Clinical features and genetic characterization of two dizygotic twins with C9orf72 expansion

The objective of the study was to present a detailed clinical, genetic, and epigenetic characterization of 2 amyotrophic lateral sclerosis (ALS) concordant dizygotic twins. The described cases underwent clinical and paraclinical examinations according to the motor neuron disease protocol of our referral center. Mutation analysis of the major causative genes related to ALS was performed. The methylation profile of the CpG island located in the promoter region of C9orf72 and in the repeat region itself was investigated by bisulfite sequencing of C9orf72 expansion carriers. The described cases presented an overlapping phenotype. Genetic analysis revealed the presence of an abnormal (>50 repeats) G₄C₂-repeat expansion in C9orf72. Both the direct bisulfite sequencing-sensitive and the methylation-sensitive HhaI assays did not reveal any DNA methylation at the CpG island 5' of the G₄C₂ repeat in C9orf72. The (G₄C₂)_n methylation assay indicated that also the expansion itself was not methylated in both twins, suggesting a probably intermediate allele expansion. This is the first report of ALS-concordant dizygotic twins carrying a C9orf72 expansion probably of intermediate length, and with a detailed clinical and genetic characterization. Twin studies add significant information about the mechanisms of C9orf72 expansion pleiotropism, probably driven by genetic, epigenetic, and environmental factors.