Aberrant somatic hypermutation and expression of activation-induced cytidine deaminase mRNA in mediastinal large B-cell lymphoma

Relationship between BCL2 mutations and follicular lymphoma outcome in the chemoimmunotherapy era

How to identify follicular lymphoma (FL) patients with low disease burden but high risk for early progression is unclear. Building on a prior study demonstrating the early transformation of FLs with high variant allele frequency (VAF) BCL2 mutations at activation-induced cytidine deaminase (AICDA) sites, we examined 11 AICDA mutational targets, including BCL2, BCL6, PAX5, PIM1, RHOH, SOCS, and MYC, in 199 newly diagnosed grade 1 and 2 FLs. BCL2 mutations with VAF ≥20% occurred in 52% of cases. Among 97 FL patients who did not initially receive rituximab-containing therapy, nonsynonymous BCL2 mutations at VAF ≥20% were associated with increased transformation risk (HR 3.01, 95% CI 1.04-8.78, p = 0.043) and a trend toward shorter event-free survival (EFS, median 20 months with mutations versus 54 months without, p = 0.052). Other sequenced genes were less frequently mutated and did not increase the prognostic value of the panel. Across the entire population, nonsynonymous BCL2 mutations at VAF ≥20% were associated with decreased EFS (HR 1.55, 95% CI 1.02-2.35, p = 0.043 after correction for FLIPI and treatment) and decreased overall survival after median 14-year follow-up (HR 1.82, 95% CI 1.05-3.17, p = 0.034). Thus, high VAF nonsynonymous BCL2 mutations remain prognostic even in the chemoimmunotherapy era.

Roles of G4-DNA and G4-RNA in Class Switch Recombination and Additional Regulations in B-Lymphocytes

Mature B cells notably diversify immunoglobulin (Ig) production through class switch recombination (CSR), allowing the junction of distant "switch" (S) regions. CSR is initiated by activation-induced deaminase (AID), which targets cytosines adequately exposed within single-stranded DNA of transcribed targeted S regions, with a specific affinity for WRCY motifs. In mammals, G-rich sequences are additionally present in S regions, forming canonical G-quadruplexes (G4s) DNA structures, which favor CSR. Small molecules interacting with G4-DNA (G4 ligands), proved able to regulate CSR in B lymphocytes, either positively (such as for nucleoside diphosphate kinase isoforms) or negatively (such as for RHPS4). G4-DNA is also implicated in the control of transcription, and due to their impact on both CSR and transcriptional regulation, G4-rich sequences likely play a role in the natural history of B cell malignancies. Since G4-DNA stands at multiple locations in the genome, notably within oncogene promoters, it remains to be clarified how it can more specifically promote legitimate CSR in physiology, rather than pathogenic translocation. The specific regulatory role of G4 structures in transcribed DNA and/or in corresponding transcripts and recombination hereby appears as a major issue for understanding immune responses and lymphomagenesis.

Impact of the microenvironment on the pathogenesis of mucosa-associated lymphoid tissue lymphomas

Approximately 8% of all non-Hodgkin lymphomas are extranodal marginal zone B cell lymphomas of mucosa-associated lymphoid tissue (MALT), also known as MALT lymphomas. These arise at a wide range of different extranodal sites, with most cases affecting the stomach, the lung, the ocular adnexa and the thyroid. The small intestine is involved in a lower percentage of cases. Lymphoma growth in the early stages is associated with long-lasting chronic inflammation provoked by bacterial infections (e.g., Helicobacter pylori or Chlamydia psittaci infections) or autoimmune conditions (e.g., Sjögren’s syndrome or Hashimoto thyroiditis). While these inflammatory processes trigger lymphoma cell proliferation and/or survival, they also shape the microenvironment. Thus, activated immune cells are actively recruited to the lymphoma, resulting in either direct lymphoma cell stimulation via surface receptor interactions and/or indirect lymphoma cell stimulation via secretion of soluble factors like cytokines. In addition, chronic inflammatory conditions cause the acquisition of genetic alterations resulting in autonomous lymphoma cell growth. Recently, novel agents targeting the microenvironment have been developed and clinically tested in MALT lymphomas as well as other lymphoid malignancies. In this review, we aim to describe the composition of the microenvironment of MALT lymphoma, the interaction of activated immune cells with lymphoma cells and novel therapeutic approaches in MALT lymphomas using immunomodulatory and/or microenvironment-targeting agents.

