IntroductionSomatic hypermutation (SHM) is a specialized process that takes place in germinal center (GC) B cells in response to T celldependent antigen stimulation. 1,2 This process introduces single nucleotide substitutions, with occasional deletions and duplications, primarily into the variable region of the immunoglobulin (IgV) heavy and light chain genes, resulting in the production of high-affinity antibodies and allowing affinity maturation of the humoral immune response. 3 SHM requires active transcription of the target locus but is not IgV sequence specific and does not depend on the V region promoter. [4][5][6][7] In fact, at least 3 non-Ig genes, including BCL6, the FAS/CD95 gene, and the genes encoding the 2 components of the BCR (B29 and mb1) have been shown to acquire somatic mutations during the normal GC reaction, indicating that this mechanism may target more genes than originally suspected. [8][9][10][11][12] The molecular basis of SHM remains largely unknown. However, studies from the past 4 years have identified the AID gene, encoding for activation-induced cytidine deaminase, as an absolute requirement for both SHM and class switch recombination (CSR) in humans and mice. 13,14 Activation-induced cytidine deaminase (AID) expression is also sufficient to initiate both events in fibroblasts expressing transcribed artificial constructs. 15,16 Because of the high homology with the RNA-editing enzyme apolipoprotein B editing catalytic subunit 1 (APOBEC-1), it has been proposed that AID may function as a cytidine deaminase to modify a preexisting mRNA into a new one, possibly encoding an endonuclease. 14,17 However, experimental evidence in Escherichia coli indicated that AID may act directly on DNA and convert deoxycytidines to uracils, which are then processed by uracil-DNA glycosylase (UNG) and endonucleases. 18 AID would thereby lead to the creation of abasic sites, which may be repaired by base excision repair and putative error-prone mechanisms. Indeed, in vitro AID exhibits cytidine deamination activity on single-stranded DNA, with a base specificity similar to that reported for SHM. [19][20][21][22] Recently, the SHM process has been shown to malfunction in about 50% of diffuse large B-cell lymphomas (DLBCLs) as well as in about 20% of AIDS-related non-Hodgkin lymphomas (NHLs) and in a significant fraction of primary central nervous system lymphomas derived from non-HIV patients. [23][24][25] In these tumors, multiple somatic mutations are introduced into the 5Ј region, including coding sequences, of several genes that do not represent physiologic SHM targets. These comprise the well-known protooncogenes PIM1, PAX5, RhoH/TTF, and cMYC, all of which have Two of the authors (R.P. and J.M.) are employed by a company (Cell Signaling Technology, Inc, Beverly, MA) whose potential product was studied in the present work.The online version of this article contains a data supplement.An Inside Blood analysis of this article appears in the front of this issue.Reprints: Laura Pasqualucci, Institute for...

Non-Hodgkin's lymphomas (NHL) form a heterogeneous group of diseases, with diffuse large B-cell lymphoma (DLBCL) comprising the largest subgroup. The commonest chromosomal translocations found in DLBCL are those affecting band 3q27. In 35% of DLBCL cases, as well as in a small fraction of follicular lymphomas, the normal transcriptional regulation of Bcl-6 is disrupted by these chromosomal translocations. In addition, about three-quarters of cases of DLBCL display multiple somatic mutations in the 5' non-coding region of Bcl-6, which occur independently of chromosomal translocations and appear to be due to the IgV-associated somatic hypermutation process. Bcl-6 is a 95-kD nuclear phosphoprotein belonging to the BTB/POZ (bric-a-brac, tramtrack, broad complex/Pox virus zinc finger) zinc finger family of transcription factors. It has been suggested that Bcl-6 is important in the repression of genes involved in the control of lymphocyte activation, differentiation, and apoptosis within the germinal center, and that its down-regulation is necessary for normal B-cells to exit the germinal center. Bcl-6 remains constitutively expressed in a substantial proportion of B-cell lymphomas. Recently, acetylation has been identified as a mode for down-regulating Bcl-6 activity by inhibition of the ability of Bcl-6 to recruit complexes containing histone deacetylases (HDAC). The pharmacologic inhibition of two recently identified deacetylation pathways, HDAC- and silent information regulator (SIR)-2-dependent deacetylation, results in the accumulation of inactive acetylated Bcl-6 and thus in cell cycle arrest and apoptosis in B-cell lymphoma cells. These results reveal a new method of regulating Bcl-6, with the potential for therapeutic exploitation. These studies also indicate a novel mechanism by which acetylation promotes transcription, not only by modifying histones and activating transcriptional activators, but also by inhibiting transcriptional repressors.