However, the rate of AID-induced mutations in genes likec-mycis reduced by error free repair by a yet to be decided UNG-dependent pathway resulting in mutation rates that are difficult to detect (Liu et al

However, the rate of AID-induced mutations in genes likec-mycis reduced by error free repair by a yet to be decided UNG-dependent pathway resulting in mutation rates that are difficult to detect (Liu et al., 2008). non-Iggenes which are direct targets of AID: this group includesmiR-142, a previously unknown micro-RNA target which is usually translocated in human B cell malignancy. We conclude that AID produces DSBs throughout the genome, which can Vitexin lead to lymphoma associated chromosome translocations in mature B cells. == Introduction == Chromosome translocations are characteristic features of Vitexin several different forms of B cell cancers in mice and humans, including lymphomas and myelomas (Kuppers, 2005;Kuppers and Dalla-Favera, 2001;Potter, 2003). These karyotypic abnormalities are etiologically important in malignancy because they either deregulate oncogene expression, or create novel oncogenes by bringing together disparate transcription units (Rabbitts, 2009). Chromosome translocations are believed to be particularly frequent in mature B lymphocytes because, in addition to V(D)J rearrangements early in development, these cells undergo two forms of programmed DNA damage in their immunoglobulin (Ig) loci during immune responses: somatic hypermutation and class switch recombination (Peled et al., 2008;Stavnezer et al., 2008;Teng and Papavasiliou, 2007). Somatic hypermutation introduces non-templated point mutations in antibody variable genes and class switch recombination replaces one constant region for another by a deletional recombination CLG4B reaction (Peled et al., 2008;Stavnezer et al., 2008;Teng and Papavasiliou, 2007). Although the two reactions are mechanistically distinct, both are initiated by a single enzyme, AID, which is believed to act by deaminating cytosine to produce U:G mismatches in target DNA (Di Noia and Neuberger, 2007;Honjo et al., 2002;Muramatsu et al., 2000). Diverse and overlapping DNA repair pathways process these lesions to produce somatic mutations, or DNA double-strand breaks (DSB), which are obligate intermediates in the class switch reaction (Peled et al., 2008). These same DSBs can also become substrates for chromosome translocations (Jankovic et al., 2007;Ramiro et al., 2007;Ramiro et al., 2006b). However, a single DSB atIgalone is not sufficient for translocation (Robbiani Vitexin et al., 2008). To account for the DSBs in non-Iggenes it has been proposed that this enzymes that create the breaks inIgloci, RAG1/2 and AID, can also cause damage in non-Iggenes (Lieber et al., 2008;Tsai et al., 2008). Consistent with this idea we found that the oncogenec-mycis a target of AID, although the frequency of DSB formation atc-mycis significantly lower than atIgH(Robbiani et al., 2008). Burkitts lymphoma, diffuse large B cell lymphoma, and multiple myeloma represent different types of mature B cells that carry clonal translocations and may have expressed AID (Kuppers, 2005;Shaffer et al., 2002). A role for AID in inducing translocations in these cancers has been suggested by studies of activated B cellsin vitroand plasmacytosis in IL-6 transgenic and pristane-treated mice, all of which develop AID dependentc-myc/IgHtranslocations (Dorsett et al., 2007;Ramiro et al., 2006a;Ramiro et al., 2004;Unniraman et al., 2004). In addition, AID expression accelerates the rate of tumor development inBcl6transgenic mice (Pasqualucci et al., 2008). Finally, transgenic AID produces mutations inc-mycand T cell receptor genes as well as epithelial and T cell malignancies (Okazaki et al., 2003). Curiously, however, deregulated AID does not cause malignancy or translocation-associated cancer in B cells (Muto et al., 2006;Okazaki et al., 2003;Shen et al., 2008). In addition, despite its obligate role inc-myc/IgHtranslocation, AID is not required for the development of plasmacytosis or plasmacytoma in IL-6 transgenic or pristane-treated mice, respectively (Kovalchuk et al., 2007;Ramiro et al., 2004). Finally, most human B cell lymphoma associated translocations do not involvec-mycand many do not involveIggenes (Kuppers, 2005). Thus, the issue of whether AID can induce a sufficiently varied genomic damage to account forIgand non-Iggene translocation associated B lymphocyte malignancyin vivoremains to be determined. == Results == == Increased switching, mutation andc-myc/IgHtranslocation in AID transgenic mice == To determine whether deregulated AID expression can destabilize the genome and induce B cell malignancyin vivo, we produced transgenic mice that express AID under the control ofIgregulatory elements (IgAID,Physique 1A). AID is expressed at low levels in resting B cells inIgAID mice, and increases in activated and germinal center B cells (Physique 1B and 1CandSupplemental Physique S1). Nevertheless, with the exception of the expected decrease in Vitexin germinal center cells (Physique 1Dand (Dorsett et al., 2008;Muramatsu et al., 2000)), B cell development and distribution is usually normal inIgAID mice and serum Ig isotypes are not increased (Supplemental Figures S2 and S3). Consistent with the increase in AID expression in stimulated B cellsin vitro,IgAID B cells switched at higher frequency to IgG1 and IgG3 (Physique 1E and 1F). Thus, Vitexin although deregulated AID expression does not significantly alter B cell developmentin vivo,it does lead to increased levels of class switch recombinationin vitro. This effect is consistent with the observation that AID concentrations are limiting for this reaction (Dorsett et al., 2008;McBride et al., 2006;Takizawa et al., 2008). == Physique 1. Increased class switching inIgAID B cells. == (A).