Molecular Pathogenesis of Multiple Myeloma

我们的主要重点是鉴定和表征多发性骨髓瘤（MM）肿瘤中IgH位点（染色体14q32）的易位。我们组装了36个EBV阴性MM细胞系，发现：1)所有36个MM细胞系（HMCL）中都存在Ig易位，包括IgH (33/36 = 92%), Iglambda（5/23 = 23%）和Igkappa (0/21)；2)在大多数情况下，克隆的IgH断点的位置与B细胞特异性机制的错误一致（主要是开关，但可能有一小部分体细胞超突变）；3)克隆的断点分散在一个大的区域，距离失调的、过表达的癌基因1mb；4) 30（50%）株中至少15株具有2（10）个或3(5)个独立的IgH易位；5)原发MM肿瘤中至少有18个（6个复发性）易位伴发者；6)除了位于8q24的c-myc（见下文），5个染色体位点（位于11q13的cyclin D1，位于6p21的cyclin D3，位于4p16.3的FGFR3酪氨酸激酶受体和MM.SET，以及分别位于16q23和20q11的c-maf mab基本zip转录因子）是复发性的；;7)在一组50例晚期肿瘤中，易位的频率稍低（58%为IgH， 15%为2个独立的IgH， 3个独立的IgH均无，Iglambda为16%，Igkappa为2%，35%为Ig无易位）。其他人对恶性肿瘤前MGUS和阴燃性MM肿瘤的分析表明，47%的恶性肿瘤前存在IgH易位。我们的工作假设是，在大约50%的肿瘤中，向Ig位点的原发性易位提供了骨髓瘤分子发病机制的初始永生化事件之一，并且通常发生在生发中心的浆细胞发育过程中。此外，涉及Ig基因座之一的继发性易位——但缺乏由B细胞特异性重组机制介导的过程的标志——在存在或不存在原发性易位的肿瘤中作为肿瘤进展事件发生。原发性和继发性易位似乎涉及不同的染色体伴侣（癌基因），尽管可能有一些重叠。

Our major focus has been to identify and characterize translocations to the IgH locus (chromosome 14q32) in multiple myeloma (MM) tumors. We assembled a panel of 36 EBV negative MM cell lines, and find that: 1) Ig translocations are present in all 36 MM cell lines (HMCL), including IgH (33/36 = 92%), Iglambda (5/23 = 23%), and Igkappa (0/21); 2) the location of cloned IgH breakpoints is consistent with errors of B cell specific mechanisms (switch mostly but perhaps somatic hypermutation in a small fraction) in most cases; 3) cloned breakpoints are scattered over a large region, as far as 1 Mb from the dysregulated, overexpressed oncogene; 4) at least 15 of 30 (50%) lines have two (10) or three (5) independent IgH translocations; 5) there are a minimum of 18 (6 recurrent) translocation partners identified by ourselves and others in primary MM tumors; 6) apart from c-myc at 8q24 (see below) five chromosomal loci (cyclin D1 at 11q13; cyclin D3 at 6p21; FGFR3 tyrosine kinase receptor and MM.SET at 4p16.3; and the c-maf or mafB basic zip transcription factors at 16q23 and 20q11,respectively) are recurrent; ; 7) in a panel of 50 advanced tumors, translocations are somewhat less frequent (IgH in 58%, 2 independent IgH in 15% 3 independent IgH in none, Iglambda in 16%, Igkappa in 2%, and no Ig translocation in 35%). Analyses of premalignant MGUS and smoldering MM tumor by others show that IgH translocations are present in 47% of pre-malignant tumors. Our working hypothesis is that primary translocations to Ig loci provide one of the initial immortalizing events in the molecular pathogenesis of myeloma in about 50% of tumors, and usually occur during plasma cell development in germinal centers. In addition, secondary translocations involving one of the Ig loci - but lacking the hallmarks of a process mediated by a B cell specific recombination mechanism - occur as a tumor progression events in tumors that do or do not have primary translocations.. It appears that primary and secondary translocations involve different chromosomal partners (oncogenes), although there might be some overlap. A second focus is to clarify the significance of our finding that there is selective expression of L-myc or one c-myc allele in 9 informative HMCL despite the apparent absence of a translocation, rearrangement, or amplification involving the c-myc locus. From a combination of FISH and SKY analyses, we have evidence for karyotypic abnormalities of L-myc (one HMCL) or c-myc locus in 28/32 (88%) HMCL that we have examined. Thus it seems clear that the selective expression of one c-myc allele is a consequence of a tumor specific, complex structural abnormality (complex translocation, insertion, duplication, inversion, with frequent involvement of 3 different chromosomes but not always an Ig locus) that alters the chromosomal context of one of the two L-myc or c-myc alleles. In all informative cases, it is clear that the myc structural abnormality was present in the primary tumor as well as in the HMCL. The incidence of c-myc