VH Gene Analysis of IgM-Secreting Myeloma Indicates an Origin From a Memory Cell Undergoing Isotype Switch Events

AID in non-Hodgkin B-cell lymphomas: The consequences of on- and off-target activity

Activation induced cytidine deaminase (AID) is a key element of the adaptive immune system, required for immunoglobulin isotype switching and affinity maturation of B-cells as they undergo the germinal center (GC) reaction in peripheral lymphoid tissue. The inherent DNA damaging activity of this enzyme can also have off-target effects in B-cells, producing lymphomagenic chromosomal translocations that are characteristic features of various classes of non-Hodgkin B-cell lymphoma (B-NHL), and generating oncogenic mutations, so-called aberrant somatic hypermutation (aSHM). Additionally, AID has been found to affect gene expression through demethylation as well as altered interactions between gene regulatory elements. These changes have been most thoroughly studied in B-NHL arising from GC B-cells. Here, we describe the most common classes of GC-derived B-NHL and explore the consequences of on- and off-target AID activity in B and plasma cell neoplasms. The relationships between AID expression, including effects of infection and other exposures/agents, mutagenic activity and lymphoma biology are also discussed.

Transgenic mouse model of IgM+ lymphoproliferative disease mimicking Waldenström macroglobulinemia

Waldenström macroglobulinemia (WM) is a low-grade incurable immunoglobulin M⁺ (IgM⁺) lymphoplasmacytic lymphoma for which a genetically engineered mouse model of de novo tumor development is lacking. On the basis of evidence that the pro-inflammatory cytokine, interleukin 6 (IL6), and the survival-enhancing oncoprotein, B cell leukemia 2 (BCL2), have critical roles in the natural history of WM, we hypothesized that the enforced expression of IL6 and BCL2 in mice unable to perform immunoglobulin class switch recombination may result in a lymphoproliferative disease that mimics WM. To evaluate this possibility, we generated compound transgenic BALB/c mice that harbored the human BCL2 and IL6 transgenes, EμSV-BCL2-22 and H2-L^d-hIL6, on the genetic background of activation-induced cytidine deaminase (AID) deficiency. We designated these mice BCL2⁺IL6⁺AID^- and found that they developed-with full genetic penetrance (100% incidence) and suitably short latency (93 days median survival)-a severe IgM⁺ lymphoproliferative disorder that recapitulated important features of human WM. However, the BCL2⁺IL6⁺AID^- model also exhibited shortcomings, such as low serum IgM levels and histopathological changes not seen in patients with WM, collectively indicating that further refinements of the model are required to achieve better correlations with disease characteristics of WM.

Waldenström macroglobulinemia: clinical and immunological aspects, natural history, cell of origin, and emerging mouse models

Waldenström macroglobulinemia (WM) is a rare and currently incurable neoplasm of IgM-expressing B-lymphocytes that is characterized by the occurrence of a monoclonal IgM (mIgM) paraprotein in blood serum and the infiltration of the hematopoietic bone marrow with malignant lymphoplasmacytic cells. The symptoms of patients with WM can be attributed to the extent and tissue sites of tumor cell infiltration and the magnitude and immunological specificity of the paraprotein. WM presents fascinating clues on neoplastic B-cell development, including the recent discovery of a specific gain-of-function mutation in the MYD88 adapter protein. This not only provides an intriguing link to new findings that natural effector IgM⁺IgD⁺ memory B-cells are dependent on MYD88 signaling, but also supports the hypothesis that WM derives from primitive, innate-like B-cells, such as marginal zone and B1 B-cells. Following a brief review of the clinical aspects and natural history of WM, this review discusses the thorny issue of WM's cell of origin in greater depth. Also included are emerging, genetically engineered mouse models of human WM that may enhance our understanding of the biologic and genetic underpinnings of the disease and facilitate the design and testing of new approaches to treat and prevent WM more effectively.

Multiple myeloma: biology of the disease

Multiple myeloma (MM) is a clonal B-cell malignancy characterized by the aberrant expansion of plasma cells within the bone marrow, as well as at extramedullary sites. Decades of scientific research are now beginning to unravel the intricate biology that underlies the pathophysiology of MM. In particular, the roles of cellular differentiation, molecular pathogenesis, and oncogenes involved in the natural history of MM are becoming clearer. This has enabled the identification of specific cytokines, adhesion molecules, and stromal cells that affect MM cell development, disease progression, and treatment responses. This review describes our current understanding regarding the biology of MM, and how this has led to a robust pipeline of novel therapeutic agents with the potential to overcome resistance to existing MM therapies and, therefore, further improve outcomes in patients with MM.

