*5.2.2.3.3 Frequencies of mutations*

*Normal and Malignant B-cell*

possible:

leading to a very high error rates.

*XRCC2/XRCC3: X-ray cross-complementing gene 2/3.*

∼1–2 kb further downstream [50] (for review, see [51–54]). Hypermutable DNA sequences are found in the regions of IG genes, preferentially in sequences containing mutational "hotspots" corresponding predominantly to RGYW/WRCYmotifs (G: C is a mutable position, **R** = purine bases [A/G], **G** = guanine, **Y** = pyrimidine bases [C/T], **W** = A/T, **C** = cytosine) [55].

• *Mutagenic repair of U:G mismatches*. Uracil (U) generated from cytosine

deamination creates U:G lesions/mismatches that will be repaired erroneously by introducing point mutations or, more rarely, insertions and deletions in V regions at one of 1000 bp per generation. Two possibilities of repair are

a.If the U is targeted by the enzyme uracil-DNA N-glycosylase (UNG), which is usually followed by components of the base-excision repair (BER) pathway (an essential DNA repair pathway), it will be excised, and an abasic site appears in DNA following the action of apurinic/apyrimidinic endonuclease (APE), an enzyme that identifies damaged apurinic/apyrimidinic sites in DNA, cuts the phosphodiester bond in the backbone of the sites, and has critical roles in the base excision pathway. This site will, in turn, be mutagenically replicated by translesion DNA synthesis (TLS) polymerases,

*DNA deamination model of IG gene diversification by AID during SHM (adapted from [51]). AID creates mutations in DNA by deamination of cytosine base, which turns it into uracil, which is then targeted by BER or MMR mechanisms or simply replicated producing mutations. AID: activation-induced deaminase, APE: apyrimidinic/apurinic endonuclease, BER: base excision repair, bp: base pairs, CSR: class switch recombination, ExoI: mismatch repair exonuclease I, IG: immunoglobulin gene, IgV: immunoglobulin variable region, MMR: DNA mismatch repair, MSH2/6: mismatch recognition proteins, NHEJ: nonhomologous end joining, pol: polymerase, pol η: DNA polymerase eta, Rev1: Y-family DNA polymerase involved in DNA damage tolerance, SHM: somatic hypermutation, TLS: translesion DNA synthesis, UNG: uracil-DNA glycosylase,* 

**38**

**Figure 8.**

The frequency of IG region V gene mutations corresponds to approximately one bp change per 1000 bp per gene and per cell division/generation, while that affecting the rest of the cell DNA is much lower, and corresponds to about one bp change per 1010 bp per cell and per division. Of note, there is approximately a 50% chance during each division of B-cells that a mutation leads to a change in BCR, since it is known that each V region is encoded by approximately 360 bp and that approximately ¾ basic changes modify the encoded amino acid [48].
