New insights into role of microenvironment in multiple myeloma
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- Tricot, G.J. Int J Hematol (2002) 76: 334. doi:10.1007/BF03165279
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Multiple Myeloma (MM) is a malignant disease of terminally differentiated B cells. It most likely originates in a B cell which has traversed the germinal center and has been exposed there extensively to antigens based on the high number of somatic mutations in the complementarity determining regions. The cell of origin is either a plasmablast, or more likely, a memory B-cell. Typically MM goes through different phases from indolent (MGUS, smoldering myeloma) to overt myeloma and then to a fulminant phase, characterized by extramedullary manifestations, high LDH, immature morphology and increased proliferation rate. In the indolent phase, the disease already has acquired major cytogenetic abnormalities as demonstrated by FISH and DNA flow cytometry. It has a gene pattern very similar to myeloma cells on gene array analysis. In the early stages of overt MM, the myeloma cells are completely dependent upon the micro-environment for their growth and survival. The interaction between myeloma cells and micro-environment causes bone disease, genetic instability and more importantly, drug-resistance, which is caused by upregulation of anti-apoptotic factors, resistance to apoptosis induced by FAS and TRAIL activation, and by cell adhesion-induced growth arrest. In this phase of the disease, MM is susceptible to chemotherapy, if delivered with adequate intensity. In the fulminant phase of MM, myeloma cells have acquired sufficient genetic alternations to become completely independent of the micro-environment which allows them to grow at extramedullary sites. Because of the many DNA breaks necessary for immature B cells to become mature plasma cells, B cells already have inherent genetic instability. DNA breaks are necessary for VDJ recombinations, somatic mutations and isotype switching and it is therefore not surprising that genetic alternations frequently occur at the Ig heavy chain site at 14q32, which is abnormal in three quarters of myeloma patients. Some of the translocations with 14q32 involve terminal fragments of chromosomes and can not be diagnosed with standard cytogenetics. Cytogenetic abnormalities are found in 30–35% of newly diagnosed patients and require sufficient proliferation of MM cells to find enough analyzable mitoses. The cytogenetic abnormalities are typically complex, involving ≥3 chromosomes in 80% of patients. Almost all chromosomes can be involved in deletions, additions or translocations of genetic material. Our group has repeatedly stressed the prognostic significance of chromosome 13 deletion by conventional cytogenetics. The role of chromosome 13 deletion by FISH is less clear. In addition to chromosome 13 deletion, the presence of a hypodiploid or hypotetraploid karyotye also carries a poor prognosis. Frequently, deletions of chromosome 13 and hypodiploidy go hand in hand. It remains unclear what specific gene confers the poor prognosis to patients with deletion 13. The issues of bone disease, drug resistance and cytogenetics will be addressed in detail during this presentation.