Ambiguous coding is required for the lethal interaction between methylated DNA bases and DNA mismatch repair.

The thiopurine 6-thioguanine (S6G) is used to treat acute leukaemia. Its cytotoxic effect requires an active DNA mismatch repair (MMR) system. S6G is incorporated into DNA where a small fraction undergoes in situ conversion to S6-thiomethylguanine (S6meG). After replication, S6meG-containing base pairs interact with MMR. This interaction is ultimately lethal and MMR-defective cells are resistant to S6G. Here, we report that growing human cells extensively incorporate the thiopyrimidine nucleoside 4-thiothymidine (S4TdR) into their DNA. The incorporated thiopyrimidine (S4T) can also undergo facile S-methylation to 4-thiomethylthymine (S4meT). The rate of methylation of S4TdR in model substrates is similar to that for the conversion of S6G to S6meG indicating that the DNA of cells grown in S4TdR will contain significant levels of S4meT. Despite this, S4TdR is not associated with MMR-related cell death. We demonstrate that, in contrast to S6meG, neither DNA S4T nor S4meT codes ambiguously. S4T retains the coding properties of unmodified T, whereas S4meT behaves like a normal cytosine and exclusively directs the incorporation of guanine. The preferred S4meT:G base pair is also a poor substrate for binding by the hMutSalpha mismatch recognition factor. We suggest that the ability of S4meT to produce a structurally acceptable base pair during replication underlies the absence of MMR-related death in cells treated with S4TdR.