Bimodal expression of RHOH during myelomonocytic differentiation: Implications for the expansion of AML differentiation therapy

RhoH is an unusual member of the Rho family of small GTP‐binding proteins in that it lacks GTPase activity. Since the RhoH protein is constantly bound by GTP, it is constitutively active and controlled predominantly by changes in quantitative expression. Abnormal levels of RHOH gene transcripts have been linked to a range of malignancies including acute myeloid leukemia (AML). One of the hallmarks of AML is a block in the normal program of myeloid differentiation. Here we investigate how myeloid differentiation is controlled by the quantitative expression of RHOH. Our analysis demonstrates that increasingly mature myeloid cells express progressively lower levels of RHOH. However, as monocytic myeloid cells terminally differentiate into macrophages, RHOH expression is up‐regulated. This up‐regulation is not apparent in AML where myeloid differentiation is blocked at stages of low RHOH expression. Nevertheless, when the up‐regulation of RHOH is forced, then terminal macrophage differentiation is induced and the Cdc42 and Wnt intracellular signalling pathways are repressed. These results indicate that RHOH induction is a driver of terminal differentiation and might represent a means of effecting AML differentiation therapy. The potential of this therapeutic strategy is supported by forced up‐regulation of RHOH reducing the ability of AML cells to produce tumours in vivo.

Activation-induced cytidine deaminase: in sickness and in health

Activation Induced cytidine Deaminase (AID) is an essential enzyme of the adaptive immune system. Its canonical activity is restricted to B lymphocytes, playing an essential role in the diversification of antibodies by enhancing specificity and changing affinity. This is possible through its DNA deaminase function, leading to mutations in DNA. In the last decade, AID has been assigned an additional function: that of a powerful DNA demethylator. Adverse cellular conditions such as chronic inflammation can lead to its deregulation and overexpression. It is an important driver of B-cell lymphoma due to its natural ability to modify DNA through deamination, leading to mutations and epigenetic changes. However, the deregulation of AID is not restricted to lymphoid cells. Recent findings have provided new insights into the role that this protein plays in the development of non-lymphoid cancers, with some research shedding light on novel AID-driven mechanisms of cellular transformation. In this review, we provide an updated narrative of the normal physiological functions of AID. Additionally, we review and discuss the recent research studies that have implicated AID in carcinogenesis in varying tissue types including lymphoid and non-lymphoid cancers. We review the mechanisms, whereby AID promotes carcinogenesis and highlight important areas of future research.

Establishment of induced pluripotent stem cells from normal B cells and inducing AID expression in their differentiation into hematopoietic progenitor cells

B cell derived induced pluripotent stem cells (BiPSCs) were recently established from peripheral blood B cells by the simultaneous transfection of Yamanaka factors (Oct3/4, Sox2, Klf4, c-Myc) and C/EBPα using a Sendai virus vector. Here, using a different method, we established BiPSCs with immunoglobulin heavy chain (IgH) gene rearrangement from normal B cells purified from lymph nodes. The critical points of our method are pre-stimulation of B cells with IL-21 and CD40-ligand (CD40L), followed by consecutive transfection of highly concentrated Yamanaka factors using a retroviral vector. Following each transfection the cells were centrifuged onto a retronectin coated plate and the activated by IL-4, IL-2, and CD40L. Furthermore, we established BiPSCs (BiPSC-A) in which activation-induced cytidine deaminase (AID) could be induced using the doxycycline-controlled. Both the parental BiPSC and BiPSC-A showed the capability of differentiating into hematopoietic progenitor cells (HPCs) based on confirmation of CD34 expression and colony-formation from CD34-positive cells. The findings that BiPSC-A can differentiate into HPCs suggest that there is a possibility that induction of AID expression would result in chromosomal translocations in the process of differentiation from BiPSCs, and therefore that these BiPSCs could be useful in elucidating the tumor origin of abnormal B cells in myelomagenesis.

