FTD associated with parkinsonism occurs frequently as a result of mutations in the C9orf72 gene and also in the genes coding for the protein associated with microtubule tau (MAPT) and progranulin (GRN) on chromosome 17 (FTDP-17).
The high prevalence of TDP-43 inclusions in the substantia nigra is a relatively new finding and is probably related to the relatively high prevalence of parkinsonism in C9ORF72-positive patients.
Expansions in the C9orf72 gene are most frequently associated with familial FTD, typically combined with motor neuron disease, but other manifestations, such as symmetrical parkinsonism, CBS and multiple system atrophy-like presentations, have been described in patients with these mutations.
C9orf72-associated parkinsonism predominantly consisted in a hypokinetic-rigid syndrome without resting tremor (61%), with both asymmetric (59%) and symmetric (41%) distributions.
Patients presenting with middle-aged onset of delusions should be screened for C9ORF72 mutations, especially if family history and parkinsonism are present.
Our findings seem to support the hypothesis that the hexanucleotide expansions of C9ORF72 gene with intermediate repetitions between 20 and 29 repetitions could be associated with typical PD with psychosis or dementia and atypical parkinsonisms with dementia (non-classical atypical parkinsonism with dementia FTD-like) or without dementia (non-classical atypical parkinsonism upper MND-like), although the causal relationship is still unclear.
There is a higher than expected incidence of parkinsonism in ALS patients with C9ORF72 expansions, and the G4C2 repeat has also been reported in other motor phenotypes, such as primary lateral sclerosis, progressive muscular atrophy, corticobasal syndrome and Huntington-like disorders.
The phenotypic spectrum of C9ORF72 hexanucleotide repeat expansion mutation has been reported to include parkinsonian syndrome, Huntington's disease-like syndrome and dementia syndrome.
Our results suggest that expanded or intermediate C9ORF72 repeats in clinically diagnosed PD or parkinsonism might be an indication of heterogeneity in clinically diagnosed PD cases.
Presence of ALS is strongly indicative of a C9ORF72 expansion; a very early age at onset (<50 years), parkinsonism and oculomotor dysfunction are indicative of MAPT mutations; whereas hallucinations, CBDS and PNFA are indicative of PGRN mutations.
This study shows that (i) although rare, C9orf72 repeat expansions may be associated with clinically typical Parkinson's disease and also with other parkinsonism; (ii) in several patients, parkinsonism was levodopa-responsive and remained pure, without associated dementia, for >10 years and (iii) interestingly, all C9orf72 repeat expansion carriers had positive family histories of parkinsonism, degenerative dementias or amyotrophic lateral sclerosis.
The lack of association between abnormal C9orf72 repeat expansion and parkinsonian syndromes might imply pathogenic mechanisms other than tau or Lewy body pathology.
To determine the histopathologic bases for the observed incidence of parkinsonism in families with C9ORF72 expansions, which typically cause amyotrophic lateral sclerosis (ALS) and/or frontotemporal dementia.
ALS was more frequent in C9ORF72 than in MAPT and PGRN patients; onset before age 50 and parkinsonism were indicative of MAPT mutations, whereas hallucinations were indicative of PGRN mutations; prioritization of genetic testing is thus possible.