Loss of reiterated DNA sequences during serial passage of human diploid fibroblasts

Abstract

A specific family of tandemly repeated DNA sequences was found to diminish in the human genome after serial passage of three strains of diploid fibroblasts. Eco RI restriction fragments of 340 and 680 bp were significantly reduced in quantity at late passage as determined by autoradiography of 14C-DNA and also by ethidium bromide fluorescence. The reduction in these closely related DNA sequences was confirmed by saturation hybridization to excess 3H-RNA transcribed from a homogeneous restriction fragment recleaved from the 340 bp DNA. The maximal fraction of DNA hybridizing to the 3H-RNA probe declined by 33–50% over 21–41 population doublings. Divergence and/or methylation of such sequences could not account for these results since the thermal stability of cRNA:DNA duplexes actually increased by 0.3°C at late passage. Total highly repetitive sequences assayed by reassociation kinetics were also substantially reduced at late passage, implying that depletion may be common to many repeat families in DNA. The denaturation temperature for such rapidly reassociated duplexes again increased slightly at late passage, possibly reflecting the minor decreases in DNA methylation which were detected in two of the cell strains. Karyotype analyses demonstrated that over 95% euploidy was maintained, with no specific chromosome loss and no visible deletions at late passage. The depletion of reiterated sequences during repeated cell division is thus attributed to numerous small DNA deletions, which may arise from unequal recombination coupled with selection or from a nonreciprocal mechanism such as excision.