With the use of novel drugs, such as thalidomide, lenalidomide and bortezomib, and risk-adapted strategies, including autologous stem cell transplantation in eligible patients, 10-year overall survival rates in the small subset of MM patients younger than 60 years have reached up to 30%.2, 3, 19
Patients who carry MM clones with hyperdiploidy or a t
(11;14) were shown to have a relatively better outcome compared with those whose tumors harbor a t
(14;16) or del 17p.20
Interestingly, MM patients carrying the t
(4;14) translocation that often leads to deregulation of FGFR3 seem to benefit from the inclusion of bortezomib into their first-line treatment21
in that the adverse prognosis may be overcome. This suggests future concepts in which the detection of molecular alterations may ultimately lead to refinement of therapeutic approaches. Nevertheless, MM currently remains largely incurable and efforts are underway to develop more individualized, molecular therapies that target critical growth and survival pathways in the tumor cells.
Novel sequencing technologies nowadays allow for the identification of somatically mutated genes in malignant tumors on a global scale. Initial genome sequencing of MM5
and published literature suggest that recurrent genetic translocations (t
(14;16)) or oncogene mutations, for example, in KRAS
, with frequencies of at best 15–20%.3
Novel mutations are detected at even lower frequencies. BRAF
, for example, is mutated in only 4% of all samples,5
in sharp contrast to the scenario in hairy cell leukemia that harbors BRAF
mutations in most, if not all tumors.22
Thus, it appears unlikely that the development of a targeted therapy against a single molecule will prove to be a successful strategy in MM. Rather, there has been a recent focus in clinical phase I/II trials to test the hypothesis whether the modulation of molecular pathways, for example, by inhibiting mitogen-activated protein kinase or AKT, or by using epigenetic agents, could represent a promising way forward.23, 24
Such an approach is supported by a recent sequencing analysis of 38 MM tumor genomes/exomes that showed clustering of somatic mutations in important cellular pathways, including the RNA-processing machinery, histone-modifying enzymes and genes involved in nuclear factor-κB signaling.5
Our discovery and validation approach, integrating altogether 43 primary MM samples (including published MM samples) and 6 MM cell lines, provides evidence that the neoplastic cells in MM are affected by multiple and heterogeneous somatic mutations in adhesion- and RTK-associated signaling molecules, which is a hitherto undescribed finding. Notably, 147 out of 201 mutated adhesion molecules were newly detected in our analysis, including the integrins ITGA6, ITGA7 and ITGB5, the cadherins CDH23, CDH18 and CDH9, and the protocadherin clusters (for example, PCDH9 and PCDHGA3) among others. Twenty mutations in adhesion molecules were found to be affected in at least 1 of the 43 primary MM samples and 1 cell line, were ‘damaging' by at least 2 functional predictors and were validated by Sanger sequencing. Available gene expression and copy number data in six investigated MM cell lines allowed us to correlate the presence of specific mutations in adhesion-associated molecules with accompanying alterations on the mRNA expression and genomic copy number level. From this analysis, it remains unclear whether mutations in adhesion molecules might increase or decrease their function, that is, whether they might act as oncogenes or as tumor suppressors.
However, the frequent occurrence of somatic mutations in adhesion molecules in primary MM emphasizes their presumably important biological role for the interaction of the neoplastic plasma cells with their BM microenvironment. For example, the interaction between MM cells and BM stromal cells was shown to provide adhesion-mediated drug resistance.8, 25, 26
In addition, this interaction may lead to the activation of signal transduction pathways that promote cell cycle progression and cell survival via expression of growth factors such as insulin-like growth factor and vascular endothelial growth factor, as well as chemokines and cytokines.8
Our results support the notion that MM cells are being selected for clones with increased numbers of mutated adhesion molecules during tumor evolution. As a consequence, therapeutic efforts to more effectively target the interaction between MM cells and their surrounding stroma appear reasonable, a strategy that is currently tested in chronic lymphocytic leukemia, in which the stromal cell-derived factor-1/C-X-C chemokine receptor type 4 axis in BM and lymph node infiltrates is targeted by the C-X-C chemokine receptor type 4antagonists, such as plerixafor and T140.27
Another important finding of our study is the enrichment of somatic mutations in RTKs in MM cells. The well-known RTKs EGFR
were affected in at least 1 of the 43 primary MM samples and in at least 1 MM cell line, and mutations in these genes were predicted to be ‘damaging' according to at least 2 functional predictors. This finding is of interest for two reasons. First, RTKs likely have a role in the pathogenesis of MM, as evidenced by the presence of FGFR3
translocations in a considerable subset of MM,2
and signaling cascades downstream of RTKs that involve the aberrant activation or deregulation of the RAS/mitogen-activated protein kinase and phosphoinositide-3-kinase/AKT pathways are frequently altered in MM.10, 28, 29
Second, these receptors may represent interesting targets for novel therapeutic approaches reinforcing initial concepts in MM to develop small-molecule inhibitors that block anti-apoptotic or pro-proliferative effects of RTK activation. RTK inhibition is already a widely used strategy for treating various types of solid cancers, including epidermal growth factor receptor inhibitors (erlotinib, gefitinib) in non-small cell lung cancer or the monoclonal antibody trastuzumab and the small-molecule inhibitor lapatinib in patients with ERBB2/HER2-neu positive breast and gastric cancer, respectively.30, 31, 32
More recently, insulin-like growth factor-1 receptor inhibitors have been under investigation in clinical trials of non-small cell lung cancer, Ewing sarcoma and prostate cancer.33, 34, 35, 36, 37
Given the increased frequency of RTK mutations in MM, therapeutic intervention using RTK inhibitors could prove useful in subsets of MM patients.
Our data, in line with published results,5
suggest that the pathogenesis and progression of MM is accompanied by the development of multiple and heterogeneous somatic mutations in various important cellular signaling cascades (RNA-processing machinery, histone-modifying enzymes, nuclear factor-κB system, adhesion- and RTK-associated molecules), leading to a profound ‘interindividual pathway redundancy'. For example, all 6 MM cell lines and approximately 95% of 43 primary MM samples carried mutations in at least 1 adhesion molecule, RTK or RTK downstream-signaling effector that all cluster within the same signaling network, indicating that pathways belonging to this network are of importance in MM pathogenesis. Approximately 50% of primary MM were affected by mutations in more than one gene of this network, and almost one third of primary MM samples carried multiple mutations (>2) in this signaling network, suggesting an ‘intra-individual pathway redundancy'. Of note, 1 MM with anaplastic morphology carried 16 mutations (including, for example, ALOX12B
). This observed pathway redundancy' underlines the necessity of comprehensive genetic approaches to make informed decisions on how to employ future targeted molecular therapies most effectively for the benefit of individual MM patients.