Molecular Pathogenesis of Multiple Myeloma

我们的主要关注点是鉴定和描述多发性骨髓瘤(MM)肿瘤中IgH基因(染色体14q32)的易位。我们组装了一组50个EBV阴性的MM细胞系，发现：1)所有分析的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)克隆的IgH断裂点的位置与大多数情况下B细胞特异性机制(主要是开关，但可能是一小部分体细胞过度突变)的错误一致；3)克隆的断裂点散布在大范围内，远离调控异常、过度表达的癌基因的1Mb；4)至少有15个(50%)系有两(10)或三(5)个独立的IgH易位；5)在原发MM肿瘤中至少有18个(6个复发)易位伙伴；6)除位于8q24的c-myc外(见下文)，有5个染色体位点(位于11q13的细胞周期蛋白d1；位于6p21的细胞周期蛋白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或mafB碱性压缩转录因子)是复发的；7)在50例晚期肿瘤中，易位的发生率较低(IgH占58%，2个独立的IGH占15%，3个独立的IGH无，Iglambda占16%，Igkappa占2%，没有Ig易位的占35%)。对MGUS癌前病变和阴燃MM瘤的分析表明，47%的癌前病变中存在IgH易位。我们的工作假设是，在大约50%的肿瘤中，原发易位到Ig基因座提供了骨髓瘤分子发病机制中最初的永生化事件之一，通常发生在生发中心的浆细胞发育期间。此外，涉及其中一个Ig基因的继发性易位--但缺乏B细胞特异性重组机制介导的过程的特征--在有或没有原发易位的肿瘤中作为肿瘤进展事件发生。原发易位和继发易位似乎涉及不同的染色体配对(癌基因)，尽管可能有一些重叠。 第二个焦点是阐明我们的发现的意义，即在9例信息性HMCL中，尽管明显缺乏涉及c-myc基因的易位、重排或扩增，但存在L-myc或一个c-myc等位基因的选择性表达。结合FISH和SKY分析，我们有证据表明在我们所检查的28/32(88%)HMCL中存在L-myc(一个HMCL)或c-myc基因座的核型异常。由此可见，一种c-myc等位基因的选择性表达是肿瘤特有的复杂结构异常的结果(复杂的易位、插入、复制、倒位，经常累及3条不同的染色体，但并不总是一个免疫球蛋白基因座)，改变了两个L-myc或c-myc等位基因之一的染色体环境。在所有提供信息的病例中，很明显，myc结构异常存在于原发肿瘤和HMCL中。在晚期原发肿瘤样本中，c-myc异常的发生率似乎要低得多(45%)。部分原发肿瘤核型异常表现为c-myc异质性，1例肿瘤核型异常为N-myc。我们假设，复杂的核型异常似乎失调的c-myc很少-如果有的话-发生在肿瘤发生的早期事件。相反，c-myc的失调似乎是一个非常晚的进展事件，最常见的是一种复杂的易位，不是由B细胞特异的DNA修饰过程介导的。 第三个重点是定义MM的其他类型的遗传和表型异常。

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 50 EBV negative MM cell lines, and find that: 1) Ig translocations are present in all 36 MM cell lines (HMCL) analyzed, 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.