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

Area of Science:

• Molecular Biology
• Cancer Therapeutics
• DNA Repair Mechanisms

Background:

• The thiopurine drug 6-thioguanine (S6G) is utilized in acute leukemia treatment.
• S6G's cytotoxic effects are dependent on an active DNA mismatch repair (MMR) system.
• S6G is incorporated into DNA and methylated to S6-thiomethylguanine (S6meG), which triggers MMR-induced cell death in sensitive cells.

Purpose of the Study:

• To investigate the cellular incorporation and DNA repair interactions of the thiopyrimidine nucleoside 4-thiothymidine (S4TdR).
• To determine if S4TdR, unlike S6G, induces MMR-related cytotoxicity.
• To elucidate the molecular mechanisms underlying the differential cellular response to S4TdR and S6G.

Main Methods:

• Cell culture of human cells with 4-thiothymidine (S4TdR).
• Analysis of S4TdR incorporation into DNA and subsequent S-methylation to 4-thiomethylthymine (S4meT).
• Assessment of base-pairing fidelity and interaction with the hMutSalpha mismatch recognition factor.

Main Results:

• Human cells extensively incorporate S4TdR into their DNA.
• Incorporated S4T can be methylated to S4meT, similar to S6G to S6meG conversion.
• Unlike S6G, S4TdR and its methylated form (S4meT) do not induce MMR-related cell death.
• S4T retains thymine's coding properties, and S4meT correctly pairs with guanine.
• The S4meT:G base pair is a poor substrate for hMutSalpha, indicating reduced MMR recognition.

Conclusions:

• S4TdR does not induce MMR-related cytotoxicity because its incorporated forms do not trigger the DNA mismatch repair pathway.
• The coding fidelity of S4T and the preferential pairing of S4meT with guanine, coupled with poor hMutSalpha binding, explain the lack of MMR-mediated cell death.
• These findings highlight the critical role of MMR in thiopurine drug efficacy and suggest potential for developing less toxic analogs.