Our major focus has been to identify and characterize translocations to the IgH locus (chromosome 14q32.3) in multiple myeloma (MM) cell lines and 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, 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, VDJ recombination, somatic hypermutation) 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) three chromosomal loci (cyclin D1 at 11q13; FGFR3 tyrosine kinase receptor and MM.SET at 4p16.3; and the c-maf basic zip transcription factor at 16q23) each account for about 10-20% of IgH translocations in MM, even though the 4;14 and 14;16 translocations are not detected by conventional karyotypes; 6) there are a minimum of 18 (6 recurrent) other translocation partners identified by ourselves and others; 7) in a panel of 30 advanced tumors, translocations are somewhat less frequent (IgH in 70%, 2 independent IgH in 20% and 3 independent IgH in none, Iglambda in 17%, Igkappa in none, and no translocation in 26%); and 8) we have evidence of heterogeneity of translocations in primary tumors. Our working hypothesis is that primary translocations to Ig loci often - but not always - provide one of the initial immortalizing events in the molecular pathogenesis of myeloma, and occur during plasma cell development in germinal centers. In addition, secondary translocations involving one of the Ig loci occur as a late event, during tumor progression.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 7 informative MM cell lines (confirmed in the corresponding tumor in 2 cases) 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 the c-myc locus in 23/28 (82%) MM cell lines that we have examined. Thus it seems clear that the selective expression of one c-myc allele in MM lines 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 parental 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 cell line. 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. 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 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 screened for ras and FGFR3 mutations in a panel of 36 MM lines, and for FGFR3 mutations in 6 of 30 primary MM tumors that have the t(4;14) translocation, with preliminary results consistent with mutation in ras or FGFR3 - but never both - contributing to tumor progression in about 40% of MM tumors. Similar to activated ras, we have shown that activated FGFR3 can transform NIH3T3 cells. Second, we are screening for p53 mutations in the cell lines. Third, with Lou Staudt, we are doing a lymphochip analysis of mRNA expression in 30 of our well-characterized MM cell lines, and comparing these results with normal plasma cells, and also MGUS and MM tumors.
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