Acta Neuropathologica, 129:39-52, 2015. PMCID: PMC4282973
C9orf72 promoter hypermethylation inhibits the accumulation of pathologies which have been postulated to be neurotoxic. We tested here whether C9orf72 hypermethyl-ation is associated with prolonged disease in C9orf72 muta-tion carriers. C9orf72 methylation was quantiﬁed from brain or blood using methylation-sensitive restriction enzyme digest-qPCR in a cross-sectional cohort of 118 C9orf72 repeat expansion carriers and 19 non-carrier family mem-bers. Multivariate regression models were used to determine whether C9orf72 hypermethylation was associated with age at onset, disease duration, age at death, or hexanucleotide repeat expansion size. Permutation analysis was performed to determine whether C9orf72 methylation is heritable. We observed a high correlation between C9orf72 methylation across tissues including cerebellum, frontal cortex, spinal cord and peripheral blood. While C9orf72 methylation was not signiﬁcantly different between ALS and FTD and did not predict age at onset, brain and blood C9orf72 hypermethyla-tion was associated with later age at death in FTD (brain:β = 0.18, p = 0.006; blood: β = 0.15, p < 0.001), and blood C9orf72 hypermethylation was associated with longer disease duration in FTD (β = 0.03, p = 0.007). Furthermore, C9orf72 hypermethylation was associated with smaller hexanucleotide repeat length (β = −16.69, p = 0.033). Finally, analysis of pedigrees with multiple mutation carriers dem-onstrated a signiﬁcant association between C9orf72 meth-ylation and family relatedness (p < 0.0001). C9orf72 hyper-methylation is associated with prolonged disease in C9orf72 repeat expansion carriers with FTD. The attenuated clinical phenotype associated with C9orf72 hypermethylation sug-gests that slower clinical progression in FTD is associated with reduced expression of mutant C9orf72. These results support the hypothesis that expression of the hexanucleotide repeat expansion is associated with a toxic gain of function.