Functional class switch recombination may occur ‘in vivo’ in Waldenström macroglobulinaemia

B-Cell-Specific Myd88 L252P Expression Causes a Premalignant Gammopathy Resembling IgM MGUS

A highly recurrent somatic L265P mutation in the TIR domain of the signaling adapter MYD88 constitutively activates NF-κB. It occurs in nearly all human patients with Waldenström’s macroglobulinemia (WM), a B cell malignancy caused by IgM-expressing cells. Here, we introduced an inducible leucine to proline point mutation into the mouse Myd88 locus, at the orthologous position L252P. When the mutation was introduced early during B cell development, B cells developed normally. However, IgM-expressing plasma cells accumulated with age in spleen and bone, leading to more than 20-fold elevated serum IgM titers. When introduced into germinal center B cells in the context of an immunization, the Myd88^L252P mutation caused prolonged persistence of antigen-specific serum IgM and elevated numbers of antigen-specific IgM plasma cells. Myd88^L252P-expressing B cells switched normally, but plasma cells expressing other immunoglobulin isotypes did not increase in numbers, implying that IgM expression may be required for the observed cellular expansion. In order to test whether the Myd88^L252P mutation can cause clonal expansions, we introduced it into a small fraction of CD19-positive B cells. In this scenario, five out of five mice developed monoclonal IgM serum paraproteins accompanied by an expansion of clonally related plasma cells that expressed mostly hypermutated VDJ regions. Taken together, our data suggest that the Myd88^L252P mutation is sufficient to promote aberrant survival and expansion of IgM-expressing plasma cells which in turn can cause IgM monoclonal gammopathy of undetermined significance (MGUS), the premalignant condition that precedes WM.

Origin of Waldenstrom's macroglobulinaemia

Waldenstrom's macroglobulinaemia (WM) is an MYD88^L265P-mutated lymphoplasmacytic lymphoma that invades bone marrow and secretes monoclonal immunoglobulin M (IgM). WM cells are usually unable to undergo class switch recombination, and have mutated IGHV, with a typical immunophenotype CD19⁺/CD22^low+/CD23^-/CD25⁺/CD27⁺/CD45⁺/CD38^low+/SmIgM⁺ (negative for CD5, CD10, CD11c, CD103). This immunophenotype matches memory B cells (smIgM^-/+/CD10^-/CD19⁺/CD20⁺/CD27⁺/CD38^low+/CD45⁺), representing 30% of B cells in the blood. Fifty percent of them have not undergone class switch recombination and are IgM⁺. These cells have suffered somatic hypermutation as WM cells. Genetic abnormalities do not abrogate the capacity to progress to plasma cells that usually belong to the clonal WM compartment, with a normal immunophenotype and functional characteristics. However, some WM cells are CD27^-, MYD88^WT, without somatic hypermutation, or with class switch recombination capable of reactivation. Thus, most data support a B-memory-cell origin for WM, but a small fraction of cases may have a different origin.

Waldenström's macroglobulinemia - a review

Waldenström's macroglobulinemia (WM) is a lymphoproliferative disease of B lymphocytes, characterized by a lymphoplasmocytic lymphoma in the bone marrow and by IgM monoclonal hypergammaglobulinemia. It was first described in 1944 by Jan Gösta Waldenström, reporting two patients with oronasal bleeding, lymphadenopathy, anemia, thrombocytopenia, high erythrocyte sedimentation rate and serum viscosity, normal radiography and bone marrow infiltrated by lymphoid cells. The WM is a rare disease with a typically indolent clinical course, affecting mainly individuals aged between 63 and 68 years. Most patients have clinical signs and symptoms related to hyperviscosity resulting from IgM monoclonal gammopathy, and/or cytopenias resulting from bone marrow infiltration by lymphoma. The differential diagnosis with other lymphomas is essential for the assessment of prognosis and therapeutic approach. Treatment of patients with asymptomatic WM does not improve the quality of life of patients, or increase their survival, being recommended, therefore, their follow-up. For the treatment of symptomatic patients, alkylating agents, purine analogs and anti-CD20 monoclonal antibodies are used. However, the disease is incurable and the response to therapy is not always favorable. Recent studies have shown promising results with bortezomib, an inhibitor of proteasomes, and some patients respond to thalidomide. In patients with relapse or refractory to therapy, autologous transplantation may be indicated. The aim of this paper is to describe in detail the current knowledge on the pathophysiology of WM, main clinical manifestations, diagnosis, prognosis and treatment.

Waldenström macroglobulinemia: clinical and immunological aspects, natural history, cell of origin, and emerging mouse models

Waldenström macroglobulinemia (WM) is a rare and currently incurable neoplasm of IgM-expressing B-lymphocytes that is characterized by the occurrence of a monoclonal IgM (mIgM) paraprotein in blood serum and the infiltration of the hematopoietic bone marrow with malignant lymphoplasmacytic cells. The symptoms of patients with WM can be attributed to the extent and tissue sites of tumor cell infiltration and the magnitude and immunological specificity of the paraprotein. WM presents fascinating clues on neoplastic B-cell development, including the recent discovery of a specific gain-of-function mutation in the MYD88 adapter protein. This not only provides an intriguing link to new findings that natural effector IgM⁺IgD⁺ memory B-cells are dependent on MYD88 signaling, but also supports the hypothesis that WM derives from primitive, innate-like B-cells, such as marginal zone and B1 B-cells. Following a brief review of the clinical aspects and natural history of WM, this review discusses the thorny issue of WM's cell of origin in greater depth. Also included are emerging, genetically engineered mouse models of human WM that may enhance our understanding of the biologic and genetic underpinnings of the disease and facilitate the design and testing of new approaches to treat and prevent WM more effectively.