The metabolomic signature of hematologic malignancies

The ongoing accumulation of knowledge raises hopes that understanding tumor metabolism will provide new ways for predicting, diagnosing, and even treating cancers. Some metabolic biomarkers are at present routinely utilized to diagnose cancer and metabolic alterations of tumors are being confirmed as therapeutic targets. The growing utilization of metabolomics in clinical research may rapidly turn it into one of the most potent instruments used to detect and fight tumor. In fact, while the current state and trends of high throughput metabolomics profiling focus on the purpose of discovering biomarkers and hunting for metabolic mechanism, a prospective direction, namely reprogramming metabolomics, highlights the way to use metabolomics approach for the aim of treatment of disease by way of reconstruction of disturbed metabolic pathways. In this review, we present an ample summary of the current clinical appliances of metabolomics in hematological malignancies.

Characterization of memory B cells from thymus and its impact for DLBCL classification

The rare memory B cells in thymus (Thy) are considered the cells of origin for primary mediastinal large B-cell lymphoma. The objectives of the present study were to characterize the normal memory B-cell compartment in Thy and to support its association with primary mediastinal B-cell lymphoma. Seven paired human tissue samples from Thy and sternum bone marrow (BM) were harvested during cardiac surgery. B-cell subsets were phenotyped by Euroflow standard and fluorescence-activated cell sorting for microarray analysis on the Human Exon 1.0 ST Arrays platform. Differentially expressed genes between Thy and BM memory B cells were identified and correlated with the molecular subclasses of diffuse large B-cell lymphoma. Within Thy, 4% (median; range 2%-14%) of the CD45(+) hematopoietic cells were CD19(+) B cells, with a major fraction being CD27(+)/CD38(-) memory B cells (median 80%, range 76%-93%). The BM contained 14% (median; range 3%-27%), of which only a minor fraction (median 5%, range 2%-10%) were memory B cells. Global gene expression analysis of the memory B-cell subsets from the two compartments identified 133 genes upregulated in Thy, including AICDA, REL, STAT1, TNF family, SLAMF1, CD80, and CD86. In addition, exons 4 and 5 in the 3' end of AICDA were more highly expressed in Thy than in BM. The Thy memory B-cell gene profile was overexpressed in primary mediastinal B-cell lymphoma compared with other diffuse large B-cell lymphoma subclasses. The present study describes a Thy memory B-cell subset and its gene profile correlated with primary mediastinal B-cell lymphomas, suggesting origin from Thy memory B cells.

Genomic Alterations in CIITA Are Frequent in Primary Mediastinal Large B Cell Lymphoma and Are Associated with Diminished MHC Class II Expression

Primary mediastinal large B cell lymphoma (PMBCL) is an aggressive non-Hodgkin's lymphoma, predominantly affecting young patients. We analyzed 45 primary PMBCL tumor biopsies and 3 PMBCL-derived cell lines for the presence of genetic alterations involving the major histocompatibility complex (MHC) class II transactivator CIITA and found frequent aberrations consisting of structural genomic rearrangements, missense, nonsense, and frame-shift mutations (53% of primary tumor biopsies and all cell lines). We also detected intron 1 mutations in 47% of the cases, and detailed sequence analysis strongly suggests AID-mediated aberrant somatic hypermutation as the mutational mechanism. Furthermore, we demonstrate that genomic lesions in CIITA result in decreased protein expression and reduction of MHC class II surface expression, creating an immune privilege phenotype in PMBCL. In summary, we establish CIITA alterations as a common mechanism of immune escape through reduction of MHC class II expression in PMBCL, with potential implications for future treatments targeting microenvironment-related biology.