Anaplastic plasmacytoma of mouse--establishing parallels between subtypes of mouse and human plasma cell neoplasia

Mouse models may provide an important tool for basic and applied research on human diseases. An ideal tumour model should replicate the phenotypic and molecular characteristics of human malignancy as well as the typical physiological effects and dissemination patterns. The histopathological and molecular genetic characterization of anaplastic plasmacytoma (APCT) in strain NSF.V(+) mice provides an example to achieve this goal for a specific lymphoma subtype. Firstly, it demonstrates that, like plasma-cell neoplasms in humans, those in mice occur as distinct subtypes. Secondly, it shows that mouse APCT exhibits striking parallels to possible human tumour counterparts for which good mouse models of de novo tumour development are sorely needed: IgM(+) multiple myeloma and Waldenström's macroglobulinaemia. Thirdly, it strongly suggests that insertional somatic mutagenesis, by either a murine leukaemia virus or an oncogenic transposon, would be an effective experimental approach to accelerating malignant transformation of mature B cells and plasma cells in mice and, thereby, tagging and uncovering cancer driver genes that may be of great relevance for the tumour initiation and progression in lymphoma.

Anaplastic plasmacytomas: relationships to normal memory B cells and plasma cell neoplasms of immunodeficient and autoimmune mice

Anaplastic plasmacytomas (APCTs) from NFS.V(+) congenic mice and pristane-induced plasmacytic PCTs from BALB/c mice were previously shown to be histologically and molecularly distinct subsets of plasma cell neoplasms (PCNs). Here we extended these comparisons, contrasting primary APCTs and PCTs by gene expression profiling in relation to the expression profiles of normal naïve, germinal centre, and memory B cells and plasma cells. We also sequenced immunoglobulin genes from APCT and APCT-derived cell lines and defined surface phenotypes and chromosomal features of the cell lines by flow cytometry and by spectral karyotyping and fluorescence in situ hybridization. The results indicate that APCTs share many features with normal memory cells and the plasma cell-related neoplasms (PLs) of FASL-deficient mice, suggesting that APCTs and PLs are related and that both derive from memory B cells. Published in 2010 by John Wiley & Sons, Ltd.

The search for multiple myeloma stem cells: the long and winding road

Recent years have brought significant breakthroughs in the understanding of tumor biology, related to discovery of cancer stem cells (CSCs) in acute myelogenous leukemia as well as in a number of solid tumors. This finding revealed that not all tumor cells are able to divide indefinitely, and that the bulk of tumor cells are expanded because of divisions and differentiation of CSC fraction. Although the CSCs identified in acute leukemia have a phenotype of early hematopoietic progenitors, it seems that CSCs in multiple myeloma (MM) may resemble the memory B cell fraction. Previous studies in patients with MM have documented the existence of cells without plasma cell characteristics expressing MM-type immunoglobulin genes--so-called "clonotypic" B cells. These cells have been characterized functionally and phenotypically as chemoresistant recirculating B cells. They have been found to self-renew and to be capable of initiating MM growth in immunocompromised animals. Controversy exists as to whether these cells truly belong to an MM clone, however; they may represent only the remaining clones of premalignant B cells. The identification of MM stem cells responsible for the recurrence of MM is of primary importance in designing targeted therapies to definitely cure this disease. This article summarizes the current state of knowledge on these hypothetical "MM stem cells."

Immunoglobulin gene rearrangements and the pathogenesis of multiple myeloma

The ability to rearrange the germ-line DNA to generate antibody diversity is an essential prerequisite for the production of a functional repertoire. While this is essential to prevent infections, it also represents the “Achilles heal” of the B-cell lineage, occasionally leading to malignant transformation of these cells by translocation of protooncogenes into the immunoglobulin (Ig) loci. However, in evolutionary terms this is a small price to pay for a functional immune system. The study of the configuration and rearrangements of the Ig gene loci has contributed extensively to our understanding of the natural history of development of myeloma. In addition to this, the analysis of Ig gene rearrangements in B-cell neoplasms provides information about the clonal origin of the disease, prognosis, as well as providing a clinical useful tool for clonality detection and minimal residual disease monitoring. Herein, we review the data currently available on both Ig gene rearrangements and protein patterns seen in myeloma with the aim of illustrating how this knowledge has contributed to our understanding of the pathobiology of myeloma.

