A characteristic feature of Burkitt's lymphoma cells is the presence of reciprocal translocations between the c-myc locus on chromosome 8 and one of the immunoglobulin gene loci on chromosome 2, 14, or 22. The most common translocation is the t(8;14). In this translocation, the c-myc gene is covalently linked to the immunoglobulin heavy chain (IgH) 1 gene. As a result of this translocation, the transcription of the translocated c-myc gene is deregulated, whereas the normal c-myc allele is silent. Furthermore, the transcripts initiated from the c-myc P1 promoter, which normally contribute to a minor (10 -20%) population of c-myc mRNA, increase to a level greater than transcripts initiated from the P2 promoter (1-3). These findings support a model in which sequences present in the IgH gene locus deregulate expression from the cis-linked c-myc allele by promoting interactions between c-myc and IgH gene regulatory elements that affect c-myc initiation and elongation. It should be noted, however, that the translocation breakpoint in many sporadic Burkitt's lymphomas separates the c-myc promoter from the coding region (4, 5). In these cases, the regulatory elements of the IgH enhancers apparently activate c-myc transcription without interaction with the c-myc promoter elements. Transcription often initiates in the first intron of c-myc in these sporadic Burkitt's lymphomas.We found that the transcription factors, Nm23H2 and NF-B, activated the c-myc promoter (6, 7). Others have also shown that NF-B is an important regulatory factor for the murine c-myc promoter (8, 9). Because the IgH 3Ј enhancers are linked to the c-myc gene in every Burkitt's lymphoma with the t(8;14) translocation, we sought to identify the transcription factors that bind to sequences in the enhancer region and activate the translocated c-myc gene.Several enhancers have been shown to be important for expression of the IgH gene. Four B cell-specific and cell stagedependent DNase I-hypersensitive sites, MHS1 to MHS4, are located 10 -35 kilobases 3Ј of the C␣ gene (10 -13). The activity of individual enhancer elements varies during B cell differentiation (10,14,15), and these regions have been shown to function as a locus control region in B cells (10). Recently, it has been shown that MHS1-MHS4 increase expression from the c-myc P2 promoter by an increase in histone acetylation. However, this increase in acetylation does not explain the MHS1-4 activation of transcription from the P1 promoter (16). Enhancers have been located downstream of two human C␣ genes (17-19), and these regions share some homology with the murine HS1-4, but only limited functional studies have been performed on the human enhancers.The 3Ј region of the IgH locus is linked to the translocated c-myc gene in all t(8;14) translocations in Burkitt's lymphoma, and it is likely that this region plays a role in the deregulated expression of the translocated c-myc gene. In this study, we show that an NF-B site in the MHS4 enhancer is required for the transcriptional activation of the tra...

SUMMARY Chemotherapy is used to treat most cancer patients, yet our understanding of factors that dictate response and resistance to such drugs remains limited. We report the generation of a quantitative chemical-genetic interaction map in human mammary epithelial cells charting the impact of the knockdown of 625 genes related to cancer and DNA repair on sensitivity to 29 drugs, covering all classes of chemotherapy. This quantitative map is predictive of interactions maintained in other cell lines, identifies DNA-repair factors, predicts cancer cell line responses to therapy, and prioritizes synergistic drug combinations. We identify that ARID1A loss confers resistance to PARP inhibitors in cells and ovarian cancer patients and that loss of GPBP1 causes resistance to cisplatin and PARP inhibitors through the regulation of genes involved in homologous recombination. This map helps navigate patient genomic data and optimize chemotherapeutic regimens by delineating factors involved in the response to specific types of DNA damage.

Ewing's family tumors are characterized by a specific t(11;22) chromosomal translocation that results in the formation of EWS-Fli1 oncogenic fusion protein. To investigate the effects of EWSFli1 on gene expression, we carried out DNA microarray analysis after specific knockdown of EWS-Fli1 through transfection of synthetic siRNAs. EWS-Fli1 knockdown increased expression of genes such as DKK1 and p57 that are known to be repressed by EWS-Fli1 fusion protein.Among other potential EWS-Fli1 targets identified by our microarray analysis, we have focused on the FOXO1 gene since it encodes a potential tumor suppressor and has not been previously reported in Ewing's cells. To better understand how EWS-Fli1 affects FOXO1 expression, we have established a doxycycline-inducible siRNA system to achieve stable and reversible knockdown of EWS-Fli1 in Ewing's sarcoma cells. Here we show that FOXO1 expression in Ewing's cells has an inverse relationship with EWS-Fli1 protein level, and FOXO1 promoter activity is increased after doxycycline-induced EWS-Fli1 knockdown. In addition, we have found that direct binding of EWS-Fli1 to FOXO1 promoter is attenuated after doxycycline-induced siRNA knockdown of the fusion protein. Together, these results suggest that suppression of FOXO1 function by EWS-Fli1 fusion protein may contribute to cellular transformation in Ewing's family tumors.