Multiple myeloma: evolving genetic events and host interactions

Abstract

Multiple myeloma, which is located at multiple sites in the bone-marrow compartment, is a malignant plasma-cell tumour that is characterized by osteolytic bone lesions. It is a slowly proliferating tumour, typically with less than 1% of tumour cells synthesizing DNA, until late in the disease, when multiple myeloma cells are often found outside the bone marrow. A pre-malignant lesion called monoclonal gammopathy of undetermined significance (MGUS), which is present in 1% of adults, progresses to malignant multiple myeloma at a rate of 1% per year. The karyotypes of multiple myeloma are complex, and more similar to those found in epithelial tumours and the blast phase of chronic myelogenous leukaemia than to those in other haematopoietic tumours. Primary translocations — mediated by errors in B-cell-specific DNA modification processes — juxtapose one or more oncogenes and immunoglobulin transcriptional regulatory regions in ∼50% of MGUS and multiple myelomas. In contrast to other B-cell malignancies, these translocations simultaneously dysregulate a variety of oncogenes, such as the genes for cyclin D1 or D3, fibroblast growth factor receptor 3 (FGFR3) combined with the nuclear protein MMSET, and the transcription factor c-MAF. Secondary translocations that do not involve B-cell-specific processes contribute to progression by dysregulating other oncogenes. Although c-MYC is dysregulated by primary translocations in some B-cell malignancies, it is dysregulated by secondary translocations, often without involvement of an immunogloublin locus, as myeloma tumours become more proliferative at a late stage of progression. Genetic changes are similar in pre-malignant MGUS and multiple myeloma, although the latter is distinguished by the presence of activating mutations of NRAS or KRAS2, and also a higher incidence of monosomy 13, indicating a possible tumour-suppressor gene on chromosome 13. Normal plasma cells, as well as MGUS and multiple myeloma cells, are dependent on the bone-marrow microenvironment for survival, growth and differentiation. These processes are, in part, mediated by paracrine interleukin-6 and insulin-like growth factor 1. The evolving interaction of multiple myeloma cells with the bone-marrow microenvironment is also involved in the secondary effects of malignancy, including osteolysis, anaemia and immunodeficiency. Multiple myeloma is an incurable malignancy for which the median survival has remained fixed at about 3 years for the past decade. Although MGUS can be efficiently diagnosed by a simple blood test, it is not possible to prevent progression or even predict when progression to myeloma will occur. Recent advances in understanding the molecular pathogenesis of these tumours indicate that improved approaches for prevention and treatment should be possible in the near future.