Cytogenetic aberrations in neuropathy associated with IgM monoclonal gammopathy

The occurrence and nature of cytogenetic aberrations in polyneuropathy associated with IgM monoclonal gammopathy was determined. Therefore, interphase fluorescence in situ hybridization (FISH) was applied in 22 patients with polyneuropathy associated with IgM monoclonal gammopathy, multiplex ligation-dependent probe amplification (MLPA) assay in 18 of these patients and genome-wide-array-based comparative genomic hybridization (CGH) in eight of these 18 patients. Four patients had 10-20% and one patient had 30% B cells with IgH rearrangements; one patient had additional loss of 14qter; one patient had amplification of 6p and loss of 6q. Cytogenetic aberrations may be found in one third of the patients with neuropathy associated with IgM monoclonal gammopathy and are mainly associated with indolent Waldenstrom's Macroglobulinemia.

Novel Analysis of Clonal Diversification in Blood B Cell and Bone Marrow Plasma Cell Clones in Immunoglobulin Light Chain Amyloidosis

Immunoglobulin light chain amyloidosis (AL) is characterized by a limited clonal expansion of plasma cells and amyloid formation. Here, we report restriction in the diversity of VL gene usage with a dominance of clonally related B cells in the peripheral blood (PB) isotype-specific repertoire of AL patients. A rigorous quantification of lineage trees reveals presence of intraclonal variations in the PB clones compared to the bone marrow (BM) clones, which suggests a common precursor that is still subject to somatic mutation. When compared to normal BM and PB B cells, AL clones showed significant but incomplete impairment of antigenic selection, which could not be detected by conventional R and S mutation analysis. Therefore, graphical analysis of B cell lineage trees and mathematical quantification of tree properties provide novel insights into the process of B cell clonal evolution in AL.

Functional class switch recombination may occur ‘in vivo’ in Waldenström macroglobulinaemia

Waldenström macroglobulinaemia (WM) malignant cells have been considered incapable of undergoing class switch recombination (CSR). However, we report a WM patient who developed an IgG M-component 4 years after diagnosis. When the second monoclonal component appeared, reverse transcription-polymerase chain reaction showed the presence of pre (Cmu) and postswitch (Cgamma) clonotypic isotypes; sequencing of these isotypes demonstrated that both corresponded to the single clone amplified at diagnosis, including the same complementarity-determining region 3 and somatic mutation pattern. This proves that WM cells can undergo a functional in vivo CSR.

Unusual evolution of Waldenstrom's macroglobulinemia into osteolytic myeloma

We report the unusual transformation of a case of Waldenström's macroglobulinemia (WM) into IgM multiple myeloma (MM). The initial clinical and biological presentation of the disease was typical smouldering WM, with lymphocytic infiltration of the bone marrow. Five years later, signs of transformation appeared: the patient presented with diffuse osteolytic bone lesions without organomegaly, and the bone marrow was infiltrated with characteristic malignant plasma cells. Electron microscopy (EM) examination showed that the endoplasmic reticulum (ER) of the dysmorphic plasma cells contained monoclonal IgM. Immunolabeling for calreticulin, a resident protein of the ER, demonstrated unequivocally that the characteristic intranuclear inclusions were indeed part of ER. Flow cytometry revealed an MM profile for the cellular proliferation. Molecular biology performed on the final marrow could only retrieve a single cellular clone. In conclusion, this is the first documented description of the transformation of typical WM into an aggressive form of MM.

Immunoglobulin gene analysis in polyneuropathy associated with IgM monoclonal gammopathy

Antineural antibody activity is the implicated pathogenic mechanism in polyneuropathy associated with monoclonal gammopathy. Recognition of antigen depends on immunoglobulin variable regions, encoded by V genes. We studied V(H)DJ(H) and V(L)J(L) gene use in monoclonal B cells by clonal analysis in 20 patients with polyneuropathy and IgM monoclonal gammopathy. V genes associated with bacterial responses appear over-represented and V(H)3-23 was preferentially used, without association with specific D, J(H) or V(L)J(L). V genes revealed somatic mutation and intraclonal variation was found in 9 of 20 patients. Polyneuropathy associated with monoclonal gammopathy may be caused by an immune response to bacterial antigens, which recruit somatically mutated autoreactive B cells.

