Increased Steady-State Mutant Huntingtin mRNA in Huntington's Disease Brain

Huntington's disease is caused by expansion of CAG trinucleotide repeats in the first exon of the huntingtin gene, which is essential for both development and neurogenesis. Huntington's disease is autosomal dominant. The normal allele contains 6 to 35 CAG triplets (average, 18) and the mutant, disease-causing allele contains >36 CAG triplets (average, 42).

We examined 279 postmortem brain samples, including 148 HD and 131 non-HD controls. A total of 108 samples from 87 HD patients that are heterozygous at SNP rs362307, with a normal allele (18 to 27 CAG repeats) and a mutant allele (39 to 73 CAG repeats) were used to measure relative abundance of mutant and wild-type huntingtin mRNA.

We used allele-specific, quantitative RT-PCR based on SNP heterozygosity to estimate the relative amount of mutant versus normal huntingtin mRNA in postmortem brain samples from patients with Huntington's disease.

In the cortex and striatum, the amount of mRNA from the mutant allele exceeds that from the normal allele in 75% of patients. In the cerebellum, no significant difference between the two alleles was evident. Brain tissues from non-HD controls show no significant difference between two alleles of huntingtin mRNAs. Allelic differences were more pronounced at early neuropathological grades (grades 1 and 2) than at late grades (grades 3 and 4).

More mutant HTT than normal could arise from increased transcription of mutant HTT allele, or decreased clearance of mutant HTT mRNA, or both. An implication is that equimolar silencing of both alleles would increase the mutant HTT to normal HTT ratio.