posted on 2024-02-23, 20:14authored byMarisa Chancharoen, Zhiyu Yang, Esha D. Dalvie, Nina Gubina, Mathuros Ruchirawat, Robert G. Croy, Bogdan I. Fedeles, John M. Essigmann
The biomarker 5-chlorocytosine
(5ClC) appears in the DNA of inflamed
tissues. Replication of a site-specific 5ClC in a viral DNA genome
results in C → T mutations, which is consistent with 5ClC acting
as a thymine mimic in vivo. Direct damage of nucleic acids by immune-cell-derived
hypochlorous acid is one mechanism by which 5ClC could appear in the
genome. A second, nonmutually exclusive mechanism involves damage
of cytosine nucleosides or nucleotides in the DNA precursor pool,
with subsequent utilization of the 5ClC deoxynucleotide triphosphate
as a precursor for DNA synthesis. The present work characterized the
mutagenic properties of 5ClC in the nucleotide pool by exposing cells
to the nucleoside 5-chloro-2′-deoxycytidine (5CldC). In both Escherichia coli and mouse embryonic fibroblasts
(MEFs), 5CldC in the growth media was potently mutagenic, indicating
that 5CldC enters cells and likely is erroneously incorporated into
the genome from the nucleotide pool. High-resolution sequencing of
DNA from MEFs derived from the gptΔ C57BL/6J
mouse allowed qualitative and quantitative characterization of 5CldC-induced
mutations; CG → TA transitions in 5′-GC(Y)-3′
contexts (Y = a pyrimidine) were dominant, while TA → CG transitions
appeared at a much lower frequency. The high-resolution mutational
spectrum of 5CldC revealed a notable similarity to the Catalogue of
Somatic Mutations in Cancer mutational signatures SBS84 and SBS42,
which appear in human lymphoid tumors and in occupationally induced
cholangiocarcinomas, respectively. SBS84 is associated with the expression
of activation-induced cytidine deaminase (AID), a cytosine deaminase
associated with inflammation, as well as immunoglobulin gene diversification
during antibody maturation. The similarity between the spectra of
AID activation and 5CldC could be coincidental; however, the administration
of 5CldC did induce some AID expression in MEFs, which have no inherent
expression of its gene. In summary, this work shows that 5CldC induces
a distinct pattern of mutations in cells. Moreover, that pattern resembles
human mutational signatures induced by inflammatory processes, such
as those triggered in certain malignancies.