Lack of PAX5 rearrangements in lymphoplasmacytic lymphomas: reassessing the reported association with t(9;14)1 1These studies were performed in the University of Pittsburgh Cancer Institute Cytogenetics Facility

[Clinical and biological characteristics of non-IgM lymphoplasmacytic lymphoma]

Objective: The study aims to explore the clinical and biological characteristics of patients with non-IgM lymphoplasmacytic lymphoma (LPL) . Methods: The clinical data of 340 patients with LPL admitted to the Institute of Hematology & Blood Diseases Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College were collected retrospectively, including 23 cases of the non-IgM LPL and 317 cases of the Waldenström's macroglobulinemia (WM) , from July 1993 to August 2020. The clinical and biological characteristics of the two groups were compared. Results: Among 23 patients with the non-IgM type LPL, two patients secreted monoclonal IgA, 14 patients secreted monoclonal IgG, and seven patients did not secrete monoclonal immunoglobulin. The median age of the non-IgM LPL and WM were both 62 (35-81) years old. Compared with the WM group, the proportion of women (56.5% vs 27.3%, P=0.007) , the proportion of splenomegaly (60.1% vs 43.8%, P=0.100) , and the proportion of extranodal invasion (21.7% vs 12.3%, P=0.672) in non-IgM LPL group were higher. Eighteen patients were tested for MYD88 gene mutation, and the overall mutation rate of MYD88 was 55.6%. In the non-IgM LPL group, a total of 17 patients received treatment, which had a comparable proportion (94.4% vs 92.7%, P=0.488) to the WM group. Sixteen patients were evaluated for efficacy, and the overall remission rate of the first-line treatment was 87.5%. The median follow-up time was 33.9 (3.5-125.1) months, and the median PFS and OS were both not reached. The 3-year PFS and OS rates were 71.4% and 68.9%, respectively. In the WM group, the median PFS was 66.2 months and the median OS was 78.1 months. Compared with the WM group, in the non-IgM group no significant differences in PFS (P=0.340) and OS (P=0.544) were seen. Conclusion: The clinical and biological characteristics of the non-IgM LPL and WM patients were similar. However, the proportion of women and extranodal involvement were higher in the non-IgM LPL group. The survival and prognosis of the non-IgM LPL patients were similar to those of the WM patients.

Diffuse large B-cell lymphoma carrying t(9;14)(p13;q32)/PAX5-immunoglobulin heavy chain gene is characterized by nuclear positivity of MUM1 and PAX5 by immunohistochemistry

We described four patients with diffuse large B-cell lymphoma (DLBCL) carrying t(9;14)(p13;q32) that places the PAX5 adjacent to the immunoglobulin heavy chain (IGH) gene. Ages ranged between 63 and 80, and three were female. One developed a nodal disease, and the other three involved extranodal organs. The lymphoma cells were CD10^- /BCL6^- /MUM1⁺ in three and CD10⁺ /BCL6⁺ /MUM1⁺ in one. BCL2 was weak or negative. All had t(9;14)(p13;q32), and three had additional 14q32/IGH translocations or +der(14)t(9;14)(p13;q32). Fluorescence in situ hybridization using the PAX5 break-apart probe showed that the locus was disrupted between the 5' and 3' probes or within the 5' probe. Immunohistochemistry (IHC) using a monoclonal antibody against PAX5 showed strong nuclear positivity in all four patients. Cell block IHC of a CD30⁺ DLBCL cell line, KIS-1, which carried the t(9;14)(p13;q32) and PAX5-IGH fusion gene, reproduced the CD10^- /BCL6^- /MUM1⁺ immunophenotype, low-level BCL2, and strong nuclear PAX5. Uniform nuclear positivity of MUM1 in all four cases and KIS-1 cells suggest that these lymphomas arose at a late stage of B-cell differentiation, where expression of PAX5 physiologically becomes downregulated. It is therefore possible that high-level PAX5 resulting from t(9;14)(p13;q32) at this stage of differentiation perturbs the plasma cell differentiation program initiated by PAX5 repression, thereby contributing to the development of a fraction of DLBCL.

An unusual case of Epstein-Barr virus-positive large B-cell lymphoma lacking various B-cell markers

Backgroud

Epstein-Barr virus (EBV) is associated with B-cell lymphoma in various conditions, such as immunodeficiency and chronic inflammation. We report an unusual case of EBV-positive diffuse large B-cell lymphoma (DLBCL) lacking the expression of many B-cell markers.

Case presentation

An 83-year-old man presented with a submandibular tumor. Histology of a lymph node biopsy specimen revealed diffuse proliferation of centroblast- or immunoblast-like lymphoid cells with plasmacytic differentiation. Scattered Hodgkin/Reed-Sternberg-like cells were also visible. A routine immunohistochemistry antibody panel revealed that the tumor cells were negative for B-cell and T-cell markers (i.e., CD3, CD19, CD20, CD38, CD45RO, CD79a, CD138, and Pax-5), but were positive for CD30 and MUM-1, not defining the lineage of tumor cells. The final diagnosis of EBV-positive DLBCL was confirmed based on the expression of B-cell-specific transcription factors (Oct-2 and BOB.1), PCR-based identification of monoclonal rearrangement of the immunoglobulin genes, and the presence of EBV-encoded small RNAs in the tumor cells (identified using in situ hybridization).

