Highly frequent frameshift DNA synthesis by human DNA polymerase μ

DNA polymerase mu (Pol mu) is a newly identified member of the polymerase X family. The biological function of Pol mu is not known, although it has been speculated that human Pol mu may be a somatic hypermutation polymerase. To help understand the in vivo function of human Pol mu, we have performed in vitro biochemical analyses of the purified polymerase. Unlike any other DNA polymerases studied thus far, human Pol mu 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 Pol mu was largely dictated by the sequence context. Human Pol mu was able to efficiently extend mismatched bases mainly by a frameshift synthesis mechanism. With the primer ends, containing up to four mismatches, examined, human Pol mu effectively realigned the primer to achieve annealing with a microhomology region in the template several nucleotides downstream. As a result, human Pol mu promoted microhomology search and microhomology pairing between the primer and the template strands of DNA. These results show that human Pol mu is much more prone to cause frameshift mutations than base substitutions. The biochemical properties of human Pol mu 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.