Multiple myeloma: Searching a novel oncogene at 12q24

Multiple myeloma (MM) is a clonal B-cell neoplasm that affects terminally differentiated plasma cells. The malignant myeloma cell corresponds to a long-lived post-germinal center plasma cell that has completed most of its differentiation. Clinically, MM may proceed through different phases: Monoclonal gammopathy of undetermined significance (MGUS), smoldering myeloma, multiple myeloma and plasma cell leukemia. MM is not curable by standard chemotherapy regimens, depending on the stage of the disease, the current median duration of survival with chemotherapy is about 3 years. The wide clinical heterogeneity of MM is also reflected by different molecular mechanisms underlying myeloma pathogenesis, MM nowadays serves as a paradigm for a multistep transformation model of a human cancer. Immunoglobulin heavy chain (IgH) associated translocations, characterized by a variety of chromosomal translocation partners, are a constant feature of MM and are thought to occur as an early step in MM pathogenesis. Some of these translocations have been dissected at the molecular level and the involved oncogenes have been identified, however, some recurring translocations have been characterized only cytogenetically, indicating that genes important to MM pathogenesis still need to be discovered. We have previously characterized a novel recurring translocation t(12; 14)(q24;q32) in MM. In a previous FWF project, we analyzed a MM cell line carrying this translocation. The translocation breakpoints were mapped by Fluorescence in situ hybridization (FISH) and Southern blotting and cloned using a probe specific for C-gamma. We confirmed the interchromosomal translocation breakpoint (derivative(12) chromosome) and unambiguously aligned the involved translocation partner sequences to chromosomal region 12q24. The goal of this project is the characterization of the reciprocal translocation breakpoint (derivative(14) chromosome) and identification of the novel candidate oncogene deregulated by this translocation.

On the genetic level many hematologic neoplasms are characterized by recurring chromosomal aberrations, e.g. reciprocal translocations, leading to the deregulation of genes in the vicinity of these translocation-breakpoints and thereby causing malignant transformation. In myeloid neoplasms, many of these translocations result in the creation of pathologic fusion RNAs and fusion proteins which are derived from different chromosomes. The detection of these fusion RNAs by means of Reverse-Transcriptase Polymerase chain reaction (RT-PCR) is a powerful tool for both the diagnostic confirmation of disease-specific translocations as well as monitoring of tumor cell mass in patients after treatment with chemotherapy (minimal residual disease, MRD). We investigated four patients with a special subtype of acute myeloid leukemia associated with a specific translocation t(8;16)(p11;p13) that leads to a fusion of the CBP and the MOZ gene on chromosome 16 and 8, respectively. So far, analysis of this specific translocation by RT-PCR has proven difficult, with only four described cases in the literature. We analyzed the genomic breakpoint region in this specific disease entity enabling us to develop a novel RT-PCR strategy for the routine detection of the CBP and MOZ RNA fusions that are associated with this translocation and we applied this RT-PCR assay also to monitor minimal residual disease in one of the patients. Our analysis led to a better understanding of the genomic breakpoint regions t(8;16)(p11;p13) positive leukemias, moreover, our results show that RT-PCR may be a valuable tool for the clinical routine detection of this specific translocation which is also associated with a dismal prognosis. Thus RT-PCR may identify high-risk patients which might benefit from more aggressive treatment strategies. In addition, for the first time in this disease entity, we have successfully monitored MRD in one of the patients investigated. Thus, our RT-PCR strategy might also be valuable to identify patients that have a high risk of relapse after chemotherapy and may help to guide treatment decisions and eventually improve the treatment outcome of this prognostically unfavorable patient group.