EVIDENCE FOR SPONTANEOUS CONVERSION OF MEX- TO MEX+ IN HUMAN LYMPHOBLASTOID-CELLS

A series of human lymphoblastoid cell lines (LCLs) called Mex- were defined by Sklar and Strauss on the basis of their inability to remove O6-methylguanine from DNA. Instability of Mex- has previously been shown as a population phenotype of LCLs. We examined whether Mex- as a cellular phenotype is spontaneously convertible or not. At the population doubling number (PDN) 23 after recloning, two out of 15 independent sibcultures derived from a Mex- LCL, AT1-1, were found to contain a small fraction of Mex+ cells after treatment with 1-(4-amino-2-methyl-5-pyr-imidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU). Three Mex+ subclones were identified without exposure to ACNU among 486 subclones from replica plating of an expanded Mex- clone (PDN30). The rate of spontaneous conversion was estimated to be in the range of 10-8-10-7 per cell per generation by the fluctuation analyses on two Mex- subclones. These results strongly support the hypothesis that Mex- as a cellular phenotype is spontaneously convertible to Mex+. © 1990 Oxford University Press.