Conclusion

The downregulation of broad band of B-cell markers in the present case with EBV-positive DLBCL posed a diagnostic dilemma, as the possible diagnoses included differentiation from anaplastic large cell lymphoma and CD20-negative B-cell lymphomas. Results of immunohistochemical panel including B-cell-specific transcription factors and gene rearrangement analyses critically support the correct diagnosis.

Genetic aberrations in small B-cell lymphomas and leukemias: molecular pathology, clinical relevance and therapeutic targets

Small B-cell lymphomas and leukemias (SBCLs) are a clinically, morphologically, immunophenotypically and genetically heterogeneous group of clonal lymphoid neoplasms, including entities such as chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), mantle cell lymphoma (MCL), follicular lymphoma (FL), lymphoplasmacytic lymphoma (LPL), marginal zone lymphoma (MZL) and hairy cell leukemia (HCL). The pathogenesis of some of these lymphoid malignancies is characterized by distinct translocations, for example t(11;14) in the majority of cases of MCL and t(14;18) in most cases of FL, whereas other entities are associated with a variety of recurrent but nonspecific numeric chromosomal abnormalities, as exemplified by del(13q14), del(11q22), and +12 in CLL, and yet others such as LPL and HCL that lack recurrent or specific cytogenetic aberrations. The recent surge in next generation sequencing (NGS) technology has shed more light on the genetic landscape of SBCLs through characterization of numerous driver mutations including SF3B1 and NOTCH1 in CLL, ATM and CCND1 in MCL, KMT2D and EPHA7 in FL, MYD88 (L265P) in LPL, KLF2 and NOTCH2 in splenic MZL (SMZL) and BRAF (V600E) in HCL. The identification of distinct genetic lesions not only provides greater insight into the molecular pathogenesis of these disorders but also identifies potential valuable biomarkers for prognostic stratification, as well as specific targets for directed therapy. This review discusses the well-established and recently identified molecular lesions underlying the pathogenesis of SBCLs, highlights their clinical relevance and summarizes novel targeted therapies.

MYD88 L265P mutation analysis helps define nodal lymphoplasmacytic lymphoma

The diagnosis of lymphoplasmacytic lymphoma is often challenging, especially in extramedullary tissues where the differential diagnosis includes nodal marginal zone lymphoma, splenic marginal zone lymphoma, or other small B-cell neoplasms with plasmacytic differentiation. The MYD88 L265P mutation has been recently identified in >90% of bone-marrow-based lymphoplasmacytic lymphoma, but the incidence of this abnormality and corresponding morphologic correlates in nodal lymphoplasmacytic lymphoma have not been established. We analyzed 87 cases of extramedullary lymphoplasmacytic lymphoma, splenic marginal zone lymphoma, unclassifiable splenic B-cell lymphomas, nodal marginal zone lymphoma with plasmacytic differentiation, and chronic lymphocytic leukemia/small lymphocytic lymphoma with plasmacytic differentiation for MYD88 L265P. Eighteen cases (21%) were positive, including 9/9 (100%) lymphoplasmacytic lymphomas with classic histologic features, 5/12 (42%) cases that met 2008 WHO criteria for lymphoplasmacytic lymphoma but with atypical morphologic features, 3/15 (20%) cases initially considered nodal marginal zone lymphoma with plasmacytic differentiation, and 1/6 (17%) unclassifiable splenic B-cell lymphomas. The presence of MYD88 L265P was associated with IgM paraprotein (P<0.001) and a trend for bone marrow involvement (P=0.09). Each of 44 splenectomy-defined splenic marginal zone lymphomas (19 with plasmacytic differentiation) and the chronic lymphocytic leukemia/small lymphocytic lymphoma with plasmacytic differentiation were negative for the mutation. Morphologic re-review with knowledge of MYD88 mutation status and all available clinical features suggested all MYD88 mutated cases were consistent with lymphoplasmacytic lymphoma (either classic or variant histology), except for one case which remained most consistent with nodal marginal zone lymphoma with plasmacytic differentiation. These results demonstrate the importance of MYD88 mutational analysis in better defining lymphoplasmacytic lymphoma as a relatively monomorphic small B-cell lymphoma with plasmacytic differentiation that may show total nodal architectural effacement and follicular colonization. Cases previously considered lymphoplasmacytic lymphoma that are more polymorphous and are often associated with histiocytes should no longer be included in the lymphoplasmacytic lymphoma category. Clinicopathologic review suggests that although MYD88 mutated non-lymphoplasmacytic lymphoma small B-cell neoplasms exist, they are very uncommon.

