Genomic instability in mouse Burkitt lymphoma is dominated by illegitimate genetic recombinations, not point mutations Lynne D Rockwood 1 , Ted A Torrey 2 , Joong Su Kim 1 , Allen E Coleman 1 , Alexander L Kovalchuk 1 , Shao Xiang 2 , Thomas Ried 3 , Herbert C Morse III 2 and Siegfried Janz* ,1 1 Laboratory of Genetics, Center for Cancer Research (CCR), NCI, Bethesda, Maryland, MD 20892, USA; 2 Laboratory of Immunopathology, NIAID, Bethesda, Maryland, MD 20892, USA; 3 Genetics Branch, CCR, NCI, NIH, Bethesda, Maryland, MD 20892, USA l-MYC-induced mouse Burkitt lymphoma (BL) harbor- ing the shuttle vector pUR288, which includes a lacZ reporter gene to study mutagenesis, was employed to assess genomic instability associated with MYC dereg- ulation. The frequency of lacZ mutations in lymphomas was elevated only 1.75-fold above that in normal tissue, indicating that mouse BL does not exhibit a phenotype of hypermutability. However, the nature of lacZ mutations was strikingly different in normal tissues and lymphomas. While point mutations comprised approximately 75% of the mutations found in normal tissues, apparent translocations, deletions and inversions constituted the majority of mutations (*65%) in lymphomas. Genomic instability in mouse BL thus seems characterized by a preponderance of illegitimate genetic rearrangements in the context of near-background mutant frequencies. SKY analyses of cell lines from primary BL tumors revealed substantial changes in chromosomal structure, confirming the lacZ studies. Bi-allelic deletions of the tumor suppressor p16 Ink4a were detected in six out of 16 cell lines, illustrating cellular selection of advantageous mutations. Together, these approaches indicate that MYC may contribute to lymphomagenesis through the dominant mutator effect of inducing chromosomal instability. The results further suggest that a phenotype of hypermutability (elevated mutant frequency) may not always be required for oncogenesis to occur. Oncogene (2002) 21, 7235 – 7240. doi:10.1038/sj.onc. 1205697 Keywords: in vivo mutant rates; MYC; mutator phenotype; B-cell neoplasia; genomic instability Deregulation of MYC, a proto-oncogene central to the coordination of cellular growth, proliferation, differ- entiation, and apoptosis, is a common feature of many forms of cancer. Although the precise mechanisms by which MYC drives the oncogenic process are not understood (Cole and McMahon, 1999), it is becoming increasingly clear that one consequence of deregulated MYC expression with a major impact on cell transformation is genomic instability (reviewed in Mushinski and Mai, 2002). MYC induced genomic instability appears to favor gene amplifications, gene rearrangements, and chromosomal aberrations (Felsher et al., 2000). This spectrum of mutations suggests that the contribution of MYC to tumor development may be primarily mediated by structural modifications of the genome, rather than point mutations. To test this hypothesis in a mouse model of MYC induced neoplasia, we decided to generate l-MYC/pUR288 doubly transgenic mice. The l-MYC transgene utilizes the 3’ enhancer and other regulatory elements of the human immunoglobulin light-chain l locus to enforce the expression of human MYC in B cells. All l-MYC mice develop lymphomas with striking similarities to human Burkitt lymphoma (BL) (Kovalchuk et al., 2000). The pUR288 transgene harbors a shuttle vector with a lacZ reporter gene to examine mutagenesis in vivo with greater ease than studying endogenous genes (Boerrigter et al., 1995). The plasmid-based pUR288 vector also permits, in contrast to previously developed shuttle vectors based on phage l (Gossen et al., 1989; Kohler et al., 1991), the detection of a broad range of recombination mutations, including rearrangements of the reporter gene with mouse genomic sequences and large deletions within the reporter gene. The combined features of the l-MYC and pUR288 transgenes rendered l-MYC/pUR288 mice uniquely valuable for assessing MYC-induced genomic instability in a mature B-cell lymphoma (mouse BL). The mutant frequency of lacZ, a mutagenesis reporter gene that confers neither positive nor negative selective pressure on the cell in which it resides, was determined in liver, spleen, and mesenteric lymph node (MLN) of three control mice (Table 1, column 5, lines 1 – 9), and liver, spleen, and lymphoma tissue from enlarged lymph nodes (MLN and peripheral lymph nodes [PLN]) of three lymphoma-bearing mice (lines 10 – 21). Histologic analysis (results not shown) revealed that the enlarged lymph nodes were comple- tely infiltrated with highly aggressive lymphomas that Received 8 January 2002; revised 15 May 2002; accepted 20 May 2002 *Correspondence: S Janz, Laboratory of Genetics, CCR, NCI, Building 37, Room 2B10, Bethesda, Maryland, MD 20892-4256, USA; E-mail: sj4s@nih.gov Oncogene (2002) 21, 7235 – 7240 ª 2002 Nature Publishing Group All rights reserved 0950 – 9232/02 $25.00 www.nature.com/onc