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

Abstract

ABSTRACT 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.