Clonal history of a human follicular lymphoma as revealed in the immunoglobulin variable region genes

A Comparison of Immunoglobulin Variable Region N-Linked Glycosylation in Healthy Donors, Autoimmune Disease and Lymphoma

N-linked glycans play an important role in immunity. Although the role of N-linked glycans in the Fragment crystallizable (Fc) region of immunoglobulins has been thoroughly described, the function of N-linked glycans present in Ig-variable domains is only just being appreciated. Most of the N-linked glycans harbored by immunoglobulin variable domain are of the complex biantennary type and are found as a result of the presence of N-linked glycosylation that most often have been introduced by somatic hypermutation. Furthermore, these glycans are ubiquitously present on autoantibodies observed in some autoimmune diseases as well as certain B-cell lymphomas. For example, variable domain glycans are abundantly found by anti-citrullinated protein antibodies (ACPA) in rheumatoid arthritis (RA) as well as by the B-cell receptors of follicular lymphoma (FL). In FL, variable domain glycans are postulated to convey a selective advantage through interaction with lectins and/or microbiota, whereas the contribution of variable domain glycans on autoantibodies is not known. To aid the understanding how these seemingly comparable phenomena contribute to a variety of deranged B-responses in such different diseases this study summarizes the characteristics of ACPA and other auto-antibodies with FL and healthy donor immunoglobulins, to identify the commonalities and differences between variable domain glycans in autoimmune and malignant settings. Our finding indicate intriguing differences in variable domain glycan distribution, frequency and glycan composition in different conditions. These findings underline that variable domain glycosylation is a heterogeneous process that may lead to a number of pathogenic outcomes. Based on the current body of knowledge, we postulate three disease groups with distinct variable domain glycosylation patterns, which might correspond with distinct underlying pathogenic processes.

B Cell Receptor Immunogenetics in B Cell Lymphomas: Immunoglobulin Genes as Key to Ontogeny and Clinical Decision Making

The clonotypic B cell receptor immunoglobulin (BcR IG) plays a seminal role in B cell lymphoma development and evolution. From a clinical perspective, this view is supported by the remarkable therapeutic efficacy of BcR signaling inhibitors, even among heavily pre-treated, relapsed/refractory patients. This clinical development complements immunogenetic evidence for antigen drive in the natural history of these tumors. Indeed, BcR IG gene repertoire biases have been documented in different B cell lymphoma subtypes, alluding to selection of B cell progenitors that express particular BcR IG. Moreover, distinct entities display imprints of somatic hypermutation within the clonotypic BcR IG gene following patterns that strengthen the argument for antigen selection. Of note, at least in certain B cell lymphomas, the BcR IG genes are intraclonally diversified, likely in a context of ongoing interactions with antigen(s). Moreover, BcR IG gene repertoire profiling suggests that unique immune pathways lead to distinct B cell lymphomas through targeting cells at different stages in the B cell differentiation trajectory (e.g., germinal center B cells in follicular lymphoma, FL). Regarding the implicated antigens, although their precise nature remains to be fully elucidated, immunogenetic analysis has offered important hints by revealing similarities between the BcR IG of particular lymphomas and B cell clones with known antigenic specificity: this has paved the way to functional studies that identified relevant antigenic determinants of classes of structurally similar epitopes. Finally, in certain tumors, most notably chronic lymphocytic leukemia (CLL), immunogenetic analysis has also proven instrumental in accurate patient risk stratification since cases with differing BcR IG gene sequence features follow distinct disease courses and respond differently to particular treatment modalities. Overall, delving into the BcR IG gene sequences emerges as key to understanding B cell lymphoma pathophysiology, refining prognostication and assisting in making educated treatment choices.

Synergistic cooperation and crosstalk between MYD88L265P and mutations that dysregulate CD79B and surface IgM

Wang et al. show cooperation between MYD88^L265P and CD79B mutations dysregulating B cell responses to self-antigen and differentiation into plasma cells. Their results reveal that CD79B and surface IgM constitute a rate-limiting checkpoint against MYD88^L265P, explaining the co-occurrence of MYD88 and CD79B mutations in human lymphomas.

CD79B and MYD88 mutations are frequently and simultaneously detected in B cell malignancies. It is not known if these mutations cooperate or how crosstalk occurs. Here we analyze the consequences of CD79B and MYD88^L265P mutations individually and combined in normal activated mouse B lymphocytes. CD79B mutations alone increased surface IgM but did not enhance B cell survival, proliferation, or altered NF-κB responsive markers. Conversely, B cells expressing MYD88^L265P decreased surface IgM coupled with accumulation of endoglycosidase H–sensitive IgM intracellularly, resembling the trafficking block in anergic B cells repeatedly stimulated by self-antigen. Mutation or overexpression of CD79B counteracted the effect of MYD88^L265P. In B cells chronically stimulated by self-antigen, CD79B and MYD88^L265P mutations in combination, but not individually, blocked peripheral deletion and triggered differentiation into autoantibody secreting plasmablasts. These results reveal that CD79B and surface IgM constitute a rate-limiting checkpoint against B cell dysregulation by MYD88^L265P and provide an explanation for the co-occurrence of MYD88 and CD79B mutations in lymphomas.

Catalog of genetic progression of human cancers: non-Hodgkin lymphoma

The recent application of next-generation sequencing technologies lead to significant improvements in our understanding of genetic underpinnings of non-Hodgkin lymphomas with identification of an unexpectedly high number of novel mutation targets across the different B-cell lymphoma entities. These recently discovered molecular lesions are expected to have a major impact on development of novel biomarkers and targeted therapies as well as patient stratification based on the underlying genetic profile. This review will cover the major discoveries in B-cell lymphomas using next-generation sequencing technologies over the last few years, highlighting alterations associated with relapse and progression of these diseases.

Common progenitor cells in mature B-cell malignancies: implications for therapy

PURPOSE OF REVIEW

This review summarizes the recent progress in defining the patterns of genetic evolution giving rise to relapse in follicular lymphoma and multiple myeloma, and discusses their implications with respect to 'personalized medicine'.

RECENT FINDINGS

High-throughput sequencing studies have uncovered a large degree of clonal heterogeneity within tumors, and found that subclones have a variable contribution to relapse. Recent studies aimed at defining patterns of clonal evolution have revealed that serial tumors in some malignancies share their ancestry in a less evolved common progenitor cell (CPC) that bears only a subset of the mutations that are present in the fully evolved tumors that present clinically. This pattern of 'divergent evolution' means that the majority of 'actionable mutations' in tumor specimens are absent within the progenitors that give rise to relapse.

SUMMARY

Follicular lymphoma and multiple myeloma are clinically, biologically and genetically distinct mature B-cell malignancies. However, recent studies have found them to share important similarities in their patterns of genetic evolution. Tumor cells that constitute subclonal populations within these tumors, or between consecutive tumors, share their origins within a genetically less evolved common progenitor cell. This pattern of evolution indicates that current therapies are unable to eradicate these less evolved populations that are at the root of relapse. This suggests that in order to obtain the best results with modern 'targeted therapies' that are directed towards 'actionable mutations', these mutations should be considered within the context of the evolutionary stage at which mutations are acquired, not simply on a presence or absence basis.