Immunoglobulin M Myeloma: Evaluation of Molecular Features and Cytokine Expression

Immunoglobulin (Ig) M myeloma is a distinct entity with features of multiple myeloma (MM) and Waldenstrom's macroglobulinemia (WM). The malignant cells in IgM myeloma have a distinctive chromosomal translocation that differentiates them from WM. These cells are postgerminal-center in origin with isotype-switch transcripts. They appear to be arrested at a point of maturation between that of WM and MM. Preliminary data indicate that a pattern of osteoclast-activating factor and osteoprotegerin expression similar to that observed in classic MM is present in IgM myeloma. Additional studies on patients with this rare tumor may provide further insight into the pathogenesis of bone disease in plasma cell dyscrasias.

Antigen-selected, immunoglobulin-secreting cells persist in human spleen and bone marrow

Plasma cells (PCs) represent the final stage of B-cell differentiation and are devoted to the production of immunoglobulin (Ig). Perturbations to their development can result in human disorders characterized by PC expansion and hypergammaglobulinemia. Ig-secreting cells (ISCs) have been identified in secondary lymphoid tissues and bone marrow (BM). Most ISCs in lymphoid tissue are short-lived; in contrast, ISCs that migrate to the BM become long-lived PCs and continue to secrete immunoglobulin for extended periods. However, a small population of long-lived PCs has been identified in rodent spleen, suggesting that PCs may persist in secondary lymphoid tissue and that the spleen, as well as the BM, plays an important role in maintaining long-term humoral immunity. For these reasons, we examined ISCs in human spleen and identified a population that appears analogous to long-lived rodent splenic PCs. Human splenic ISCs shared morphologic, cellular, molecular, and functional characteristics with long-lived PCs in BM, demonstrating their commitment to the PC lineage. Furthermore, the detection of highly mutated immunoglobulin V region genes in splenic ISCs suggested they are likely to be antigen-selected and to secrete high-affinity immunoglobulin. Thus, our results suggest that splenic ISCs have an important role in humoral immunity and may represent the affected cell type in some B-cell dyscrasias.

Clonotypic IgM V/D/J sequence analysis in Waldenstrom macroglobulinemia suggests an unusual B-cell origin and an expansion of polyclonal B cells in peripheral blood

Analysis of clonotypic immunoglobulin M (IgM) from 15 patients with Waldenstrom macroglobulinemia (WM) showed a strong preferential use of the VH3/JH4 gene families. Identification of the WM IgM V/D/J was validated using single-cell analysis, confirming its presence in most B cells. Despite the extensive hypermutated VH genes in 13 of 15 patients, statistical analysis of framework/complementary-determining region (FR/CDR) mutation patterns suggests that they might have escaped antigenic selection. Neither intraclonal diversity nor isotype switching was detectable. Membranous and secreted forms of clonotypic IgM transcripts were present in bone marrow and blood. Single-cell analysis showed that clonotypic B cells coexpress CD20, surface IgM (sIgM), and sIgD but that they lack CD138. Most B cells lacked memory marker CD27 despite their hypermutated variable regions otherwise suggestive of memory status. At diagnosis, circulating B cells in WM are largely clonotypic. However, when monoclonal IgM levels are decreased, clonotypic frequencies are substantially reduced despite elevated CD20+ cells, shown to be polyclonal by DNA sequencing and CDR3 fragment analysis. Thus, WM includes the expansion of circulating, polyclonal B cells. Overall, this work suggests that WM may originate from a largely VH3-restricted, somatically mutated, predominantly CD27(-)IgM(+)IgD+ population that cannot undergo class switching, suggestive of B cells that might have bypassed the germinal center.

