BMC Cancer 2006, 6:2 http://www.biomedcentral.com/1471-2407/6/2 chromosomes were only randomly retained. To map the region of chromosome 22 that is over-represented in BCNU-resistant cells, we performed FISH analyses using pooled BAC probes. The over-represented region was fur- ther defined by use of individual BACs. This work estab- lished that this region encompassed 22q12.3–13.32. When cells from recurrent tumor were further selected in vitro for BCNU resistance, the percentage of cells contain- ing the retained region of chromosome 22 increased, sug- gesting a role in BCNU resistance and/or cell survival for a gene or genes mapped to this region. One gene mapped to this region is the gene encoding the B-chain of platelet- derived growth factor (PDGF-B) [19]. We have previously demonstrated that this gene is over-expressed in cells selected for resistance to BCNU and is probably involved in the autocrine growth of these cells [10]; however, there is no evidence that expression of this gene is directly involved in chemotherapy resistance. Differential mRNA display and cDNA microarray analyses have not demon- strated consistent over-expression of additional specific genes mapped to this area in cells selected for resistance to BCNU. This suggests that sequences in this region may be contributing to a novel mechanism of resistance. We have recently begun studies of microRNA (miRNA) expression in cells selected for resistance to BCNU. Pre- liminary results suggest that hsa-let-7b is over-expressed in ME cells selected for BCNU resistance and MER cells before and after selection for BCNU resistance. This miRNA is mapped to chromosome 22q13.31, the region we have found to be over-represented in BCNU resistance cells. Other miRNAs mapped to this region include hsa- let-7a-3 which is just centromeric to let-7b and hsa-mir-33 which is mapped to 22q13.2. Neither of these miRNAs are over-expressed in these cells, suggesting that this is not simply a non-specific results of over-representation. While the precise function of these miRNAs has not been eluci- dated, the hsa-let7 family has been implicated in the reg- ulation of ras [20], and reduced expression of hsa-let7 is associated with poorer survival in lung cancer [21]. Our current studies are aimed at the identification of the role, if any, of sequences mapped to chromosome 22 in BCNU resistance. Conclusion In summary, we have found that cells selected for resist- ance to BCNU either in vitro or in vivo (recurrent tumor) have over-representation of a specific region of chromo- some 22 encompassing 22q12.3–13.32. There are no genes known to be involved in resistance mapped to 22q12.3–13.32, suggesting the presence of a gene or DNA sequence involved in a novel mechanism contributing to the growth of resistant cells. Further studies are underway to identify a gene(s) or non-coding RNA on chromosome 22 that is involved in chemotherapy resistance. List of abbreviations BCNU, 1,3-bis(2-chloroethyl)-1-nitrosourea FISH, fluorescent in situ hybridization BAC, bacterial artificial chromosome MAB, Waymouth MAB 87/3 medium FCS, fetal calf serum WCP, whole chromosome paint DAPI, 4',6-Diamidino-2-phenylindole PDGF-B, platelet-derived growth factor, B chain Competing interests The author(s) declare that they have no competing inter- ests. Authors' contributions NCH performed all FISH analyses, statistical analyses and helped draft the manuscript. JRS supervised the isolation of the cell lines and per- formed all karyotyping. ACS conceived the study, directed its design and coordina- tion, obtained and grew the BACs and finalized the man- uscript. All authors read and approved the final manuscript. Acknowledgements The authors gratefully acknowledge the generous gift of chromosome 22 BACs from Dr. I. Dunham at The Sanger Institute, Cambridge, UK. We thank M.H. Almeida, K. Kretkowski, Dr. John F. Stone and Dr. Avery A. Sandberg for helpful discussions. We thank Dr. Stephen W. Coons (BNI) and Drs. Engelson and Farhood (MSKCC) for their neuropathology exper- tise and Dr. Mark Reiser for his biostatistics expertise. We also thank the neurosurgeons and operating room staff of the Barrow Neurological Insti- tute and Memorial Hospital for providing the patient samples. This work was supported by NCI CA25956 and the Barrow Neurological Foundation. References 1. Shapiro WR, Shapiro JR, Walker RW: Central Nervous System. In Clinical Oncology Edited by: Abeloff MD, Armitage JO, Lichter AS, Niederhuber JE. New York: Churchill Livingstone; 1995:851-912. 2. Shapiro JR: Cellular characterization and BCNU resistance of freshly resected and early passage human glioma cells. Progress in Neuropathology 1986, 6:133-143. 3. Watkins D, Ruttledge MH, Sarrazin J, Rangaratnam S, Poisson M, Delattre JY, Rouleau GA: Loss of heterozygosity on chromo- some 22 in human gliomas does not inactivate the neurofi- bromatosis type 2 gene. Cancer Genet Cytogenet 1996, 92:73-78. Page 9 of 10 (page number not for citation purposes)