Foreign and self-cytoplasmic DNA are recognized by numerous DNA sensor molecules leading to the production of type I interferons. Such DNA agonists should be degraded otherwise cells would be chronically stressed. Most human APOBEC3 cytidine deaminases can initiate catabolism of cytoplasmic mitochondrial DNA. Using the human myeloid cell line THP-1 with an interferon inducible APOBEC3A gene, we show that cytoplasmic DNA triggers interferon α and β production through the RNA polymerase III transcription/RIG-I pathway leading to massive upregulation of APOBEC3A. By catalyzing C→U editing in single stranded DNA fragments, the enzyme prevents them from re-annealing so attenuating the danger signal. The price to pay is chromosomal DNA damage in the form of CG→TA mutations and double stranded DNA breaks which, in the context of chronic inflammation, could drive cells down the path toward cancer.
The human APOBEC3A and APOBEC3B genes (A3A and A3B) encode DNA mutator enzymes that deaminate cytidine and 5-methylcytidine residues in single-stranded DNA (ssDNA). They are important sources of mutations in many cancer genomes which show a preponderance of CG->TA transitions. Although both enzymes can hypermutate chromosomal DNA in an experimental setting, only A3A can induce double strand DNA breaks, even though the catalytic domains of A3B and A3A differ by only 9% at the protein level. Accordingly we sought the molecular basis underlying A3B attenuation through the generation of A3A-A3B chimeras and mutants. It transpires that the N-terminal domain facilitates A3B activity while a handful of substitutions in the catalytic C-terminal domain impacting ssDNA binding serve to attenuate A3B compared to A3A. Interestingly, functional attenuation is also observed for the rhesus monkey rhA3B enzyme compared to rhA3A indicating that this genotoxic dichotomy has been selected for and maintained for some 38 million years. Expression of all human ssDNA cytidine deaminase genes is absent in mature sperm indicating they contribute to somatic mutation and cancer but not human diversity.
Highlights d Cas9 ribonucleoproteins (RNPs) allow robust gene knockout in primary human monocytes d Cas9 RNP nucleofected monocytes can differentiate into macrophages or dendritic cells d Cas9 RNP nucleofected macrophages appropriately phagocytose M. tuberculosis d Knockout of the host factor SAMHD1 increased HIV infection of macrophages >503
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