New insights into the pathophysiology of multiple myeloma

For understanding of the pathophysiology of multiple myeloma, features of the malignant clone and changes induced by the bone-marrow microenvironment are equally important. Multiple myeloma plasma cells, which originate from postfollicular B cells, are characterised by complex chromosomal aberrations. Among the earliest genetic events are translocations of the immunoglobulin heavy-chain gene locus, which leads to dysregulation of oncogenes at translocation partner regions (cyclin D1 at 11q13, FGFR3/MMSET at 4p16.3, c-MAF at 16q23, and cyclin D3 at 6p21), and deletions of 13q14, the site of a putative tumour suppressor gene, which is an adverse prognostic indicator. Additional molecular events include epigenetic changes and activation of oncogenes (mutations of N-RAS and K-RAS, and changes in c-MYC), which are usually associated with disease progression. Bone-marrow stromal cells support growth and survival of multiple myeloma cells via various cytokines. Osteoclast activity factors (in particular MIP1alpha) and imbalances between RANKL and osteoprotegerin are major factors for the development of myeloma bone disease. Further characterisation of crucial events in the development of monoclonal gammopathies by novel techniques such as global gene expression profiling will contribute to a molecular classification of multiple myeloma and foster future therapeutic approaches.

Probing the specificity of human myeloma proteins with a random peptide phage library

Human myeloma proteins (HMPs) from 10 patients with multiple myeloma (MM) were used to affinity-select peptides from a random phage-display peptide library. Binding peptides were identified for the 10 analysed antibodies (eight, immunoglobulin G (IgG), and two, immunoglobulin A (IgA)). The specificity of the binding was confirmed by competitive experiments using phages and chemically synthesized peptides. Interestingly, some phage-displayed peptides were immuno-selected with HMPs isolated from different patients. Sequence alignments and homology searches revealed a significant homology with human proteins (e.g. neural cell adhesion proteins) and pathogen-derived proteins (e.g. herpes simplex virus capsid proteins). The selected peptides could be useful as targeting agents for myeloma cells expressing surface immunoglobulins.

14q32 Translocations discriminate IgM multiple myeloma from Waldenstrom's macroglobulinemia

Even though the diagnosis of Waldenstrom's macroglobulinemia WM is usually clear, the differential diagnosis with IgM multiple myeloma (MM) might be possible. IgM MM is usually characterized by the accumulation of small mature plasma cells within the bone marrow, and the detection of a monoclonal IgM in the serum. However, in contrast with classical MM, IgM MM is rarely associated with these patients' extensive osteolytic lesions. We analyzed eight cases of IgM MM. None presented with extensive bone lesions. All cases were characterized by the presence of small mature plasma cells within the bone marrow. Molecular cytogenetic analysis revealed a t(11;14) in seven of the eight cases. In contrast, a similar analysis in 17 WM cases failed to detect any t(11;14) cases. We performed further fluorescence in situ hybridization (FISH) experiments, focused on the 14q32 region, and especially on the IgH gene. In contrast to MM (in which illegitimate IgH rearrangements are common), we did not detect any abnormality in the WM cases. In conclusion, even though the cells of origin in WM and MM are mature heavily mutated cells, they differ by the IgH gene rearrangements. Especially in IgM MM, the search for t(11;14) might be useful in difficult cases to discriminate with WM.

Origins of the malignant clone in typical Waldenstrom's macroglobulinemia

Histopathology and phenotypic considerations in IgM-secreting B-cell tumors allow distinction between different diseases, but insight into pathogenesis is restricted. Here, variable region (V) gene analysis can complement classification but, more importantly, can also reveal disease origins and clonal history. We used V(H) gene analysis to probe origins in Waldenstrom's macroglobulinemia (WM), and contrasted these with the less malignant counterpart, IgM-secreting monoclonal gammopathy of undetermined significance (MGUS). Limited data on WM sequences had previously shown evidence for somatic mutation, but with conflicting analysis of intraclonal variation in tumor sequences. To further the investigation, we analyzed seven cases of WM and compared these with three cases of IgM MGUS. In both diseases, V(H) genes were somatically mutated with no evidence of intraclonal variation, even at the MGUS stage. A sensitive V(H) gene-probe assay revealed no evidence for isotype switch transcripts in any of the WM and IgM MGUS cases. These findings reveal an origin of WM and IgM MGUS from a IgM cell, which transforms after cessation of somatic mutation but without initiating switch events. In contrast, IgM-secreting multiple myeloma arises at a later stage in differentiation, when isotype switch mechanisms have been engaged.

