Ligase-based detection of mononucleotide repeat sequences

Up to 15% of all colorectal cancers are considered to be replication error positive (RER(+)) and contain mutations at hundreds of thousands of microsatellite repeat sequences. Recently, a number of intragenic mononucleotide repeat sequences have been demonstrated to be targets for inactivating genes in RER(+)colorectal tumors. In this study, thermostable DNA ligases were tested for the ability to detect alterations in microsatellite sequences in colon tumor samples. Ligation profiles on mononucleotide repeat sequences were determined for four related thermostable DNA ligases, Thermus thermophilus ( Tth ) ligase, Thermus sp. AK16D ligase, Aquifex aeolicus ligase and the K294R mutant of the Tth ligase. While the limit of detection for point mutations was one mutation in 1000 wild-type sequences, the ability to detect a single base deletion in a 10 base mononucleotide repeat was one mutation in 100 wild-type sequences. Furthermore, the misligation error increased exponentially as the length of the mono-nucleotide repeat increased, and was 10% of the correct signal for a 19 base mononucleotide repeat. A fluorescent ligase-based assay [polymerase chain reaction/ligase detection reaction (PCR/LDR)] correlated with results obtained using a radioactive assay to detect instability within the TGF-beta Type II receptor gene. PCR/LDR was also used to detect the APCI1307K mononucleotide repeat allele which has a carrier frequency of 6.1% in Ashkenazi Jewish individuals. In a blind study, 30 samples that had been typed for the presence of the APCI1307K allele were tested. The PCR/LDR results correlated with those obtained using sequencing and allele-specific oligonucleotide hybridization for 16 samples carrying the mutation and 13 wild-type samples. Ligation assays that characterize mononucleotide repeats can be used to rapidly detect somatic mutations in tumors, and to screen for individuals who have a hereditary predisposition to develop colon cancer.