AID-associated DNA repair pathways regulate malignant transformation in a murine model of BCL6-driven diffuse large B-cell lymphoma

Somatic hypermutation and class-switch recombination of the immunoglobulin (Ig) genes occur in germinal center (GC) B cells and are initiated through deamination of cytidine to uracil by activation-induced cytidine deaminase (AID). Resulting uracil-guanine mismatches are processed by uracil DNA glycosylase (UNG)-mediated base-excision repair and MSH2-mediated mismatch repair (MMR) to yield mutations and DNA strand lesions. Although off-target AID activity also contributes to oncogenic point mutations and chromosome translocations associated with GC and post-GC B-cell lymphomas, the role of downstream AID-associated DNA repair pathways in the pathogenesis of lymphoma is unknown. Here, we show that simultaneous deficiency of UNG and MSH2 or MSH2 alone causes genomic instability and a shorter latency to the development of BCL6-driven diffuse large B-cell lymphoma (DLBCL) in a murine model. The additional development of several BCL6-independent malignancies in these mice underscores the critical role of MMR in maintaining general genomic stability. In contrast, absence of UNG alone is highly protective and prevents the development of BCL6-driven DLBCL. We further demonstrate that clonal and nonclonal mutations arise within non-Ig AID target genes in the combined absence of UNG and MSH2 and that DNA strand lesions arise in an UNG-dependent manner but are offset by MSH2. These findings lend insight into a complex interplay whereby potentially deleterious UNG activity and general genomic instability are opposed by the protective influence of MSH2, producing a net protective effect that promotes immune diversification while simultaneously attenuating malignant transformation of GC B cells.

Molecular Pathogenesis of MALT Lymphoma

Approximately 8% of all non-Hodgkin lymphomas are extranodal marginal zone B cell lymphoma of mucosa associated lymphoid tissue (MALT), also known as MALT lymphoma, which was first described in 1983 by Isaacson and Wright. MALT lymphomas arise at a wide range of different extranodal sites, with the highest frequency in the stomach, followed by lung, ocular adnexa, and thyroid, and with a low percentage in the small intestine. Interestingly, at least 3 different, apparently site-specific, chromosomal translocations and missense and frameshift mutations, all pathway-related genes affecting the NF-κB signal, have been implicated in the development and progression of MALT lymphoma. However, these genetic abnormalities alone are not sufficient for malignant transformation. There is now increasing evidence suggesting that the oncogenic product of translocation cooperates with immunological stimulation in oncogenesis, that is, the association with chronic bacterial infection or autoaggressive process. This review mainly discusses MALT lymphomas in terms of their genetic aberration and association with chronic infections and summarizes recent advances in their molecular pathogenesis.

BCL2 mutations are associated with increased risk of transformation and shortened survival in follicular lymphoma

Follicular lymphoma (FL), an indolent neoplasm caused by a t(14;18) chromosomal translocation that juxtaposes the BCL2 gene and immunoglobulin locus, has a variable clinical course and frequently undergoes transformation to an aggressive lymphoma. Although BCL2 mutations have been previously described, their relationship to FL progression remains unclear. In this study, we evaluated the frequency and nature of BCL2 mutations in 2 independent cohorts of grade 1 and 2 FLs, along with the correlation between BCL2 mutations, transformation risk, and survival. The prevalence of BCL2 coding sequence mutations was 12% in FL at diagnosis and 53% at transformation (P < .0001). The presence of these BCL2 mutations at diagnosis correlated with an increased risk of transformation (hazard ratio 3.6; 95% CI, 2.0-6.2; P < .0001) and increased risk of death due to lymphoma (median survival of 9.5 years with BCL2 mutations vs 20.4 years without; P = .012). In a multivariate analysis, BCL2 mutations and high FL international prognostic index were independent risk factors for transformation and death due to lymphoma. Some mutant Bcl-2 proteins exhibited enhanced antiapoptotic capacity in vitro. Accordingly, BCL2 mutations can affect antiapoptotic Bcl-2 function, are associated with increased activation-induced cytidine deaminase expression, and correlate with increased risk of transformation and death due to lymphoma.

