Expression of error-prone polymerases in BL2 cells activated for Ig somatic hypermutation

High affinity antibodies are generated in mice and humans by means of somatic hypermutation (SHM) of variable (V) regions of Ig genes. Mutations with rates of 10(-5)--10(-3) per base pair per generation, about 10(6)-fold above normal, are targeted primarily at V-region hot spots by unknown mechanisms. We have measured mRNA expression of DNA polymerases iota, eta, and zeta by using cultured Burkitt's lymphoma (BL)2 cells. These cells exhibit 5-10-fold increases in heavy-chain V-region mutations targeted only predominantly to RGYW (R = A or G, Y = C or T, W = T or A) hot spots if costimulated with T cells and IgM crosslinking, the presumed in vivo requirements for SHM. An approximately 4-fold increase pol iota mRNA occurs within 12 h when cocultured with T cells and surface IgM crosslinking. Induction of pols eta and zeta occur with T cells, IgM crosslinking, or both stimuli. The fidelity of pol iota was measured at RGYW hot- and non-hot-spot sequences situated at nicks, gaps, and double-strand breaks. Pol iota formed T x G mispairs at a frequency of 10(-2), consistent with SHM-generated C to T transitions, with a 3-fold increased error rate in hot- vs. non-hot-spot sequences for the single-nucleotide overhang. The T cell and IgM crosslinking-dependent induction of pol iota at 12 h may indicate an SHM "triggering" event has occurred. However, pols iota, eta, and zeta are present under all conditions, suggesting that their presence is not sufficient to generate mutations because both T cell and IgM stimuli are required for SHM induction.

A new class of errant DNA polymerases provides candidates for somatic hypermutation

The mechanism of somatic hypermutation of the immunoglobulin genes remains a mystery after nearly 30 years of intensive research in the field. While many clues to the process have been discovered in terms of the genetic elements required in the immunoglobulin genes, the key enzymatic players that mediate the introduction of mutations into the variable region are unknown. The recent wave of newly discovered eukaryotic DNA polymerases have given a fresh supply of potential candidates and a renewed vigour in the search for the elusive mutator factor governing affinity maturation. In this paper, we discuss the relevant genetic and biochemical evidence known to date regarding both somatic hypermutation and the new DNA polymerases and address how the two fields can be brought together to identify the strongest candidates for further study. In particular we discuss evidence for the in vitro biochemical misincorporation properties of human Rad30B/Pol iota and how it compares to the in vivo somatic hypermutation spectra.

The expanding polymerase universe

Over the past year, the number of known prokaryotic and eukaryotic DNA polymerases has exploded. Many of these newly discovered enzymes copy aberrant bases in the DNA template over which 'respectable' polymerases fear to tread. The next step is to unravel their functions, which are thought to range from error-prone copying of DNA lesions, somatic hypermutation and avoidance of skin cancer, to restarting stalled replication forks and repairing double-stranded DNA breaks.

Sloppier copier DNA polymerases involved in genome repair

When chromosomal replication is impeded in the presence of DNA damage, members of a newly discovered UmuC/DinB/Rev1/Rad30 superfamily of procaryotic and eucaryotic DNA polymerases catalyze translesion synthesis at blocked replication forks. Although these polymerases share sequence elements essentially unrelated to the standard replication and repair enzymes, some of them (such as the SOS-induced Escherichia coli pol V) catalyze 'error-prone' translesion synthesis leading to large increases in mutation, whereas others (an example being the Xeroderma pigmentosum variant gene product XPV pol eta) carry out aberrant, yet nonmutagenic translesion synthesis. Ongoing studies of these low fidelity polymerases could provide new insights into the mechanism of somatic hypermutation, a key element in the immune response.