Nodal and leukemic small B-cell neoplasms

The small B-cell neoplasms represent some of the most frequently encountered lymphoproliferative disorders in routine surgical pathology practice. This report reviews the current diagnostic criteria for classifying small B-cell neoplasms and distinguishing them from newly recognized precursor conditions that do not appear to represent overt lymphomas. Newly available immunohistochemical stains and molecular studies that may assist in the diagnosis and classification of these neoplasms are also discussed.

Biphenotypic B-lymphoid/myeloid cells expressing low levels of Pax5: potential targets of BAL development

The expression of Pax5 commits common lymphoid progenitor cells to B-lymphoid lineage differentiation. Little is known of possible variations in the levels of Pax5 expression and their influences on hematopoietic development. We have developed a retroviral transduction system that allows for the study of possible intermediate stages of this commitment by controlling the levels of Pax5 expressed in Pax5-deficient progenitors in vitro and in vivo. Retroviral transduction of Pax5-deficient pro-/pre-B cell lines with a doxycycline-inducible (TetON) form of the human Pax5 (huPax5) gene yielded cell clones that could be induced to different levels of huPax5 expression. Clones inducible to high levels developed B220+/CD19+/IgM+ B cells, while clones with low levels differentiated to B220+/CD19−/CD11b+/Gr-1− B-lymphoid/myeloid biphenotypic cells in vitro and in vivo. Microarray analyses of genes expressed at these lower levels of huPax5 identified C/ebpα, C/ebpδ, Pu.1, Csf1r, Csf2r, and Gata-3 as myeloid-related genes selectively expressed in the pro-/pre-B cells that can develop under myeloid/lymphoid conditions to biphenotypic cells. Therefore, reduced expression of huPax5 during the induction of early lymphoid progenitors to B-lineage–committed cells can fix this cellular development at a stage that has previously been seen during embryonic development and in acute lymphoblastic lymphoma–like biphenotypic acute leukemias.

Gamma heavy-chain disease: defining the spectrum of associated lymphoproliferative disorders through analysis of 13 cases

Gamma heavy-chain disease (gHCD) is defined as a lymphoplasmacytic neoplasm that produces an abnormally truncated immunoglobulin gamma heavy-chain protein that lacks associated light chains. There is scant information in the literature regarding the morphologic findings in this rare disorder, but cases have often been reported to resemble lymphoplasmacytic lymphoma (LPL). To clarify the spectrum of lymphoproliferative disorders that may be associated with gHCD, this study reports the clinical, morphologic, and phenotypic findings in 13 cases of gHCD involving lymph nodes (n=7), spleen (n=2), bone marrow (n=8), or other extranodal tissue biopsies (n=3). Clinically, patients showed a female predominance (85%) with frequent occurrence of autoimmune disease (69%). Histologically, 8 cases (61%) contained a morphologically similar neoplasm of small lymphocytes, plasmacytoid lymphocytes, and plasma cells that was difficult to classify with certainty, whereas the remaining 5 cases (39%) showed the typical features of one of several other well-defined entities in the 2008 WHO classification. This report demonstrates that gHCD is associated with a variety of underlying lymphoproliferative disorders but most often shows features that overlap with cases previously reported as "vaguely nodular, polymorphous" LPL. These findings also provide practical guidance for the routine evaluation of small B-cell neoplasms with plasmacytic differentiation that could represent a heavy-chain disease and give suggestions for an improved approach to the WHO classification of gHCD.

CD19 is a major B cell receptor-independent activator of MYC-driven B-lymphomagenesis

PAX5, a B cell-specific transcription factor, is overexpressed through chromosomal translocations in a subset of B cell lymphomas. Previously, we had shown that activation of immunoreceptor tyrosine-based activation motif (ITAM) proteins and B cell receptor (BCR) signaling by PAX5 contributes to B-lymphomagenesis. However, the effect of PAX5 on other oncogenic transcription factor-controlled pathways is unknown. Using a MYC-induced murine lymphoma model as well as MYC-transformed human B cell lines, we found that PAX5 controls c-MYC protein stability and steady-state levels. This promoter-independent, posttranslational mechanism of c-MYC regulation was independent of ITAM/BCR activity. Instead it was controlled by another PAX5 target, CD19, through the PI3K-AKT-GSK3β axis. Consequently, MYC levels in B cells from CD19-deficient mice were sharply reduced. Conversely, reexpression of CD19 in murine lymphomas with spontaneous silencing of PAX5 boosted MYC levels, expression of its key target genes, cell proliferation in vitro, and overall tumor growth in vivo. In human B-lymphomas, CD19 mRNA levels were found to correlate with those of MYC-activated genes. They also negatively correlated with the overall survival of patients with lymphoma in the same way that MYC levels do. Thus, CD19 is a major BCR-independent regulator of MYC-driven neoplastic growth in B cell neoplasms.