AID expression in B-cell lymphomas causes accumulation of genomic uracil and a distinct AID mutational signature

The most common mutations in cancer are C to T transitions, but their origin has remained elusive. Recently, mutational signatures of APOBEC-family cytosine deaminases were identified in many common cancers, suggesting off-target deamination of cytosine to uracil as a common mutagenic mechanism. Here we present evidence from mass spectrometric quantitation of deoxyuridine in DNA that shows significantly higher genomic uracil content in B-cell lymphoma cell lines compared to non-lymphoma cancer cell lines and normal circulating lymphocytes. The genomic uracil levels were highly correlated with AID mRNA and protein expression, but not with expression of other APOBECs. Accordingly, AID knockdown significantly reduced genomic uracil content. B-cells stimulated to express endogenous AID and undergo class switch recombination displayed a several-fold increase in total genomic uracil, indicating that B cells may undergo widespread cytosine deamination after stimulation. In line with this, we found that clustered mutations (kataegis) in lymphoma and chronic lymphocytic leukemia predominantly carry AID-hotspot mutational signatures. Moreover, we observed an inverse correlation of genomic uracil with uracil excision activity and expression of the uracil-DNA glycosylases UNG and SMUG1. In conclusion, AID-induced mutagenic U:G mismatches in DNA may be a fundamental and common cause of mutations in B-cell malignancies.

Activation induced deaminase: how much and where?

Activation induced deaminase (AID) plays a central role in adaptive immunity by initiating the processes of somatic hypermutation (SHM) and class switch recombination (CSR). On the other hand, AID also predisposes to lymphoma and plays a role in some autoimmune diseases, for which reasons AID expression and activity are regulated at various levels. Post-translational mechanisms regulating the amount and subcellular localization of AID are prominent in balancing AID physiological and pathological functions in B cells. Mechanisms regulating AID protein levels include stabilizing chaperones in the cytoplasm and proteins efficiently targeting AID to the proteasome within the nucleus. Nuclear export and cytoplasmic retention contribute to limit the amount of AID accessing the genome. Additionally, a number of factors have been implicated in AID active nuclear import. We review these intertwined mechanisms proposing two scenarios in which they could interact as a network or as a cycle for defining the optimal amount of AID protein. We also comparatively review the expression levels of AID necessary for its function during the immune response, present in different cancers as well as in those tissues in which AID has been implicated in epigenetic remodeling of the genome by demethylating DNA.

Expression of activation-induced cytidine deaminase in malignant lymphomas infiltrating the bone marrow

Somatic hypermutation of immunoglobulin genes and class switch recombination are pivotal processes in the germinal center (GC) reaction and have been implicated in the development of malignant B-cell lymphoma. Both processes require the enzyme activation-induced cytidine deaminase (AID). Expression of AID is largely restricted to GC B cells and B cells that undergo class switch recombination outside the GC. AID is also expressed in many B-cell lymphomas. This study investigates the expression of AID of malignant lymphomas infiltrating the bone marrow. Bone marrow trephines (n=130) with infiltration of Hodgkin lymphoma and non-Hodgkin lymphoma of B cell and T-cell type and trephines with reactive lymphoid follicles (n=16) were analyzed immunohistochemically for AID protein. AID is expressed in bone marrow infiltrates of malignant lymphomas. AID was generally detected in lymphomas of GC origin. Tumor cells of hairy cell leukemia are mostly AID. There is apparently no different expression of AID found in bone marrow infiltrates of malignant lymphomas compared with a control group with nodal malignant lymphoma infiltrates (n=105). These results suggest that the expression pattern of AID in lymphoma infiltrates in the bone marrow reflects that of extramedullary lymphoma infiltrates.