Isotype-switched follicular lymphoma displays dissociation between activation-induced cytidine deaminase expression and somatic hypermutation

In B-cells, activation-induced cytidine deaminase (AID) is required for somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin genes. AID introduces mutations in immunoglobulin variable regions (IGV) during B-cell receptor affinity maturation, but may also introduce aberrant mutations into non-immunoglobulin genes, most commonly BCL6. Follicular lymphoma (FL) B-cells constitutively express AID and undergo CSR, SHM and aberrant SHM. We have studied AID expression, the presence of SHM mutations, CSR, and aberrant SHM in BCL6 in a cohort of 75 FL patients. Whereas IgM-expressing (non-switched) FL were characterized by an expected positive correlation between AID and IGV and BCL6 mutations, isotype-switched FL showed dissociation between AID expression and aberrant SHM, and inverse correlation between SHM and AID expression. Our results unveil two manifest biological subgroups of FL and indicate that the specific dissociation between AID and SHM after isotype switch may correlate with the clinical outcome of this heterogeneous disease.

IGHV gene usage and mutational status in follicular lymphoma: Correlations with prognosis and patient age

Follicular lymphoma (FL) is a heterogeneous disease with some patients developing progressively or transformed disease early, whereas others follow an indolent clinical course. We evaluated the prognostic value of immunoglobulin heavy chain variable (IGHV) gene usage and mutational status in FL patients. One hundred and four IGH sequences were obtained in tumour samples from 99 patients. The IGHV3 subgroup had the highest usage frequency (57.7%) with IGHV3-23 being the most common sequence. Patients with the IGHV5 subgroup or IGHV sequences from more than one subgroup had significantly less favourable prognosis with an estimated 5-year survival of 62.5 and 50.0%, respectively, as compared with a 5-year survival of 95.1% for patients with other IGHV subgroups (P=0.013 and P<0.001, log-rank). The poor survival associated with IGHV5 or >1 IGHV subgroup usage was an independent prognostic factor in Cox multivariate analysis (P=0.005). IGHV genes were unmutated showing >98% homology in 15.2% of cases. Contrasting the situation in chronic lymphocytic leukaemia (CLL), the presence of unmutated sequences did not yield prognostic information, although unmutated sequences were associated with age at diagnosis >60 years (P=0.022, Fisher's exact). In conclusion, our results indicate that analysis of IGHV gene usage might aid in predicting prognosis for FL patients.

Ultradeep analysis of tumor heterogeneity in regions of somatic hypermutation

Tumor heterogeneity is of growing importance in the treatment of cancers. Mutational hot spots are prime locations for determining number and proportions of low variant allele frequency (VAF) tumor subclones by next generation sequencing. Low VAF detection is complicated by poor mapping efficiency in regions with high mutation density. Our Deep-Drilling with iterative Mapping (DDiMAP) method retains variant allele patterns to aid in single nucleotide variation detection and generation of additional reference alleles, with remapping increasing coverage of highly mutated regions to capture data critical to heterogeneity analysis and enhancing sensitivity. DDiMAP outputs variant patterns with frequencies, enabling rapid phylogenetic analysis of ongoing mutation.

Intraclonal diversity in follicular lymphoma analyzed by quantitative ultradeep sequencing of noncoding regions

Cancers are characterized by genomic instability, and the resulting intraclonal diversity is a prerequisite for tumor evolution. Therefore, metrics of tumor heterogeneity may prove to be clinically meaningful. Intraclonal heterogeneity in follicular lymphoma (FL) is apparent from studies of somatic hypermutation (SHM) caused by activation-induced deaminase (AID) in IGH. Aberrant SHM (aSHM), defined as AID activity outside of the IG loci, predominantly targets noncoding regions causing numerous "passenger" mutations, but it has the potential to generate rare significant "driver" mutations. The quantitative relationship between SHM and aSHM has not been defined. To measure SHM and aSHM, ultradeep sequencing (>20,000-fold coverage) was performed on IGH (~1650 nt) and nine other noncoding regions potentially targeted by AID (combined 9411 nt), including the 5' untranslated region of BCL2. Single-nucleotide variants (SNVs) were found in 12/12 FL specimens (median 136 SHMs and 53 aSHMs). The aSHM SNVs were associated with AID motifs (p < 0.0001). The number of SNVs at BCL2 varied widely among specimens and correlated with the number of SNVs at eight other potential aSHM sites. In contrast, SHM at IGH was not predictive of aSHM. Tumor heterogeneity is apparent from SNVs at low variant allele frequencies; the relative number of SNVs with variable allele frequency < 5% varied with clinical grade, indicating that tumor heterogeneity based on aSHM reflects a clinically meaningful parameter. These data suggest that genome-wide aSHM may be estimated from aSHM of BCL2 but not SHM of IGH. The results demonstrate a practical approach to the quantification of intratumoral genetic heterogeneity for clinical specimens.

Molecular analysis of light-chain switch and acute lymphoblastic leukemia transformation in two follicular lymphomas: Implications for lymphomagenesis

We observed novel transformations of follicular lymphoma (FL), first, a switch in immunoglobulin (Ig) light chain, and second, transformation of FL to acute lymphoblastic leukemia (ALL). Each set of tumors shared a common clonal origin, as demonstrated by expression of identical, unique CDR IIIH sequences, shared somatic mutations in JH, and identical bcl-2 translocation breakpoints of microdissected ALL cells. Molecular analysis of lambda V-gene expression demonstrated lambda-bearing cells in the original kappa tumor, while expansion of the lambda subclone at relapse occurred after active immunotherapy targeting the Ig receptor. These exceptional cases are compatible with a more contemporary model of lymphomagenesis in which critical events originate from genetic mechanisms which normally occur in germinal center (GC) B cells and challenge the current paradigm of parallel generation of subclones from an early, pre-GC precursor. It is also possible that the outgrowth of these variants was a consequence of immunoselection.

Transformation of follicular lymphoma to diffuse large B-cell lymphoma may occur by divergent evolution from a common progenitor cell or by direct evolution from the follicular lymphoma clone

To investigate the cell of origin linking follicular (FL) and transformed (t-FL) lymphomas, we analyzed the somatic hypermutation (SHM) pattern of the variable region of the immunoglobulin heavy gene (IgH-VH) in 18 sequential FL/t-FL samples and a father (donor) and son (recipient), who developed FL and t-FL, after transplantation. Genealogic trees showed a pattern compatible with a common progenitor cell (CPC) origin in 13 cases. The identification of the t-FL clonotype in the previous FL sample and of the putative CPC sequence in both the FL/t-FL biopsies showed that the intraclonal diversity of FL and t-FL germinal centers (GCs) is more intricate than previously described, and all 3 clonotypes (CPC, FL, t-FL) may occur simultaneously within the same lymph node. On the basis of the father/son model, this CPC must be long-lived, providing a possible explanation for the incurable nature of this disease.

S(mu) mutation patterns suggest different progression pathways in follicular lymphoma: early direct or late from FL progenitor cells

Follicular lymphoma (FL) is a B-cell malignancy characterized by the t(14;18) translocation. Although sensitive to treatment, the disease remains incurable and the reason why tumor cells invariably evade treatment, leading to clinical relapse, is still unknown. Here, we tracked the clonal history of tumor cells by studying mutations introduced by activation-induced cytidine deaminase on the switch mu region of the der(14)t(14;18) during the early phase of the class-switch recombination (CSR) process. We observed frequent intraclonal variations, suggesting that CSR often remains active after the acquisition of the fully transformed phenotype. However, mutations only rarely accumulated over time, but instead showed complex evolutionary scenarios and 2 different progression pathways. The first pathway was a direct and rapid evolution from the dominant clone. The second was indirect, arising from earlier subclones usually after years of remission. A better understanding of these mechanisms might influence the future choice of treatment strategies.

