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Amyotrophic lateral sclerosis (ALS) is characterised by a loss of motor neurons, leading to paralysis. Several autosomal dominant genes were implicated in ALS pathogenesis, such as SOD1, with over 180 reported mutations (http://alsod.iop.kcl.ac.uk/), including a p.T137A substitution.1 ,2 Recently, ARHGEF28, encoding an RNA binding protein involved in the aggregation of light neurofilaments in ALS, was suggested as a novel ALS gene (a heterozygous K280M>fs40X mutation was detected in three patients).3 ,4
Risk of ALS may be modulated by environmental factors, sex and ageing, which could be linked to epigenetic events (eg, DNA methylation). Monozygotic (MZ) twins provide the best opportunity to investigate environmental/epigenetic factors in disease development.5 Hence, we studied ALS-discordant MZ twins, including the evaluation of DNA methylation (DNAm) age, which is an accurate predictor of chronological age across different tissues.6 It is possible that DNAm age better reflects biological age than chronological age does, since age acceleration (DNAm age minus chronological age) was associated with several disorders and mortality.
Participants were recruited from the ALS Clinic at the Sunnybrook Health Sciences Centre. Genetic and epigenetic analyses were conducted as described in the online supplementary methods, including mutation analysis of SOD1 and C9orf72, NeuroX array, reverse transcription PCR (RT-PCR), whole genome DNA methylation array (HumanMethylation450 BeadChip) and bisulfite pyrosequencing.
The MZ twins from a Canadian PED24 family of Italian origin were 52 years old at last examination and ALS-discordant for 17 years, with the second-born twin (9377) diagnosed with ALS at 35 years (figure 1 …