Cell-type-specific alternative splicing in spinocerebellar ataxia type 6

Area of Science:

• Neuroscience
• Genetics
• Molecular Biology

Background:

• Spinocerebellar ataxia type 6 (SCA6) is caused by mutations in the alpha1A voltage-dependent calcium-channel (Ca(v)2.1) gene.
• Alternative splicing of the Ca(v)2.1 gene produces two major mRNA isoforms, one of which is a "toxic isoform" containing a polyglutamine-encoding repeat.

Purpose of the Study:

• To investigate if an expanded CAG repeat in the Ca(v)2.1 gene increases the relative amount of the toxic isoform in Purkinje cells.
• To determine if this increase is specific to Purkinje cells in SCA6.

Main Methods:

• Independent isolation of Purkinje and granule cells from the brains of SCA6 patients and normal controls.
• Quantification of total and toxic Ca(v)2.1 mRNA isoforms using real-time reverse transcription (RT)-PCR.

Main Results:

• The ratio of toxic Ca(v)2.1 mRNA to total Ca(v)2.1 mRNA was significantly increased in Purkinje cells from SCA6 brains compared to controls.
• This increase was specific to Purkinje cells and not observed in granule cells from SCA6 brains.
• Total Ca(v)2.1 mRNA levels were similar between cell types and groups.

Conclusions:

• The expanded CAG repeat in the Ca(v)2.1 gene leads to a Purkinje cell-specific increase in the toxic mRNA isoform.
• This selective accumulation of the toxic isoform in Purkinje cells is a potential mechanism underlying SCA6-associated neurodegeneration.