Molecular pathways in follicular lymphoma

Follicular lymphoma (FL) is one of the most common B-cell non-Hodgkin's lymphomas. The initiating genetic event found in approximately 90% of FL is the t(14;18), causing constitutive expression of the antiapoptotic BCL-2 protein. The exact secondary alterations leading to full FL development are still poorly defined. In this review, we address (i) the genetic pathways associated with tumorigenesis and progression of FL, (ii) the role of micro-environmental factors with emphasis on B-cell receptor ligands and (iii) lymphoma models in mice and what they teach us about lymphomagenesis in man.

Clonal selection in the bone marrow involvement of follicular lymphoma

To characterize the pathways of bone marrow (BM) involvement of follicular lymphoma (FL), we performed morphological and immunophenotypical analysis of tumor cells from lymph nodes (LNs) and corresponding BMs in 21 patients with FL. In three cases, genealogical trees were constructed based on the immunoglobulin variable region heavy chain (IgV(H)) gene sequences of tumor clones from LNs and BMs. Results showed that FLs within the BMs display identical or lower cytological grades than in the LNs. In the majority of cases, different proportions of tumor cells expressed bcl-2, CD10 and Ki67 in LNs and BMs. Tumor cells in the BM showed ongoing somatic hypermutation of the IgV(H) genes; the distribution of these mutations was highly consistent with antigen selection. The topology of the genealogical trees revealed that different subclones populate the LN and BM and BM infiltration may occur at different points of the clonal evolution of FL. Early descendants of the original tumor clone and derivatives of diversified tumor clones may invade the BM. These results suggest that the BM involvement of FL is associated with intensive clonal selection of tumor cells, and the BM provides a microenvironment similar to the germinal centers of LNs, where tumor cells retain their biological nature.

Effect of Helicobacter pylori eradication on ongoing mutation of immunoglobulin genes in gastric MALT lymphoma

Gastric low-grade mucosa-associated lymphoid tissue (low-grade MALT) lymphomas has been associated with Helicobacter pylori (H. pylori) infection. Although infiltrating T cells with specificity for H. pylori are known to stimulate the development of MALT lymphomas, the effect of H. pylori eradication on rearranged immunoglobulin heavy chain (IgH) genes of low-grade gastric MALT lymphomas is unclear. Gastric biopsies from five cases were investigated by cloning and sequence analysis of rearranged IgH genes before and after the treatment for H. pylori. In all cases, IgH genes were mutated from their germline counterpart. The frequency of intraclonal sequence heterogeneity before the eradication of H. pylori varied from 0.25 to 0.49%. Clones obtained from the tumours before the eradication of H. pylori in cases 1 and 2 showed a tendency to display a mutation pattern by positive antigen selection and their monoclonarity disappeared after the eradication. The frequency of intraclonal sequence heterogeneity of the clones obtained from cases 3, 4 and 5 (0.12% in case 3, 0.10% in 4 and 0.18% in 5) after the eradication of H. pylori was lower than that in tumours before the eradication (0.30% in case 3, 0.49% in 4 and not determined in 5). These findings suggest that low-grade gastric MALT lymphomas expand due to the persistent presence of H. pylori in vivo. The characteristic feature of tumour clones obtained from the tumours after the eradication of H. pylori is a very low intraclonal heterogeneity, which may potentially be independent of H. pylori.

The immunoglobulin genes and chronic lymphocytic leukemia (CLL)

The somatic hypermutation (SMH) status of the immunoglobulin (Ig) V(H) genes can divide chronic lymphocytic leukemia (CLL) into two prognostic subsets, with mutated V(H) genes display superior survival compared to unmutated cases. Biased V(H) gene usage has also been reported in CLL which may reflect antigen selection. In a V(H) gene analysis of 265 CLL cases we confirmed the prognostic impact of the V(H) mutation status and found preferential V(H) gene usage in both the mutated and unmutated subset. Interestingly, CLL cases rearranging one particular V(H) gene, V(H)3-21, displayed poor outcome despite that two-thirds showed mutated V(H) genes. Many of the V(H)3-21 utilizing cases expressed lambda light chains, rearranged a Vlambda2-14 gene, and had homologous complementarity determining region 3s (CDR3s), implying recognition of a common antigen epitope. We thus believe that the cases rearranging the V(H)3-21 gene comprises an additional CLL entity. We further analyzed the V(H) gene rearrangements and, specifically, the heavy chain CDR3 sequences in 346 CLL cases to investigate the role of antigens in CLL. We identified six new subgroups with similar HCDR3 features and restricted VL gene usage as in the V(H)3-21-using group. Our data indicate a limited number of antigen recognition sites in these subgroups and give further evidence for antigen selection in the development of CLL. Different mutational cutoffs have been used to distinguish mutated CLL in addition to the 2% cutoff. Using three levels of somatic mutations we divided 323 CLLs into subsets with divergent survival (<2%, 2-5% and >5% mutations). This division revealed a low-mutated subgroup (2-5%) with inferior outcome that would have been masked using the traditional 2% cutoff. A 1513A/C polymorphism in the P2X(7) receptor gene was reported to be more frequent in CLL, but no difference in genotype frequencies was revealed in our 170 CLL cases and 200 controls. However, CLL cases with the 1513AC genotype showed superior survival than 1513AA cases and this was in particular confined to CLL with mutated VH genes. In summary, we could define new prognostic subgroups in CLL using Ig gene rearrangement analysis. This also allowed us to gain insights in the biology and potential role of antigen involvement in the pathogenesis of CLL.

CD27 distinguishes two phases in bone marrow infiltration of splenic marginal zone lymphoma

AIMS

To investigate CD27 expression in splenic marginal zone lymphoma (SMZL), an indolent low-grade B-cell lymphoma with constant involvement of the bone marrow, especially with an intrasinusoidal pattern. It is not clear if the neoplastic clone is composed of virgin or somatically mutated B cells. CD27 is reported to be a hallmark of memory B cells.

METHODS AND RESULTS

We evaluated 64 bone marrow biopsy specimens (BMBs) from 36 patients with SMZL for the expression of CD27. For comparison, splenectomy specimens of patients with traumatic splenic rupture or with SMZL were used. All BMBs showed lymphomatous infiltration. When located in the marrow sinusoids, neoplastic cells were CD27- in all cases and therefore corresponded to naive B cells. In nodular/interstitial infiltration, the cells were CD27+ and therefore corresponded to memory B cells. No difference in immunohistochemical expression of B and T antibodies was found between intrasinusoidal and interstitial/nodular infiltration. CD27 was constantly expressed in the splenic marginal zone of normal spleen, surgically removed for trauma, and in seven out of 10 spleens with SMZL.

CONCLUSION

We propose the existence of two different phases of neoplastic progression with, first, expansion of a virgin B clone in the bone marrow and, following exposure to antigen, a re-colonization of the bone marrow.

