Complete nucleotide sequence of Ig V genes in three cases of Burkitt lymphoma associated with AIDS

Analysis of the IGHV region in Burkitt's lymphomas supports a germinal center origin and a role for superantigens in lymphomagenesis

The analysis of immunoglobulin heavy chain variable (IGHV) region may disclose the influence of antigens in Burkitt's lymphomas (BL). IGHV sequences from 38 patients and 35 cell lines were analyzed. IGHV3 subset genes were the most used and IGHV4-34 gene was overrepresented. IGHV genes were mutated in 98.6% of the cases, 36% acquired potential glycosylation sites, and in 52% somatic-hypermutation-process was ongoing. Binding motifs for superantigens like Staphylococcal protein A and carbohydrate I/i were preserved in 89% of the cases. IGHV analysis of BL cells supports a germinal center origin and points toward a role for superantigens in lymphomagenesis.

Adult Burkitt leukemia and lymphoma

The World Health Organization Classification of Lymphoid Neoplasms identifies Burkitt lymphoma/leukemia as a highly aggressive mature B-cell neoplasm consisting of endemic, sporadic, and immunodeficiency-associated variants. These subtypes share many morphologic and immunophenotypic features, but differences exist in their clinical and geographic presentations. All of these subtypes possess chromosomal rearrangements of the c-myc oncogene, the genetic hallmark of Burkitt lymphoma that contributes to lymphomagenesis through alterations in cell cycle regulation, cellular differentiation, apoptosis, cellular adhesion, and metabolism. Brief-duration, high-intensity chemotherapy regimens containing aggressive central nervous system prophylaxis have had remarkable success in the treatment of this disease, with complete remission rates of 75% to 90% and overall survivals reaching 50% to 70% in adults. Although Burkitt lymphoma cells are extremely chemosensitive, biologically targeted therapies should be developed because current treatment options are suboptimal for patients with poor prognostic features or in the setting of relapsed disease.

Incidence of novel N-glycosylation sites in the B-cell receptor of lymphomas associated with immunodeficiency

Novel N-glycosylation sites are introduced by somatic mutation into the V genes of the majority of follicular lymphomas. Sites are positively selected and rare in normal memory B cells, indicating a potential role in tumour survival in the germinal centre (GC). The incidence of c. 40% in diffuse large B-cell lymphomas (DLBCL) parallels the known heterogeneity of the disease. Immunodeficiency-related non-Hodgkin's lymphomas (NHL) include post-transplant lymphoproliferative disorders (PTLD) and acquired immunodeficiency syndrome-related NHL (AIDS-NHL). Most PTLD derive from B cells that carry mutated VH genes and that have completed the GC reaction. All AIDS-NHL carry mutated VH genes and variable features of GC or post-GC cells. To determine if N-glycosylation is a feature of immunodeficiency-related lymphomas, we analysed the VH genes of 19 PTLD and 36 AIDS-NHL. Novel sites were rare in PTLD (4/19), similar to memory B cells (P = 0.15). AIDS-NHL, including DLBCL and Burkitt's lymphomas (BL), showed heterogeneity with 16 of 36 (44%) having novel sites. The findings indicate no selection of N-glycosylation sites in PTLD, consistent with post-GC features. The variable incidence of N-glycosylation sites in AIDS-NHL mirrors that in DLBCL and sporadic BL of immunocompetent hosts, supporting the known heterogeneity of these disorders, and possibly pointing to distinct routes of tumour development.

Diversity and somatic hypermutation of the Ig VHDJH, V kappa J kappa, and V lambda J lambda gene segments in lymphoma B cells: relevance to the origin of the neoplastic B cell clone

