When
B cells are activated after
immunization or
infection, they exchange the gene encoding the Ig H chain C region by
class switch recombination (CSR). CSR generally occurs by an intrachromosomal deletional recombination within switch (S) region sequences. However, approximately 10% of CSR events occur between
chromosome homologs (trans- or interallele CSR), suggesting that the
homologous chromosomes are aligned during CSR. Because the
Mut S homolog 4 (Msh4) and Msh5 bind to
Holliday junctions and are required for
homologous recombination during
meiosis in
germ cells, we hypothesized these proteins might be involved in trans-chromosomal CSR (trans-CSR). Indeed, Msh4-Msh5 has recently been suggested to have a role in CSR. However, we find a large variety of alternative
splice variants of Msh5
mRNA in
splenic B cells rather than the full-length form found in
testis. Most of these
mRNAs are unlikely to be stable, suggesting that Msh5 might not be functional. Furthermore, we find that msh5 nullizygous
B cells undergo CSR normally, have unaltered levels of trans-CSR, normal levels of
DNA breaks in the Smu region, and normal S-S junctions. We also show that the S-S junctions from cis- and trans-CSR events have similar lengths of junctional microhomology, suggesting trans-CSR occurs by nonhomologous end joining as does intrachromosome (cis)-CSR. From these data, we conclude that Msh5 does not participate in CSR.
