Improved sizing of fragile X CCG repeats by nested polymerase chain reaction.

We have developed an improved method for polymerase chain reaction (PCR)-based sizing of the CCG repeat region at the fragile X locus, FMR-1. This method is designed to optimize denaturation and replication of long repeats with high G + C content, which are otherwise refractory to amplification. The method utilizes nested PCR primers to increase sensitivity and specificity. Alkaline denaturation of the genomic template DNA, combined with addition of glycerol and deaza-dGTP, facilitates strand separation. Labeled PCR products are sized on denaturing polyacrylamide gels. For alleles in the normal-to-premutation size range, strong reproducible signals are routinely obtained from small amounts of rapidly prepared DNA. This allows precise determination of the CCG repeat number, providing data related to the expansion potential of the repetitive segment. Detection of large premutations and some full mutations is also enhanced by the improved procedure.