Unstable spinocerebellar ataxia type 10 (ATTCT*(AGAAT) repeats are associated with aberrant replication at the ATX10 locus and replication origin-dependent expansion at an ectopic site in human cells

Spinocerebellar ataxia type 10 (SCA10) is associated with expansion of (ATTCT)_n repeats (where n is the number of repeats) within the ataxin 10 (ATX10/E46L) gene. The demonstration that (ATTCT)_n tracts can act as DNA unwinding elements (DUEs) in vitro has suggested that aberrant replication origin activity occurs at expanded (ATTCT)_n tracts and may lead to their instability. Here, we confirm these predictions. The wild-type ATX10 locus displays inefficient origin activity, but origin activity is elevated at the expanded ATX10 loci in patient-derived cells. To test whether (ATTCT)_n tracts can potentiate origin activity, cell lines were constructed that contain ectopic copies of the c-myc replicator in which the essential DUE was replaced by ATX10 DUEs with (ATTCT)_n. ATX10 DUEs containing (ATTCT)₂₇ or (ATTCT)₄₈, but not (ATTCT)₈ or (ATTCT)₁₃, could substitute functionally for the c-myc DUE, but (ATTCT)₄₈ could not act as an autonomous replicator. Significantly, chimeric c-myc replicators containing ATX10 DUEs displayed length-dependent (ATTCT)_n instability. By 250 population doublings, dramatic two- and fourfold length expansions were observed for (ATTCT)₂₇ and (ATTCT)₄₈ but not for (ATTCT)₈ or (ATTCT)₁₃. These results implicate replication origin activity as one molecular mechanism associated with the instability of (ATTCT)_n tracts that are longer than normal length.