Predominance of Mex= cells in newly-established human lymphoblastoid cell lines

Abstract

Previous studies have demonstrated that approximately onethird of human lymphoblastoid cell lines (LCLs) are deficient in removing O⁶-methylguanine residues because of the lack of O⁶-alkylguanine-DNA alkyltransferase (O⁶-AGT) activity. Such LCLs have been designated Mex⁻, while the proficient LCLs are Mex⁺. Our determinations of O⁶-AGT activity as a function of cellular protein concentration on 37 previouslyestablished LCLs discolsed that the expression of the enzyme was high in 14 (Mex⁺) and barely detectable in 16 (Mex⁻). The other seven LCLs showed intermediate activity of the enzyme. By contrast, all of the 28 LCLs that we newly established contained high enzyme activity, implying that they consisted of mainly Mex⁺ cells. Since the conventional O⁶-AGT w y on Mex⁺ cell populations was not capable of detecting the co-existence of Mex⁻ cells as a minor component, we attempted to determine the proportion of Mex⁻ phenotype in newly-immortalized lymphoblastoid cell clones which had been established directly on semisolid agar. All of the 15 independent clones derived from a single blood sample also showed high O⁶-AGT activity, rendering it unlikely that Epstein - Barr virus transformation per se was responsible for the generation of Mex⁻ LCLs. These results collectively indicate that Mex⁺ in cells predominate in LCLs shortly after establishment and also suggest that the possible growth advantage for Mex⁻ cells should play an important role in the subsequent development of Mex⁻ LCIs during the long-term culture in vifro.