Waldenström macroglobulinemia: a review of the entity and its differential diagnosis

The definition of Waldenström macroglobulinemia (WM), originally described in 1944, has been refined substantially over time. The current fourth edition of the World Health Organization of lymphoid neoplasms, in large part, adopted criteria proposed for WM at a consensus conference in 2002. WM is defined as lymphoplasmacytic lymphoma involving the bone marrow associated with a serum immunoglobulin (Ig) M paraprotein of any concentration. Morphologically, WM is composed of a variable mixture of lymphocytes, plasmacytoid lymphocytes, and plasma cells. Immunophenotypically, the neoplastic cells express monotypic IgM and light chain: B lymphocytes express pan-B-cell antigens and surface Ig are usually negative for CD5 and CD10; and plasma cells are typically positive for CD138, CD38, CD45, cytoplasmic Ig, and CD19 (in a substantial subset of cases). The putative cell of origin of WM is a postantigen selected memory B-cell that has undergone somatic hypermutation. The most common cytogenetic abnormality in WM is del(6q), usually in the region 6q23-24.3, present in 40% to 50% of cases. IGH gene translocations are rare and recurrent chromosomal translocations or gene aberrations have not been identified in WM. Here, we provide a historical perspective of WM, review clinical and pathologic aspects of the disease as it is currently defined, and discuss some practical issues in the differential diagnosis of WM that pathologists encounter in the signout of cases.

Lymphoplasmacytic lymphoma and other non-marginal zone lymphomas with plasmacytic differentiation

Small B-cell lymphomas with plasmacytic differentiation frequently present diagnostic challenges. Session 3 of the 2009 Society for Hematopathology/European Association for Haematopathology Workshop focused on lymphoplasmacytic lymphoma (LPL). The submitted cases illustrated classic examples of bone marrow-based and nodal LPL and cases with atypical features, including unusual phenotypes or involvement of extranodal sites. Several cases showed varying degrees of overlap with marginal zone lymphoma, and, as acknowledged in the 2008 World Health Organization classification, a definitive distinction between these 2 possibilities cannot always be established. Session 6 of the workshop focused on other non-marginal zone lymphomas that may display plasmacytic differentiation. This session highlights the wide variety of neoplasms that enter into the differential diagnosis of small B-cell lymphomas with plasmacytic differentiation and demonstrates the use of clinical features and ancillary studies in establishing an appropriate diagnosis by 2008 World Health Organization criteria.

Pax5: a master regulator of B cell development and leukemogenesis

The B cell lineage of the hematopoietic system is responsible for the generation of high-affinity antibodies, which provide humoral immunity for protection against foreign pathogens. B cell commitment and development depend on many transcription factors including Pax5. Here, we review the different functions of Pax5 in regulating various aspects of B lymphopoiesis. At B cell commitment, Pax5 restricts the developmental potential of lymphoid progenitors to the B cell pathway by repressing B-lineage-inappropriate genes, while it simultaneously promotes B cell development by activating B-lymphoid-specific genes. Pax5 thereby controls gene transcription by recruiting chromatin-remodeling, histone-modifying, and basal transcription factor complexes to its target genes. Moreover, Pax5 contributes to the diversity of the antibody repertoire by controlling V(H)-DJ(H) recombination by inducing contraction of the immunoglobulin heavy-chain locus in pro-B cells, which is likely mediated by PAIR elements in the 5' region of the V(H) gene cluster. Importantly, all mature B cell types depend on Pax5 for their differentiation and function. Pax5 thus controls the identity of B lymphocytes throughout B cell development. Consequently, conditional loss of Pax5 allows mature B cells from peripheral lymphoid organs to develop into functional T cells in the thymus via dedifferentiation to uncommitted progenitors in the bone marrow. Pax5 has also been implicated in human B cell malignancies because it can function as a haploinsufficient tumor suppressor or oncogenic translocation fusion protein in B cell precursor acute lymphoblastic leukemia.

The basis and rational use of molecular genetic testing in mature B-cell lymphomas

An increasing number of neoplasms are associated with variably specific genetic abnormalities. This is best exemplified by hematological malignancies, in which there is a growing list of entities that are defined by their genetic lesion(s); this is not (yet) the case in mature B-cell lymphomas. However, enhanced insights into the pathogenesis of this large and diverse group of lymphomas have emerged with the ongoing unraveling of a plethora of fascinating genetic abnormalities. The purpose of this review is to synthesize well-recognized data and nascent discoveries in our understanding of the genetic basis of a spectrum of mature B-cell lymphomas, and how this may be applied to contemporary clinical practice. Despite the explosion of new and exciting knowledge in this arena, with the potential for enhanced diagnostic and prognostic strategies, it is essential to remain cognizant of the limitations (and complexity) of genetic investigations, so that assays can be developed and used both judiciously and rationally.

