Chapter 1 – Pathophysiology

Contributors: Hervé Avet-Loiseau and Jill Corre

1 – Chromosomal aberrations

Cytogenetics

As with cancer in general, multiple myeloma (MM) is characterized by the occurrence of many genetic changes, either at the chromosomal level or at the DNA level (mutations). If karyotype analyses have been crucial in our understanding of leukemia oncogenesis, it is clearly not the case in MM. Several reasons may explain these differences. The most important one is probably the low proliferative index of plasma cells, preventing the generation of clonal metaphases in vitro. A second reason is that the quality of bone marrow samples sent to cytogenetic labs for analysis is frequently poor, partly due to the patchy distribution of plasma cells within the bone marrow. Nevertheless, even with a success rate inferior to 30%, cytogenetics can reveal some recurrent abnormalities [1].

MM karyotypes can be divided in two subgroups: hyperdiploid versus pseudo/hypodiploid with a 50:50 distribution. Amongst structural chromosomal abnormalities, mostly observed in non-hyperdiploid karyotypes, gains of 1q and 6p and losses of 1p, 6q, 8p, 14q, and 16q are most frequently observed. Numerical abnormalities are mainly gains with the exception of chromosome 13 monosomies, observed in approximately half of patients with MM. Hyperdiploidy is characterized by the nonrandom gains of chromosomes 3, 5, 7, 9, 11, 15, 19, and 21. The causes of this selectivity in gained chromosomes are totally unknown, especially if compared with another B-cell malignancy, for example childhood acute lymphoblastic leukemia where hyperdiploidy affects other chromosomes. Older studies did suggest that patients with hyperdiploidy displayed a better outcome than others with hypo/pseudodiploidy [2]. However, it is important to note that this kind of evaluation is by definition restricted to patients with clonal metaphases at karyotype, ie, less than 30% of patients.

Molecular cytogenetics

Using Southern-blot techniques, Bergsagel et al showed the highly recurrent presence of translocations involving the immunoglobulin H (IGH) gene at 14q32 in human myeloma cell lines (HMCLs) [3]. The authors first showed that ~25% of the HMCLs displayed the t(11;14)(q13;q32), similar to the translocation observed in mantle cell lymphoma [4]. This translocation systematically upregulates the cyclin D1 (CCND1) gene. They also identified two novel 14q32 translocations specific to MM, the t(4;14)(p16;q32) and the t(14;16)(q32;q13) [5,6]. The t(4;14) is unique in B-cell malignancies by upregulating two genes, fibroblast growth factor receptor 3 (FGFR3) and MM set domain (MMSET). Furthermore, the translocation forms a chimeric transcript on the derivative chromosome 4, Eμ-MMSET. The t(14;16) upregulates the avian musculoaponeurotic fibrosarcoma (MAF) proto-oncogene. These two translocations were each found in ~25% of the HCMLs. Based on these findings, several investigators aimed to check these incidences in patient primary plasma cells. By using interphase fluorescence in situ hybridization (FISH), they confirmed the recurrence of these three translocations, but with a lower incidence of 15–20%, 10–15%, and 3%, respectively for t(11;14), t(4;14) and t(14;16).

Subclones

One of the most important discoveries in the recent years was the description of molecular subclones in almost all patients with MM [7–9]. These subclones are present at the time of diagnosis (and probably already at the monoclonal gammopathy of undetermined significance [MGUS] stage), and derive from a common ancestral clone. The differences between the subclones can be due to mutations, but can also be the result of larger chromosomal gains or deletions. It is not known how these subclones evolve during the course of the disease. In one publication discussing a single patient, the distribution of the subclones at different relapse times varied even with the same treatment, ruling out selection pressure due to therapy [10]. However, a second report found a higher incidence of subclones in patients treated with bortezomib-dexamethasone as compared with broader chemotherapy [7]. It is thus not possible to choose between the therapeutic pressure and natural history selection hypotheses. Larger systematic analyses are warranted to address this question. This issue may have important implications for patient management, particularly, the choice of treatment at relapse.

Next generation sequencing

Three major papers have addressed the issue of the mutational landscape of MM using next generation sequencing (NGS) [11–13]. The first study analyzed 38 patients at different stages of the disease (for example, diagnosis and relapse), and showed that the distribution of mutations is widely distributed between patients, confirming the large heterogeneity observed at the clinical or biological levels. No common mutation was observed (in contrast to other B-cell malignancies such as Waldenström macroglobulinemia or hairy cell leukemia) [11]. The median number of mutations per genome is about 60, with a large range (21–488). When compared with other tumors, MM is in the middle between low mutated tumors such as leukemias and highly mutated carcinogen-induced tumors [12]. Furthermore, all three reports showed that only a few mutations are recurrent, such as neuroblastoma RAS viral oncogene homolog (NRAS) and v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS) mutations. Some other mutations are observed in approximately 3% of the patients, but less than 30 genes are recurrently mutated in at least 5% of the patients. Of note, many of these mutations are observed only in subclones, including genes that are supposed to act as drivers (for instance NRAS, KRAS, and v-raf murine sarcoma viral oncogene homolog B1 [BRAF]). These data have important implications for the treatment of patients with specific BRAF or mitogen-activated protein kinase kinase (MEK) inhibitors. Their therapeutic effect should be maximal only in cases where these driver mutations are present in all tumor cells.

Prognostic significance

As demonstrated in most hematological malignancies, genetic changes play a major role in prognostication in MM. However, in contrast with leukemias, no ‘favorable-risk’ abnormalities have been described so far. Amongst the high-risk chromosomal abnormalities, the most powerful ones are del(17p), t(4;14), and del(1p32) [14,15]. These abnormalities have a significant impact on both progression free survival (PFS) and overall survival (OS). Of note, these high-risk factors do not have an impact on response to therapy, including the del(17p). This finding favors the hypothesis that tumor protein 53 (TP53) is not the main target of del(17p). Other chromosomal changes do have an impact on survival, such as 1q gains, del(12), or t(14;16). However, their prognostic significance is either low (1q gains), or not confirmed in all studies (del(12p) and t(14;16)) [16]. Regarding other recurrent chromosomal changes, such as t(11;14) or hyperdiploidy, they are associated with a standard risk, although hyperdiploidy is probably heterogeneous, and may contain some ‘good-risk’ combinations. So far, the identification of ‘good-risk’ patients is essentially based on the absence of high-risk genetic features, associated with a low b2-microglobulin level. In an ongoing single nucleotide polymorphism (SNP)-array study (unpublished observations; manuscript in preparation), we showed that genetic abnormalities represent the major prognostic value, representing 75% of the OS prediction. Preliminary analyses of NGS data did not suggest that mutations display specific prognostic value, although larger systematic studies are warranted to clarify this point.