Cognitive Impairment in a Complex Family With AAGGG and ACAGG Repeat Expansions in RFC1 Detected by ExpansionHunter...

We investigated the genetic basis and brain metabolism and blood flow of a Japanese family with spinocerebellar degeneration (SCD), with multiple affected members for 3 generations.

After excluding DNA repeat expansion (RE) of common SCD genes by fragment analysis, we performed whole-exome sequencing (WES) and whole-genome sequencing (WGS). Homozygosity mapping was performed using these data. REs were investigated with WGS data using ExpansionHunter Denovo and Expansion Hunter.

WES and WGS were unable to identify likely pathogenic variants, and homozygosity mapping failed to narrow down the locus. However, ExpansionHunter Denovo detected REs in intron 2 of the RFC1 gene and led us to the diagnosis of RFC1-related disorders. Subsequent repeat-primed PCR and Southern blot hybridization analyses revealed that 3 of 6 patients and 1 suspected individual had expansions of AAGGG ((AAGGG)_exp) and (ACAGG)_exp repeats in a compound heterozygous state and 3 had a homozygous (ACAGG)_exp. The patients showed a variety of clinical features, including adult-onset ataxia, sensorimotor neuropathy, head tremor, parkinsonism, dystonia, and cognitive impairment. A comparison of previous reports with those of the family in study suggested that motor neuropathy could be a feature of compound heterozygous patients and biallelic (ACAGG)_exp patients. Cognitive function tests showed cognitive impairment with a predominance of frontal lobe dysfunction. Examination of MRI, SPECT, and ¹⁸F-fluorodeoxyglucose-PET showed clear cortical damage with frontal lobe predominance in 1 case, but no cerebral damage was evident in the other 2 cases.

Our report shows the usefulness of WGS and RE detection tools for SCD of unknown cause. The studied family with RFC1-related disorders included patients with (ACAGG)_exp and (AAGGG)_exp in a compound heterozygous state and was characterized by motor neuropathy. Based on the results of cognitive function tests and imaging studies, 1 patient presented with cognitive impairment due to frontal lobe metabolic changes, but there were also patients who presented with cognitive impairment without apparent cerebral metabolic or blood flow, suggesting that other factors are also associated with cognitive impairment.