The clonogenic precursor cell in multiple myeloma

In multiple myeloma, bone-marrow lymphocytes harboring the same chromosomal abnormalities as autologous plasma cells predict poor survival

Chromosomal abnormalities in plasma cells (PCs) from multiple myeloma (MM) provide a clonal signature to identify malignant cells. BM-lymphocytes from MM aspirates, defined by stringent criteria, were screened for the same chromosomal abnormalities as autologous PCs, including translocations, deletions, and amplifications. For 200 MM patients, we evaluated BM mononuclear cells to identify lymphocytes and autologous PCs on the same slide, followed by interphase fluorescence in situ hybridization to characterize their chromosomal abnormalities. Of all patients having a given chromosomal abnormality(s) in PCs, 45% showed that same abnormality(s) in 2–37% (median = 5%) of BM-lymphocytes. Most translocations, amplifications, and deletions found in MM PCs were also detected in lymphocytes, above the healthy-donor “cut-off.” In patients having chromosomally abnormal CD20⁻ PCs, chromosomally abnormal lymphocytes were found among CD20+ cells confirming them as B cells. Exceptions were amplification of 1q21 or p53 deletion, which characterize PCs but were undetectable in BM-lymphocytes, suggesting that processes leading to these abnormalities may be exclusive to PCs. For a set of 75 patients whose BM-lymphocytes and PCs were analyzed by all six probe sets, 58% of those with abnormal PC also had abnormal BM-lymphocytes harboring from one to five different abnormalities. Confirming the clinical significance of chromosomally abnormal BM-lymphocytes, MM patients having abnormalities in both lymphocytes and PC had significantly worse survival than those with abnormalities only in PC (HR = 2.68). The presence of at least one chromosomal abnormality in BM-lymphocytes appears to have greater clinical significance than particular abnormalities. Chromosomally abnormal BM-lymphocytes correlate with poor outcome and by extrapolation with more aggressive disease.

Phenotyping studies of clonotypic B lymphocytes from patients with multiple myeloma by flow cytometry

CONTEXT

Clonotypic B lymphocytes, monoclonal B lymphocytes sharing identical, rearranged IGH-CDR3 sequences with the patient's myeloma cells, have been detected in the peripheral blood of patients with multiple myeloma. These cells have been postulated to act as a therapy-resistant tumor reservoir that drives recurrence.

OBJECTIVE

To characterize clonotypic B lymphocytes for future investigation of their role in myeloma pathogenesis.

DESIGN

Harvests of cryopreserved peripheral blood stem-cells from 20 myeloma patients were enriched for clonotypic B lymphocytes. Cytoplasmic immunoglobulin light chain and surface immunophenotype were analyzed by flow cytometry. IGH-CDR3 gene-rearrangement pattern was performed to determine clonality. Posttransplant remission rate was compared with the percentage of clonotypic B lymphocytes.

RESULTS

Clonotypic B lymphocytes expressing CD34(+/-), CD38(+), CD184(+), CD31(+/-), CD50(+/-), CD138(-), CD19(-), CD20(-), and the same immunoglobulin light chain as the patients' known myeloma cells were identified in 12 of 20 patients (60%). Progenitor B lymphocytes expressing similar surface immunophenotype but opposite light chains were identified in the same patients. Polymerase chain reaction for IGH rearrangement showed clonal rearrangement pattern in clonotypic lymphocytes but not in B lymphocytes expressing light chains opposite to myeloma cells. There was no statistically significant correlation between the percentage of clonotypic B lymphocytes and response to autologous transplant.

CONCLUSIONS

Clonotypic B lymphocytes expressing CD34, but not CD19, were identified in stem cell harvests from patients with myeloma and could represent progenitor cells of neoplastic plasma cells. However, the same or similar immunophenotyping can be detected in both clonotypic B lymphocytes and benign progenitor B cells, suggesting clonality analysis might be needed to determine clonotypic B lymphocytes in patients with myeloma. Further studies are warranted to study the role of clonotypic B lymphocytes in the pathogenesis of myeloma.