Activation-induced cytidine deaminase expression in follicular lymphoma: association between AID expression and ongoing mutation in FL

Activation-induced cytidine deaminase (AID) is required for somatic hypermutation (SHM) and class switch recombination (CSR) of the immunoglobulin (Ig) gene. AID has been reported to be specifically expressed in the germinal center (GC). Follicular lymphoma (FL) cells are known to be exposed to GC reaction, as characterized by a high degree of SHM with some heterogeneity in terms of intraclonal microheterogeneity and antigen selection. The heterogeneity of SHM pattern in FL intrigued us to investigate the AID expression. AID expression was investigated in 19 FL materials consisting of 15 cases of FL fresh cells and four cell lines. In all, 10 fresh cells and three cell lines expressed AID, but the others did not. SHM was investigated in 12 fresh cells and four cell lines. The ongoing mutation was significantly different between AID-positive and AID-negative FL fresh cells (unpaired Student's t-test, P=0.047). Ongoing mutation was not seen in any of the cell lines. AID expression was associated with the ongoing mutation in FL fresh cells (two-tailed Pearson's coefficient correlation, r=0.899, P=0.01). The switch off of AID expression may start in the B-lineage differentiation stage counterpart of FL after optimizing SHM, indicated by the cessation of the ongoing mutation in AID-negative FL fresh cells.

Comparative studies of somatic and ongoing mutations in immunoglobulin heavy-chain variable region genes in diffuse large B-cell lymphomas of the stomach and the small intestine

CONTEXT

Many diffuse large B-cell lymphomas (DLBCLs) of the stomach are believed to represent high-grade transformation of low-grade marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue type, which is of memory B-cell origin, displaying evidence for positive antigen selection and a low level of ongoing somatic mutation of the rearranged immunoglobulin heavy-chain variable region (V(H)) genes. The pattern of somatic mutation has been studied little in intestinal DLBCLs.

OBJECTIVE

To assess evidence for antigen selection and the levels of ongoing mutation, we analyzed the ratio of replacement to silent mutations, as well as the frequency of intraclonal sequence variation in gastric and small intestinal DLBCLs that showed no concomitant low-grade component.

DESIGN

Genomic DNA was extracted from formalin-fixed paraffin blocks of gastric (n = 6) and small intestinal (n = 6) DLBCLs. The complementarity-determining region 2 and framework region 3 sequences (<200 base pairs) of the rearranged immunoglobulin V(H) gene were obtained from polymerase chain reaction-amplified product, and the ratio of replacement-to-silent mutations and the frequency of intraclonal sequence variation were determined.

RESULTS

Clustering of replacement mutations in complementarity-determining region 2 with a high (>2.9) ratio of replacement-to-silent mutations was observed in 5 gastric DLBCLs, whereas it was recognized in only 1 intestinal DLBCL. Intraclonal sequence variation was observed in 6 intestinal and 5 gastric DLBCLs. The frequency of ongoing mutation was much higher in the intestinal (median, 0.33%) than in the gastric DLBCLs (median, 0.13%), but the difference was not statistically significant (P =.09).

CONCLUSIONS

The mutation pattern was consistent with positive antigen selection in gastric DLBCLs, but not in the intestinal tumors. Ongoing mutation was much more frequent in the intestinal than in the gastric DLBCLs. These findings suggest that positive antigen selection plays a major role in a significant proportion of gastric tumors, whereas germinal center reaction with aberrant mutation is important in small intestinal DLBCLs.

A subset of t(11;14) lymphoma with mantle cell features displays mutated IgVH genes and includes patients with good prognosis, nonnodal disease

We analyzed lymphocyte morphology, histology, immunophenotype, immunoglobulin heavy chain (IgVH) gene mutations, and clinical course in 80 unselected patients presenting with circulating t(11;14) lymphocytes. Of the 80 patients, 43 had peripheral lymphadenopathy (nodal group), and histology confirmed mantle cell lymphoma (MCL) in all. There were 37 patients with no lymphadenopathy (nonnodal group); 13 of 37 had histology, all showing MCL. IgVH genes were unmutated in 28 (90%) of 31 nodal and 15 (44%) of 34 nonnodal cases (P =.0001); CD38 was positive in 32 (94%) of 34 nodal and 16 (48%) of 33 nonnodal cases (P <.001); 41 (95%) of 43 nodal patients required immediate treatment compared with 18 (49%) of 37 nonnodal patients who had indolent disease (P <.0001). Median survival (95% confidence interval) was 30 months (10-50) in the nodal group and 79 months (22-136) in the nonnodal group (P =.005). Mutation status did not statistically affect survival, but of 6 long-term survivors (> 90 months) all were nonnodal and 5 of 5 had mutated IgVH genes. Lymphocyte morphology was heterogeneous in both groups: typical MCL in 56 cases (34 nodal, 22 nonnodal), blastoid MCL in 8 cases (3 nodal, 5 nonnodal), and small-cell MCL in 16 cases (6 nodal, 10 nonnodal, P =.12). Matutes immunophenotyping score was 1 in 65 cases and 2 in 15 (8 nodal, 7 nonnodal). We find no evidence against a diagnosis of MCL in the nonnodal group and suggest that mutated IgVH genes may help identify patients with indolent disease.

Patterns of somatic mutations in VH genes reveal pathways of clonal transformation from MGUS to multiple myeloma

Monoclonal gammopathy of undetermined significance (MGUS) can transform to multiple myeloma (MM). In myeloma, mutated V(H) genes with sequence homogeneity reveal a postfollicular origin. Previously, some MGUS cases showed mutated V(H) genes with intraclonal variation, indicating an earlier stage of arrest. We investigated progression from 2 of 2 MGUS to MM, in which V(H) genes confirmed clonal evolution. In one MGUS case, intraclonal heterogeneity was evident, and transformation to myeloma occurred rapidly with apparent homogeneity in the emergent clone. However, residual MGUS-derived sequences were detectable at this time. Heterogeneity in MGUS does not associate with benign disease, but it indicates an origin from a tumorigenic cell, most likely surface immunoglobulin(+), undergoing somatic mutation. The remaining case displayed intraclonal homogeneity at the MGUS stage, conceivably resulting from a self-cloning outgrowth from MGUS with heterogeneity. Transformation can occur at either MGUS stage, but it involves a single cell in which somatic mutation is then silent.

V(H) gene analysis of splenic marginal zone lymphomas reveals diversity in mutational status and initiation of somatic mutation in vivo

Tumors of the splenic marginal zone can present in spleen or blood. The maturational status of the neoplastic B cells from each site appears heterogeneous, with either unmutated or mutated variable-region heavy chain (V(H)) genes. To determine an influence of tissue location, we assessed matched blood and splenic tumor cells from 4 patients and found them identical. However, one patient with unmutated V(H) genes in blood and spleen developed a clonally related diffuse large B-cell lymphoma in the chest wall. Strikingly, this subclone had undergone significant somatic mutation, with clear intraclonal heterogeneity. To our knowledge, this is the first case of a B-cell tumor showing initiation of somatic mutation in vivo. The finding emphasizes that the tissue microenvironment can influence tumor cell behavior and possibly affect disease progression. Importantly, because several replacement mutations were located within or close to the complementarity-determining regions (CDRs), it raises the question of a role for antigen in driving tumor growth.

Malignant B cells and antigenic receptor: necessity or habit?

