5-Aza-4'-thio-2'-deoxycytidine induces C>G transversions in a specific trinucleotide context and leads to acute lymphoid leukemia.

DNA methyltransferase inhibitors (DNMTi), most commonly cytidine analogs, are compounds that are used clinically to decrease 5'-cytosine methylation, with the aim of re-expression of tumor suppressor genes. We used a murine pre-clinical model of myelodysplastic syndrome based on transplantation of cells expressing a NUP98::HOXD13 transgene to investigate 5-Aza-4'-thio-2'-deoxycytidine (Aza TdCyd or ATC), a thiol substituted DNMTi, as a potential therapy. We found that ATC treatment led to lymphoid leukemia in wild-type recipient cells; further study revealed that healthy mice treated with ATC also developed lymphoid leukemia. Whole exome sequencing revealed thousands of acquired mutations, almost all of which were C > G transversions in a previously unrecognized, specific 5'-NCG-3' context. These mutations involved dozens of genes well-known to be involved in human lymphoid leukemia, such as Notch1, Pten, Pax5, Trp53 , and Nf1 . Treatment of human cells in vitro showed thousands of acquired C > G transversions in a similar context. Deletion of Dck , the rate-limiting enzyme for the cytidine salvage pathway, eliminated C > G transversions. Taken together, these findings demonstrate that DNMTi can be potent mutagens in human and mouse cells, both in vitro and in vivo .