Glycosylation of IgG B cell receptor (IgG BCR) in multiple myeloma: relationship between sialylation and the signal activity of IgG BCR

Little is known about the glycosylation of the isotype switched B cell receptor (BCR) in multiple myeloma, and the way it might affect receptor function. In this work IgG BCRs isolated from the individual lysates of peripheral blood lymphocytes (PBL) of 32 patients with IgG multiple myeloma and healthy controls were investigated for the expression of sialic acid (SA), galactose (Gal) and N-acetylglucosamine (GlcNAc), the sugars known to specify the glycoforms of human serum IgG. The degree of glycosylation and signaling status of all 32 isolated myeloma IgG BCRs were correlated and compared with the glycosylation of the IgG paraproteins isolated from sera of the same patients. It was shown that BCR IgG in myeloma is more heavily sialylated when compared with normal controls, that the increased sialylation of IgG BCR is associated with higher levels of tyrosine phosphorylation (signaling activity) of the IgG BCR supramolecular complex and that BCR IgG and serum IgG paraprotein from the same patient differed in all cases in the levels of terminal sugar expression. The results suggest that the development of the malignant clone in MM from post-switch B cells expressing IgG BCR at their surfaces to plasma cells secreting IgG paraprotein may be followed by permanent glycosylation changes in the IgG molecules.

Signaling status of IgG B cell receptor (IgG BCR) is indicative for an activated state of circulating B cells in multiple myeloma

Circulating post-switch B cells have been proposed as proliferative and disseminating progenitors in multiple myeloma. It is unclear whether the class-switched antigen receptor expressed at the surface of these cells plays a role in their expansion. In this work, the signaling status of IgG B cell receptor (BCR) isolated from the lysates of peripheral blood lymphocytes of 32 patients with IgG multiple myeloma, at the time of diagnosis, was investigated by examining whether phosphorylation of BCR Igalpha and Igbeta signal transducer factors (co-receptors) or other signaling molecules was abnormal in these cells when compared with healthy controls. In IgG BCR of normal controls, weak phosphorylation of 56 and 61 kDa Src kinase-related proteins and unphosphorylated co-receptors were found. In myeloma, p56 and p61 kDa proteins, co-receptors, and other IgG BCR-associated proteins from the signal cascade were phosphorylated. Myeloma patients can be classified into subgroups by IgG BCR phosphorylation profiles which characteristically coordinated with the level of IgG paraprotein in serum and the stage of disease. There was a correlative trend between the extent of phosphorylation reduction and advanced stage of disease. Reduced phosphorylation was more pronounced with advanced stages of multiple myeloma.

A biased Gm haplotype and Gm paraprotein allotype in multiple myeloma suggests a role for the Gm system in myeloma development

The association between a particular Gm haplotype and susceptibility to multiple myeloma (MM) is not clear. The reason is probably because no investigations have so far been carried out on the relationship between the Gm haplotype, which represents the inherited combination of IgG Gm allotypes, and the Gm allotype expressed at the IgG paraprotein (M-component), which reflects the enhanced gene expression within the haplotype in MM. We studied the incidence of Gm allotypic markers present in IgG subclasses in the serum from 52 patients with MM and in parallel with the isolated IgG paraproteins. The results showed that 84.6% of the patients were heterozygous for haplotypes Gm(a; z; n-; g;)/Gm(f; n+/n-; b1; b0; b5) and 15.3% were homozygous for Gm(f; n/n-; b1; b0; b5), while no homozygous Gm(a; z; n-; g) individuals were found among the studied patients. The incidence of these combinations in the healthy population in Serbia is 34%, 66% and < 1%, respectively. Subjects with Gm(a; z; n-; g)/Gm(f; n+/n-; b1; b0; b5) combination are over 10 times [odds ratio (OR) = 10.69; 95% confidence interval 1.67-68] as likely to be affected by the disease as the subjects with homozygous Gm(f; n+/n-; b1; b0; b5) combination (OR = 0.35, 95% confidence interval 0.06-2.23). However, despite the Gm heterozygosity, most of the Gm(a; z; n-; g;)/Gm(f; n+/n-; b1; b0; b5) positive patients with MM (86.3%) had IgG paraprotein with the allotypic marker from the Gm(f; n+/n-; b1; b0; b5) haplotype. Together with patients homozygous for this haplotype, the relative number of patients with serum IgG paraprotein carrying allotypic marker from the Gm(f; n/n-; b1; b0; b5) haplotype was 88.5%. These results suggest that the development of an M-component could be related to a disturbance on chromosome 14q32 carrying the Gm (f; n+/n-; b1; b0; b5) set of genes.

