Combined Bisulfite Restriction Analysis (COBRA)

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Most molecular biological techniques used to analyze specific loci in complex genomic DNA involve some form of sequence-specific amplification, whether it is biological amplification by cloning in Escherichia coli, direct amplification by polymerase chain reaction (PCR), or signal amplification by hybridization with a probe that can be visualized. Since DNA methylation is added postreplicatively by a dedicated maintenance DNA methyltransferase that is not present in either E. coli or in the PCR reaction, the methylation information is lost during molecular cloning or PCR amplification. Molecular hybridization does not discriminate between methylated and unmethylated DNA, since the methyl group on the cytosine does not participate in base pairing. The lack of a facile way to amplify the methylation information in complex genomic DNA has been a significant impediment to DNA methylation research.