Myotonic dystrophy type 1 (DM1) is associated with an expansion of CTG repeats in the 3'UTR of the DMPK gene. It is accepted, as in other trinucleotide diseases, that the number of the repeats is correlated with age at onset and severity of the disease. However, assessment of genotype-phenotype correlation in DM1 is complicated with the expansion-biased somatic instability of mutant alleles over time and difficulties in precise assessment of the number of repeats by standard Southern blot hybridization. In order to clarify this issue we defined DM1 expansion size in lymphocytes by three parameters: size of progenitor, average, and largest allele, using a more precise small-pool/long-range PCR technique. We found a negative linear correlation of age at onset and average expansion size in juvenile-adult DM1 patients (35 out of 46) whose progenitor allele is less than 245 repeats long. Our result favors the hypothesis of the existence of a threshold in the progenitor allele size beyond which number of CTG repeats does not influence age at onset. Potential clinical significance is that the average allele size could be a useful indicator for the age at onset in juvenile-adult DM1 patients with relatively short progenitor allele. To test whether somatic instability of mutant alleles influences the progression of DM1, patients were divided in three phenotypic classes according to the severity of neuromuscular symptoms. We showed that the largest expansion in each DM1 phenotypic class reflects somatic instability of mutant allele over time independently of progenitor allele size and patient's age at sampling. The mean of the largest expansion was significantly different between phenotypic classes, implying the possible association between expansion-biased somatic instability of mutant alleles over time and progression of neuromuscular symptoms.

Spinocerebellar ataxia type 8 (SCA8) is a slowly progressive ataxia causally associated with untranslated CTG repeat expansion on chromosome 13q21. However, the role of the CTG repeat in SCA8 pathology is not yet well understood. Therefore, we studied the length of the SCA8 CTA/CTG expansions (combined repeats, CRs) in 115 patients with ataxia, 64 unrelated individuals with non-triplet neuromuscular diseases, 70 unrelated patients with schizophrenia, and 125 healthy controls. Only one patient with apparently sporadic ataxia was identified with an expansion of 100 CRs. He had inherited the expansion from his asymptomatic father (140 CRs) and transmitted the mutation to his son (92 CRs). Paternal transmission in this family produced contractions of 40 and 8 CRs, respectively. None of the subjects from other studied groups had an expansion at the SCA8 locus. In the control group the number of CRs at the SCA8 locus ranged from 14 to 34. Our findings support the notion that allelic variants of the expansion mutation at the SCA8 locus can predispose to ataxia.

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