Aberrant somatic hypermutations in thyroid lymphomas

To determine a possible role of aberrant somatic hypermutation (ASHM) in the pathogenesis of thyroid lymphoma (TL), mutational status of genes affected by ASHM, including c-MYC, PIM-1, PAX-5 and RhoH/TTF, was analyzed. Tumor specimens from 33 patients with thyroid B-cell lymphoma and 14 with chronic lymphocytic thyroiditis (CLTH), an autoimmune thyroiditis known to provide a basis for TL development, was examined. Mutations of at least one of these genes was detected in 16 of 33 (48.5%) patients with TL and in 2 of 14 (14.3%) CLTH. Occurrence of ASHM in PIM-1, RhoH/TTF, and c-MYC was a constant finding in follicular lymphoma (FL) (all of 11 cases) but not so frequent in diffuse large B-cell lymphoma (DLBCL) (4 (33.3%) of 12 cases) and Marginal zone B-cell lymphoma (MZBCL) (1 (10.0%) of 10 cases). ASHM activity is ongoing in most of FL and DLBCL because intraclonal variants were found. FL was also unique in its lower expression level of activation-induced cytidine deaminase, a main player in DNA-modifying processes during SHM, compare to DLBCL and MZBCL.

The small GTPase RhoH is an atypical regulator of haematopoietic cells

Rho GTPases are a distinct subfamily of the superfamily of Ras GTPases. The best-characterised members are RhoA, Rac and Cdc42 that regulate many diverse actions such as actin cytoskeleton reorganisation, adhesion, motility as well as cell proliferation, differentiation and gene transcription. Among the 20 members of that family, only Rac2 and RhoH show an expression restricted to the haematopoietic lineage.

RhoH was first discovered in 1995 as a fusion transcript with the transcriptional repressor LAZ3/BCL6. It was therefore initially named translation three four (TTF) but later on renamed RhoH due to its close relationship to the Ras/Rho family of GTPases. Since then, RhoH has been implicated in human cancer as the gene is subject to somatic hypermutation and by the detection of RHOH as a translocation partner for LAZ3/BCL6 or other genes in human lymphomas. Underexpression of RhoH is found in hairy cell leukaemia and acute myeloid leukaemia.

Some of the amino acids that are crucial for GTPase activity are mutated in RhoH so that the protein is a GTPase-deficient, so-called atypical Rho GTPase. Therefore other mechanisms of regulating RhoH activity have been described. These include regulation at the mRNA level and tyrosine phosphorylation of the protein's unique ITAM-like motif. The C-terminal CaaX box of RhoH is mainly a target for farnesyl-transferase but can also be modified by geranylgeranyl-transferase. Isoprenylation of RhoH and changes in subcellular localisation may be an additional factor to fine-tune signalling.

Little is currently known about its signalling, regulation or interaction partners. Recent studies have shown that RhoH negatively influences the proliferation and homing of murine haematopoietic progenitor cells, presumably by acting as an antagonist for Rac1. In leukocytes, RhoH is needed to keep the cells in a resting, non-adhesive state, but the exact mechanism has yet to be elucidated. RhoH has also been implicated as a regulatory molecule in the NFκB, PI3 kinase and Map kinase pathways. The recent generation of RhoH knockout mice showed a defect in thymocyte selection and TCR signalling of thymic and peripheral T-cells. However, RhoH-deficient mice did not develop lymphomas or showed obvious defects in haematopoiesis.

Primary cutaneous marginal zone B-cell lymphomas are targeted by aberrant somatic hypermutation

A mechanism inducing genetic instability, termed aberrant somatic hypermutation (ASHM), has been described in diffuse large B-cell lymphoma. To further investigate whether ASHM also occurs in primary cutaneous marginal zone B-cell lymphoma (PCMZL), we studied the mutational profile of PAX5, RhoH/TTF, cMYC, and PIM1 in 11 PCMZLs. A total of 17 sequence variants were found in 8 of 11 lymphomas cases (72.7%), and they displayed the molecular features typical for the ASHM. Further, two mutations, one mutation in PIM1 and one in cMYC, led to amino-acid substitution with potential functional consequences. These data indicate that ASHM is associated with PCMZLs. By mutating regulatory and coding sequences of the targeted genes, ASHM may represent a major contributor to their pathogenesis.