B-cell malignancies account for the majority of lymphoid tissue neoplasia. Similar to normal B cells, malignant B cells in most Hodgkin's and non-Hodgkin's types of lymphomas express B-cell receptor (BCR) on their membrane. Since neoplastic B cells retain the capacity to respond to microenvironmental signals, and in many respects still behave as normal B cells, it does not seem bizarre that the BCR, which dominates the biology of normal B cells, can remain equally important for some malignant B cells. Indirect evidence suggests that retained BCR expression, and in certain cases coupled with stimulation by antigen (Ag), may be necessary for the viability of some B-cell tumors. The aim of this review is to consider the evidence regarding the role of the BCR in tumorigenesis of B-cell lymphomas, and discuss different approaches used in evaluating this role in the persistence and progression of these malignancies. The diversity in B-cell lymphomas prevents easy classification of these cancers based on their dependence on BCR expression. It seems likely that some malignant B cells need BCR expression, or additionally, stimulation by Ag in order to survive. However, through accumulation of additional genetic changes, the original tumor can give rise to a clone that no longer requires signals from the BCR to survive. Thus, most B-cell lymphomas may initially retain dependence on BCR expression that governs normal B-cell physiology and may lose it only at later stages of tumor progression, through the accumulation of additional transforming events.

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.

In situ analysis of the variable heavy chain gene of an IgM/IgG-expressing follicular lymphoma: evidence for interfollicular trafficking of tumor cells

It is generally assumed that follicular lymphomas (FL) not only morphologically resemble normal germinal centers but have retained some functional characteristics of their non-neoplastic counterparts as well. Recent IgV gene analyses on a panel of FLs however, strongly suggested that FLs do not retain the capacity of somatic hypermutation and are not being selected on basis of the quality of their mIgV regions. To extend these findings, we investigated the follicular organization and class switching in a FL that consisted of both IgM- and IgG-expressing tumor cells with a high somatic mutation load and significant intraclonal V(H) gene diversity. V(H)-C(mu) and V(H)-Cgamma gene transcripts were amplified and sequenced from samples of approximately 50 tumor cells, isolated from frozen tissue sections by laser microdissection. We identified many different subclones and obtained limited evidence of subclone dominance in individual follicles. Remarkably, several subclones were found scattered over different follicles. All samples contained IgM- and IgG-expressing tumor cells with, in general, non-identical mutation patterns, which is not in support of ongoing class switching. Accordingly, no switch circle recombination products were found. The findings indicate that the neoplastic follicles lack the organization and functions typical of reactive germinal centers.

Primary cutaneous follicle center cell lymphomas and large B cell lymphomas of the leg descend from germinal center cells. A single cell polymerase chain reaction analysis

Primary cutaneous B cell lymphomas are defined as non-Hodgkin lymphomas that occur in the skin without extracutaneous involvement for 6 mo after diagnosis. They are characterized by a less aggressive course and better prognosis than their nodal counterparts. According to the European Organization for Research and Treatment of Cancer classification, the major subentities of primary cutaneous B cell lymphoma are follicle center cell lymphomas, immunocytomas, and large B cell lymphomas of the leg, which differ considerably regarding their clinical behavior, the former two being indolent, the latter being of intermediate malignancy. In this study, we applied a single cell polymerase chain reaction approach to analyze immunoglobulin V(H)/V(L) genes in 532 individual B lymphocytes from histologic sections of four follicle center cell lymphomas localized on the head and trunk, and four large B cell lymphomas on the leg. We found: (i) in six of eight patients a clonal heavy chain, and in seven of eight patients a clonal light chain rearrangement, all being potentially productive; (ii) no bias in VH gene usage, in four of seven light chain rearrangements the V kappa germline gene IGVK3-20*1 was used; (iii) no biallelic rearrangements; (iv) all V(H)/V(L) genes are extensively mutated (mutation rate 5.4-16.3%); (v) intraclonal diversity in six of eight cases (three of each group); and (vi) low replacement vs silent mutation ratios in framework regions indicating preservation of antigen-receptor structure, as in normal B cells selected for antibody expression. Our data indicate a germinal center cell origin of primary cutaneous follicle center cell lymphomas and large B cell lymphomas independent of those belonging to one of these subentities.

Variable heavy-chain gene analysis of follicular lymphomas: subclone selection rather than clonal evolution over time

To investigate B-cell receptor evolution in follicular lymphomas (FLs), immunoglobulin variable heavy chain (V(H)) gene regions of 3 FLs were analyzed at different time points. One FL with a high somatic mutation load and intraclonal V(H) gene diversity was investigated in situ. V(H) gene transcripts were amplified and sequenced from samples of approximately 50 tumor cells isolated from frozen tissue sections by laser microdissection. Interestingly, the mutation pattern of the prevalent subclone in the relapse biopsy was virtually identical to that of a subclone isolated by microdissection from the presentation biopsy 9 years earlier. In a second FL, proof was obtained that the subclone that dominated the relapse sample had already been present in the initial biopsy. The finding that subclones found in the relapses of these FLs had not evolved over time but were preexistent, challenges the concept of antigen-driven B-cell receptor evolution during disease course.

Immunoglobulin heavy chain variable region (VH) genes of B cell chronic lymphocytic leukemia cells from lymph nodes show somatic mutations and intraclonal diversity irrespective of follicular dendritic cell network

We analyzed the immunoglobulin heavy chain variable region (VH) gene in 4 Japanese cases of B cell chronic lymphocytic leukemia (B-CLL) with enlarged lymph nodes to clarify the presence of somatic mutations and intraclonal diversity. We also attempted to determine the role of the follicular dendritic cell (FDC) network in some proliferation centers, where tumor cells are mitotically active. Immunohistochemical studies revealed that all 4 cases showed the typical immunophenotype: CD5+, CD23+, IgM+ and IgD+. DNA was extracted from paraffin sections (lymph node) and rearranged VH gene was amplified by PCR. All but one exhibited a moderate number of somatic mutations, with percentages ranging from 4.1 to 9.5, and one of which indicated the effect of antigen selection on its VH gene. Multiple clone analysis of whole tissues showed intraclonal diversity in one case, whose VH gene carried a somatic mutation but the effect of antigen selection was not apparent. We further examined microdissected tissues to elucidate the relationship between FDC network and VH gene status in 2 cases. In one case, intraclonal diversity was not apparent irrespective of FDC network, however, both tumor cells around the FDC network and those apart from the FDC showed signs of intraclonal diversity in another case, suggesting that intraclonal diversity was not related to the FDC network in B-CLL. Here we demonstrate that some cases of B-CLL involved in lymph node carried mutated VH genes and showed intraclonal diversity like the tumor cells in the peripheral blood. However, the significance of the FDC network in the proliferation center still remains to be resolved.

Cells of the marginal zone--origins, function and neoplasia

Splenic marginal zone B cells of humans and mice are anatomically positioned with specialized macrophages, dendritic and endothelial cells. Together, they function as the first line of defense against blood borne pathogens with a low triggering threshold for B cells providing a rapid proliferative and antibody response to infections. In humans, B cells with similar cytology and physical relations to follicles are found in lymph nodes and Peyer's patches. However, they also develop in mucosa-associated lymphoid tissue (MALT) and other sites, such as the thyroid and salivary gland, that normally lack organized lymphoid tissue. Chronic antigenic stimulation at these sites or in response to infection with Hepatitis C provides the milieu for mutations at FAS, API2/ML, TP53 and INK4a/p19ARF and the development of marginal zone lymphomas (MZL) in node, spleen and MALT. Only splenic MZL are seen in mice. A reduced threshold for triggering to proliferation may predispose the marginal zone B cell to neoplasia with mutations in genes regulating apoptosis playing a leading role.