Epstein-Barr virus-positive diffuse large B-cell lymphoma carrying a t(9;14)(p13;q32) translocation

We, herein, report a 75-year-old man with lymphoma who initially presented with disseminated disease involving the lung, followed by temporal regression, and finally died of disease progression. Lymph-node biopsy showed a morphology of diffuse large B-cell lymphoma (DLBCL), containing CD30(+) Reed-Sternberg-like cells. The lymphoma cells were stained by in situ hybridization (ISH) for Epstein-Barr virus (EBV)-encoded RNA, and the presence of the EBV genome was confirmed by the polymerase chain reaction. A cytogenetic study showed that the lymphoma cells carried a t(9;14)(p13;q32) translocation, and rearrangement of the PAX5 gene was determined by fluorescence ISH using a split signal probe. This case report is the first to identify t(9;14)(p13;q32) in EBV(+) DLBCL of the elderly, which was very recently listed among subtypes of DLBCL.

Fluorescence immunophenotypic and interphase cytogenetic characterization of nodal lymphoplasmacytic lymphoma

Lymphoplasmacytic lymphoma (LPL) is a small B-cell lymphoma with plasmacytic differentiation that does not fulfill the criteria for any other small B-cell lymphoma. Cytogenetic characterization of nodal LPL is limited and the distinction from marginal zone lymphomas with plasmacytic differentiation can be problematic. Thus, 17 cases of lymph node-based LPL were studied with fluorescence immunophenotypic and interphase cytogenetics for the investigation of neoplasia (FICTION) using a CD79a antibody and probes to detect trisomies of chromosomes 3 (15 cases), 12 (16 cases), and 18 (17 cases); rearrangements (R) of IgH (10 cases), BCL6 (6 cases), PAX5 (7 cases), and MALT1 (16 cases); and deletion 6q21 (7 cases). Cases with IgH R were further studied with an IgH/BCL2 probe. In cases without FICTION studies, previously reported fluorescence in situ hybridization results for IgH, PAX5, and deletion 6q21 were available from prior studies. The histopathology, immunophenotype, and available clinical data were also reviewed. Three pathologic categories were recognized: 5 classic LPL, 5 vaguely nodular polymorphous (VN-P), and 7 other. Among the classic LPL, 4/4 had an IgM paraproteinemia, 5/5 had bone marrow involvement (BM+), and 1/5 had +MALT1. One of one VN-P LPL had an IgM paraprotein, 2/4 were IgM+, 2/4 IgG+, 1/3 had BM+, and 1/5 had an IgH R. Among the other cases, 2/3 had a paraprotein, 2/7 were IgM+, 5/7 IgG+, and 0/3 had BM+. Of these cases, 1 showed +12, 1 +18, and 1 IgH/BCL2 rearrangement plus +18. None of the 17 cases had a 6q21 deletion or +3. Therefore, with rare exception, lymph node-based LPL with classic or more varied histopathologic features does not have the cytogenetic abnormalities frequently associated with bone marrow-based LPL/Waldenstrom macroglobulinemia or many of the marginal zone lymphomas. The search for better objective inclusionary criteria for LPL must continue.

B cell activator PAX5 promotes lymphomagenesis through stimulation of B cell receptor signaling

The presumed involvement of paired box gene 5 (PAX5) in B-lymphomagenesis is based largely on the discovery of Pax5-specific translocations and somatic hypermutations in non-Hodgkin lymphomas. Yet mechanistically, the contribution of Pax5 to neoplastic growth remains undeciphered. Here we used 2 Myc-induced mouse B lymphoma cell lines, Myc5-M5 and Myc5-M12, which spontaneously silence Pax5. Reconstitution of these cells with Pax5-tamoxifen receptor fusion protein (Pax5ER(TAM)) increased neoplastic growth in a hormone-dependent manner. Conversely, expression of dominant-negative Pax5 in murine lymphomas and Pax5 knockdown in human lymphomas negatively affected cell expansion. Expression profiling revealed that Pax5 was required to maintain mRNA levels of several crucial components of B cell receptor (BCR) signaling, including CD79a, a protein with the immunoreceptor tyrosine-based activation motif (ITAM). In contrast, expression of 2 known ITAM antagonists, CD22 and PIR-B, was suppressed. The key role of BCR/ITAM signaling in Pax5-dependent lymphomagenesis was corroborated in Syk, an ITAM-associated tyrosine kinase. Moreover, we observed consistent expression of phosphorylated BLNK, an activated BCR adaptor protein, in human B cell lymphomas. Thus, stimulation of neoplastic growth by Pax5 occurs through BCR and is sensitive to genetic and pharmacological inhibitors of this pathway.

Diagnostic uses of Pax5 immunohistochemistry

Pax5, or B-cell-specific activator protein, is a nuclear protein in the paired-box containing (PAX) family of transcription factors involved in control of organ development and tissue differentiation. Pax5 is mostly expressed in B lymphocytes and B-cell lymphomas, although recent data have shown expression in the developing central nervous system, some neuroendocrine tumors, and occasional myeloid leukemias. Pax5 immunohistochemistry shows robust nuclear staining, and has become a valuable tool in the diagnosis and subclassification of lymphomas. Pax5 staining is positive in most Hodgkin and B-cell non-Hodgkin lymphomas, and also precursor B-cell lymphoblastic neoplasms. Plasma cell neoplasms, multiple myeloma, and plasmablastic lymphomas typically are negative. T-cell lymphomas are, to date, consistently negative. Recently, Pax5 expression has been described in the majority of small cell carcinomas and Merkel cell carcinomas. Rare cases of Pax5 expression in other carcinomas have been reported. With these exceptions, Pax5 immunohistochemistry is fairly specific for B-cell lineage and is a valuable addition to the armamentarium of markers available for lymphoma subtyping.