The malignant clone in Waldenstrom's macroglobulinemia

The unique IgM VDJ sequence that characterizes the malignant clone in Waldenstrom's macroglobulinemia (WM), termed clonotypic, was identified for 12 WM patients. The majority of WM patients (92%) had a clonotypic IgM from the VH3 family, with predominantly long CDR3 regions, characteristic of those found in antigen-stimulated populations. Clonotypic IgM transcripts were detected in both blood and bone marrow (BM), clearly identifying a blood-borne compartment of WM. Abnormal numbers of CD20(+) B cells were usually detectable and expressed surface IgM. In most cases these cells also expressed surface IgD. Most WM patients lacked detectable CD138(+) plasma cells in either blood or BM. Longitudinal analysis suggests that phenotypic identification of B cells in blood of WM patients is insufficient for monitoring disease. Although serum IgM had decreased and clonotypic transcripts were very weak for one patient, the number of CD20(+) B cells increased dramatically. The lack of clonotypic transcripts suggests that the majority of these circulating B cells were polyclonal and were not part of the WM clone, indicating that monitoring of clonotypic IgM provides the most accurate identifier of WM cells.

Translocation t(11;14)(q13;q32) is the hallmark of IgM, IgE, and nonsecretory multiple myeloma variants

In an attempt to address the issue of cytogenetic features of multiple myeloma (MM) variants, we have analyzed a series of 8 IgM, 9 IgD, 2 IgE, and 14 nonsecretory (NS) MM cases using fluorescence in situ hybridization. A very high incidence (83%) of t(11;14)(q13;q32) was detected in the IgM (7 of 8), IgE (2 of 2), and NS (11 of 14) MM cases, but not in the IgD cases (2 of 9). Of note, no t(4;14) was observed in this cohort of patients. This increased incidence of t(11;14) was associated with 2 dominant features in these variants, namely, a "lymphoplasmacytic" presentation mainly in IgM MM and a lower secreting capacity in the others, 2 features previously associated with t(11;14). Of major interest, t(11;14) was never observed in Waldenström macroglobulinemia or in IgG/IgA "lymphoplasmacytic" lymphomas. Thus, for unknown reasons, t(11;14) is the hallmark of IgM, IgE, and NS MM, (but not IgD MM), with a 5-fold increase of its incidence compared to that of IgG and IgA MM.

Typical Waldenstrom macroglobulinemia is derived from a B-cell arrested after cessation of somatic mutation but prior to isotype switch events

There exists a wide spectrum of IgM-secreting B-cell tumors with different clinical behavior. Knowledge of the VH gene status can reveal their origin and clonal history. For Waldenstrom macroglobulinemia (WM), a distinct subtype of lymphoplasmacytic lymphoma, early data on limited sequences showed evidence for somatic mutation. A recent report of one case demonstrated intraclonal mutational activity occurring after transformation, a characteristic of germinal center lymphomas. To extend the investigation, we have analyzed 7 cases of WM. VH genes were somatically mutated with no evidence of intraclonal variation in all cases. In contrast to IgM-secreting multiple myeloma, there was no evidence for isotype switch transcripts in any of the cases. These data support the concept that typical WM is derived from a B cell that has undergone somatic mutation prior to transformation, at a point where isotype switch events have not been initiated.

Isotype class switching and the pathogenesis of multiple myeloma

Translocations at the immunoglobulin heavy chain locus (14q32) are now considered the commonest karyotypic change in multiple myeloma. These translocations are thought to be intimately involved in the pathogenesis of this disease. The heavy chain locus is strongly transcriptionally active in B and plasma cells and transfer of a potential oncogene to 14q32 would result in its dysregulation. Molecular characterization suggests that the majority of these breakpoints cluster in switch regions within the heavy chain locus. Switch regions are normally involved in the regulated process of isotype switching so that in myeloma the rearrangements are believed to be a result of so-called illegitimate (aberrant) switch recombination and are likely to be an early event in myeloma development. A legitimate switch recombination event occurs between two switch regions producing a hybrid switch; this is necessary for class switching to proceed on a productive allele. In this review we describe the process of isotype switching and how illegitimate class switching may be related to the pathogenesis of multiple myeloma.

