Azanucleoside treatment leads to B cell leukemia in Rag1 deficient mice

5-Aza-4’-thio-2’-deoxycytidine (ATC) is an azanucleoside cytidine analog used in preclinical studies for solid tumors as a promising DNA methyltransferase 1 (DNMT1) inhibitor. Repeated treatment with ATC has previously been shown to result in acute lymphoblastic leukemia (ALL) of both B-cell and T-cell origin in mice. Herein, Rag1 knockout mice were treated with ATC to determine if ATC could be oncogenic in non-lymphoid cells. However, ATC treatment targeted early B progenitors and invariably led to B-lineage ALL, with a gene expression signature similar to human B cell precursor (BCP) ALL. Whole exome sequencing revealed numerous single base substitutions of cytosine, primarily C>G transversions at CpG dinucleotides, within genes important for BCP-ALL. Bisulfite sequencing and treatment with a non-covalent DNMT1 inhibitor indicated that methylated cytosines were preferred targets for mutagenesis. This study reveals that ATC exposure leads to both DNMT1 dependent and independent mutagenesis and provides a direct link from ATC exposure to complex mutation signature to malignant transformation