—  PULMONARY PATHOLOGY SOCIETY   —

New Insights into the Molecular Pathogenesis of Pulmonary MALT Lymphomas


Ahmet Dogan
Mayo Clinic
Rochester, MN


Introduction
Since the first description of lymphomas of mucosa associated lymphoid tissue (MALT) in 1983 by Isaacson and Wright [1] , there have been rapid advances in our understanding of the pathogenesis and molecular pathology of these tumours. [2, 3] Initial analysis concentrated on the histological characterisation of the lymphoma and the confirmation that it represented a distinct lymphoma entity separate from those lymphomas most commonly encountered within lymph nodes. [4] This is now fully accepted with the adoption of the term "extranodal marginal-zone B cell lymphoma of MALT-type" in the recent WHO Classification of Tumours of the Hematopoietic And Lymphoid Tissues. [5]

Most MALT lymphomas arise in mucosal sites devoid of organised lymphoid structure. Development of MALT lymphoma is often preceded by a chronic inflammatory process which leads to the acquisition of lymphoid tissue. This forms the background for the emergence of the lymphoma. The examples of chronic inflammatory processes which are associated with MALT lymphoma development include Helicobacter pylori-associated gastritis in the stomach, [6, 7] Sjogren's syndrome in the salivary glands and Hashimoto's thyroiditis in the thyroid.

MALT lymphomas are cytologically low-grade lymphoid neoplasms, and are thought to arise form marginal zone B-cell compartment of the mucosal lymphoid follicles. The cytological features and the architecture of the tumour often mimic the features of normal mucosal organised lymphoid tissues such the Peyer's patches. For this reason many of the cases have been mistakenly diagnosed as pseudo-lymphomas before the application of clonality and genetic studies.

MALT lymphomas tend to remain localised at the site of origin for many years and when they disseminate they tend to disseminate to other sites mucosal sites; a phenomenon thought to be a result of homing programming. Clinically most cases have an indolent course and respond to locally directed therapies such as H.pylori eradication, surgery or radiotherapy.

Primary lymphomas of the lung are rare, accounting for 1% of all lymphomas. [8] Histologically half of these cases are MALT lymphomas [8] and approximately 10% of all MALT lymphomas present in the lung. [9, 10] Unlike MALT lymphomas of other sites, no preceding chronic inflammatory process or association with an infectious agent have been reported for pulmonary MALT lymphomas. Morphological, immunophenotypic and genetic features of pulmonary MALT lymphomas are similar to the MALT lymphomas of other sites, in particular to those arising in the stomach. [8, 11]

Immunology of the neoplastic B-cell in MALT lymphomas
The neoplastic B-cells of the MALT lymphomas immunologically resemble postfollicular memory B-cells. The rearranged immunoglobulin genes show evidence of somatic hypermutation indicating that the tumor cells have gone through a germinal center reaction and the pattern of the mutations suggest that the tumor cells have been selected by antigen recognition. [12, 13] Phenotypically the tumor cells mimic memory B-cells. They typically express IgM but not IgD, the expression of follicle centre markers such as CD10 and Bcl-6 is downregulated. A third of the cases show plasma cell differentiation. [8] The tumor cells may express homing receptors such as α4β7 integrin favoring recirculation and dissemination through other mucosal sites. [14, 15]

Genetic alterations in Pulmonary MALT lymphomas
A number of recurrent chromosomal changes characteristic of MALT lymphomas have been identified. [16, 17, 18, 19, 20, 21] These fall into two groups; the translocations and the numerical changes in chromosomes (aneuploidy).

The translocations and their frequencies in MALT lymphomas of different sites are summarized in Tables 1 and 2.

Table 1: Recurrent translocations in MALT lymphomas.(16,17,18,19,20,21)
Translocation Genes involved Consequence
t(11;18)(q21;q21) API2 and MALT1 A novel fusion gene and protein API2-MALT1
Nuclear localization of BCL10
t(14;18)(q32;q21) IgH and MALT 1 Overexpression of MALT1
t(1;14)(p22;q32) BCL10 and IgH Overexpression of BCL10
Nuclear localization of BCL10
t(1; 2)(p22; p12) BCL10 and IgK Overexpression of BCL10
Nuclear localization of BCL10
t(3;14)(p14.1;q32) FOXP1 and IgH Overexpression of FOXP1

The translocations appear to be essentially specific for MALT lymphoma. They have not been reported in other low grade B-cell lymphomas but they have been occasionally observed in cases of diffuse large B-cell lymphomas, often arising at extranodal sites. [22, 23] The characteristic translocations are seen over half of the MALT lymphomas originating in the lungs, by far the highest proportion in MALT lymphomas of various sites. The reasons for the increased frequency of translocations in lung lymphomas are not known.