Electrophoretic characteristics of monoclonal immunoglobulin G of different subclasses

Monoclonal IgG are commonly observed in various B cell disorders, of which multiple myeloma is the most clinically relevant. In a series of serum samples, we identified by immunofixation 73 monoclonal IgG, including 63 IgG(1), 4 IgG(2), 5 IgG(3), and 1 IgG(4). The light chains were of kappa type in 45 cases, and of lambda type in 28 cases. These monoclonal IgG were further characterized by high resolution two-dimensional polyacrylamide gel electrophoresis (2-DE) in various isoelectric focusing conditions, as well as by 3-DE (2-DE of the proteins extracted from agarose after serum protein agarose electrophoresis). After 2-DE, 38 out of 73 monoclonal gamma chains (52%) were visualized using immobilized pH 3-10 gradients for isoelectric focusing. In 6 cases (8%), gamma chains were only detected using alkaline immobilized pH 6-11 gradients. In 3 cases (4%), 3-DE revealed monoclonal gamma chains hidden by polyclonal gamma chains. Finally, in 26 cases (36%), no monoclonal gamma chains were clearly visualized. Sixty-one monoclonal light chains (84%) were detected using immobilized pH 3-10 gradients, whereas 12 (16%) were not. Monoclonal gamma chains and light chains were highly heterogeneous in terms of pI and M(r). However, a statistically significant correlation (P<0.05) was observed between the position of the monoclonal IgG in agarose gel and the pI of their heavy and light chains (R=0.733, multiple linear regression). Because of the extreme diversity of their heavy and light chains, it appears that a classification of monoclonal IgG based only on their electrophoretic properties is not possible.

Acquisition of potential N-glycosylation sites in the immunoglobulin variable region by somatic mutation is a distinctive feature of follicular lymphoma

Most patients with follicular lymphoma (FL) have somatically mutated V genes with intraclonal variation, consistent with location in the germinal center site. Using our own and published sequences, we have investigated the frequency of potential N-glycosylation sites introduced into functional V(H) genes as a consequence of somatic mutation. FL cells were compared with normal memory B cells or plasma cells matched for similar levels of mutation. Strikingly, novel sites were detected in 55 of 70 (79%) patients with FL, compared to 7 of 75 (9%) in the normal B-cell population (P <.001). Diffuse large B-cell lymphoma (DLCL) showed an intermediate frequency (13 of 32 [41%] patients). Myeloma and the mutated subset of chronic lymphocytic leukemia showed frequencies similar to those of normal cells in 5 of 64 (8%) patients and 5 of 40 (13%) patients, respectively. In 3 of 3 random patients with FL, immunoglobulin was expressed as recombinant single-chain Fv in Pichia pastoris, and glycosylation was demonstrated. These findings indicate that N-glycosylation of the variable region may be common in FL and in a subset of DLCL. Most novel sites are located in the complementarity-determining regions. V(H) sequences of nonfunctional V(H) genes contained few sites, arguing for positive selection in FL. One possibility is that the added carbohydrate in the variable region contributes to interaction with elements in the germinal center environment. This common feature of FL may be critical for tumor behavior.

The occurrence and significance of V gene mutations in B cell-derived human malignancy

The classification of B cell tumors has relevance for refining and improving clinical strategies. However, consensus has been difficult to establish, and although a scheme is now available, objective criteria are desirable. Genetic technology will underpin and extend current knowledge, and it is certain to reveal further subdivisions of current tumor categories. The Ig variable region genes of B cell tumors present a considerable asset for this area of investigation. The unique sequences carried in neoplastic B cells are easily isolated and sequenced. In addition to acting as clone-specific markers of each tumor, they indicate where the cell has come from and track its history following transformation. There is emerging clinical value in knowing whether the cell of origin has encountered antigen and has moved from the naive compartment to the germinal center, where somatic mutation is activated. This is amply illustrated by the subdivision of chronic lymphocytic leukemia into two subsets, unmutated or mutated, each with very different prognosis. Other tumors may be subdivided in a similar way. Microarray technology is developing rapidly to probe gene expression and to further divide tumor categories. All these genetic analyses will provide objective data to enhance both our understanding of B cell tumors and our ability to treat them.

