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

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• The APOBEC3A (A3A) and APOBEC3B (A3B) genes encode DNA cytidine deaminase enzymes.
• These enzymes are significant contributors to mutations observed in various cancer genomes, particularly CG to TA transitions.
• While both A3A and A3B can hypermutate DNA, only A3A induces double-strand DNA breaks, despite minor differences in their catalytic domains.

Purpose of the Study:

• To investigate the molecular mechanisms responsible for the attenuated activity of A3B compared to A3A.
• To identify specific domains and mutations that influence the DNA mutator activity and double-strand break induction.
• To understand the evolutionary conservation of this functional difference between A3A and A3B orthologs.

Main Methods:

• Generation and analysis of A3A-A3B chimeras and mutants.
• Assessment of enzyme activity, including DNA deamination and double-strand break induction.
• Comparative analysis of human and rhesus monkey APOBEC3 enzymes.

Main Results:

• The N-terminal domain of A3B plays a role in facilitating its activity.
• Specific substitutions within the C-terminal domain, affecting single-stranded DNA (ssDNA) binding, attenuate A3B activity relative to A3A.
• A similar functional attenuation of A3B compared to A3A is observed in rhesus monkeys, suggesting ancient evolutionary selection.

Conclusions:

• The differential activity between A3A and A3B is primarily determined by their N-terminal domain and specific residues in the C-terminal catalytic domain that impact ssDNA binding.
• This genotoxic dichotomy between A3A and A3B has been evolutionarily conserved for at least 38 million years.
• The absence of these enzyme expressions in mature sperm suggests their role in somatic mutation and cancer, not in human genetic diversity.