Background:Previous small scale studies indicate that DNA polymerase ␤ variants are present in 30% of human tumors. Results: 40% of samples in a large human colorectal tumor collection harbor coding region variants, many of which exhibit altered function. Conclusion: Aberrant activity or fidelity phenotypes exhibited by variants may contribute to tumorigenesis. Significance: Expression of variants in human tumors plays a role in driving carcinogenesis.

Background: Accurate bypass of DNA damage by translesion DNA polymerases is critical for cell survival.Results: Wild-type human DNA polymerase ι incorporates ribonucleotides, and a steric gate mutant increases both ribonucleotide incorporation and deoxyribonucleotide selectivity.Conclusion: A single amino acid residue in DNA polymerase ι limits incorporation of ribonucleotides into DNA.Significance: DNA polymerase ι may incorporate ribonucleotides during translesion DNA synthesis.

Background: Mutations in the POLB gene are present in 40% of human colorectal tumors. Results: The G231D variant is a slow polymerase that induces genomic instability and cellular transformation. Conclusion:The slow G231D variant induces cellular transformation due to its inability to fill in single nucleotide gaps. Significance: Slow pol ␤ variants may drive tumorigenesis.

Human RAD51 protein catalyzes DNA pairing and strand exchange reactions that are central to homologous recombination and homology-directed DNA repair. Successful recombination/repair requires the formation of a presynaptic filament of RAD51 on ssDNA. Mutations in BRCA2 and other proteins that control RAD51 activity are associated with human cancer. Here we describe a set of mutations associated with human breast tumors that occur in a common structural motif of RAD51. Tumor-associated D149N, R150Q and G151D mutations map to a Schellman loop motif located on the surface of the RecA homology domain of RAD51. All three variants are proficient in DNA strand exchange, but G151D is slightly more sensitive to salt than wild-type (WT). Both G151D and R150Q exhibit markedly lower catalytic efficiency for adenosine triphosphate hydrolysis compared to WT. All three mutations alter the physical properties of RAD51 nucleoprotein filaments, with G151D showing the most dramatic changes. G151D forms mixed nucleoprotein filaments with WT RAD51 that have intermediate properties compared to unmixed filaments. These findings raise the possibility that mutations in RAD51 itself may contribute to genome instability in tumor cells, either directly through changes in recombinase properties, or indirectly through changes in interactions with regulatory proteins.