Burkitt's lymphoma (BL) is a malignancy of B cells characterized by chromosomal translocations involving the immunoglobulin (Ig) and c-MYC gene loci. To address the putative role of antigen in the clonal expansion of these neoplastic B cells, we analyzed the VHDJH and VLJL gene segments expressed by the established cell lines derived from six endemic BL and six sporadic BL. Eight BL cell lines used genes of the VH3 family, two of the VH4, and two of the VH1. Eight VL chains were kappa (four members of the V kappa3, two of the V kappa1, and two of the V kappa2 subgroups) and four lambda (three members of the V lambda1 and one of the Vl ambda3 subgroup). The VH gene utilization was stochastic (i.e., it reflected the relative representation of the different VH gene family members in the human haploid genome). In contrast, the VL gene utilization was skewed toward the overutilization of the V kappa3 and V lambda1 gene subgroups. When compared with those of the respective germline genes, the sequences of the expressed Ig VDJ genes displayed nucleotide differences that resulted from somatic hypermutation. In three endemic and three sporadic BL cells, nucleotide changes yielding amino acid substitutions segregated within the complementarity determining region, indicating the application of a positive pressure for replacement mutations and suggesting that these neoplastic lymphocytes underwent a process of clonal selection driven by antigen, perhaps emerging from or transitioning through germinal centers.

Incidence of potential glycosylation sites in immunoglobulin variable regions distinguishes between subsets of Burkitt's lymphoma and mucosa-associated lymphoid tissue lymphoma

Recently, a high incidence of novel N-glycosylation sites introduced by somatic mutation was observed in the immunoglobulin variable region genes of follicular lymphoma. As these are positively selected and are uncommon in normal B cells, they may have a role in tumour growth and behaviour. Sites are not characteristic of chronic lymphocytic leukaemia or myeloma, but are detectable in approximately 50% of diffuse large cell lymphomas. Another feature of the variable region genes of certain lymphomas is ongoing somatic mutation. To determine whether glycosylation is associated with this phenomenon, we analysed variable region gene sequences of Burkitt's lymphoma (BL) and mucosa-associated lymphoid tissue (MALT) lymphoma. Novel sites were common in endemic BL (82%) and in 4/5 patients with Iranian BL. However, sporadic BL had a lower incidence (43%). Patients with MALT lymphoma had a low frequency (9%) of novel sites, comparable to normal B cells. These findings distinguish glycosylation sites from ongoing mutation and may reflect different environmental influences on these tumours.

Molecular biology of Burkitt's lymphoma

The diagnostic category of Burkitt's lymphoma encompasses a closely related group of aggressive B-cell tumors that includes sporadic, endemic, and human immunodeficiency virus-associated subtypes. All subtypes are characterized by chromosomal rearrangements involving the c-myc proto-oncogene that lead to its inappropriate expression. This review focuses on the roles of c-myc dysregulation and Epstein-Barr virus infection in Burkitt's lymphoma. Although the normal function of c-Myc remains enigmatic, recent data indicate that it has a central role in several fundamental aspects of cellular biology, including proliferation, differentiation, metabolism, apoptosis, and telomere maintenance. We discuss new insights into the molecular mechanisms of these c-Myc activities and their potential relevance to the pathogenesis of Burkitt's lymphoma and speculate on the role of Epstein-Barr virus.

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.

Cytokines in the evolution and treatment of AIDS-lymphoma

AIDS-lymphoma is a heterogeneous disease that most likely results from the complex interaction of several contributing factors, including chronic antigenic activation of B lymphocytes, dysregulated cytokine and co-stimulatory networks, infection with potentially oncogenic viruses (human herpesvirus-8 [HHV-8], Epstein-Barr virus), and accumulation of secondary genetic mutations. Cytokines are believed to play an important role in the immunologic decline that favors opportunistic infection and malignancy in advanced HIV infection. Recent work has provided some evidence that cytokine therapy can partially reverse the immune dysregulation seen in AIDS. This suggests that cytokines are likely to have an important role in both the pathogenesis and treatment or prevention of AIDS-lymphoma.

