Summary A panel of doxorubicin-resistant sublines of the human small-cell lung carcinoma cell line GLC4 displays decreasing DNA topoisomerase Ila (TopoIla) mRNA levels with increasing resistance. In the present study we describe how this decrease may be regulated. No significant differences in TopoIla mRNA stability or gene arrangement were found, using mRNA slot-blotting and Southern blotting, in the most resistant cell line compared with the parental cell line. To investigate if Topolla gene copy loss contributed to the mRNA decrease, fluorescence in situ hybridisation using a TopoIllx-specific probe was performed. During doxorubicin resistance development, the composition of the population in each cell line shifted with increasing resistance, from a population in which most cells contain three TopoIla gene copies (GLC4) to a population in which most cells contain only two copies. A partial revertant of the most resistant cell line displayed a shift back to the original situation. We conclude that the Topolla gene copy number decrease per cell line is in good agreement with the decreased TopolIa mRNA and protein levels, and Topoll activity levels in these cell lines which were described previously.

Telomere length is maintained by two known mechanisms, activation of telomerase or alternative lengthening of telomeres (ALT). The ALT pathway is more commonly activated in tumours of mesenchymal origin, although the mechanisms involved in the decision of a cell to activate either telomerase or ALT are unknown at present and no molecular markers exist to define the ALT phenotype. We have previously shown an association between chromatin remodelling, telomerase gene expression and ALT in cell line models. Here, we evaluate these findings and investigate their prognostic significance in a panel of liposarcoma tissue samples to understand the biology underlying the ALT phenotype. Liposarcoma samples were split into three groups: telomerase positive (Tel þ ); ALT positive; ALTÀ/TelÀ. Differences in telomerase gene expression were evident between the groups with increased expression of hTR in ALT and Tel þ compared to ALTÀ/TelÀ samples and increased hTERT in Tel þ samples only. Investigation of a small panel of chromatin modifications revealed significantly increased binding of acetyl H3 in association with hTR expression. We confirm that the presence of the ALT phenotype is associated with poor prognosis and in addition, for the first time, we show a direct association between hTR expression and poor prognosis in liposarcoma patients.

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