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

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. Studied 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. 