Molecular genetic analysis of haematological malignancies II: mature lymphoid neoplasms

Molecular genetic techniques have become an integral part of the diagnostic assessment for many lymphomas and other chronic lymphoid neoplasms. The demonstration of a clonal immunoglobulin or T cell receptor gene rearrangement offers a useful diagnostic tool in cases where the diagnosis is equivocal. Molecular genetic detection of other genomic rearrangements may not only assist with the diagnosis but can also provide important prognostic information. Many of these rearrangements can act as molecular markers for the detection of low levels of residual disease. In this review, we discuss the applications of molecular genetic analysis to the chronic lymphoid malignancies. The review concentrates on those disorders for which molecular genetic analysis can offer diagnostic and/or prognostic information.

Rare expression of BSAP (PAX-5) in mature T-cell lymphomas

Lineage determination in lymphomas is based on the assessment of lineage-specific markers, such as the B-cell-specific activator protein of the paired box family (BSAP, PAX-5) for the B-cell lineage. BSAP is thought to be expressed exclusively in B cells from the pro-B- to the mature B-cell stage and then silenced in plasma cells. BSAP has oncogenic potential and experimental evidence shows that the T-cell lineage is prone to this effect. Herein, we report on a BSAP-positive peripheral T-cell lymphoma with monoclonal T-cell receptor gamma-gene rearrangement. To assess the relative frequency of BSAP expression in mature T-cell lymphomas, we constructed and examined a tissue microarray consisting of 43 angioimmunoblastic T-cell lymphomas and peripheral T-cell lymphomas and detected no additional BSAP-positive cases. To conclude, BSAP can probably contribute to T-cell lymphomagenesis not only in vitro, but also in vivo. It is rarely expressed in peripheral T-cell lymphoma, thus its detection on lymphoid malignancies cannot be considered definitively lineage specific.

Oncogenic role of Pax5 in the T-lymphoid lineage upon ectopic expression from the immunoglobulin heavy-chain locus

Four of 9 PAX transcription factor genes have been associated with chromosomal translocations in human tumors, although their oncogenic potential has not yet been demonstrated in transgenic mouse models. The B-lymphoidPAX5 gene participates in the generation of the t(9;14)(p13;q32) translocation in germinal center B cells, which leads to deregulated PAX5 expression under the control of the immunoglobulin heavy-chain (IgH) locus in a subset of B-cell non-Hodgkin lymphomas. Here we reconstructed a human t(9;14) translocation in a knock-in mouse by inserting a PAX5 minigene into the IgH locus. The IgHP5ki allele, which corresponds to a germline rather than somatic mutation, is activated in multipotent hematopoietic progenitors and is subsequently expressed in dendritic cells (DCs) and in natural killer (NK), T, and B cells. Ectopic Pax5 expression interferes with normal T-cell development and causes immature T-lymphoblastic lymphomas in IgHP5ki/+ and IgHP5ki/P5ki mice. Aggressive T-cell lymphomas develop even faster in IkPax5/+ mice expressing Pax5 from the Ikaros locus. Pax5 expression in thymocytes activates B-cell–specific genes and represses T-lymphoid genes, suggesting that Pax5 contributes to lymphomagenesis by deregulating the T-cell gene-expression program. These data identify Pax5 as a potent oncogene and demonstrate that the T-lymphoid lineage is particularly sensitive to the oncogenic action of Pax5.

Cytogenetics of lymphomas

Cytogenetic analysis is now a routine part of the diagnosis and management of a significant number of lymphoid malignancies. Whilst conventional cytogenetics remains the most comprehensive method for assessing chromosome abnormalities, the technical difficulties associated with conventional cytogenetics in most lymphomas has resulted in increased use of fluorescence in situ hybridisation (FISH) to identify specific abnormalities that are useful in either the diagnosis or management of these disorders. The finding of one of the Burkitt's translocations is of major importance in the diagnosis of Burkitt's and Burkitt's-like lymphomas, whereas the t(14;18), although seen in most follicular lymphomas (FL), is not usually required to make a diagnosis. Thus, whilst cytogenetics may be of interest in FL, it is not an essential part of the diagnostic work-up. Conventional cytogenetics may be useful for identifying markers of resistance to Helicobacter pylori therapy in MALT lymphomas. In disorders such as Hodgkin lymphoma, hairy cell leukaemia and lymphoplasmacytoid lymphoma, although many cytogenetic abnormalities have been observed, no consistent or specific abnormalities have been identified and so, at this point in our knowledge of the genetics of these disorders, cytogenetics cannot be considered a useful test for either diagnosis or prognosis. In contrast, the diagnosis of mantle cell lymphoma is now dependent upon the identification of the 11;14 translocation that results in cyclin D1 up-regulation. It is widely acknowledged that FISH is the most consistently useful test to identify the juxtaposition of the CCND1 and IGH genes in mantle cell lymphoma and is regarded as the 'gold standard'. FISH also has a role in identifying genetic abnormalities of prognostic significance in chronic lymphocytic leukaemia. Given the wealth of genetic and cytogenetic abnormalities that are continuing to be found in chronic lymphoid malignancies, it will be some time before the optimal use of both conventional cytogenetics and FISH is established in the diagnosis and management of lymphomas.

