C9ORF72 hexanucleotide expansions of 20–22 repeats are associated with frontotemporal deterioration

Expansions of more than 30 hexanucleotide repetitions in the C9ORF72 gene are a common cause of frontotemporal dementia (FTD) or amyotrophic lateral sclerosis (ALS). However, the range of 20-30 repetitions is rarely found and still has unclear significance. A screening of our cohort of cases with FTD (n = 109) revealed 4 mutation carriers (>30 repetitions) but also 5 probands with 20-22 confirmed repetitions. This study explored the possible pathogenic correlation of the 20-22 repeats expansion (short expansion).Comparison of clinical phenotypes between cases with long vs short expansions; search for segregation in the families of probands with short expansion; analysis of the presence of the common founder haplotype, described for expansions >30 repeats, in the cases having the short expansion; and analysis of the distribution of hexanucleotide repeat alleles in a control population.No different patterns were found in the clinical phenotype or aggressiveness of the disease when comparing cases with long or short expansions. Cases in both groups had psychiatric symptoms during 1-3 decades before evolving insidiously to cognitive deterioration. The study of the families with short expansion showed clear segregation of the 20-22 repeats allele with the disease. Moreover, this 20-22 repeats allele was associated in all cases with the pathogenic founder haplotype. None of 216 controls had alleles with more than 14 repetitions.Description of these families suggests that short C9ORF72 hexanucleotide expansions are also related to frontotemporal cognitive deterioration.