B cells are lymphocytes that have several important roles in the adaptive immune system. When activated by the presence of "foreign" molecules, they can synthesise antibodies; present peptides on their surface for recognition by T cells; and eventually become "memory cells" that are specific to the antigen encountered.

If mutations are allowed to accumulate in B cells they can be transformed into the rapidly proliferating cells of B-cell lymphoma or chronic lymphocytic leukaemia. The series of oncogenic transformations that leads to carcinogenesis arises following the same immune stimulation that under normal circumstances simply activates the B cells.

Resting B cells are in a state of quiescence, arrested until immunostimulation in the G0 phase of the cell cycle, and these cells are known to be tolerant of DNA damage because they down-regulate the "guardian of the genome", p53.

Another mechanism that is very important for maintaining genome integrity, the nuclear excision repair (NER) pathway, is known to be down-regulated in terminally differentiated cells, such as neurons, that are permanently arrested in G0 and cannot re-enter the cell cycle.

Thierry Nouspikel and his colleagues at the Institute of Cancer Studies, University of Sheffield, UK set out to investigate whether this pathway is also deficient in quiescent B cells that are only temporarily arrested in G0.

Nouspikel and co-workers first isolated human B lymphocytes obtained from donors, purified and classified them, finding the majority to be naïve mature B cells with a minor component of memory cells; all cells were as expected in the G0 phase of the cell cycle. Lymphoblasts used as control cells had a standard cell cycle profile.

The researchers then stimulated the B cells to proliferate using two separate antigen-specific signals, and tested the effect of ultra-violet radiation, which is known to cause DNA lesions that are repaired by the NER pathway, on that proliferation. Repair of ultra-violet light induced lesions was found to be attenuated across the genome, but only in the quiescent cells.

The researchers then investigated whether genes that are active in quiescent cells are repaired proficiently by this pathway. Using the constitutively expressed gene DHFR (dihydrofolate reductase) as an example, they found repair of UV-damaged DHFR genes to be normal in these cells. In contrast, DNA damage was found to accumulate in non-transcribed genes. As this repair phenotype matches that observed in terminally differentiated cells, Nouspikel and co-workers then investigated whether the same mechanism could be involved.

Loss of the NER pathway in terminally differentiated cells is known to be associated with incomplete phosphorylation of the enzyme Ube1, which activates ubiquitin; exposure of quiescent B cells to an Ube1 inhibitor did not reduce NER repair further, indicating that the same mechanism does apply in these cells. Furthermore, when B cells exposed to UV radiation while quiescent re-entered the cell cycle after immune stimulation, a number of genes – including BCL6 and Cyclin D6, which are involved in cell replication and differentiation – were found to have accumulated mutations.

Taken together, these results show that there is a strong, mechanistic link between the attenuation of the NER DNA repair pathway when B cells are in the quiescent phase and mutagenesis when those cells are stimulated. Mutations accumulate in quiescent B cells after exposure to carcinogens such as UV radiation, cigarette smoke and atmospheric pollution and this may drive the development of lymphomas and other B cell malignancies once those B cells have been stimulated by antigens.

Reference: Hyka-Nouspikel, N., Lemonidis, K., Lu, W-T., and Nouspikel, T. (2011). Circulating human B lymphocytes are deficient in nucleotide excision repair and accumulate mutations upon proliferation. Blood 117, 6277-6286