A genomic approach to the study of chemoresistance
This study evaluated comparative genomic hybridisation (CGH) as a tool to detect candidate regions of the genome associated with chemoresistance. Using a variation on conventional CGH, DNA from three cell lines that were resistant to thymidylate synthase (TS) inhibitors (tomudex [TDX] or 5-fluorouracil [5-FU]) and their sensitive parent cells were evaluated. In MCF-7 and H630, cells that were resistant to TDX, a specific TS inhibitor with no other known cytotoxic potential, only a single region of change (18p gain) was apparent. The third cell line H630R10, which was resistant to 5-FU, had changes in several genomic regions following the acquisition of resistance, including 18p. Gain in the chromosomal region containing the TS gene (18pll.32) was detected by CGH in all three resistant cell lines. However, additional novel regions of interest were identified in the cells that were resistant to 5-FU, a cytotoxic agent known to have several other modes of cytotoxicity besides TS inhibition. These results suggested that CGH is of potential use in the detection of regions of the genome involved in chemoresistance. Having shown the potential of CGH as a tool for assessing chemoresistance at the genomic level, steps toward clinical application of this technique were evaluated. A prerequisite for study in archival pathology samples was successful DNA extraction and universal amplification of tumour DNA from paraffin-embedded tumour sections for CGH analysis. Degenerate oligonucleotide primed - polymerase chain reaction (DOP-PCR) was performed on minute quantities (50ngs) of fresh cell line DNA (H630R10) and tumour DNA (osteosarcoma), as well as paraffin-embedded DNA from the same case. The results of these DOP-PCR CGH reactions were compared with conventional CGH using l|0.g quantities of fresh DNA from both H630R10 cell line and osteosarcoma. The CGH profiles of the conventional CGH and DOP-PCR CGH did not show a high level of concordance, only 55% of the gains and 83.3% of losses detected by conventional CGH were detected by DOP-PCR CGH The use of universal amplification by DOP-PCR in paraffin-embedded sections was not taken forward into clinical evaluation. A study of colorectal cancer (CRC) was initiated which involved the microdissection of 29 Dukes' C CRC tumours from fresh frozen material for CGH analysis. This conventional CGH analysis of CRC tumours involved assessing each tumour twice by reversal of fluorochromes. Only genomic regions that were detected as changed in both forward and reverse profiles were accepted. This approach detected several regions of genome as changed across the 29 tumours. In all, 108 gains (a mean number of 3.7 aberrations per tumour, range 1-12) and 85 losses (a mean number of 2.9 aberrations per tumour, range 0-11) were detected in the 29 tumours. CGH analyses identified certain chromosomal regions as more likely to be changed than others. The most frequent aberrations detected across the 29 tumours was a loss of chromosomal arm 18q, seen in 31% of the tumours assessed. Gain was also common at some sites throughout the genome, for example, gain of chromosomal arms, 13q and 20q was seen in 27.6% of cases. Mann-Whitney U tests investigating the association between specific chromosomal aberrations such as gain of 20q or loss of 18q and known markers of CRC tumourigenesis (p53, p27, p21, Rb, cyclin Dl, PCNA, P-catenin, e-cadherin, c-erbB-2, bcl2, EGFR and c-erbB-2) assessed by immunohistochemistry (IHC) in 29 tumours found no association. Testing of the total number of genomic aberrations detected (loss + gain = genetic grade) rather than the frequency of aberration at specific chromosomal loci also found no association with the CRC tumour markers. Finally, the association between the chromosomal aberrations detected by CGH was investigated in relation to patient survival. This thesis has demonstrated the value of a global approach to the study of chemoresistance and tumourigenesis through the application of powerful technology such as CGH.