CAG repeat instability at SCA2 locus: anchoring CAA interruptions and linked single nucleotide polymorphisms.

Area of Science:

• Genetics
• Neurodegenerative Disorders
• Molecular Biology

Background:

• Spinocerebellar ataxia 2 (SCA2) is an autosomal dominant neurodegenerative disorder caused by expanded CAG repeats in the SCA2 gene.
• Normal alleles have 14-31 CAG repeats, often interrupted by CAA triplets, while expanded alleles have 34-59 uninterrupted repeats.

Purpose of the Study:

• To identify and characterize novel single nucleotide polymorphisms (SNPs) in the SCA2 gene.
• To investigate the association of these SNPs and their haplotypes with SCA2 alleles.
• To understand the role of CAA interruptions in SCA2 repeat instability.

Main Methods:

• Identification of two novel biallelic SNPs in exon 1 of the SCA2 gene.
• Characterization of these SNPs and their associated haplotypes in 215 normal and 64 expanded SCA2 chromosomes.
• Analysis of CAA interspersion patterns in relation to repeat length and haplotype.

Main Results:

• Two predominant haplotypes, GT and CC, were identified.
• All expanded SCA2 alleles were associated with the CC haplotype.
• The CC haplotype was found in 29.3% of normal chromosomes, often lacking proximal CAA interruptions.
• Repeat length variation primarily occurred at the 5' end of the repeat.

Conclusions:

• CAA interruptions are crucial for SCA2 repeat stability; their absence predisposes alleles to instability and expansion.
• The identified novel haplotypes provide valuable tools for studying SCA2 mutational history and mechanisms of repeat instability.