older subjects (mean age, with Alzheimer's disease
3.5 versus without Alzheimer's disease
3.5), future studies are needed to explore whether or not the C9ORF72
mutation is associated with an increased risk for Alzheimer's disease. The fact that the Alzheimer's disease pathology was somewhat atypical, with more abundant tau-immunoreactive neuritic and glial inclusions, may indicate an influence of the C9ORF72
mutation on the development of Alzheimer's disease pathology. These cases also demonstrate that the finding of abundant tau-immunoreactive pathology in a case with clinical FTD does not necessarily restrict the diagnosis to FTLD-tau; there may still be coexistent TDP-43 immunoreactive pathology and an associated C9ORF72
Finally, the most intriguing pathological finding in our subjects was the consistent presence of inclusions in certain neuronal populations, such as the granule cells of the cerebellar cortex, that were immunoreactive for ubiquitin and p62 but negative for TDP-43. Similar pathological changes have been described previously and seem to be specific for the spectrum of FTLD-TDP and ALS where they are more commonly found in familial cases (Pikkarainen et al., 2010
; King et al., 2011
). Our findings further suggest that this pathological change may be specifically related to the C9ORF72
mutation, which implies that this genetic abnormality results in the abnormal intracellular accumulation of some unknown protein(s) that is not TDP-43 or C9ORF72.
In summary, expansion of the GGGGCC hexanucleotide repeat in intron 1 of C9ORF72 is an important genetic cause of both FTD and ALS with TDP pathology and probably accounts for most families in which both conditions occur. Patients with this mutation show a high degree of heterogeneity in clinical presentation but the final phenotype usually includes a combination of frontotemporal lobar dysfunction and motor abnormalities. Discovery of the C9ORF72 mutation is the strongest evidence to date that FTD and ALS represent a clinicopathological spectrum of disease with overlapping molecular pathogenesis.