NOVEL MUTATIONS IN THE XLRS1 GENE MAY BE CAUSED BY EARLY OKAZAKI FRAGMENT SEQUENCE REPLACEMENT

PURPOSE. TO determine whether two families diagnosed with X-linked ret inoschisis contained mutations in the XLRS1 gene. METHODS. DNA from th e patients was obtained from blood lymphocytes using commercially avai lable kits. Single-strand conformation assay was performed in an elect rophoresis apparatus using 10% acrylamide TBE gels at 10 degrees C. Th e gels were stained with SYB green II and were scanned in a phosphoima ger. DNA was sequenced using an automated fluorescence sequencer. RESU LTS. A deletion that eliminates exon 2 was found in one family. An abn ormal sequence replacement in exon 4 was found in the other family. Bo th mutations have severe effects in the coding region by inserting pre mature stop codons. CONCLUSIONS. Bath of the families have mutations i n the XLRS1 gene. One of these mutations points to a novel mechanism. The mutation is caused by a replacement of 17 bp of a normal sequence with 20 bp of a sequence originating from two different places in the antisense strand. This suggests that early Okazaki fragments were inco rporated into the sense strand of exon 4, replacing the normal sequenc e.