Expression of Specialized Error-Prone DNA Polymerases in Chronic Lymphocytic Leukemia (CLL).

Specialized DNA polymerases are low fidelity enzymes required to bypass DNA lesions that are also involved in somatic hypermutation. Deregulation of these enzymes can lead both to an increased mutation rate and to an enhanced translesion synthesis capability. DNA polymerase Kappa (DNA pol Kappa) has been found to promote tumorigenesis and to be overexpressed in some cancers. Expression levels of specialized DNA polymerases Iota, Eta, Zeta and Mu were analyzed in primary B-cells from 12 CLL patients (6 with IgV_H mutated genes [M] and 6 with unmutated [UM] genes). Expression of DNA pol Kappa was also assessed in 27 CLL samples (13 M and 14 UM), in B-cell lines derived from pre- and post-germinal center malignancies (NALM-6 [preB-acute lymphoblastic leukemia], JVM-2 [prolymphocytic leukemia], Granta, NCEB and REC [mantle-cell lymphoma] and Daudi [Burkitt’s lymphoma] and in normal B-cells from peripheral blood and tonsil. Expression of DNA polymerases was measured by Quantitative RT-PCR using Ramos (a constitutively hypermutating Burkitt’s lymphoma cell line) as a reference, considering an arbitrary unit of 1 for each gene. Although somatic hypermutation is absent in UM CLL cells, there were no differences in the expression of the DNA polymerases analyzed according to the mutational status. Expression of DNA pols Iota and Zeta was similar in CLL cells and Ramos (mean value: 0.95 and 1.3, respectively), whereas expression of DNA pol Mu was slightly higher (mean 3.24) and DNA pol Eta was underexpressed (mean 0.19). In contrast, among all the hematological malignancies analyzed, significant overexpression of DNA pol Kappa was only found in CLL samples (mean value; 8.43;range: 1.42–19.47). Induction of AID expression by stimulating CLL cells with CD40 ligand and IL-4 did not modulate DNA pol Kappa expression. Finally, expression of DNA pol Kappa in normal B cells from peripheral blood was similar to that in Ramos (0.87) and slightly higher in B cells from tonsil (2.02). In conclusion, no differences in DNA polymerases were observed between M and UM CLL. As previously described in some solid tumors, CLL cells showed an increased expression of DNA pol Kappa, regardless the mutational status of the IgV_H genes, suggesting that the expression of this polymerase is not related to the process of somatic hypermutation that occurs in some CLL cells. Taken together, these results support the concept that DNA pol Kappa may behave as an oncogene in CLL and that its regulation may be crucial for maintaining genomic stability.