Clonal evolution in Waldenstrom macroglobulinemia highlights functional role of B-cell receptor

The course of clonal evolution of 2 related clones in the blood of a patient with Waldenstrom macroglobulinemia (WM) indicates the functional importance for the expression of the B-cell receptor for the survival of these malignant cells. Protein and nucleotide sequencing of the paraproteins' variable regions revealed 2 predominant Vlambda and 2 VH sequences, each set comprised in the ratio 1:1.5. The 2 VH sequences and 2 Vlambda sequences shared the same VDJ and VJ junctional sequences, respectively, indicating that 2 malignant clones had evolved from a common ancestor. This is the first report on intraclonal heterogeneity in WM. Comparison of the Vlambda and VH sequences with the closest matching known germline genes showed that they contained approximately 10 somatic mutations each. The distribution and type of mutations demonstrate that mutations have continued to accumulate in the malignant clones and that selection has been operating to preserve immunoglobulin structure.

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.

Clinical relevance of immunoglobulin mutation analysis

The precise diagnosis of malignant B-cell lymphoproliferations is essential for their appropriate clinical management. Histologic and immunophenotypical features alone are often insufficient in discriminating between various lymphoma entities and subtypes. The analysis of clonally rearranged immunoglobulin chain genes of the tumor cells was therefore suggested as an additional parameter. This article reviews current knowledge and describes which conclusions can be drawn for lymphoma diagnosis based on the type of immunoglobulin chain gene rearrangement, as well as on the number and distribution of somatic immunoglobulin mutations. Although there are striking correlations between the clinical outcome and the immunoglobulin mutations in some entities, many additional studies are required in order to draw final conclusions for most types of lymphoma.

Molecular insights into the immunopathogenesis of follicular lymphoma

Follicular lymphoma is caused by the transformation of a germinal-center-derived B cell with a t(14;18) chromosomal translocation. The distribution of somatic mutations within immunoglobulin genes indicates that follicular-lymphoma cells can interact with antigen. In addition, nonimmunoglobulin genes such as BCL6 seem to undergo somatic hypermutation. Here, Kostas Stamatopoulos and colleagues relate the molecular data about immunoglobulin genes and the protooncogenes BCL2 and BCL6 to the pathogenesis and evolution of follicular lymphoma.

Variable heavy chain gene analysis of follicular lymphomas: correlation between heavy chain isotype expression and somatic mutation load

The expansion of follicular lymphomas (FLs) resembles, both morphologically and functionally, normal germinal center B-cell growth. The tumor cells proliferate in networks of follicular dendritic cells and are believed to be capable of somatic hypermutation and isotype switching. To investigate the relation between somatic mutation and heavy chain isotype expression, we analyzed the variable heavy (VH) chain genes of 30 FL samples of different isotypes. The VH genes of the FLs were heavily mutated (29.3 mutations on average). In addition, isotype-switched lymphomas contained more somatic mutations than immunoglobulin M–positive lymphomas (33.8 mutations per VH gene versus 23.0, respectively). In all but one of the FLs, the ratios of replacement versus silent mutations in the framework regions were low, independent of the absolute number of somatic mutations and the level of intraclonal variation. Analysis of relapse samples of 4 FLs showed no obvious increase in somatic mutation load in most FLs and a decrease in intraclonal variation in time. In 3 of 4 cases, we obtained evidence for selection of certain subclones, rather than clonal evolution. Our findings question if intraclonal variation is always a reflection of ongoing somatic hypermutation. This may have implications for the concept of antigen-driven lymphomagenesis.

Quality control and sensitivity of polymerase chain reaction techniques for the assessment of immunoglobulin heavy chain gene rearrangements from fixed- and paraffin-embedded samples

Frozen tissue is considered the gold standard if DNA is to be extracted for polymerase chain reaction (PCR) analysis. In molecular studies from paraffin-embedded material, only positive results are usually taken into account. Our goal was to evaluate both the sensitivity and the specificity of PCR techniques for immunoglobulin heavy chain (IgH) gene rearrangement according to various lengths and types of fixative before paraffin embedding. One set of studies compared IgH rearrangement in a case of mantle cell lymphoma tissue that had been fixed in 14 different ways before paraffin embedding and frozen tissue. Formalin fixation was found not to be deleterious for DNA, amplification being possible up to 15 days after fixation with good sensitivity. In contrast, the performance of PCR decreased for samples fixed in Bouin's liquid for longer than 6 hours or after 48 hours of incubation in a vacuum infiltration processor (in which Bouin's liquid-fixed and formalin-fixed samples are mixed). In addition, we undertook a retrospective study of 20 routinely processed B-cell lymphomas, with frozen formalin-fixed and Bouin's liquid-fixed tissues for each case. Of the 14 positive cases on frozen material, 13 were also clonal from paraffin-embedded tissues. Whatever the IgH locus analyzed, each time the adapted control was positive, results from paraffin-embedded material were identical to results obtained from frozen tissue. In this study, we showed that the use of paraffin-embedded tissue is efficient for the study of IgH gene rearrangement. Whenever adapted controls are used, it is even possible to assess negative results.

Clonal evolution in a primary cutaneous follicle center B cell lymphoma revealed by single cell analysis in sequential biopsies

B cell neoplasias descending from germinal center cells harbor the hallmark of intraclonal diversity resulting from ongoing mutation in the variable parts of their immunoglobulin-encoding genes. To characterize a primary cutaneous follicle center B cell lymphoma in more detail, we analyzed the respective VH and VL genes in single cells mobilized from four sequential biopsies, three taken from the skin and one obtained after internal dissemination from a retrobulbar infiltrate. The lymphoma cells were found to contain V5-51/D6-12/JH5b (heavy chain) and A27/Jkappa2 (light chain) gene rearrangements detected on both the genomic and the transcriptional level. To provide an accurate mutation analysis, the specific VH gene counterpart (V5-51UK) was cloned from the patient's germline. Analyzing 226 single cells, we found: (i) complete nucleotide identity when VH and VL genes of lymphoma cells from one particular biopsy were compared among each other; (ii) intraclonal diversity due to ongoing mutation comparing the sequences obtained from sequential biopsies; (iii) both VH and VL genes to be highly mutated. Deducing from the sequence data, we propose a scenario of the clonal evolution of the B cell tumor in this patient. From the molecular-biological point of view, this primary cutaneous follicle center B cell lymphoma shows the features of a germinal center cell lymphoma. To draw this conclusion from single cell PCR data, however, a sample of sequential biopsies had to be analyzed.