abnormalities appears to be much lower (45%) in advanced, primary tumor samples. Some primary tumors show heterogeneity of the karyotypic abnormalities of c-myc, and one tumor had a karyotypic abnormality of N-myc. We have hypothesized that the complex karyotypic abnormalites that appear to dysregulate c-myc rarely - if ever- occur as an early event in tumorigenesis. Instead it appears that the dysregulation of c-myc occurs as a very late progression event, most often a complex translocation that is not mediated by B cell specific DNA modification processes. A third focus is to define other kinds of genetic and phenotypic abnormalities in MM. First, we have shown that K- or N-RAS mutations are present in 17/36(45%) HMCL and approximately 30% of untreated primary MM tumors (29/99), consistent with other studies suggesting that RAS mutations are associated with the transition from MGUS to MM, with some RAS mutations occurring during progression of MM. Second, for HMCL and primary MM tumors that overexpress FGFR3 due to the t(4;14) translocation, we find about 10% with mutations of FGFR3 and about 45% with mutations of RAS, but none with mutations in both. Similar to activated RAS, we have shown that activated FGFR3 can transform NIH3T3 cells. Third, we have identified bi-allelic deletion of p18INK4c in 32% of HMCL but only about 2-3% of unselected tumors. In one case bi-allelic deletion was present in relapsed tumor but not early in tumorigenesis, indicating that this is a very late progression event. Fourth, we have investigated p16INK4A expression by Taqman assays and also assessed methylation of the p16INK4A gene, with our preliminary results suggesting that p16INK4A expression is absent or very low in approximately 70% of untreated primary MM tumors despite the fact that methylation of the gene is present in only about 25% of the untreated primary MM tumors that we analyzed. A final focus is to analyze expression arrays of MM cell lines and primary tumors to gain further insight into mechanisms of pathogenesis. These studies, which have been done collaboratively with Drs. P.L. Bergsagel, J. Shaughnessy, and L. Staudt, have enabled us to propose that a unifying molecular event in the pathogenesis of most MGUS and MM tumors is dysregulation of a cyclin D gene despite the very low proliferative index of these tumors. Each of the recurrent translocations, which together appear to be initiating events in about 40% of tumors, are associated with dysregulation of one of the three cyclin D genes: 11q13 and 6p21 translocations directly dysregulate cyclin D1 and cyclin D3, respectively; 16q23 and 20q11 translocations indirectly dysregulate cyclin D2 by the action of the c-maf and mafB transcription factors that are dysregulated by these translocations; the 4p16 translocation is associated with up-regulation of cyclin D2, although the mechanism is unclear. Although cyclin D1 is not expressed at significant levels in normal B cells, about 40% of MM or MGUS tumors ectopically express cyclin D1 despite the absence of translocations or other structural abnormalities of the cyclin D1 locus; the ectopic expression of cyclin D1 in the absence of an Ig translocation does not appear to occur in a large panel of lymphoma tumors. Curiously, the tumors that ectopically express cyclin D1 in the absence of a translocation are not represented in our panel of HMCL. Most of the remaining tumors express high levels of cyclin D2. We are currently focusing our efforts to determine the causes and consequences of the various kinds of cyclin D dysregulation in MM tumors. In addition, we have developed a novel classification of MM tumors based on the presence of absence of the five recurrent Ig translocations and cyclin D expression, and are attempting to determine how this correlates with prognosis, response to treatment, etc.