Multiple myeloma biology: lessons from the 5TMM models

Multiple myeloma (MM) is a B cell neoplasm characterized by the monoclonal proliferation of plasma cells in the bone marrow, the development of osteolytic lesions and the induction of angiogenesis. These different processes require three-dimensional interactions, with both humoral and cellular contacts. The 5TMM models are suitable models to study these interactions. These murine models originate from spontaneously developed myeloma in elderly mice, which are propagated by in vivo transfer of the myeloma cells into young syngeneic mice. In this review we report on studies performed in the 5TMM models with special emphasis on the homing of the myeloma cells, the characterization of the migrating and proliferating clone and the identification of the isotype switch variants. The bone marrow microenvironment was further targeted with osteoprotegerin (OPG) to block the RANK/RANKL/OPG system and with potent bisphosphonates. Both treatments resulted in a significant protection against myeloma-associated bone disease, and they decreased myeloma disease, as evidenced by a lower tumor load and an increased survival of the mice. These different studies demonstrate the strength of these models, not only in unraveling basic biological processes but also in the testing of potentially new therapeutic targets.

Myeloma clonotypic B cells are hampered in their ability to undergo B-cell differentiation in vitro

In the peripheral blood (PB) of multiple myeloma (MM) patients, clonotypic B cells are present that express the identical V(D)J rearrangements as the malignant plasma cells in the bone marrow. In the present study, the proliferative capacity of clonotypic B cells from MM patients (n = 10) and the ability to differentiate in vitro was determined using the CD40-culturing system. For six patients, the presence of clonotypic B cells expressing variant immunoglobulin (Ig) isotypes was assessed by Ig isotype-specific allele-specific oligonucleotide reverse transcription polymerase chain reaction (ASO-RT-PCR) after culturing with CD40L and interleukin 4 (IL-4). In three out of six patients, clonotypic B cells expressing variant isotypes were detected both before and after culturing. The ability of clonotypic B cells to undergo B-cell differentiation was studied by abrogating CD40 signalling accompanied by IL-10 and IL-2 stimulation, enhancing differentiation towards Ig-secreting cells. The numbers of clonotypic B cells were determined by quantitative ASO-PCR. An increase in cell number was observed upon CD40L and IL-4 stimulation, whereas the relative number of clonotypic B cells was unaltered. In contrast, upon B-cell differentiation the relative number of clonotypic B cells decreased. In conclusion, clonotypic B cells can be cultured and isolated in vitro using the CD40 system. Clonotypic B cells responded to CD40 triggering in a similar fashion as to non-clonotypic normal B cells. However, the ability of clonotypic B cells to undergo in vitro activation and differentiation into Ig-secreting cells is hampered.

The biology and cytogenetics of multiple myeloma

Despite the advances in our knowledge of myeloma cell biology, our understanding of myeloma pathogenesis is still incomplete. In this review, we present a summary of the cellular and molecular aspects of B-cell development and immunoglobulin (lg) gene rearrangement which have been important in defining the characteristics of the myeloma plasma cell (MPC). The PMC has undergone variable gene recombination, somatic hypermutation and isotype switching, and is therefore at a postgerminal center stage of development. The finding of preswitch clonal cells and isotype variants have raised interesting questions about the cell of origin of myeloma, for which no conclusive data is as yet available. However much information has been obtained about the chromosomal and genetic aberrations in myeloma, including monosomy 13, Ig heavy chain (IgH) switch region translocations, numerical abnormalities and a multitude of heterogeneous changes. A variety of techniques have been developed to overcome the insensitivity of conventional karyotyping, utilizing molecular cytogenetic strategies ranging from the delineation of precise loci by fluorescent in situ hybridization, a more "global" assessment of the genome by multicolor spectral karyotyping, to the quantitation of chromosomal material of specific origin by comparative genomic hybridization. Whether the abnormalities detected represent oncogenic insults, are involved in disease progression or are simply "by-products" of genetic instability is still unclear. For IgH translocations, the role of candidate genes such as Cyclin D1 and FGFR3 has been studied extensively by quantitating their expression and assessment of their oncogenicity (e.g. for FGFR3) in animal models. The significance of other aberrations such as c-myc, ras and p53 has also been investigated. With the advent of oligonucleotide microarrays, the expression of thousands of genes can be efficiently examined. So far, this approach seems promising in defining subgroups of different disease behavior, and may highlight specific genes and molecular mechanisms which are important in myeloma pathogenesis.

