- Nov 9, 2017
Introduction: Spinocerebellar ataxia type 17 (SCA17) is an inherited cerebellar degeneration associated with trinucleotide repeat expansions in the TATA-binding protein gene (TBP). Low-range expansions of TBP have recently been described in association with Parkinson's disease (PD). However, these low-range expansion alleles were also observed in healthy individuals. Prior distinct findings may result from reduced penetrance or age-dependent susceptibility, which may influence phenotypic expression.
Methods: A case-control study of 456 PD patients and 374 control subjects was conducted. Data and blood samples were collected during 2008-2013. Control subjects were individuals over 65 years old without parkinsonism. Sizes of TBP trinucleotide repeats were analyzed. All available carriers of the TBP repeat of ≥40 repeats were re-examined.
Results: A high prevalence of carriers of TBP repeat expansion ≥41 developed PD, mainly at an advanced age. Half of these carriers had onset after 70 years of age (range 34-84). Seven participants carried expansion alleles of ≥42, and all had PD. Fourteen participants (six patients and eight controls) carried a heterozygous 41-repeat allele. At the current mean age of 79 years and mean follow-up period of 4 years, three out of the eight control carriers of the 41-repeat allele developed PD, while none of the thirteen asymptomatic carriers of the 40-repeat allele did.
Conclusions: A high prevalence of PD was observed in carriers of low-range expansions of TBP (41-45 repeats), especially in elderly. This finding suggests that cut-off value for pathological TBP repeat expansion appear to be 41.
Keywords: Parkinson's disease; Spinocerebellar ataxia type 17; TATA box-binding protein gene; TBP; Trinucleotide repeat expansion.
Spinocerebellar ataxia (SCA) type 17 (SCA17), currently referred to as ATX-TBP by the International Parkinson and Movement Disorder Society Task Force on Genetic Nomenclature in Movement Disorders,1 is an autosomal dominant cerebellar ataxia caused by a polyglutamine-encoding CAG/CAA repeat expansion within the TATA box-binding protein (TBP) gene.2, 3 The average age of onset is in the fourth or fifth decade of life, and the typical phenotype includes cerebellar ataxia associated with one or more of the following clinical manifestations: chorea, dystonia, parkinsonism, pyramidal signs, cognitive impairment, and psychiatric symptoms such as psychosis and depression.4, 5 ATX-TBP is one of the Huntington's disease–like syndromes.6, 7
In 2010, the cutoff values for repeat expansions within TBP reported by the European Molecular Genetics Quality Network best practice guidelines for molecular genetic testing of SCAs8 were set as normal (25–42 CAG/CAA repeats), reduced penetrance (RP; 43–48 repeats), and full penetrance (FP; 49–66 repeats), respectively. However, establishing an unequivocal cutoff for a disease-causing repeat expansion has been a challenging task in ATX-TBP because many reports suggested that the threshold for pathological expansions is lower.9-13 Also, atypical phenotypes have been reported for ATX-TBP, such as the (inconsistent) finding of repeat expansions in patients considered to have Parkinson's disease (PD),14-17 casting doubt on the breadth of the phenotypic spectrum of ATX-TBP. These controversial findings motivated us to systemically evaluate the reported ATX-TBP patients to study the full phenotypic spectrum, extract potential genotype–phenotype correlations, and reconsider the cutoff values for expanded TBP repeats.