Gene and human tumour cell line specific differencesin nitrogen mustard induced DNA alkylation and interstrand crosslinking frequencies.

The levels of N-alkyl purine and DNA interstrand crosslink formation, produced by the clinically used nitrogen mustard antitumour drug mechlorethamine (HN2), were quantitated at the level of specific genes in a panel of human tumour cell lines using modified Southern blotting methods. When purified genomic DNA was treated with HN2 in vitro, no significant difference in the extent of N-alkyl purine or interstrand crosslink formation in the N-ras, c-myc or CD3delta genes was observed. When the cell lines LS174T, Colo320HSR, J6 and U937 were treated with HN2, however, there was significant heterogeneity in the levels of N-alkyl purine formation in the three genes. The rank order of the extent of damage in the three genes was also different in the cell lines. The level of alkylation did not correlate with either the transcriptional activity of a gene or drug sensitivity. Crosslinks were not detectable in the N-ras or c-myc genes of LS174T, J6 or U937 cells treated with HN2, and only detectable in the amplified c-myc gene of the Colo320HSR cell line. In the related cell line Colo320DM, which has both native and translocated c-myc alleles which are both amplified and episomal, crosslinks were detected in the amplified native and rearranged c-myc alleles, and also in the N-ras gene which is also amplified in this cell line. For bifunctional alkylating agents such as HN2, therefore, heterogeneity of DNA damage can occur between different genes in human cells and can also vary for different lesions produced by the same agent. In addition, this heterogeneity can differ between human tumour cell lines.