Abstract
Mantle cell lymphoma (MCL) is an aggressive non-Hodgkin’s lymphoma, originating from naïve B-cells. The blastoid MCL tumors often show complex cytogenetic aberrations. In this review, we summarized the data available on immunoglobulin heavy-chain (IgH) genes rearrangement for their importance in suggesting the MCL normal counterpart B-cell. Some new data suggesting an antigen selection process were also presented in this review.
keywords
Introduction
Mantle cell lymphoma (MCL) accounts for 4%–10% of all non-Hodgkin’s lymphomas (NHL). It occurs primarily among elderly individuals with a median age of approximately 60 years (range 29–85). The neoplasm has a male preference with male:female ratio around 2-7:1[1]. Two main morphological MCL subgroups occur: typical and blastoid variant(BV)MCL. The blastoid cases constitute approximately 10% of all MCL patients and are clinical more aggressive[2]. MCL cells typically express CD5 and CD20 and lack CD23 and CD10 expression[3]. Virtually all MCLs exhibit the translocation (11;14) (q13;q32), leading to deregulated ectopic expression of cyclin D1[4,5].
Most patients are diagnosed with widespread disease with disseminated lymphadenopathy and frequent involvement of the bone marrow (BM), spleen, and gastrointestinal (GI) tract[6,7]. MCL is considered one of the most aggressive lymphoid neoplasms with poor responses to conventional chemotherapy and a median survival time of 3 to 5 years. To date, there are no standard treatments for MCL.
In this review, we focused on the data available on immunoglobulin heavy-chain (IgH) genes rearrangement for their importance in suggesting the MCL normal counterpart B-cell. We also presented some new data suggesting an antigen selection process.
The immunoglobulin gene rearrangement in the development of B lymphocytes
B lymphocytes usually undergo an extensive genomic rearrangement within their Ig loci to express functional Igs[8–11]. The diversity of Ig molecules required for recognition of the tremendous range of different antigens a B-cell may encounter is created on different levels, of which the first is recombination of gene segments coding for the variable part of the Ig heavy chain molecule, corresponding to the antigen recognition site. This process is called VDJ recombination and takes place in the bone marrow during early B-cell maturation[12]. At the early pro-B stage one Diversity (D) segments is first joined to one Joining (J) regions. At the late pro-B stage, the D-J unit is joined to one Variable (V) genes. A rearranged region with a DNA sequence that is unique in each mature B lymphocyte is formed. Up to 46 functional IGHV genes, 23 IGHD genes and 6 IGHJ genes can be recombined in different combinations, creating an enormous variability in the IGHV-D-J sequence[2]. A mature Ig molecule also consists of one of two possible light chains – κ or λ, respectively – each also being the product of recombination of numerous light-chain V and J gene segments.
This apparently random recombination of the multiple V, D and J segments, in addition to excision of bases and insertion of non-germline ‘N’ nucleotides at the V–D and D–J junctions, generates a virtually unlimited antibody repertoire[13].
B-cell bearing Ig-molecules on their surface can move from the bone marrow to the periphery and circulate in the blood stream and the lymphatic system, ready for activation by antigen encounter. The antigen binds the Ig molecule in a region known as the complementarity-determining region 3 (CDR3), which is the most hypervariable region of the Ig molecule and is encoded by the unique DNA sequence spanning the IGHV-D-J rearrangement[14].
Most antigens that enter the body are caught in the lymph nodes, where circulating B-cells meet the antigen matching its specificity. Upon antigen-antibody binding, the B-cell is activated and migrates into the lymph node follicle and due to massive cell proliferation, creates a structure known as the GC. In the germinal centres, under the influence of antigen-specific CD4+ T-cells and follicular dendritic cells, B cells undergo proliferation, somatic hypermutation within V regions, and isotype class switch in order to even further increase antibody affinity[15]. Somatic hypermutation is dependent on the presence of the enzyme activation-induced cytidine deaminase (AID), which is expressed exclusively in the GC. AID mediates the substitution of cytidine to uracil, introducing base pair mismatch[16]. The result of somatic hypermutation is that some B-cells obtain a higher affinity to the antigen and are positively selected to differentiate to memory or plasma cells, while others get reduced antigen specificity and undergo apoptosis[17]. Altogether, the processes of V (D) J recombination, combination of heavy and light chains, junctional modifications and somatic hypermutation, create unique B-cell receptors on B-cells, not one similar to another. It is very helpful for us to identify the cellular origins of B cell lymphomas.
