Molecular basis of the attenuated phenotype of human APOBEC3B DNA mutator enzyme.

Vincent Caval,Anu Bashamboo,Jean-Pierre Vartanian,Marie-Charlotte Dumargne,Rodolphe Suspène,Mohamed S Bouzidi,Hélène Laude,Simon Wain-Hobson,Thomas Krey

doi:10.1093/nar/gkv935

Abstract

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

Mutation-selection drives the oncogenic process [1,2]
The non-canonical uracil bases are removed by uracil N-glycosylase (UNG) generating abasic sites in single stranded DNA (ssDNA), which in turn are cleaved by apurinic/apyrinidic endonucleases, APE1 and APE2
The A3 enzymes were initially described as innate immune restriction factors for many DNA viruses, retroviruses and retroelements, A3A and A3B have emerged as sources of the large number of CG→TA somatic mutations in cancer genomes and nuclear DNA in experimental settings [2,4,5,6,7,8,9]

Summary

Introduction

Mutation-selection drives the oncogenic process [1,2]. While replication errors and exogenous sources of DNA damage such as UV light, mutagens and free radicals have long been appreciated to be sources of somatic diversity driving oncogenesis [3], the contribution of two endogenous DNA mutator enzymes has only recently come to the fore [2,4,5,6,7,8,9]. Epidemiological data links A3A to the development of breast, ovarian and hepatitis B virus associated liver cancer [7,14,15,16]. While both A3A and A3B can deaminate 5-methylcytidine residues [6,17], the two enzymes are not orthologous for A3B overexpression fails to produce detectable double stranded DNA breaks (DSBs), unlike A3A [6]

Methods

Results

Conclusion