There is now ample evidence, both clinical and in vitro that patients
with Down syndrome (DS) are unusually sensitive to the dihydrofolate reductase
inhibitor: methotrexate (MTX). Investigating the pathophysiological mechanisms
responsibly: for their characteristic increased MTX sensitivity, we studied the
"de novo" and the "salvage" pathways of thymidine monophosphate (TMP) synthesis
in 9 patients with DS and 9 healthy control sibs. Using an in vitro assay
preciously described (1), we added FUdR (0.1 x 10-8M, 2 X 10
-8 M, 20 X 10 -8M) , an inhibitor of thymidylat
synthetase to peripheral blood lymphocyte cultures to study de novo TMP
synthesis. At the different doses of FUdR, there was a significant dose-related
decrease in mitotic index (MI) in both DS and control lymphocytes, which was
similar in both groups. The decrease was only partially alleviated by the
addition of exogenous thymidine. The salvage pathway for TMP synthesis (via
thymidine kinase) was investigated by adding exogenous thymidine (15 X
10-6 M) to rescue lymphocyte cultures from MTX cytotoxicity (1.2 X
10-8M and 2.4 X 10-8 M). The adjunction of thymidine
alone to the culture medium did not modify the MI in either DS or control
cultures. In the presence of MTX, exogenous thymidine rescued both normal and
DS lymphocytes from thymineless death. These findings suggest that there Is no
underlying intrinsic defect of either thymidylate synthetase or of thymidine
kinase which could account for the increased MTX toxicity observed in patients
with DS.