Ig VH gene mutation and B cell lymphomas
The Ig gene rearrangement in each B-cell is thus exclusive and since B-cell lymphomas and leukemias arise from monoclonally expanded B-cells, this can be used as markers of tumor clonality. Analysis of somatic hypermutation of the immunoglobulin variable heavy chain (IgVH) genes has provided valuable insight into the origin of B-cell lymphomas[18,19]. The presence of somatically acquired IgVH mutations is considered a consequence of the exposure of B cells to the microenvironment of the germinal center. By looking at the somatic mutation pattern of the Ig gene rearrangements expressed by lymphoma cells, normal B cell counterparts have been hypothesized for individual lymphoma subtypes, such as follicle center B cell for follicular lymphomas[20]. The mutational status of the Ig variable heavy chain (VH) genes of neoplastic B-cells reflects the point of maturation reached by the B-cell of origin, prior to transformation. Therefore, a mutated VH gene in a B-cell lymphoma indicates GC or post-GC B-cell origin, whereas an unmutated VH gene indicates derivation from a pre-GC B cell that has proliferated independently of T-cell help[21,22].
The cellular origin of MCL and somatic hypermutation
Up to now, the cell of origin of MCL is still unknown[1,23]. The t (11;14) (q13;q32), occurring in over 90% of the cases, is considered the MCL primary genetic event[24,25]. The analysis of the breakpoints of the derivative chromosomes suggests that this lesion takes place very early during the B-cell ontogenesis, in an early B-cell at the pre-B stage of differentiation, as an error during the the recombination of the Ig gene V(D)J segments[26]. The distribution of tumor cells surrounding germinal centers and the expression of several genes normally detected in naïve and normal follicular mantle zone B cells had supported the relationship of this tumor with B-cells of the primary lymphoid follicle or the mantle cells of secondary follicles[1]. Indeed, MCL cells were considered derived from naïve B cells, which are characterized by a lack of somatic mutations within their IgVH genes, i.e. pre-germinal centre B cells not yet exposed to antigen. Additionally, since MCL cells co-express the CD5, normal CD5 positive B lymphocytes, representing a small fraction of naïve B cells present in fetal blood and decreasing with age[27], was also considered as possible MCL normal counterparts.
These possible MCL normal counterparts have been more recently questioned. MCL cases with mutated IgH V-genes have been reported by different groups[28–37], including our groups[18,19]. Somatic hypermutations in IgH V-genes can be detected in 15%–40% of MCLs (Fig.1) indicating that some tumors originate in cells that have undergone the influence of the mutational machinery of the follicular germinal centre. In our two published studies, 56/61 cases (92%) had unmutated IgH (homology with closest VH gene ≥ 98%) and 16/61 (26%) had mutated IgH (homology with closest VH gene < 98%) (Fig.2). However, the load of somatic mutations is lower than in other B-cell tumors bearing mutated IgH. Less than 10% of MCL cases presented a somatic mutation rate higher than 4% (Fig.2). These data suggest that MCL cells derive from antigen-experienced B cells. However, due to the small load of mutations, it has been suggested that their normal counterpart might be marginal zone or peripheral blood memory B-cells which have undergone an extra-follicular T-cell independent antigen response.
Chronic lymphocytic leukemia (CLL) and MCL share many similarities, such as morphology, immunophenotype and recurrent genetic lesions[20]. About half of CLL cases have evidence of somatic hypermutations in the immunoglobulin heavy-chain variable-region (IgVH) genes. CLL patients with mutated IgVH genes have a much better clinical outcome than patients carrying unmutated IgH[38–40]. Mutated MCL cases show only marginally better outcome that unmutated cases[2,35,41]. Differently from CLL, in MCL the analysis of the IgH mutational status is not part of the diagnostic routine for newly diagnosed patients.
Biased usage of IgVH genes in MCL
Studies on CLL have revealed a biased use of VH genes with a frequency of particular VH genes higher than expected. VH1-69, VH3-21, VH4-34, VH3-30 and VH3-23 appear as the most commonly used[40,42].
A biased VH usage is also present in MCL (Fig.3). Here, VH3-21 is the most commonly observed gene. Interestingly, in CLL, VH3-21 positive cases are usually mutated, associated with the Ig lambda light chain containing the V2-14, and have a poor clinical outcome[39,40,43,44]. Instead, in MCL VH3-21 is usually unmutated, combined with a Ig kappa light chain V3-19, and associated with a better outcome[1,2,23,34,36]. Underlining genetics also seem different. While CLL-VH321 positive cases often carry 11q and 17p deletions, both very well known poor prognostic markers, MCL-VH3-21 positive cases seem to have less genomic lesions than VH3-21 negative patients.