AIDS Primary Central Nervous System Lymphoma: Molecular Analysis of the Expressed VH Genes and Possible Implications for Lymphomagenesis

AIDS-associated primary central nervous system lymphomas are late events that have an extremely poor prognosis. Despite different hypotheses, the brain localization of these B cell lymphomas remains an enigma. To better define the cell origin of the lymphomas and the possible role of the B cell receptor (BCR) in the brain localization and/or in the oncogenic transformation, we analyzed the V region genes of the Ig heavy chain expressed by lymphoma cells in five randomly selected patients. After amplifying the rearranged VHDJH DNA by PCR, cloning, and sequencing of the amplified products, we observed that: 1) of the five lymphomas analyzed, four were clearly monoclonal; 2) there was no preferential use of one peculiar VH family or one peculiar segment of gene; 3) the mutation analysis showed that an Ag-driven process occurred in at least two cases, probably before the oncogenic event; and 4) there was no intraclonal variability, suggesting that the hypermutation mechanism is no longer efficient in these lymphoma B cells. Taken together, our results suggest that distinct Ags could be recognized by the BCR of the lymphoma cells in different patients and that, if the Ags are responsible for the brain localization of these B cells bearing mutated BCR, other factors must be involved in B cell transformations in primary central nervous system lymphoma.

Strong intrinsic biases towards mutation and conservation of bases in human IgVH genes during somatic hypermutation prevent statistical analysis of antigen selection

Immunoglobulin V region genes acquire point mutations during affinity maturation of the T-cell-dependent B-cell response. It has been proposed that both selection by antigen and characteristics of the DNA sequence are involved in determining the distribution of mutations along the genes. There is a tendency for replacement mutations to occur in the complementarity-determining regions and for silent mutations to accumulate in the framework regions of used genes. By analysing a group of highly mutated human IgVH4-34 (VH4. 21) and family 5 genes derived from human gut-associated lymphoid tissues, which were out-of-frame between VH and JH (and therefore not used) we have investigated the distribution of mutations acquired in the absence of selection. We observed that these genes may show the statistical hallmarks of selected genes, suggesting that intrinsic biases alone may be enough to give the appearance of selection. These data suggest that analysis of the distribution of mutations in IgVH genes cannot be used reliably to state whether antigenic selection of the B-cell carrying the genes occurred. In-frame genes had more silent mutations than the out-of-frame genes and lacked stop codons. These characteristics were considered to be indicative of selection in the in-frame genes derived from human gut-associated lymphoid tissue.

Aberrant and Unstable Expression of Immunoglobulin Genes in Persons Infected With Human Immunodeficiency Virus

We examined the IgM VH gene subgroup use-distribution in serial blood samples of 37 human immunodeficiency virus (HIV)-infected patients and a group of HIV-seronegative healthy adults. The IgM VH gene repertoires of healthy adults were relatively similar to one another and were stable over time. In contrast, individuals infected with HIV had IgM VH gene repertoires that were significantly more heterogeneous and unstable. Persons at early stages of HIV infection generally had abnormal expression levels of Ig VH3 genes and frequently displayed marked fluctuations in the relative expression levels of this VHgene subgroup over time. In contrast, persons with established acquired immunodeficiency syndrome (AIDS) had a significantly lower incidence of abnormalities in Ig VH3 expression levels, although continued to display abnormalities and instability in the expression levels of the smaller Ig VH gene subgroups. Moreover, the skewing and/or fluctuations in the expressed-IgM VHgene repertoire appeared greatest for persons at earlier stages of HIV infection. These studies show that persons infected with HIV have aberrant and unstable expression of immunoglobulin genes suggestive of a high degree humoral immune dysregulation and ongoing humoral immune responses to HIV-associated antigens and superantigens.

© 1998 by The American Society of Hematology.

Aberrant and Unstable Expression of Immunoglobulin Genes in Persons Infected With Human Immunodeficiency Virus

We examined the IgM VH gene subgroup use-distribution in serial blood samples of 37 human immunodeficiency virus (HIV)-infected patients and a group of HIV-seronegative healthy adults. The IgM VH gene repertoires of healthy adults were relatively similar to one another and were stable over time. In contrast, individuals infected with HIV had IgM VH gene repertoires that were significantly more heterogeneous and unstable. Persons at early stages of HIV infection generally had abnormal expression levels of Ig VH3 genes and frequently displayed marked fluctuations in the relative expression levels of this VHgene subgroup over time. In contrast, persons with established acquired immunodeficiency syndrome (AIDS) had a significantly lower incidence of abnormalities in Ig VH3 expression levels, although continued to display abnormalities and instability in the expression levels of the smaller Ig VH gene subgroups. Moreover, the skewing and/or fluctuations in the expressed-IgM VHgene repertoire appeared greatest for persons at earlier stages of HIV infection. These studies show that persons infected with HIV have aberrant and unstable expression of immunoglobulin genes suggestive of a high degree humoral immune dysregulation and ongoing humoral immune responses to HIV-associated antigens and superantigens.

