Candidate DNA replication initiation regions at human trinucleotide repeat disease loci

Human Molecular Genetics12, 1021–1028 (2003)

The direction of transcription we used to establish the orientation of the CAG tract in the SCA-7 gene is not correct and is the opposite to that shown in Figure 2. This further emphasizes our original conclusion that a complex relationship exists between trinucleotide repeat tract instability and orientation and/or proximity in humans.

As a consequence of this correction two changes in the text need to be made.

1. The last paragraph of the section ‘Analysis of the SCA-7 locus’ should read:

   The SCA-7 CAG strand is the sense strand and codes for poly-glutamine. The candidate IR is located between 700 and 900 bp downstream of the repeats (relative to the direction of transcription). This defines the CTG strand as the template for Okazaki fragment synthesis during DNA replication in cells with an unexpanded repeat tract (Fig. 2).

2. The second paragraph of the Discussion should read:

   Relative to its candidate IR, the HD repeats are in the orientation that makes the CAG strand the template for Okazaki fragment synthesis. In a preliminary study on the HD locus we showed that expanded alleles also have an IR at the same position. In the case of normal SCA-7 alleles the CTG tract is the template for Okazaki fragment synthesis. Thus, the extensive germline expandability of the disease genes at these two loci does not appear to fit well with a simple repeat tract orientation model. Recent studies using an SV40 virus model system also question the importance of repeat tract orientation and suggest that the location of Okazaki fragment initiation relative to the repeat tract is a critical determinant of repeat instability (11).