Molecular biology of lymphoma in the microarray era

This review will focus on the molecular biology of lymphoproliferative disorders with emphasis on lymphomas. The spectrum of known recurrent gene rearrangements found in lymphomas will be outlined and their relevance to diagnosis and subclassification of disease will be discussed. Finally, a survey of the current trends in gene expression profiling of lymphomas by microarray technology will be presented with reference to implications for diagnosis, classification, prognosis and treatment.

Rarity of IgH translocations in Waldenström macroglobulinemia

Comparatively little is known of the cytogenetics of Waldenström macroglobulinemia (WM). This is primarily due to the low proliferation of the clonal B cells, which precludes conventional karyotyping in many cases. Translocations involving the immunoglobulin heavy chain (IGH) gene at 14q32 are characteristic of many B-cell lymphomas and myelomas. Initial reports suggested that the t(9;14) was characteristic of lymphoplasmacytic lymphoma (the underlying pathological diagnosis in WM), but subsequent studies have failed to confirm the uniqueness of the translocation. To clarify this, we examined 69 cases of WM with interphase fluorescence in situ hybridization and failed to demonstrate an IgH translocation in 67 (97%). We conclude that IGH translocations are not a feature of WM, and the implications of this finding are discussed.

Low-grade B-Cell lymphomas with plasmacytic differentiation lack PAX5 gene rearrangements

The chromosomal translocation t(9;14)(p13;q32) has been reported in association with lymphoplasmacytic lymphoma (LPL). Although this translocation involving the paired homeobox-5 (PAX5) gene at chromosome band 9p13 and the immunoglobulin heavy chain (IgH) gene at 14q32 has been described in approximately 50% of LPL cases, the actual number of cases studied is quite small. Many of the initial cases associated with t(9;14)(p13;q32) were actually low-grade B-cell lymphomas with plasmacytic differentiation other than LPL. Thus, we analyzed a series of low-grade B-cell lymphomas for PAX5 gene rearrangements. We searched records from the Department of Pathology, Stanford University Medical Center for low-grade B-cell lymphomas, with an emphasis on plasmacytic differentiation, that had available paraffin blocks or frozen tissue. We identified 37 cases, including 13 LPL, 18 marginal zone lymphomas (nodal, extranodal, splenic, and alpha-heavy chain disease), and 6 small lymphocytic lymphomas. A novel dual-color break-apart bacterial artificial chromosome probe was designed to flank the PAX5 gene, spanning previously described PAX5 breakpoints, and samples were analyzed by interphase fluorescence in situ hybridization. All cases failed to demonstrate a PAX5 translocation, indicating that t(9;14)(p13;q32) and other PAX5 translocations are uncommon events in low-grade B-cell lymphomas with plasmacytic differentiation. This study also confirms recent reports that found an absence of PAX5 rearrangements in LPL, suggesting the reassessment of PAX5 rearrangements in LPL.

Deletion 6q is not a characteristic marker of nodal lymphoplasmacytic lymphoma

Lymphoplasmacytic lymphoma (LPL) is a small B-cell neoplasm with plasmacytic differentiation that does not fulfill the criteria for any other type of B-cell leukemia or lymphoma. In many cases, LPL is associated with Waldenström macroglobulinemia (WM), although WM may also be associated with other types of lymphoma. Recent studies have demonstrated that del(6q) is the most common structural abnormality in patients with bone marrow-based LPL. It is unknown whether del(6q) might also be associated with nodal LPL. We, therefore, examined 10 well-characterized LPL involving lymph nodes or other extramedullary tissues for del(6q) using paraffin section interphase fluorescence in situ hybridization (FISH). Dual-color FISH was performed using a chromosome 6 centromere probe (CEP6) and a probe for 6q21 (RP11-91C23). The latter probe has previously been reported as deleted in up to 63% of cases of bone marrow-based LPL. In contrast, no nuclei containing a del(6q) pattern were identified in any case of extramedullary LPL examined in this study, and 89-98.5% of nuclei contained a normal signal pattern. These results indicate that del(6q) is at least uncommon in nodal LPL, and cannot be employed as a diagnostic marker to identify this type of lymphoma. Furthermore, these findings suggest that nodal LPL and bone marrow-based cases of LPL may be associated with different cytogenetic findings.

