Profiling DNA methylation by melting analysis

The idea of modifying DNA with bisulfite has paved the way for a variety of polymerase chain reaction (PCR) methods for accurately mapping 5-methylcytosine at specific genes. Bisulfite selectively deaminates cytosine to uracil under conditions where 5-methylcytosine remains unreacted. Following conventional PCR amplification of bisulfite-treated DNA, original cytosines appear as thymine while 5-methylcytosines appear as cytosine. Because the relative thermostability of a DNA duplex increases with increasing content of G:C base pairs, PCR products originating from DNA templates with different contents of 5-methylcytosine differ in melting temperature, i.e., the temperature required to convert the double helix into random coils. We describe two methods that resolve differentially methylated DNA sequences on the basis of differences in melting temperature. The first method integrates PCR amplification of bisulfite-treated DNA and subsequent melting analysis by using a thermal cycler coupled with a fluorometer. By including in the reaction a PCR-compatible, fluorescent dye that specifically binds to double-stranded DNA, the melting properties of the PCR product can be examined directly in the PCR tube by continuous fluorescence monitoring during a temperature transition. The second method relies on resolution of alleles with different 5-methylcytosine contents by analysis of PCR products in a polyacrylamide gel containing a gradient of chemical denaturants. Optimal resolution of differences in melting temperature is achieved by a special design of PCR primers. Both methods allow resolution of "heterogeneous" methylation, i.e., the situation where the content and distribution of 5-methylcytosine in a target gene differ between different molecules in the same sample.