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Calculating predictive values for the large repeat alleles at the SCA8 locus in patients with ataxia
  1. V Juvonen1,2,
  2. V Kairisto2,3,
  3. M Hietala4,
  4. M-L Savontaus1,5
  1. 1Department of Medical Genetics, University of Turku, Finland
  2. 2Turku University Hospital Laboratories, Turku, Finland
  3. 3Department of Clinical Chemistry, University of Turku, Finland
  4. 4Clinical Genetics Unit, Turku University Hospital, Finland
  5. 5Department of Biology, University of Turku, Finland
  1. Correspondence to:
 Dr V Juvonen, Turku University Hospital, Laboratory Department 931, Kiinamyllynkatu 4–8, FIN-20520 Turku, Finland;

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Since the original description of SCA8 in 1999,1 several reports have been published on the occurrence of the SCA8 gene CTG repeat expansion in various populations. These studies have shown that, in addition to finding the SCA8 expansion in familial and sporadic ataxia patients, expanded alleles can also be found in non-ataxic subjects, psychiatric patients, and in patients with various other neurological diseases with a known aetiological cause.2–6 Owing to the exceptionally low penetrance of the mutation, questions have been raised about the pathogenic role of the expansion, and there exists speculation about alternative mechanisms, such as linkage disequilibrium of the CTG expansion with another as yet unidentified causal mutation. Accordingly, caution in interpreting mutation findings in clinical samples has been stressed and some investigators discourage genetic testing for SCA8 until a pathological mechanism has been established.3,6


We have investigated the occurrence of the SCA8 repeat expansion in 251 unrelated Finnish SCA patients and found 22 of them with the expansion. Based on our present and previous studies on controls and patients, we define the size range of 15-40 combined repeats as normal alleles and above 80 up to 800 as expanded alleles. Intermediate alleles between these groups are sometimes seen and we interpret them to be most likely non-pathogenic. We used a combination of PCR and Southern blotting methods as described previously5 in order to reach 100% sensitivity for the detection of expansions. None of the 22 SCA8 positive patients …

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