© 1998 by The American Society of Hematology.

Insight into Burkitt's lymphoma from immunoglobulin variable region gene analysis

Analysis of usage of V(H) and V(L) genes, and the degree and pattern of somatic mutation, has been used to investigate the cell of origin and clonal history in cases of Burkitt's lymphoma (BL). Tumor cell lines and biopsy material from patients with endemic, sporadic and AIDS-associated BL have been compared. V(H) genes were most commonly derived from the V(H)3 (52%) and V(H)4 (39%) families. This shows a similar gene usage of the V(H)3 family to that seen in the normal peripheral blood repertoire (55%), but a biased usage of the V(H)4 family (22% in normal). There was no restriction in V(L) gene usage. This overall distribution was similar in all subsets of BL. In all categories, there was significant somatic mutation in both V(H) and V(L) sequences. There was no evidence for accumulation of mutations in cell lines cultured in vitro indicating that all mutations in BL-derived cell lines have accumulated in vivo. The mean percentage level of mutation +/- standard deviation was greater in endemic BL (V(H) = 7.7 +/- 4.0, V(L) = 5.3 +/- 2.2) and AIDS-associated BL (V(H) = 7.5 +/- 3.6, V(L) = 3.9 +/- 1.9) than in sporadic BL (V(H) = 4.0 +/- 2.5, V(L) = 2.2 +/- 1.2). The pattern of somatic hypermutation was similar in V(H) and V(L) sequences of the different types of BL although the light chain genes were less mutated. Mutational patterns in the V(H) genes did not reveal a conventional role for antigen in selection of tumor cell sequences in 23/25 V(H) genes analysed. In contrast, patterns in V(L) sequences were consistent with a role for antigen in 8/13 sBL +/- eBL cases and 8/17 cases overall. The presence of EBV did not seem to influence the quantity or pattern of somatic mutations. Evidence for intraclonal variation was seen in uncloned cell lines from cases of eBL and AIDS-associated BL and confirmed in biopsy material in some, but not all cases of eBL, sBL and AIDS-associated BL examined. These common features indicate that the B-cells involved in all types of BL are derived from cells that have traversed the germinal centre, and that the somatic mutation mechanism may still be operative following neoplastic transformation. Overall, in 10/30 cases, there was evidence of significant clustering of replacement amino acids, in CDRs, particularly in V(L), indicating that the B-cell of origin is likely to have been selected by antigen.

The immunology of AIDS-associated lymphomas

Lymphomas that occur in patients with human immunodeficiency virus (HIV) infection are predominantly of B-cell origin and subsets show evidence for Epstein-Barr virus (EBV) infection or chromosomal translocations in the c-myc locus. The only subset of lymphoma clearly related to the immunodeficiency caused by HIV infection (similar to transplantation-associated lymphomas) is the EBV+ primary central nervous system lymphoma. The systemic AIDS-related lymphomas (ARLs) represent a complex set of disease processes histologically categorized as large cell or small non-cleaved (Burkitt's-like) lymphomas. Molecular analyses of the ARLs have demonstrated polyclonal lymphomas as likely early representatives of monoclonal immunoglobulin (Ig)-expressing B-cell lymphomas. Variable region analysis of lymphoma-associated Ig has shown evidence for extensive somatic mutation with little evidence for appropriate affinity maturation. These observations suggest that abnormal control of B-cell maturation in response to polyclonal antigenic stimulation may play a central role in the pathogenesis of ARL. The recent finding of clonal HIV integrated within macrophages in a subset of early lymphomas also provides evidence for abnormalities outside the B-cell compartment playing roles in this disease. Overall, ARLs generally appear to be outgrowths of antigen-driven B-cells with significant growth control influence provided by abnormal T-cell and antigen-presenting cell processes.