The integrity of the U12 snRNA 3′ stem–loop is necessary for its overall stability

Disruption of minor spliceosome functions underlies several genetic diseases with mutations in the minor spliceosome-specific small nuclear RNAs (snRNAs) and proteins. Here, we define the molecular outcome of the U12 snRNA mutation (84C>U) resulting in an early-onset form of cerebellar ataxia. To understand the molecular consequences of the U12 snRNA mutation, we created cell lines harboring the 84C>T mutation in the U12 snRNA gene (RNU12). We show that the 84C>U mutation leads to accelerated decay of the snRNA, resulting in significantly reduced steady-state U12 snRNA levels. Additionally, the mutation leads to accumulation of 3'-truncated forms of U12 snRNA, which have undergone the cytoplasmic steps of snRNP biogenesis. Our data suggests that the 84C>U-mutant snRNA is targeted for decay following reimport into the nucleus, and that the U12 snRNA fragments are decay intermediates that result from the stalling of a 3'-to-5' exonuclease. Finally, we show that several other single-nucleotide variants in the 3' stem-loop of U12 snRNA that are segregating in the human population are also highly destabilizing. This suggests that the 3' stem-loop is important for the overall stability of the U12 snRNA and that additional disease-causing mutations are likely to exist in this region.