Screening for the C9ORF72 expansion in Greek Huntington Disease phenocopies and controls and meta-analysis of current data

Expanding the spectrum of C9ORF72-related neurodegenerative disorders in the Greek population

The C9ORF72 hexanucleotide repeat expansion is an established cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) and has also been associated with Huntington disease (HD)-like syndromes and rarely with Parkinson's disease (PD) and Alzheimer's disease (AD). In the present study we aimed to investigate the genotypic and phenotypic profile of C9ORF72-related disorders in Greece. For this reason, 957 patients (467 with ALS, 53 with HD-like syndromes, 247 with dementia, 175 with PD and 15 with hereditary spastic paraplegia, HSP) and 321 controls were tested for the C9ORF72 repeat expansion. Forty-nine patients with ALS (10.5%), 2 with HD-like syndromes (3.8%), 13 with FTD (11.5%), 1 with AD (1.6%), and 2 with PD (1.1%) were expansion-positive. The expansion was not detected in the HSP or control groups. The results of this study provide an update on the spectrum of C9ORF72-related neurodegenerative diseases, emphasizing the importance of C9ORF72 genetic testing in Greek patients with familial and sporadic ALS and/or FTD and HD-like syndromes.

Huntington's disease: lessons from prion disorders

Decades of research on the prion protein and its associated diseases have caused a paradigm shift in our understanding of infectious agents. More recent years have been marked by a surge of studies supporting the application of these findings to a broad array of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Here, we present evidence to suggest that Huntington's disease, a monogenic disorder of the central nervous system, shares features with prion disorders and that, it too, may be governed by similar mechanisms. We further posit that these similarities could suggest that, like other common neurodegenerative disorders, sporadic forms of Huntington's disease may exist.

C9orf72 Hexanucleotide Repeat in Huntington-Like Patients: Systematic Review and Meta-Analysis

Introduction: Patients with Huntington-Like disorders (HLD) comprise a variety of allelic disorders sharing a Huntington phenotype. The hexanucleotide repeat expansion of the C9orf72 gene could explain part of the HLD etiology. We aimed to conduct a systematic review and meta-analysis looking for the frequency of the hexanucleotide repeat expansion of the C9orf72 gene in HLD patients. Methods: The protocol was registered on the International Prospective Register of Systematic Reviews database (PROSPERO) (registration number: CRD42018105465). The search was carried out in Medline, Scopus, Web of Science, and Embase in April 2018, and updated in July 2020. Observational studies reporting patients with HLD carrying the hexanucleotide repeat expansion in the C9orf72 gene were selected and reviewed; this process was duplicated. The cutoff threshold for considering the hexanucleotide expansion as a pathogenic variant was equal to or >30 G4C2 repeats. Cases with intermediate alleles with 20-29 repeat are also analyzed. Pooled frequency and 95% CI were calculated using random-effects models. Results: Nine out of 219 studies were selected, reporting 1,123 affected individuals with HLD. Among them, 18 individuals carried C9orf72 expansion, representing 1% (95% CI: 0-2%, I 2 = 0%) of the pooled frequency. Seven selected studies came from European centers, one was reported at a US center, and one came from a South-African center. We identified five individuals carrying intermediate alleles representing 3% (95% CI: 0-14%, I 2 = 78.5%). Conclusions: The frequency of C9orf72 unstable hexanucleotide repeat expansion in HLD patients is very low. Further studies with more accurate clinical data and from different ethnic backgrounds are needed to confirm this observation.