Table 2: Frequency of recurrent translocations in MALT lymphomas

Site t(11;18) t(1;14) t(14;18) t(3;14)* t(neg)
Stomach 22% >1% 3% - 74%
Orbita 7% 0% 12% 20% 61%
Salivary gland 2% 1% 5% - 92%
Lung 42% 8% 6% - 44%
Skin 4% 0% 8% 10% 78%
Intestine 15% 10 0% - 75%

* Preliminary data, only a small number of cases have been tested so far

The precise mechanisms involved in the development of these translocations remain essentially unknown. The ones involving the immunoglobulin heavy and light chain chains genes are likely to be caused by errors during physiological Ig gene remodeling, including V(D)J recombination and somatic hypermutation. [24] Whereas the genomic breakpoints of t(11;18)(API2/MALT1), which involves two non-Ig genes, are not associated with any known or putative sequence motif that may associate with chromosomal recombination. Extensive sequence alterations including deletions, duplications and non-template-based insertions occur commonly at the fusion junction. [25] It has been proposed that t(11;18)(API2/MALT1) may result from illegitimate non-homologous end joining following double strand breaks. Such double strand breaks may be a consequence of genotoxic insults generated by the chronic inflammatory process preceding MALT lymphoma. [26]

The numerical chromosomal abnormalities seen in MALT lymphomas include trisomy 3, 12 and 18. [9, 11] Aneuploidy can be seen the in the presence or absence of the translocations. However interestingly the MALT lymphomas carrying the t(11;18)(API2/MALT1) usually do not show aneuploidy.

Additionally mutations involving oncogenes such as c-myc and a number of tumour suppressor genes such as p53, fas and p16 have been reported in MALT lymphomas, in particular those with a large cell component.

Biological consequences of genetic alterations in MALT lymphomas
Most of the translocations, t(11;18)(API2/MALT1), t(14;18)(IgH/MALT1), t(1;14)(BCL10/IgH) and t(BCL10/Igκ) in MALT lymphoma affect the same molecular pathway critical in antigen mediated T and B-cell activation, through the nuclear factor κB (NF-κB) family of transcription factors. Members of NF-κB family are key regulators of the expression of genes that are essential for lymphocyte activation, proliferation and generation of immune responses.

In normal B and T cells, antigen receptor mediated activation is thought to depend on the interaction between cell membrane receptor associated kinases and a trimolecular complex involving membrane-associated guanylate kinase (MAGUK) family member CARMA1, the adaptor protein BCL-10 and the human paracaspase MALT1. (Figure 1) Following antigen-receptor stimulation, CARMA1 induces BCL10 oligomerization through caspase recruitment domain (CARD)–CARD interactions. BCL10 then binds the Ig-like domain of MALT1 through its CARD and induces MALT1 oligomerization. Oligomerized MALT1 is thought to induce activation of the inhibitor of NF-κB kinase (IKK) complex.


Figure 1: The structure of molecules thought to be involved in antigen receptor-mediated activation of B-cells and MALT lymphomas. In API2-MALT1 fusion protein formed as the result of t11(11;18) the fusion product always retains the baculovirus IAP repeats (BIR) domain of the AIP2 and caspase recruitment domain (CARD) and at least one of the immunoglobulin-like (IG) domains of MALT1.The arrows indicate domains involved in molecular interactions.