Somatic mutation of the 5' noncoding region of the BCL-6 gene is associated with intraclonal diversity and clonal selection in histological transformation of follicular lymphoma

Follicular lymphoma (FL) is a B cell non-Hodgkin's lymphoma (NHL) that frequently displays a t(14;18) translocation. Clonal evolution and histological transformation of FL is frequently associated with the accumulation of secondary genetic alterations. It has been demonstrated that the BCL-6 gene can be altered by chromosomal rearrangements and by mutations clustering in its 5' noncoding region in a significant fraction of FL and diffuse large cell lymphoma (DLCL). To elucidate the role of the BCL-6 gene alterations in the histological transformation and clonal progression of FL, we analyzed serial biopsy specimens from 12 patients with FL. Two cases of FL showed no histological alteration in the second biopsy, and 10 cases of FL showed morphological transformation to DLCL in the second biopsy. Southern blot analysis was used to detect rearrangement of the BCL-6 gene, polymerase chain reaction-single strand conformation polymorphism and sequence analysis were performed for identification of mutations in the 5' noncoding region of the BCL-6 gene, and immunohistochemical analysis was applied to reveal the BCL-6 protein expression. No BCL-6 gene rearrangement was detected in any of the samples, but a total of 58 mutations were found in the 5' noncoding region of the BCL-6 gene in seven cases. In five cases, both the FL and the clonally related FL or DLCL, and in two cases only the DLCL samples were mutated. The mutations were identical in multiple biopsy specimens of FL that did not show morphological transformation. In six patients where FL cells underwent morphological transformation, considerable intraclonal sequence heterogeneity was observed, indicating an ongoing type of somatic mutation. Based on the pattern of shared and nonshared mutations, the genealogical relationship of neoplastic clones could be established. In all of these cases, the histological transformation of FL was associated with the emergence of a subpopulation marked by new sites of mutations in the BCL-6 5' noncoding sequences. In three of these six cases, the histological transformation is also associated with the reduced expression of the BCL-6 protein. These findings demonstrate that mutation of the 5' noncoding region of the BCL-6 gene developed in the clonal evolution of FL, and at different time points in the lymphoma evolution different clonotypes dominate.

Precursor B-lymphoblastic transformation of grade I follicle center lymphoma

Part of the natural history of follicle center lymphoma (FCL) is transformation to a more aggressive neoplasm, almost always a diffuse large B-cell lymphoma. We describe a rare example of a precursor B-lymphoblastic transformation of grade I FCL occurring in a 45-year-old woman 12 years after initial presentation and 3 years after successful treatment for a diffuse large cell transformation. The lymphoblastic lymphoma shared the same immunoglobulin heavy chain gene rearrangement as the FCL as assessed by polymerase chain reaction amplification and direct sequencing, as well as identical kappa light chain gene rearrangements by Southern blot analysis. The immunoglobulin heavy chain variable gene sequences of both tumors showed numerous identical base substitutions compared with germline sequences and 3 additional mutations in the lymphoblastic lymphoma not present in the low-grade FCL. These results indicate origin of the lymphoblastic process from the mature follicle center B-cell clone, rather than divergent origin of the 2 tumors from a common immature B-cell precursor.

The genetic variability of the VH genes in follicular lymphoma: the impact of the hypermutation mechanism

Follicular lymphoma (FL) cells have inherited an activated hypermutation mechanism from their origin of germinal centre B cells. Based on today's knowledge of the intrinsic properties related to this mechanism and the VH base composition, reconsideration of previous reports should be made on a broader range of samples. The present study examined the mutation pattern of the VH genes expressed by 55 cases of FL. FL VH genes showed evidence of antigenic selection in 30% of cases with 88% carrying a functional sIg and 78.2% showing intraclonal variation. VH family and gene segment utilization was found to be roughly similar to that of normal B lymphocytes. FL VH genes revealed extensive variations. 17% of the VH exons harboured a total of five deletions, three duplications and two insertions as compared to the most homologous germline counterpart. The VH genes of one tumour displayed three populations with varying CDR3 length at diagnosis. At relapse, emergence of a differently mutated gene, additional mutations reminiscent of ongoing mutations or no variation was prominent. From this study the heterogeneity of FLs is well established and ongoing mutations are seen in the scope of the activated status of the hypermutation mechanism rather than antigen-stimulated tumour growth.

VH gene sequences from a novel tropical splenic lymphoma reveal a naive B cell as the cell of origin

The prevalence of malaria and other infections in tropical Africa provides a setting for the emergence of B-cell tumours distinct from that in Western countries. Attempts to draw comparisons with Western lymphomas have led to difficulties, with so-called African chronic lymphocytic leukaemia (CLL) having a different pattern of incidence from Western CLL. Splenomegaly is common in African CLL, and this has posed diagnostic problems in differentiating the tumour from malaria-associated hyper-reactive malarial splenomegaly (HMS). One feature of the splenomegalic form of African CLL is that the tumour cells often possess short but fine cytoplasmic projections reminiscent of those observed in Western splenic lymphoma with villous lymphocytes (SLVL). Analysis of Ig VH genes both facilitates discrimination between clonal B-cell tumours and HMS, and reveals the differentiation status of the cell of origin. This study indicated that VH genes of nine cases of clonal splenic B-cell tumours with villous lymphocytes from Ghana were relatively unmutated, consistent with an origin from a naive B cell. These features differ from SLVL which arises from a post-follicular antigen-selected B cell. One possibility is that these splenic B-cell tumours derive from a splenic T-independent B cell, with malaria infection as a potential influence.

Unmutated Ig VH Genes Are Associated With a More Aggressive Form of Chronic Lymphocytic Leukemia

Despite having several characteristics of naı̈ve B cells, chronic lymphocytic leukemia (CLL) cells have been shown in some cases to have somatically mutated Ig variable region genes, indicating that the cell of origin has passed through the germinal center. A previous study of patients with CLL found an association between lack of somatic mutation and trisomy 12 and, therefore, possibly with a less favorable prognosis. We have sequenced the Ig VH genes of the tumor cells of 84 patients with CLL and correlated our findings with clinical features. A total of 38 cases (45.2%) showed ≥ 98% sequence homology with the nearest germline VH gene; 46 cases (54.8%) showed >2% somatic mutation. Unmutated VH genes were significantly associated with V1-69 and D3-3 usage, with atypical morphology; isolated trisomy 12, advanced stage and progressive disease. Survival was significantly worse for patients with unmutated VH genes irrespective of stage. Median survival for stage A patients with unmutated VH genes was 95 months compared with 293 months for patients whose tumors had mutated VHgenes (P = .0008). The simplest explanation is that CLL comprises 2 different diseases with different clinical courses. One, arising from a memory B cell, has a benign course, the other, arising from a naı̈ve B cell, is more malignant.

Unmutated Ig VH Genes Are Associated With a More Aggressive Form of Chronic Lymphocytic Leukemia

Despite having several characteristics of naı̈ve B cells, chronic lymphocytic leukemia (CLL) cells have been shown in some cases to have somatically mutated Ig variable region genes, indicating that the cell of origin has passed through the germinal center. A previous study of patients with CLL found an association between lack of somatic mutation and trisomy 12 and, therefore, possibly with a less favorable prognosis. We have sequenced the Ig VH genes of the tumor cells of 84 patients with CLL and correlated our findings with clinical features. A total of 38 cases (45.2%) showed ≥ 98% sequence homology with the nearest germline VH gene; 46 cases (54.8%) showed &gt;2% somatic mutation. Unmutated VH genes were significantly associated with V1-69 and D3-3 usage, with atypical morphology; isolated trisomy 12, advanced stage and progressive disease. Survival was significantly worse for patients with unmutated VH genes irrespective of stage. Median survival for stage A patients with unmutated VH genes was 95 months compared with 293 months for patients whose tumors had mutated VHgenes (P = .0008). The simplest explanation is that CLL comprises 2 different diseases with different clinical courses. One, arising from a memory B cell, has a benign course, the other, arising from a naı̈ve B cell, is more malignant.