IgG-Secreting B-Cell Lymphoplasmocytoid Leukaemia : Molecular Characterization of Igh Rearrangements

Purpose : Elevated serum M component of IgG isotype is typically associated with multiple myeloma (MM). However, our group has previously reported cases with an elevated serum monoclonal IgG and a leukaemic B-cell lymphoplasmocytoid lymphoma (LPL) similar to Waldenstrom’s macroglobulinaemia (WM). The aim of this study was to extend analysis of IgH locus events in a larger series of IgG-secreting LPL.

Patients and Methods : We investigated 20 patients with an elevated serum monoclonal IgG (>20g/l) and LPL (IgG-LPL). Morphological classification and immunophenotyping analysis were performed at diagnosis (serum IgG>4 g/l, CD19+ cells >30%, presence of lymphoplasmocytoid cells in blood and/or bone marrow). Histological classification and FISH analysis were performed when possible to further characterize those cases. Analysis of VH genes was carried out from RNA with VHLeader and CH primers. 14 patients were examined for both IgG and IgM transcripts; VH-Cμ and VH-Cg transcripts could be compared in 9 patients.

Results : Of 25 Ig VH rearrangement sequences, 23 were functional and expressed in each case. VH3 family members appeared to be over-represented (19/21 patients (90.5%) as compared to 40/71 (56.3%) in normal B-cell repertoire (1). VH3-23 was the most frequently used segment (10/21 patients) and is frequently utilized in normal B-cells. IgG-LPL JH family use resembled the normal B-cell repertoire (predominance of JH4 and JH6 segments). The median CDR3 length was 10 amino acids [5–19]. However, and in contrast with features seen in other leukaemia, there was no evidence of homologous CDR3 motifs. All VH genes revealed highly somatically mutated sequences, with a median mutation rate 8.8% [0.7 – 11.1%] (IMGT database(2)). We compared pre (VH-Cμ) and post-switch (VH-Cg) transcripts, and 4/9 patients had identical clonally-derived sequences, and two 2/9 had divergent sequences.

Interpretation and conclusion : This intended study of IgG-LPL reveals consistent features that argue for common origins of IgG-LPL with Waldenstrom’s macroglobulinemia. One feature is extensive somatic mutations in VH genes, suggesting origins from a cell that may have undergone successive rounds of mutation. Patterns of mutations in pre-and post-switched clonally derived sequences suggest that the final neoplastic event has occurred in a IgM+ memory cell undergoing isotype switch. However, no aberrant chromosomal translocation accrue at the IgH locus, as apparent from FISH data. This indicates that, unlike in typical MM, switch activity does not generate 14q32 abnormalities nor that such lesions play a role in the pathogenesis of LPL. Extensive mutations are also seen in VH genes in WM, and there is evidence that WM cells can undergo class switch events in vivo. In this tumor, switching occurs at a low subclonal level in some cells, which in rare cases can lead to the emergence of a dual population of clonally identical IgM and IgG expressing WM tumor cells at a later stage of disease (7). In typical WM, 14q32 abnormalities are also generally not seen. These data suggest that IgG-LPL and WM could be two variants of the same entity, with IgG-LPL exposed to persistent switch stimuli following transformation.

Waldenström macroglobulinaemia

Over time, Waldenström macroglobulinaemia (WM) has evolved conceptually from a clinical syndrome to a distinct clinicopathological entity. Progress is being made in standardization of the disease definition and treatment response criteria, although nosologic controversies persist. According to the Second International Workshop on WM, the disease is defined as a B-cell neoplasm characterized by a lymphoplasmacytic infiltrate in the bone marrow, with an associated immunoglobulin (Ig) M paraprotein. Disease symptoms are often divided into those related to tumour infiltration and those related to the rheological effects of the monoclonal IgM. As with other low-grade lymphomas, asymptomatic patients are observed only, with treatment reserved for symptomatic patients. There is no standard treatment for WM and choices include rituximab, alkylating agents, purine nucleoside analogues, alone or in combination, as well as autologous peripheral blood stem cell transplant in eligible patients. Novel treatments, such as bortezomib, oblimersen sodium, perifosine and others are being evaluated.

Waldenström macroglobulinemia

In the past 36 months, new developments have occurred both in the understanding of the biology of Waldenström macroglobulinemia (WM) and in therapeutic options for WM. Here, we review the classification, clinical features, and diagnostic criteria of the disease. WM is a B-cell neoplasm characterized by lymphoplasmacytic infiltration of the bone marrow and a monoclonal immunoglobulin M (IgM) protein. The symptoms of WM are attributable to the extent of tumor infiltration and to elevated IgM levels. The most common symptom is fatigue attributable to anemia. The prognostic factors predictive of survival include the patient's age, β2-microglobulin level, monoclonal protein level, hemoglobin concentration, and platelet count. Therapy is postponed for asymptomatic patients, and progressive anemia is the most common indication for initiation of treatment. The main therapeutic options include alkylating agents, nucleoside analogues, and rituximab. Studies involving combination chemotherapy are ongoing, and preliminary results are encouraging. No specific agent or regimen has been shown to be superior to another for treatment of WM. Novel agents such as bortezomib, perifosine, atacicept, oblimersen sodium, and tositumomab show promise as rational targeted therapy for WM.