CNR1 variation is associated with the age at onset in Huntington disease

Huntington disease (HD) is caused by the expansion of a CAG repeat within exon 1 of the HTT gene. Although the variation in age at onset (AO) is partly explained by the length of the expanded repeat blocks, the unexplained variation in AO is highly heritable, emphasizing the role of modifier genes on disease expression. Since down-regulation of type 1 cannabinoid (CB1) receptors is a key pathogenic event in HD, it has been suggested that activation of these receptors in patients may attenuate disease progression. In order to evaluate whether variations in the cannabinoid receptor 1 (CNR1) gene encoding the CB1 receptor protein have modifying effects on the AO of HD, we performed an association study between CNR1 polymorphisms and AO in HD patients. A (AAT)n repeat and a total of nine single nucleotide polymorphisms (SNPs) in the CNR1 gene were selected for genotyping in a cohort of 473 German HD patients recruited in the Huntington Center NRW in Bochum. The AO was significantly associated with the longest alleles (≥17 AAT) of the (AAT)n repeat polymorphism downstream of the CNR1 gene (p = 0.007) as well as with one SNP in the 3'UTR of CNR1 (rs4707436, p = 0.05). Interestingly, the allelic variation of rs4707436 affects different microRNA (miRNA) binding sites which could alter gene regulation and consequently influence protein expression. These findings support the idea that CNR1 variation may have modifying effects on the AO in HD.