C9orf72 repeat expansions are a rare genetic cause of parkinsonism

Abstract

The recently identified C9orf72 gene accounts for a large proportion of amyotrophic lateral sclerosis and frontotemporal lobar degenerations. As several forms of these disorders are associated with parkinsonism, we hypothesized that some patients with Parkinson's disease or other forms of parkinsonism might carry pathogenic C9orf72 expansions. Therefore, we looked for C9orf72 repeat expansions in 1446 unrelated parkinsonian patients consisting of 1225 patients clinically diagnosed with Parkinson's disease, 123 with progressive supranuclear palsy, 21 with corticobasal degeneration syndrome, 43 with Lewy body dementia and 25 with multiple system atrophy-parkinsonism. Of the 1446 parkinsonian patients, five carried C9orf72 expansions: three patients with typical Parkinson's disease, one with corticobasal degeneration syndrome and another with progressive supranuclear palsy. 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. This study also provides the tools for identifying parkinsonian patients with C9orf72 expansions, with important consequences for genetic counselling.

Figure 1

Molecular genetic analyses of hexanucleotide repeat expansions in the seven C9ORF72 patients with pathogenically expanded or intermediate alleles. (A) Repeat-primed PCR products separated on an ABI 3730 genetic analyzer (Applied Biosystems); data were analyzed using Peak Scanner software version v1.0 (Applied Biosystems). Similar to the positive control (FTLD patient with a C9ORF72 expansion), stutter amplifications characteristic of C9ORF72 repeat expansions were seen in patients PD1, PD2, PD3, PD4 and PD5. The electrophoregrams of patients PD6 and PD7with intermediate length alleles and the negative control with a non-expanded allele are also shown. (B) Fluorescent fragment length analysis of PCR products containing GGGGCC hexanucleotide repeats, the numbers of which are indicated under the peaks. Patients PD1, PD2, PD3, PD4 and PD5 as well as the FTLD patient had a single peak and potentially unamplifiable repeat expansions. Patients PD6 and PD7 showed heterozygous peaks corresponding to 2/26 and 2/30 hexanucleotide repeats, respectively. (C) Southern blot profiles of a control with non-expanded alleles (C−) at the expected size (~825 base pairs), a FTLD patient with a known C9ORF72 expansion (C+), PD7 with 2 and 30 hexanucleotide repeats, and patients PD1, PD2, PD3, PD4 and PD5 with additional expanded alleles >12 kilobases in size.

Figure 2

Pedigree structures of families with repeat expansions that had family histories of parkinsonism, degenerative dementias or ALS are shown. Patients with PD are represented by black symbols, those with parkinsonism, including PSP, CBDS, and degenerative dementias or ALS by grey symbols, unaffected individuals by open symbols. The arrows point to the probands. *DNA available. AD = age at death, ALS = amyotrophic lateral sclerosis, AO = age at onset, CBDS = corticobasal degeneration syndrome, D = dementia, FTLD = frontotemporal lobar degenerations, P = parkinsonism, PD = Parkinson’s disease, PSP = progressive supranuclear palsy.