Highly Frequent Frameshift DNA Synthesis by Human DNA Polymerase μ

DNA polymerase mu (Polmu) is a newly identified member of the polymerase X family. The biological function of Polmu is not known, although it has been speculated that human Polmu may be a somatic hypermutation polymerase. To help understand the in vivo function of human Polmu, we have performed in vitro biochemical analyses of the purified polymerase. Unlike any other DNA polymerases studied thus far, human Polmu catalyzed frameshift DNA synthesis with an unprecedentedly high frequency. In the sequence contexts examined, -1 deletion occurred as the predominant DNA synthesis mechanism opposite the single-nucleotide repeat sequences AA, GG, TT, and CC in the template. Thus, the fidelity of DNA synthesis by human Polmu was largely dictated by the sequence context. Human Polmu was able to efficiently extend mismatched bases mainly by a frameshift synthesis mechanism. With the primer ends, containing up to four mismatches, examined, human Polmu effectively realigned the primer to achieve annealing with a microhomology region in the template several nucleotides downstream. As a result, human Polmu promoted microhomology search and microhomology pairing between the primer and the template strands of DNA. These results show that human Polmu is much more prone to cause frameshift mutations than base substitutions. The biochemical properties of human Polmu suggest a function in nonhomologous end joining and V(D)J recombination through its microhomology searching and pairing activities but do not support a function in somatic hypermutation.