Clonal evolution as judged by immunoglobulin heavy chain gene rearrangements in relapsing precursor-B acute lymphoblastic leukemia

Oligoclonality and ongoing clonal evolution are common features in patients with precursor-B (pre-B) acute lymphoblastic leukemia (ALL), as judged by immunoglobulin heavy chain (IgH) gene rearrangement analysis. These features are considered to be results of secondary rearrangements after malignant transformation or emergence of new tumor clones. In the present study we analyzed the IgH gene rearrangement status in 18 cases with relapsing pre-B ALL using variable heavy chain (V(H)) gene family specific polymerase chain reaction (PCR) amplification and single stranded conformation polymorphism (SSCP) analysis. Clonal IgH rearrangements were displayed in all leukemias but one, and altered rearrangement patterns occurred in five cases (29%), which were selected for detailed nucleotide sequence analysis. In one case, multiple subclones at diagnosis were suggested to be derived from a progenitor clone through joining of different V(H) germline gene segments to a pre-existing D-J(H) complex (V(H) to D-J(H) joining). Evidence for V(H) gene replacement with identical N-sequences at the V(H)-D junction and a common D-J(H) region was observed in one case. Diversification at the V(H)-D junction consisting of heterogeneous N-sequences were observed in one case. This molecular modification of the V(H)-D region could fit a hypothesized "open-and-shut" mechanism. Nevertheless, despite these ongoing events at least one IgH rearrangement remained unchanged throughout the disease in most patients, indicating that the immunoglobulin heavy chain locus can be a suitable marker for detection of minimal residual disease (MRD).

VH Gene Sequences From Primary Central Nervous System Lymphomas Indicate Derivation From Highly Mutated Germinal Center B Cells With Ongoing Mutational Activity

Primary central nervous system lymphoma (PCNSL) represents 1% to 3% intracranial tumors. Most PCNSL are located in the brain, and 75% are large B-cell lymphomas. The largest subgroup of these tumors contains cells that resemble centroblasts and has been labelled diffuse centroblastic (polymorphous) lymphoma. To investigate the cell of origin and the clonal history of these tumors, we have analyzed VH gene of 5 cases of PCNSL, all confirmed by histological studies to be Epstein-Barr virus (EBV)-negative, high-grade diffuse B-cell lymphomas. The V4-34 gene of the VH4 family was used in 4 of 5 cases. All VHgenes were found to have accumulated very high levels of somatic mutation (14% to 25%). In 3 of 5 cases, intraclonal nucleotide heterogeneity, including codon deletion in some clones in 1 case, was observed, indicating that the VH genes were still under the influence of the somatic hypermutation mechanism. Analysis of the distribution of silent and replacement mutations showed evidence for preservation of immunoglobulin structure in all cases. These results suggest that, although there is no evidence for germinal center formation in the brain tissue, PCNSL is derived from a B cell with features associated with location in a germinal center environment.

Somatic mutations and intraclonal variations in the rearranged Vkappa genes of B-non-Hodgkin's lymphoma cell lines

Three established Burkitt's lymphoma (BL) cell lines (Daudi, Raji and DG-75) and three B-non-Hodgkin's lymphoma (B-NHL) of other types (Pfeiffer, Farage and Toledo) were analyzed with respect to the presence of somatic point mutations in their rearranged immunoglobulin Vkappa genes. Two of the Vkappa sequences of BL and two of those of the B-NHL were heavily mutated (up to 11%), when compared with their closest germline variable region counterparts ("clonal mutations"). Only one of the six cell lines contained an unmutated germline Vkappa sequence. The clonal mutations have features characteristic of the mutation machinery operating in the course of the T-dependent immune response, such as a preference of mutations in purine bases, more transitions than transversions and targeting to CDR and to known "hotspot" motifs. Sequence variations among different Vkappa PCR clones isolated from each of the cell lines ("intraclonal mutations") showed that the Vkappa of Toledo exhibited about 5-fold higher mutation frequency (MF) than the background level of Taq polymerase error (approximately 0.12% mut/bp). Similarly, the MF of Vkappa of two of the BL cell lines was 3-4-fold higher than the Taq polymerase misincorporation rate. In contrast, the mutation frequencies of the Vkappa of DG-75, Farage and Pfeiffer did not significantly exceed the level of Taq polymerase error. Our combined results show that 5 out of the 6 B-cell lines studied originated from B-cells that have already somatically mutated in vivo their rearranged Vkappa genes. Moreover, two of the Burkitt's and one of the B-NHL cell lines exhibit intraclonal variation indicating that the process of somatic hypermutation continued following the neoplastic event, either in vivo or in culture. These results are in accord with the presumed origin of the majority of the BL and some types of the B-NHL, from centrocytes or centroblasts of the germinal centers in which the process of somatic hypermutation is taking place.

VH Gene Sequences From Primary Central Nervous System Lymphomas Indicate Derivation From Highly Mutated Germinal Center B Cells With Ongoing Mutational Activity

Primary central nervous system lymphoma (PCNSL) represents 1% to 3% intracranial tumors. Most PCNSL are located in the brain, and 75% are large B-cell lymphomas. The largest subgroup of these tumors contains cells that resemble centroblasts and has been labelled diffuse centroblastic (polymorphous) lymphoma. To investigate the cell of origin and the clonal history of these tumors, we have analyzed VH gene of 5 cases of PCNSL, all confirmed by histological studies to be Epstein-Barr virus (EBV)-negative, high-grade diffuse B-cell lymphomas. The V4-34 gene of the VH4 family was used in 4 of 5 cases. All VHgenes were found to have accumulated very high levels of somatic mutation (14% to 25%). In 3 of 5 cases, intraclonal nucleotide heterogeneity, including codon deletion in some clones in 1 case, was observed, indicating that the VH genes were still under the influence of the somatic hypermutation mechanism. Analysis of the distribution of silent and replacement mutations showed evidence for preservation of immunoglobulin structure in all cases. These results suggest that, although there is no evidence for germinal center formation in the brain tissue, PCNSL is derived from a B cell with features associated with location in a germinal center environment.

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

IgM-secreting plasma cell tumors are rare variants of typical isotype-switched multiple myeloma with a similar disease outcome. To probe the origin and clonal history of these tumors, we have analyzed VH gene sequences in 6 cases. Potentially functional tumor-derived VH genes were all derived from VH3, with the V3-7 gene segment being used by 4 of 6. All were somatically mutated, with a mean deviation from germline sequence of 5.2% (range, 3.1% to 7.1%). The distribution of replacement mutations was consistent with antigen selection in 4 of 6 cases, and no intraclonal heterogeneity was observed. Clonally related switched isotype transcripts were sought in 4 cases, and Cγ transcripts with tumor-derived CDR3 sequence were identified in 2 of 4. These findings indicate that IgM-secreting myelomas are arrested at a postfollicular stage at which somatic mutation has been silenced. Isotype switch variants show the cell of origin to be at the IgM to IgG switch point. These features indicate that the final neoplastic event has occurred at a stage immediately before that of typical isotype-switched myeloma. One possibility is that IgM myeloma involves the previously identified precursor cell of typical myeloma.