Lymphoplasmacytic lymphoma/waldenstrom macroglobulinemia: an evolving concept

The concept of Waldenstrom macroglobulinemia has evolved from the original description of a clinical syndrome to its more recent designation as a distinct clinicopathologic entity, that is, lymphoplasmacytic lymphoma/Waldenstrom macroglobulinemia (LPL/WM), in the World Health Organization (WHO) classification and by the participants of consensus meetings on WM. The diagnosis of LPL/WM, however, remains a challenge in daily practice. Distinguishing LPL/WM from other B-cell lymphomas, especially marginal zone B-cell lymphomas, which share overlapping morphologic features, is difficult. The traditional practice of separating LPL/WM from other lymphomas by an arbitrary level of serum IgM is no longer considered valid. The characteristic immunophenotype described for LPL/WM by the WHO classification, that is, CD5(-)CD10(-)CD23-, is observed in 60-80% of neoplasms, but variations from this pattern of antigen expression are common, with CD23 being detected in up to 40% of cases. Lack of a distinct molecular genetic hallmark complicates the distinction of LPL/WM from other B-cell lymphomas. Although the t(9;14) is stated to be present in 50% of cases in the WHO classification, translocations involving the Ig heavy chain including the t(9;14) are actually rare in LPL/WM. Deletion of 6q21-q23, a nonspecific finding, is the most common aberration reported in 40-70% of patients. At the molecular level, the neoplastic clone in most cases has undergone Ig variable gene mutation, but not isotype switching, and the clone retains the capability of plasmacytic differentiation. Currently, the diagnosis of LPL/WM can only be established by incorporating clinical and pathologic findings and excluding alternative diagnoses. In some cases, in our opinion, distinguishing LPL/WM from marginal zone B-cell lymphomas seems arbitrary using currently recommended criteria.

Clinicopathologic Features of Waldenström's Macroglobulinemia and Marginal Zone Lymphoma: Are They Distinct or the Same Entity?

Waldenstrom's macroglobulinemia (WM) is considered in the World Health Organization classification as a clinical syndrome associated with monoclonal immunoglobulin (Ig) M secretion, mainly observed in patients with lymphoplasmacytic lymphoma (LPL) and occasionally with other small B-cell lymphomas. Some authors consider it a rare distinct lymphoproliferative disorder with primary bone marrow infiltration and IgM monoclonal gammopathy. As LPL shares important morphologic and immunophenotypic overlaps with marginal zone B-cell lymphomas (MZLs) in cases showing plasmacytic maturation, it remains unclear if they constitute unique or distinct entities. Both diseases are composed of lymphocytes, lymphoplasmacytoid cells, and tumoral plasma cells with a surface (s) IgM-positive sIgD+/ cytoplasmic IgMpositive CD19+ CD20+ CD27+/ CD5 CD10 CD23 phenotype, without a specific marker. Extranodal mucosa-associated lymphoid tissue (MALT) lymphoma, nodal MZL (NMZL), and splenic MZL (SMZL) are distinct entities displaying common morphologic, immunophenotypic, and genetic characteristics. MALT lymphoma is clearly distinct from LPL, although bone marrow infiltration and IgM paraprotein are not rare. Splenic MZL and NMZL are incompletely characterized, but a plasmacytoid/plasmacytic differentiation, autoimmune manifestations, and monoclonal component are frequent in both diseases. Bone marrow involvement is constant in SMZL and present in 60% of NMZLs. Molecular IgVH gene analysis has confirmed this heterogeneity, particularly within SMZL, with mutated and unmutated cases. Further studies are needed to clarify the pathogenesis of these MZLs and their relationship with LPL.

PAX5/IGH rearrangement is a recurrent finding in a subset of aggressive B-NHL with complex chromosomal rearrangements

We present an extensive characterization of 10 B-cell lymphomas with a t(9;14)(p13;q32). The presence of the PAX5/IGH gene rearrangement was demonstrated by fluorescence in situ hybridization (FISH) using a validated probe set, whereas complex karyotypic changes were reassessed by multiplex-FISH (M-FISH). Pathologic and clinical review revealed the presence of this rearrangement in 4 histiocyte-rich, T-cell-rich B-cell lymphomas (HRTR-BCLs) and 2 posttransplantation diffuse large B-cell lymphomas (PTLD-DLBCLs). In contrast to initial observations describing this translocation in lymphoplasmacytic lymphoma (LPL) and LPL-derived large B-cell lymphoma, our data showed a wide morphologic and clinical spectrum associated with the PAX5/IGH rearrangement, pointing to an association between this aberration and a subset of de novo DLBCLs presenting with advanced disease and adverse prognosis. In addition, the recurrent incidence of this rearrangement in both HRTR-BCL (4 cases) and PTLD-DLBCL (2 cases) was previously unrecognized and is intriguing.