Myeloma isotype-switch variants in the murine 5T myeloma model: evidence that myeloma IgM and IgA expressing subclones can originate from the IgG expressing tumour

Isotype-switch variants can easily be detected in a significant proportion of multiple myeloma (MM) patients. The biological significance of these isotype-switch variants remains obscure. Therefore, we studied the appearance of these isotype-switch variants in two murine MM models, 5T2MM and 5T33MM, both of IgG isotype. With a MM-specific PCR assay we could detect isotype-switch variants in the bone marrow of both the 5T2MM and the 5T33MM bearing mice, reflecting again the close resemblance of this mouse model to the human MM. These isotype-switch variants were not found in an in vitro stroma-independent variant of the 5T33MM line. However, when this 5T33MMvitro line was injected into young syngeneic mice, isotype-switch variants appeared thereafter in the isolated tumour cells. These isotype-switch variants could only originate from the MM-IgG expressing cell since IgG subclones from the 5T33MMvitro line again gave rise to isotype-switch variants. The appearance of IgA cells can be explained by down-stream switching of IgG to IgA, while the emergence of IgM cells have to occur via trans-switching to the sister chromatid as the Cmu region is deleted from the CIS-chromosome. This study demonstrates that isotype-switch variants originate from the major tumour clone suggesting no role for the MM-IgM expressing cell as a pre-switch precursor MM cell. The appearance of isotype-switch variants should be considered as a rare but normal event now becoming visible due to the high number of clonal cells present in MM.

Immunophenotypic Aberrations, DNA Content, and Cell Cycle Analysis of Plasma Cells in Patients with Myeloma and Monoclonal Gammopathies

We describe the immunophenotypic and gross DNA defects in 55 patients with myeloma and 50 patients with monoclonal gammopathy and review the literature on this subject (MedLine, 1994-2000). Our data confirmed previous reports indicating that in myeloma nearly all marrow plasma cells are abnormal (98.7 +/- 8.1%). In monoclonal gammopathy the fraction of abnormal plasma cells was 35.0 +/- 32.8%. In both myeloma and monoclonal gammopathy, the most frequent aberrant phenotypic features consisted of absence of expression of CD19, strong expression of CD56, and decreased intensity of expression of CD38; aberrant expression of CD10, CD20, CD22, or CD28 was observed in less than one-third of myeloma cases. The vast majority of cases had two or more phenotypic aberrations. In the DNA studies, 7% of myeloma cases were biclonal and 93% of cases were monoclonal. In those studies with only one plasma cell mitotic cycle, 37% had normal DNA content and 63% were aneuploid (hyperploid, 61%; hypoploid, 2%). The mean percentages of plasma cells in S- and G2M phases were 4.9 +/- 8.5 and 4.4 +/- 6.9%, respectively. Thirty-eight percent of cases had more than 3% of plasma cells in S phase. In monoclonal gammopathy, the DNA index of abnormal plasma cells ranged from 0.89 to 1.30 and the percentage of diploid (31%) and aneuploid (69%) cases was not different from the results found in myeloma. The differences in percentage of abnormal plasma cells in S- (7.4 +/- 8.6%) and G2M-phases (2.4 +/- 1.7%) in patients with monoclonal gammopathy were not statistically significant.

