Differences in the incorporation of thymidine into DNA of normal and cystic fibrosis fibroblasts in vitro

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Abstract
Although similar fractions of cells were in the S phase of the cell cycle, normal human skin fibroblasts were shown to incorporate more than twice the 3HTdR into their DNA in vitro than did cells obtained from individuals with cystic fibrosis (CF). Obligate heterozygotes incorporated an intermediate amount of the DNA precursor. Studies were initiated to determine the basis of the differential incorporation of 3HTdR among the genotypes. An analog of thymidine, BUdR, produced varied effects on the growth kinetics of the three genotypes. The growth of cells in BUdR resulted in a 50% increase in the population doubling times of all three genotypes, and caused the cell morphology to change from a spindle shape to one in which the cells became broadened and flat, with numerous cytoplasmic projections extending for distances of several cell diameters. The activities of thymidine kinase and the participation of the exogenous and de novo pathways in the synthesis of TMP were found to be approximately the same in all three genotypes. The data suggest that an alteration in the transport of thymidine into the cells may account for the differences in TdR incorporation into DNA, and this may be associated with other changes in cystic fibrosis that are apparently membrane associated.