Analysis of C9orf72 Intermediate Alleles in a Retrospective Cohort of Neurological Patients: Risk Factors for Alzheimer's Disease?

Background: C9orf72 hexanucleotide GGGGCC (G4C2) large repeat expansions within the first intron of the gene are a major cause of familial frontotemporal dementia, but also of apparently sporadic cases. Alleles with > 30 repeats are often considered pathogenic, but the repeat length threshold is still undefined. It is also unclear if C9orf72 intermediate alleles (9-30 repeats) have clinically significant effects. Objectives: We correlated the presence of C9orf72 intermediate alleles with clinical diagnoses in a perspective cohort referred to a secondary memory clinic. Methods: All samples were genotyped with AmplideXPCR/CE C9ORF72 Kit (Asuragen, Inc), an optimized C9orf72 PCR amplification reagent. Results: We showed that in patients with Alzheimer's disease (AD) the frequency of the intermediate repeat alleles was significantly increased versus controls (34/54, 63­ versus 16/39, 41%CTRLs, *p = 0.01, OR 2.91 CI 95%1.230-6.077), whereas no significant differences (p > 0.05) were observed when comparing all other dementias with non-demented individuals. Conclusion: Our findings suggest that C9orf72 intermediate repeat units may represent a genetic risk factor, contributing to the occurrence of AD. Nevertheless, further longitudinal studies, including larger cohort of subjects with intermediate alleles with long-term follow-up, would be needed to confirm these results.