Comparison of spontaneous mutagenesis in early-passage human mammary cells from normal and malignant tissues

Spontaneous mutant frequencies were determined in early-passage epithelial-cell strains derived from either normal or malignant human breast tissues, as well as a non-tumorigenic immortalized human mammary epithelial cell line (184B5) derived from normal cells. Mutations at the hypoxanthine-guanine phophoribosyltransferase (HPRT) locus were quantified by determining the 6-thioguanine resistance. Mutant frequencies in human mammary epithelial cells (HMEC) from 4 normal and 5 carcinoma tissue samples did not differ significantly. In contrast, the mutant frequency in the immortalized HMEC was approximately 10 times higher than in average normal HMEC and normal non-immortalized cells from early-passage cultures of the same cell lineage. Our data suggest that the progression of normal breast cells to invasive carcinoma cells does not necessarily involve the establishment of a general genetic instability during this progression.