Mutant huntingtin messenger RNA forms neuronal nuclear clusters in rodent and human brains

Abstract

Mutant messenger RNA (mRNA) and protein contribute to the clinical manifestation of many repeat-associated neurological disorders, with the presence of nuclear RNA clusters being a common pathological feature. Yet, investigations into Huntington's disease-caused by a CAG repeat expansion in exon 1 of the <i>huntingtin</i> (<i>HTT</i>) gene-have primarily focused on toxic protein gain-of-function as the primary disease-causing feature. To date, mutant <i>HTT</i> mRNA has not been identified as an <i>in vivo</i> hallmark of Huntington's disease. Here, we report that, in two Huntington's disease mouse models (YAC128 and BACHD-97Q-ΔN17), mutant <i>HTT</i> mRNA is retained in the nucleus. Widespread formation of large mRNA clusters (∼0.6-5 µm³) occurred in 50-75% of striatal and cortical neurons. Cluster formation was independent of age and driven by expanded repeats. Clusters associate with chromosomal transcriptional sites and quantitatively co-localize with the aberrantly processed N-terminal exon 1-intron 1 mRNA isoform, <i>HTT1a</i>. <i>HTT1a</i> mRNA clusters are observed in a subset of neurons from human Huntington's disease post-mortem brain and are likely caused by somatic expansion of repeats. In YAC128 mice, clusters, but not individual <i>HTT</i> mRNA, are resistant to antisense oligonucleotide treatment. Our findings identify mutant <i>HTT</i>/<i>HTT1a</i> mRNA clustering as an early, robust molecular signature of Huntington's disease, providing <i>in vivo</i> evidence that Huntington's disease is a repeat expansion disease with mRNA involvement.