Quantifying the role of aberrant somatic hypermutation in transformation of follicular lymphoma

Somatic hypermutation (SHM) aberrantly targets proto-oncogenes in various non-Hodgkin's lymphoma. To test the association of SHM with transformation of follicular lymphoma (FL), we sequenced mutational hot spots in five proto-oncogenes (BCL6, PAX5, RHOH, MYC and PIM1) in 32 low-grade FL (lgFL) with follicular histology and 26 transformed FL (tFL) with diffuse large cell histology. No difference was detected in the fraction of specimens mutated (75% of lgFL and 77% of tFL) or in the mutation load (0.08 for lgFL vs. 0.06 mutations/100 bp/allele for tFL). Serial specimens were examined from 25 patients showing stable low-grade FL (slgFL; n=6) or a low-grade FL that later transformed into diffuse large cell lymphoma (tFL; n=19). slgFL and tFL patients accumulated similar numbers of mutations in the interval between biopsies. These data indicate that mutations attributable to aberrant SHM occur with similar frequency in low-grade and transformed FL; transformation is not associated with a higher rate of aberrant SHM. Moreover, the frequency of mutations attributable to aberrant SHM in tFL was significantly lower than that reported for de novo diffuse large B cell lymphoma, suggesting differing oncogenic mechanisms in transformed follicular lymphoma and de novo diffuse large B cell lymphoma.

Target Sequence Accessibility Limits Activation-Induced Cytidine Deaminase Activity in Primary Mediastinal B-Cell Lymphoma

Activation-induced cytidine deaminase (AID) initiates somatic hypermutation (SHM) and class switch recombination (CSR) in activated B lymphocytes and is potentially implicated in genomic instability of B-cell malignancies. For unknown reasons, B-cell neoplasms often lack SHM and CSR in spite of high AID expression. Here, we show that primary mediastinal B-cell lymphoma (PMBL), an immunoglobulin (Ig)-negative lymphoma that possesses hypermutated, class-switched Ig genes, expresses high levels of AID with an intact primary structure but does not do CSR in 14 of 16 cases analyzed. Absence of CSR coincided with low Ig germ-line transcription, whereas high level germ-line transcription was observed only in those two cases with active CSR. Interleukin-4/CD40L costimulation induced CSR and a marked up-regulation of germ-line transcription in the PMBL-derived cell line MedB-1. In the PMBL cell line Karpas 1106P, CSR was not inducible and germ-line transcription remained low on stimulation. However, Karpas 1106P, but not MedB-1, had ongoing SHM of the Ig gene and BCL6. These genes were transcribed in Karpas 1106P, whereas transcription was undetectable or low in MedB-1 cells. Thus, accessibility of the target sequences seems to be a major limiting factor for AID-dependent somatic gene diversification in PMBL.

Richter's and prolymphocytic transformation of chronic lymphocytic leukemia are associated with high mRNA expression of activation-induced cytidine deaminase and aberrant somatic hypermutation

Chronic lymphocytic leukemia (CLL) is an indolent B-cell non-Hodgkin's lymphoma that may transform into higher-grade lymphoma. The transformation involves an increased number of prolymphocytic cells, termed prolymphocytic transformation (PLT) or the development of diffuse large B-cell lymphoma (DLBL), also referred to as Richter's transformation (RT). To analyze whether activation-induced cytidine deaminase (AID), which is essential for somatic hypermutation (SHM) of normal B-cells, and malfunction of SHM termed aberrant somatic hypermutation (ASHM) are associated with higher-grade transformation of CLL, AID mRNA expression and the mutation pattern of c-MYC, PAX-5 and RhoH genes were analyzed in eight cases of CLL without transformation and in 21 cases that showed RT or PLT. Chronic lymphocytic leukemia cases, which showed no transformation or eventually transformed into higher-grade lymphoma, showed low levels of AID mRNA expression and low frequency of mutations of c-MYC, PAX-5 and RhoH genes. In both RT and PLT, high-levels of AID mRNA expression and high-frequency mutations of c-MYC, PAX-5 and RhoH genes were detected. These results indicate that AID expression and ASHM are associated with higher-grade transformation of CLL and provide further evidences that AID expression and ASHM may be activated during the clonal history of B-cell lymphomas.