Spinocerebellar ataxia type 17: report of a family with reduced penetrance of an unstable Gln49 TBP allele, haplotype analysis supporting a founder effect for unstable alleles and comparative analysis of SCA17 genotypes

Abstract

Background: Spinocerebellar ataxia type 17 (SCA17), a neurodegenerative disorder in man, is caused by an expanded polymorphic polyglutamine-encoding trinucleotide repeat in the gene for TATA-box binding protein (TBP), a main transcription factor. Observed pathogenic expansions ranged from 43-63 glutamine (Gln) codons (Gln43-63). Reduced penetrance is known for Gln43-48 alleles. In the vast majority of families with SCA17 an expanded CAG repeat interrupted by a CAA CAG CAA element is inherited stably.

Results: Here, we report the first pedigree with a Gln49 allele that is a) not interrupted, b) unstable upon transmission, and c) associated with reduced penetrance or very late age of onset. The 76-year-old father of two SCA17 patients carries the Gln49 TBP allele but presents without obvious neurological symptoms. His children with Gln53 and Gln52 developed ataxia at the age of 41 and 50. Haplotype analysis of this and a second family both with uninterrupted expanded and unstable pathological SCA17 alleles revealed a common core genotype not present in the interrupted expansion of an unrelated SCA17 patient. Review of the literature did not present instability in SCA17 families with expanded alleles interrupted by the CAA CAG CAA element.

Conclusion: The presence of a Gln49 SCA17 allele in an asymptomatic 76-year-old male reams the discussion of reduced penetrance and genotypes producing very late disease onset. In SCA17, uninterrupted expanded alleles of TBP are associated with repeat instability and a common founder haplotype. This suggests for uninterrupted expanded alleles a mutation mechanism and some clinical genetic features distinct from those alleles interrupted by a CAA CAG CAA element.

Figure 1

Pedigrees and haplotype analysis. Pedigrees of families A and B, including six-locus haplotypes based on five 6q-telomeric microsatellite markers (D6S1719, D6S264, D6S281, D6S446, D6S1590) and closely downstream located polymorphic poly-Gln encoding trinucleotide stretch within TBP. Alleles of the microsatellites are denoted by numbers 1 – 8, TBP alleles are denoted by the number of Gln-encoding trinucleotides. SCA17-linked alleles are boxed as well as the disease haplotype D6S446 and D6S1590.