Molecular aspects of multiple myeloma

Multiple myeloma (MM) is a B-cell neoplasm characterized by bone marrow infiltration with malignant plasma cells, which synthesize and secrete monoclonal immunoglobulin (Ig) fragments. Despite the considerable progress in the understanding of MM biology, the molecular basis of the disease remains elusive. The initial transformation is thought to occur in a postgerminal center B-lineage cell, carrying a somatically hypermutated Ig heavy chain (IGH) gene. This plasmablastic precursor cell colonizes the bone marrow, propagates clonally and differentiates into a slowly proliferating myeloma cell population, all under the influence of specific cell adhesion molecules and cytokines. Production of interleukin-6 by stromal cells, osteoblasts and, in some cases, neoplastic cells is an essential element of myeloma cell growth, with the cytokine stimulus being delivered intracellularly via the Jack-STAT and ras signaling pathways. While karyotypic changes have been identified in up to 50% of MM patients, recent molecular cytogenetic techniques have revealed chromosomal abnormalities in the vast majority of examined cases. Translocations mostly involve illegal switch rearrangements of the IGH locus with various partner genes (CCND1, FGFR3, c-maf). Such events have been assigned a critical role in MM development. Mutations in coding and regulatory regions, as well as aberrant expression patterns of several oncogenes (c-myc, ras) and tumor suppressor genes (p16, p15) have been reported. Key regulators of programmed cell death (BCL-2, Fas), tumor expansion (metalloproteinases) and drug responsiveness (topoisomerase II alpha) have also been implicated in the pathogenesis of this hematologic malignancy. A tumorigenic role for human herpesvirus 8 (HHV8) was postulated recently, following the detection of viral sequences in bone marrow dendritic cells of MM patients. However, since several research groups were unable to confirm this observation, the role of HHV8 remains unclear. Translation of the advances in MM molecular biology into novel therapeutic strategies is essential in order to improve disease prognosis.

Bcl-1 rearrangement and cyclin D1 protein expression in multiple myeloma precursor cells

The rearrangement of Bcl-1 gene (Bcl-1/IgH rearrangement) and expression of cyclin D1 in multiple myeloma (MM) precursor cells were studied and the role of cyclin D1 in the pathogenesis of MM was investigated. The BCL-1 rearrangement and cyclin D1 protein expression in 15 cases of MM were detected. By using hemi-nested polymerase chain reaction (PCR) the genomic DNA from fresh peripheral blood and bone marrow was amplified and the expression of cyclin D1 in the smears was detected by using immunohistochemical method. Ten volunteer with normal bone marrow served as control group. The results showed Bcl-1 rearrangement was detectable in 3/15 (20%) MM patients and cyclin D, expression in 4/15 (27%) MM patients. Bcl-1 rearrangement and cyclin D1 protein expression were also detected in MM precursor cells. No overexpression of cyclin D1 or the rearrangement of the Bcl-1 gene was found in the 10 volunteers. It was concluded that Bcl-1 rearrangement and cyclin D1 protein overexpression were detected in MM precursor cells, speculating that overexpression of cyclin D1 protein may play an initial (critical) role in the pathogenesis of MM.

Growth control mechanisms in multiple myeloma

Interleukin-6 (IL-6) is the major growth factor for the malignant plasma cell clone in patients with multiple myeloma (MM). Although interferon-alpha (IFN-alpha) has been widely used as maintenance therapy in MM, controversy exists as to its clinical utility. This review summarizes data showing that cell growth arrest brought about by type I (IFNs-alpha/beta) and type II (IFN-gamma) IFNs occurs in part through utilization/modification of various components of the otherwise stimulatory Jak-STAT and Ras signaling pathways triggered by IL-6. Recent experimental results indicating that IFN-alpha acts as a survival factor for certain myeloma cell lines and frequently induces endogenous IL-6 expression may help to explain the conflicting clinical findings obtained in this heterogeneous disease with this usually potent growth inhibitor. By comparison, consistent antiproliferative activity exhibited by IFN-gamma on IL-6-dependent myeloma cell lines and primary myeloma cells from patients suggests that further investigation of the possible value of this cytokine in the treatment of MM may be warranted.

Soluble IL-6R in plasma cell dyscrasias

The understanding of the biology of multiple myeloma has advanced significantly in the past few years. The identification of the pivotal role of Interleukin-6 (IL-6), the soluble IL-6 receptor (sIL-6R), and how the ligand receptor complex interacts with the signal transducer gp130 has provided new biological insights into plasma cell disorders. Some studies have suggested that sIL-6R levels may have prognostic significance in MM, however this is not a consistent finding. Here the biology and function of IL-6 and sIL-6R are reviewed and the clinical significance of sIL-6R discussed.