The origin of O⁶-methylguanine-DNA methyltransferase in Chinese hamster overy cells transfected with human DNA

Transfection of Chinese hamster ovary (CHO) cells with human DNA has been shown in several laboratories to produce clones which stably express the DNA-repair protein, O⁶-methylguanine-DNA methyltransferase (MGMT), that is lacking in the parent cell lines (Mex^- Phenotype). We have investigated the genetic origin of the MGMT in a number of such MGMT-positive (Mex⁺) clones by using human MGMT cDNA and anti-human MGMT antibodies as probes. None of the five independently isolated Mex⁺ lines has human MGMT gene sequences. Immunoblot analysis confirmed the absence of the human protein in the extracts of these cells. The MGMT mRNA in the lines that express low levels of MGMT (0.6-1.4×10⁴ molecules/cell) is of the same size (1.1 kb) as that present in hamster liver. One cell line, GC-1, with a much higher level of MGMT (4×10⁴ molecules/cell) has two MGMT mRNAs, a major species of 1.3 kb and a minor species of 1.8 kb. It has also two MGMT polypeptides (32 and 28 kDa), both of which larger than the 25 kDa MGMT present in hamster liver and other Mex⁺ transfectants. These results indicate that the MGMT in all Mex⁺ CHO cell clones is encoded by the endogenous gene. While spontaneous activation of the MGMT geen cannot be ruled otu in the Mex⁺ cell clones, the intervention of human DNA sequences may be responsible for activation fo the endogenous gene in the GC-1 line.