Evidence for reduced copying fidelity of DNA polymerases alpha, delta, and epsilon from Novikoff hepatoma cells

Family A and B DNA Polymerases in Cancer: Opportunities for Therapeutic Interventions

DNA polymerases are essential for genome replication, DNA repair and translesion DNA synthesis (TLS). Broadly, these enzymes belong to two groups: replicative and non-replicative DNA polymerases. A considerable body of data suggests that both groups of DNA polymerases are associated with cancer. Many mutations in cancer cells are either the result of error-prone DNA synthesis by non-replicative polymerases, or the inability of replicative DNA polymerases to proofread mismatched nucleotides due to mutations in 3'-5' exonuclease activity. Moreover, non-replicative, TLS-capable DNA polymerases can negatively impact cancer treatment by synthesizing DNA past lesions generated from treatments such as cisplatin, oxaliplatin, etoposide, bleomycin, and radiotherapy. Hence, the inhibition of DNA polymerases in tumor cells has the potential to enhance treatment outcomes. Here, we review the association of DNA polymerases in cancer from the A and B families, which participate in lesion bypass, and conduct gene replication. We also discuss possible therapeutic interventions that could be used to maneuver the role of these enzymes in tumorigenesis.

DNA replication fidelity and cancer

Cancer is fueled by mutations and driven by adaptive selection. Normal cells avoid deleterious mutations by replicating their genomes with extraordinary accuracy. Here we review the pathways governing DNA replication fidelity and discuss evidence implicating replication errors (point mutation instability or PIN) in carcinogenesis.

A mutation in subunit B of the DNA polymerase alpha-primase complex from Novikoff hepatoma cells concomitant with a conformational change and abnormal catalytic properties of the DNA polymerase alpha-primase complex

Mutated constituents of the DNA replication complex might contribute to the mutational load of the genome during tumor development by impairing DNA synthesis as well as cell cycle-related control of DNA replication. To prove or disprove this hypothesis, we looked for mutations in the cDNA sequences of the four subunits of DNA polymerase alpha-primase from both highly malignant Novikoff hepatoma cells and regenerating normal rat liver and compared physicochemical and catalytic properties of the DNA polymerase alpha-primase complexes purified from both sources. Sequence analysis showed two mutations in subunit B from Novikoff cells: one in nucleotide position 855 (CCG-->CCA) that did not result in an amino acid exchange and one in position 862 (GTG-->ATG) that caused a change of valine to methionine in codon 288. No mutation was found in the three other subunits. The wild-type and mutated sequences of subunit B were cloned and expressed in vitro. Sedimentation analysis of the expressed polypeptides revealed different sedimentation constants, indicating that the amino acid exchange affected the conformation of subunit B. The analysis of the purified DNA polymerase alpha-primase complexes showed a sedimentation value that was significantly higher for the enzyme complex from normal liver than for that from Novikoff cells. In addition, DNA polymerase alpha-primase complexes from Novikoff cells showed higher sensitivity to camptothecin, topotecan, and structurally related compounds (such as (R,S)-7-ethyl-10-hydroxy camptothecin, 9-aminocamptothecin, and 10-hydroxycamptothecin) than the enzyme from normal rat liver. Thus, the amino acid change found in subunit B appears to result in a conformational change of the DNA polymerase alpha-primase complex from Novikoff hepatoma cells. Whether this mutation influences genetic instability or tumor development needs to be explored.

Detection of mutations in the DNA polymerase delta gene of human sporadic colorectal cancers and colon cancer cell lines

To test the hypothesis whether DNA polymerases acquire mutator properties during tumor development (mutator hypothesis), we examined DNA polymerase delta mRNA in 6 colon cancer cell lines (DLD-1, HCT116, SW48, HT29, SW480 and SW620) and 7 sporadic human colorectal cancers. For analysis we used amplification of cDNA by polymerase chain reaction, single-strand conformation polymorphism and sequencing techniques. In 5 of the cell lines, 9 mutations leading to changes of the amino acid sequence of DNA polymerase delta were detected. Most mutations were found in the cell lines DLD-1, HCT116 and SW48 for which defects in mismatch repair genes had been identified previously. In the majority of cases, wild type and mutated sequences were present. In 2 cell lines (HCT116 and SW48), a single-nucleotide deletion occurred at the same position. This resulted in a premature termination codon by which the DNA interaction domain of the enzyme was eliminated. Furthermore, sequence deviations were found in the tumor tissues of 4 colon cancer patients. Wild-type and altered sequences were present simultaneously. The deviations included missense mutations (2 cases) and silent mutations (2 cases). The missense mutations and one of the silent mutations were found in normal mucosa as well. In addition, the mutation clustered region of a tumor suppressor gene, often found to be defective in colon cancer, the adenomatous polyposis coli (APC) gene, was investigated in surgical specimens and cell lines. One carcinoma and 2 cell lines exhibited amino acid changes in both the DNA polymerase delta gene and in the mutation clustered region of the APC gene. Since most of the mutations detected in the DNA polymerase delta mRNA are likely to alter the structure of the protein, the enzyme is expected to be functionally impaired. In particular, copying fidelity might be decreased, thus contributing to the high mutation rate observed in colorectal cancer.