Isotype switch variants reveal clonally related subpopulations in diffuse large B-cell lymphoma

Primary diffuse large B-cell lymphomas (DLBCLs) are aggressive tumors accounting for approximately 40% of B-cell malignancies. The immunoglobulin (Ig) variable region genes have undergone rearrangement and are commonly somatically mutated. The majority show intraclonal variation which indicates that somatic mutation has continued after transformation. Typically, cells of DLBCLs express Ig of a single isotype, but there may be accompanying cells that express alternative isotypes. To probe the status of the isotype switch process in DLBCL, 4 cases of tumor-derived constant region transcripts of all isotypes were investigated. Following the identification of the VDJ sequences, the presence of the major isotype expected from immunohistochemical analysis was confirmed at the RNA level. Another 3-4 alternative isotypes were revealed in all cases, some of which could also be detected by immunohistochemistry. All cases were somatically mutated with an intraclonal variation. In 2 cases there were clearly distinct patterns of somatic mutation between isotypes, which was consistent with independent evolution of the tumor subpopulations. There was apparent clustering of mutational patterns into either an IgMD/IgG3/IgA set or an IgG1/IgA set, indicating that the switch to IgA can occur by different routes. Alternative isotype expression is evident in DLBCL at both the RNA and protein levels. The pattern of mutation indicates that switching is occurring in subpopulations of the tumor after malignant transformation. The findings support the concept that isotype switch events may be a feature of DLBCL.

Isotype switch variants reveal clonally related subpopulations in diffuse large B-cell lymphoma

Primary diffuse large B-cell lymphomas (DLBCLs) are aggressive tumors accounting for approximately 40% of B-cell malignancies. The immunoglobulin (Ig) variable region genes have undergone rearrangement and are commonly somatically mutated. The majority show intraclonal variation which indicates that somatic mutation has continued after transformation. Typically, cells of DLBCLs express Ig of a single isotype, but there may be accompanying cells that express alternative isotypes. To probe the status of the isotype switch process in DLBCL, 4 cases of tumor-derived constant region transcripts of all isotypes were investigated. Following the identification of the VDJ sequences, the presence of the major isotype expected from immunohistochemical analysis was confirmed at the RNA level. Another 3-4 alternative isotypes were revealed in all cases, some of which could also be detected by immunohistochemistry. All cases were somatically mutated with an intraclonal variation. In 2 cases there were clearly distinct patterns of somatic mutation between isotypes, which was consistent with independent evolution of the tumor subpopulations. There was apparent clustering of mutational patterns into either an IgMD/IgG3/IgA set or an IgG1/IgA set, indicating that the switch to IgA can occur by different routes. Alternative isotype expression is evident in DLBCL at both the RNA and protein levels. The pattern of mutation indicates that switching is occurring in subpopulations of the tumor after malignant transformation. The findings support the concept that isotype switch events may be a feature of DLBCL.

IgG-secreting lymphoplasmacytoid leukaemia: a B-cell disorder with extensively mutated VH genes undergoing Ig isotype-switching frequently associated with trisomy 12

We investigated 16 patients with elevated serum monoclonal IgG and a leukaemic B-cell lymphocytic disorder different from multiple myeloma. Their clinical history was that of a non-aggressive disease with dominant splenomegaly and long survival. Whereas abnormal blood and bone marrow cells were predominantly small lymphocytes with a few lymphoplasmacytoid cells, histopathological features included a lymphoplasmacytic infiltrate in eight cases. Most frequently, abnormal blood cells displayed a CD19+CD5-CD23+/- immunophenotype different from that of chronic lymphocytic leukaemia, except in two cases with a CD19+CD5+CD23+ phenotype. Interestingly, a coexistent serum monoclonal IgM and/or surface IgMG+ with identical light chain was identified in 10 patients, whereas in the remaining six patients only IgG expression was determined. VH gene analysis was performed in eight patients to investigate the clonal origins of tumour cells. All cases utilized the VH3 family, with evidence of extensive somatic mutations and intraclonal homogeneity in all cases. VH gene analysis indicated a clonal relationship between cells expressing IgM and IgG, with one case being biclonal. Cytogenetic evaluation showed a high incidence of trisomy 12 (60%) and 13q14 deletion (40%). In conclusion, we have described an unusual subset of low-grade lymphoma with high-serum IgG and frequent lymphoplasmacytoid features in which tumour cells derive from post-follicular memory B cells undergoing isotype switching with some cases arrested at both the IgM and IgG stage and others as IgG-positive cells only.