As a whole, the observed biased VH usage suggests that MCL tumor cells may originate from specific subsets of B cells, possibly selected by antigens.
Stereotyped CDR3s in mantle cell lymphoma?
A fraction of CLL patients carry highly homologous B cell receptors characterized by non-random stereotyped combinations of specific VH genes and CDR3, often associated with a restricted number of Ig light kappa or lambda chain genes, suggesting that CLL cells are selected by specific exogenous of auto-antigens[45–51].
In MCL, the amino acid sequences in the CDR3 are different among cases using the IGHV3-21 gene, suggesting antigen binding outside the CDR3 such as on the framework regions. Interestingly, unique among all B-cell lymphoid neoplasms, MCL more commonly express Ig lambda than kappa light chains, and this could also be due to the presence of antigens (super antigens) binding outside the CDR3.
To further evaluate the possible presence of an antigens election in MCL, we applied clustering algorithms to 48 MCL CDR3 amino acid sequences[18,19], analyzing them together with 2150 CLL, 1973 normal B-cells, 563 immuno-disfunction, 1247 auto-reactive B-cells, 259 B-cell neoplasia, 486 HIV-positive and post-transplant patients. MCL cases presented a certain degree of homology with CLL-derived cases, but the highest homology was restricted to the D and J segments, and not the VH part of the CDR3. MCL showed homology with normal B-cells. Similarly to what previously described for CLL, this homology was higher with sequences derived from neonatal lymphocytes than from other development stage, including other neoplastic mature B-cells or B-cells associated to immuno-disfunction or auto-reactivity. Despite a low numbers of samples analyzed, we could observe that MCL cases tend to align together, and that individual clusters, carrying almost the same VDJ could be identified. Interestingly, these clusters were not overlapping with known CLL clusters.
Conclusion
MCL consists of one major subset with unmutated IGHV genes and another smaller subset with mutated IGHV genes[21,36,37]. The latter represents about 20%–30% of cases and the presence of mutations suggests either exposure to a GC environment or alternatively, that somatic hypermutations have been acquired in a non-GC place, such as in the marginal zone. A possible role of an unknown antigen selecting cells which then might give origin to MCL can be hypothesized based upon the observed biased VH usage, as well by the presence of small groups of MCL cases sharing the same CDR3.
For more recent analysis, different subsets in MCL are suggested as a common origin arising from the marginal zone, which surrounds the mantle zone[2]. Furthermore, the IGHV gene usage is restricted in MCL as well as in CLL. IGHV3-21 and IGHV4-34 are the two most predominant IGHV genes utilized in MCL and interestingly, IGHV3-21+ cases also share the same light chain in most cases analyzed. This indicates that antigen stimulation might be a part of the pathogenic process in MCL development, where future studies hopefully will reveal if antigens play a role or not in pathogenesis. Using array-CGH, it has been found that losses in 9p and gains in 15q were exclusively detected in non-IGHV3-21 and the IGHV3-21+cases displayed only half as many genetic alterations compared to MCLs with other IGHV genes. Thus, these evidences support the hypothesis that IGHV3-21+ tumours constitute a separate subentity within MCL, possibly with antigen stimulation involved in the pathogenesis, and provide some genetic explanation to account for the less aggressive clinical behaviour of these MCL tumours.
The possibility is that continuous proliferation stimuli by antigens might increase the risk of the fast dividing B-cells to undergo oncogenetic events, such as chromosomal rearrangements. The identification of potential antigens involved in lymphoma development is of high significance to search the real pathogenesis in the haematological disease.
Conflict of interest statement
No potential conflicts of interest were disclosed.
Acknowledgments
Dr. Hui-lai Zhang is recipient of a fellowship from the Swiss State Secretariat for Education and Research (SER). Work partially supported by the Krebsforschung Schweiz and the Swiss Group for Clinical Research (SAKK).
- Received March 11, 2011.
- Accepted June 4, 2011.
- Copyright © 2011 by Tianjin Medical University Cancer Institute & Hospital and Springer
References
In this issue
Jump to section
- Article
- Abstract
- Introduction
- The immunoglobulin gene rearrangement in the development of B lymphocytes
- Ig VH gene mutation and B cell lymphomas
- The cellular origin of MCL and somatic hypermutation
- Biased usage of IgVH genes in MCL
- Stereotyped CDR3s in mantle cell lymphoma?
- Conclusion
- Conflict of interest statement
- Acknowledgments
- References
- Figures & Data
- Info & Metrics
- References
Related Articles
Cited By...
- No citing articles found.