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 sometimes 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) four chromosomal loci (cyclin D1 at 11q13; cyclin D3 at 6p21; FGFR3 tyrosine kinase receptor and MM.SET at 4p16.3; and the c-maf basic zip transcription factor at 16q23) each account for about 5-20% 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 60%, 2 independent IgH in 20% and 3 independent IgH in none, Iglambda in 17%, Igkappa in none, and no Ig translocation in 26%). Analyses of premalignant MGUS and malignant MM tumor by ourselves and others show that IgH translocations are present in 47% of MGUS tumors and 55 to 70% of MM tumors, perhaps related to the stage of the disease. 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 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 event. 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 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 ras mutations are present in 17/36(45%) HMCL, consistent with the incidence in primary MM tumors. 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 and about 10% of unselected tumors, and suggest that this is a progression event in a subset of MM tumors. Finally, we are continuing studies in collaboration with Lou Staudt to use the lymphochip microarray to determine the patterns of gene expression in our panel of HMCL, with a goal of determining whether different translocations result in different patterns of gene expression and different tumor phenotypes.
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