Analysis of short tandem repeats linked to polyglutamine diseases from whole-genome sequencing reveals intermediate alleles of HTT associated with an early disease onset in C9orf72 carriers

Carriers of the GGGGCC pathogenic expansion in C9orf72 can develop symptoms of frontotemporal dementia and/or amyotrophic lateral sclerosis, with variable and unpredictable ages at onset. Previous studies aiming to decipher the genetic bases of the clinical variability in this rare disease included bi-allelic polymorphisms, excluding short tandem repeats. Whole-genome sequencing data of 195 C9orf72 patients were used to consider all short tandem repeats linked to polyglutamine disorders as potential genetic modifiers given the existing links between C9orf72 and polyglutamine diseases. Intermediate alleles of HTT encoding huntingtin were associated with an earlier age at onset among C9orf72 carriers in the discovery cohort (n = 195, P = 0.0003) and in a European replication cohort (n = 145, P = 0.006). In the merged cohort (n = 340), the average difference of age at disease onset was 9.42 ± 2.14 years (P = 1.3 × 10e-5) between carriers and non-carriers of HTT-intermediate alleles. Neuropathology of one C9orf72 case heterozygous for HTT-intermediate allele showed typical TDP-43 inclusions related to the C9orf72 pathogenic expansion and was negative for polyglutamine inclusion. No somatic expansion of HTT was detected in blood of all C9orf72exp/HTT-intermediate carriers. If this study reinforces potential biological links between huntingtin and C9orf72 that remain to be explored, the results also illustrate the interest of considering short tandem repeats from whole-genome data in association studies which paves the way to more exhaustive approaches to explore the trait heritability due to short-tandem-repeats still hidden in the genome.