NF-κB proteins are normally sequestered in the cytoplasm in an inactive form, bound to inhibitory κB (IκB) proteins. [29] Activation of the IKK complex leads to phosphorylation and subsequent ubiquitylation and degradation of IkB releasing NF-κB. NF-κB then translocates to the [27] nucleus and transactivates genes important for cellular activation, proliferation and survival, and induction of effector functions of lymphocytes. (Figure 2)

In MALT lymphomas the translocations involving BCL10 or MALT-1 is thought to induce constitutive NF-κB activation without a need for antigen receptor mediated upstream signaling. [28, 29, 30, 31, 32, 33, 34] In cases with t(1;14)(BCL10/IgH), BCL10 is placed under the regulation of the Ig-heavy-chain gene enhancers and is overexpressed. It is thought that excess BCL10 can form oligomers through its CARD domain without the need of upstream signals and engage MALT1 for the downstream events, leading to NF-κB activation. In cases with t(14;18)(IgH/MALT1), MALT1 is overexpressed and spontaneous oligomerization occurs possibly with the involvement of BCL10 . In MALT lymphomas with t(11;18)(API2/MALT1), the resulting API2–MALT1 fusion protein can bypass the requirement for BCL10 and oligomerizes through BIR domains of the API2 molecule, therefore leading to NF-κB nuclear translocation.


Figure 2: The antigen-receptor-mediated NFkB activation through the CARMA-1, BCL10 and MALT1 in normal B-cells and the genetic alterations of the same pathway seen in MALT lymphomas and their consequences.

The most recent translocation that has been discovered in MALT lymphomas, t(3;14)(p14.1;q32) involves IgH gene locus and forkhead box protein P1 (FOXP1). [35] FOXP1 is a member of the FOXP subfamily(FOXP1-4) of transcription factors, characterized by a commonDNA-binding, winged-helix or forkhead domain, together withN-terminal zinc finger and leucine zipper domains. FOXP1 is expressed in normal and activated B cells. However, the physiologicrole of FOXP1 in lymphocytes remains unclear. In MALT lymphomas, the translocation juxtaposes FOXP1 gene close to the IgH gene enhancers leading to overexpression of the protein. How this contributes to neoplastic transformation is not yet known. Interestingly, in diffuse large B-cell lymphomas strong expression of FOXP1 identifies a distinct subset of patients with poor clinical outcome. [36, 37] FOXP1 translocation have not been observed in lung MALT lymphomas as yet.

Clinical consequences of molecular alterations in MALT lymphomas
Diagnosis:
The genetic alterations in MALT lymphomas, being largely specific for MALT lymphoma, have become useful diagnostic tools. All of the translocation can be detected by interphase FISH in paraffin embedded material, either using sections or isolated whole nuclei. [10, 11, 18] A number of robust commercially available probes are already available. t(11;18)(API2/MALT1), by far the most frequent translocation in MALT lymphoma can be identified by an RT-PCR strategy as the translocation leads to development of a unique fusion transcript. [38, 39] Again the methods work well in paraffin embedded tissue blocks.

The alterations in protein expression patterns of BCL10 and MALT-1 can also be used as surrogate markers for the translocation. In cases with the t(1;14)(BCL10/IgH) involving the BCL10 gene, strong overexpression of BCL10 can be detected by immunohistochemistry. BCL10 is weakly expressed in the cytoplasm of normal B-cells. [40] The translocation leads not only to overexpression of BCL10 but also to nuclear localization of the BCL10 protein. The biological significance of nuclear localization is not known. Interestingly, (11;18)(API2/MALT1) cases also show nuclear BCL10 expression albeit weakly. [39] Similarly cases with (14;18)(IgH/MALT1) show overexpression of MALT1. [10]

Unfortunately the alterations in protein expression patterns are neither specific for individual translocations nor to MALT lymphoma. The clinical utility of BCL10-MALT1 immunohistochemistry remains limited.

Prognosis:
Helicobacter pylori eradication has emerged as the first line treatment for management of early stage gastric lymphoma. [41] Although most of the patients respond to this treatment, approximately 25% of the cases are resistant. It has now been shown that these resistant cases almost always carry the t(11;18)(API2/MALT1). [42] It is likely that the tumor cells are not reliant to the signals from the microenvironment since the molecular changes override the normal signaling pathways. The prognostic significance of other rarer translocations is not known. The prognostic significance of t(11;18)(API2/MALT1) in lung lymphomas has not been studied due to relative rarity of this tumor and overall good prognosis of these patients.

Management:
The molecular alterations in MALT lymphomas are attractive targets for development of new treatment strategies interfering either BCL10-MALT1 pathway or downstream NF-κB activation.

References

  1. Isaacson P, Wright DH. Malignant lymphoma of mucosa-associated lymphoid tissue. A distinctive type of B-cell lymphoma. 1983;52:1410-1416.

