To further investigate the SCA8 repeat instability and the paternal bias towards d(CTG) repeat contraction, we examined sperm samples from men who had expansions ranging in size from 80 to 800 repeats in the blood (Moseley et al. 2000; Mosemiller et al. 2003). Southern blot analysis on sperm DNA from two unrelated individuals showed that each expanded allele underwent a massive contraction—into a size range less often associated with ataxia (from 500 to approximately 90 and from 800 to approximately 110)
Fig. 7 En masse contraction of SCA8 alleles in sperm. a Dramatic repeat length changes in patients 1 and 2 detected by Southern blotting. The repeat length of patients 1 and 2 contracts from 500 and 800 repeats in blood (B) to approximately 80 and 100 repeats in sperm (S), respectively. The probe used did not contain the CTG repeat. b Southern blots of blood and sperm DNA from patients with smaller expansions in their blood reveal the same trend of contractions of the expanded allele in sperm to repeat sizes that are less often associated with ataxia (below approximately 100 repeats). Again, the equal intensities of the bands representing the normal and expanded alleles indicate that repeat contractions occurred in all or nearly all of the sperm with expanded alleles. c PCR analysis of SCA8 contractions in two patients from a family with paternal disease transmission. Although contraction of repeats in sperm is again observed, the resulting alleles remain within a more penetrant size range (more than 100 CTGs). (Reproduced from Moseley et al. SCA8 CTG repeat: en masse concentrations in sperm and intergenerational sequence changes may play a role in reduced penetrance. Human Molecular Genetics (2000) 9(14):2125-2130 with permission from Oxford University)
(Fig. 7a). A similar trend was also observed for individuals with smaller somatic expansions; the expanded allele contracted in sperm to a non-pathogenic range, generally below approximately 100 repeats (Fig. 7b). The equal intensities of the bands representing the normal and expanded alleles indicate that all or nearly all of the expanded allele in the sperm contracted. The tendency for the SCA8 expanded allele to contract in sperm most likely contributes to the reduced penetrance and maternal bias observed in some SCA8 families (Moseley et al. 2000; Silveira et al. 2000).
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