  2. Wotherspoon AC, Dogan A, Du MQ. Mucosa-associated lymphoid tissue lymphoma. 2002;9:50-55.

  3. Isaacson PG, Du MQ. MALT lymphoma: from morphology to molecules. Nat Rev Cancer 2004;4:644-53.

  4. Isaacson PG, Spencer J. Malignant lymphoma of mucosa-associated lymphoid tissue. 1987;11:445-462.

  5. Jaffe ES, Harris NL, Stein H, et al. World Health Organisation Classification of Tumours. Pathology and Genetics of Tumours of Haematopoietic and Lymphoid Tissues. Lyon: IARC Press; 2001.

  6. Wotherspoon AC, Ortiz-Hidalgo C, Falzon MR, et al. Helicobacter pylori-associated gastritis and primary B-cell gastric lymphoma. Lancet 1991;338:1175-6.

  7. Doglioni C, Wotherspoon AC, Moschini A, et al. High incidence of primary gastric lymphoma in northeastern Italy. 1992;339:834-835.

  8. Kurtin PJ, Myers JL, Adlakha H, et al. Pathologic and clinical features of primary pulmonary extranodal marginal zone B-cell lymphoma of MALT type. 2001;25:997-1008.

  9. Streubel B, Simonitsch-Klupp I, Mullauer L, et al. Variable frequencies of MALT lymphoma-associated genetic aberrations in MALT lymphomas of different sites. Leukemia 2004;18:1722-6.

  10. Ye H, Gong L, Liu H, et al. MALT lymphoma with t(14;18)(q32;q21)/IGH-MALT1 is characterized by strong cytoplasmic MALT1 and BCL10 expression. J Pathol 2005:in press.

  11. Remstein ED, Kurtin PJ, Einerson RR, et al. Primary pulmonary MALT lymphomas show frequent and heterogeneous cytogenetic abnormalities, including aneuploidy and translocations involving API2 and MALT1 and IGH and MALT1. Leukemia 2004;18:156-60.

  12. Du M, Diss TC, Xu C, et al. Ongoing mutation in MALT lymphoma immunoglobulin gene suggests that antigen stimulation plays a role in the clonal expansion. Leukemia 1996;10:1190-1197.

  13. Kurosu K, Yumoto N, Furukawa M, et al. Low-grade pulmonary mucosa-associated lymphoid tissue lymphoma with or without intraclonal variation. Am J Respir Crit Care Med 1998;158:1613-9.

  14. Dogan A, Du M, Koulis A, et al. Expression of lymphocyte homing receptors and vascular addressins in low-grade gastric B-cell lymphomas of mucosa- associated lymphoid tissue. 1997;151:1361-1369.

  15. Du MQ, Peng HZ, Dogan A, et al. Preferential dissemination of B-cell gastric mucosa-associated lymphoid tissue (MALT) lymphoma to the splenic marginal zone. Blood 1997;90:4071-4077.

  16. Willis TG, Jadayel DM, Du MQ, et al. Bcl10 is involved in t(1;14)(p22;q32) of MALT B cell lymphoma and mutated in multiple tumor types. Cell 1999;96:35-45.

  17. Dierlamm J, Baens M, Wlodarska I, et al. The apoptosis inhibitor gene API2 and a novel 18q gene, MLT, are recurrently rearranged in the t(11;18)(q21;q21)p6ssociated with mucosa-associated lymphoid tissue lymphomas. Blood 1999;93:3601-9.

  18. Streubel B, Lamprecht A, Dierlamm J, et al. T(14;18)(q32;q21) involving IGH and MALT1 is a frequent chromosomal aberration in MALT lymphoma. Blood 2003;101:2335-9.

  19. Wotherspoon AC, Finn TM, Isaacson PG. Trisomy 3 in low-grade B-cell lymphomas of mucosa-associated lymphoid tissue. 1995;85:2000-2004.

  20. Dierlamm J, Pittaluga S, Wlodarska I, et al. Marginal zone B-cell lymphomas of different sites share similar cytogenetic and morphologic features [see comments]. Blood 1996;87:299-307.

  21. Streubel B, Vinatzer U, Raderer M, et al. t(13;14)(p14.1;q32) involving IgH and FOXP1 is a recurrent chromosomal aberration in MALT lymphoma. Blood 2004;104:159a.

  22. Chuang SS, Lee C, Hamoudi RA, et al. High frequency of t(11;18) in gastric mucosa-associated lymphoid tissue lymphomas in Taiwan, including one patient with high-grade transformation. Br J Haematol 2003;120:97-100.

  23. Cook JR, Sherer M, Craig FE, et al. T(14;18)(q32;q21) involving MALT1 and IGH genes in an extranodal diffuse large B-cell lymphoma. Hum Pathol 2003;34:1212-5.

  24. Kuppers R, Dalla-Favera R. Mechanisms of chromosomal translocations in B cell lymphomas. Oncogene 2001;20:5580-94.

  25. Liu H, Hamoudi RA, Ye H, et al. t(11;18)(q21;q21) of mucosa-associated lymphoid tissue lymphoma results from illegitimate non-homologous end joining following double strand breaks. Br J Haematol 2004;125:318-29.

  26. Ye H, Liu H, Attygalle A, et al. Variable frequencies of t(11;18)(q21;q21) in MALT lymphomas of different sites: significant association with CagA strains of H pylori in gastric MALT lymphoma. Blood 2003;102:1012-8.

  27. Thome M. CARMA1, BCL-10 and MALT1 in lymphocyte development and activation. Nat Rev Immunol 2004;4:348-59.

  28. Ruland J, Duncan GS, Elia A, et al. Bcl10 is a positive regulator of antigen receptor-induced activation of NF-κB and neural tube closure. Cell 2001;104:33-42.

  29. Ruland J, Duncan GS, Wakeham A, et al. Differential requirement for Malt1 in T and B cell antigen receptor signaling. Immunity 2003;19:749-58.

  30. McAllister-Lucas LM, Inohara N, Lucas PC, et al. Bimp1, a MAGUK family member linking protein kinase C activation to Bcl10-mediated NF-κB induction. J Biol Chem 2001;276:30589-97.

  31. Lucas PC, McAllister-Lucas LM, Nunez G. NF-κB signaling in lymphocytes: a new cast of characters. J Cell Sci 2004;117:31-9.

  32. Lucas PC, Yonezumi M, Inohara N, et al. Bcl10 and MALT1, independent targets of chromosomal translocation in malt lymphoma, cooperate in a novel NF-κB signaling pathway. 2001;276:19012-19019.

  33. Che T, You Y, Wang D, et al. MALT1/paracaspase is a signaling component downstream of CARMA1 and mediates T cell receptor-induced NF-κB activation. J Biol Chem 2004;279:15870-6.

  34. Ho L, Davis RE, Conne B, et al. MALT1 and the API2-MALT1 fusion act between CD40 and IKK and confer NF-κB dependent proliferative advantage and resistance against FAS-induced cell death in B cells. Blood 2004; epub.

  35. Banham AH, Beasley N, Campo E, et al. The FOXP1 winged helix transcription factor is a novel candidate tumor suppressor gene on chromosome 3p. Cancer Res 2001;61:8820-9.

  36. Barrans SL, Fenton JA, Banham A, et al. Strong expression of FOXP1 identifies a distinct subset of diffuse large B-cell lymphoma (DLBCL) patients with poor outcome. Blood 2004;104:2933-5.

  37. Hans CP, Weisenburger DD, Greiner TC, et al. Confirmation of the molecular classification of diffuse large B-cell lymphoma by immunohistochemistry using a tissue microarray. Blood 2004;103:275-82.

  38. Liu H, Ruskon-Fourmestraux A, Lavergne-Slove A, et al. Resistance of t(11;18) positive gastric mucosa-associated lymphoid tissue lymphoma to Helicobacter pylori eradication therapy. Lancet 2001;357:39-40.

  39. Liu H, Ye H, Dogan A, et al. T(11;18)(q21;q21) is associated with more advanced MALT lymphoma that expresses nuclear BCL10. Blood 2001;98:1182-7.

  40. Ye HT, Dogan A, Willis TG, et al. Bcl10 Expression in Normal and Neoplastic Lymphoid Tissue. 2000;157:1147-54.

  41. Zucca E, Bertoni F, Roggero E, et al. The gastric marginal zone B-cell lymphoma of MALT type. 2000;96:410-419.

  42. Liu H, Ye H, Ruskone-Fourmestraux A, et al. T(11;18) is a marker for all stage gastric MALT lymphomas that will not respond to H. pylori eradication. Gastroenterology 2002;122:1286-94.