Determining the specific activity of thymidine phosphorylase in leukocytes of patients with MNGIE and the plasma thymidine level by RP-HPLC

Thymidine phosphorylase (TP) catalyses the conversion of thymidine into thymine. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive disease which is caused by mutations in the nuclear gene encoding TP, bringing about severe impairment of TP-enzyme specific activity and accumulation of thymidine in plasma. The clinical manifestations of MNGIE are recognizable and homogenous, but not in the early stages of the disease. In patients who are suspected of having MNGIE, determination of TP-specific activity in leukocytes and thymidine levels in plasma are diagnostic. The methods that are usually used for the measurement of TP activity and plasma thymidine are not rapid or accurate enough and lack sensitivity. The specific activity of TP was measured by RP-HPLC in leukocytes of both the controls and the patients exhibiting clinical features suggestive of MNGIE. Moreover, plasma thymidine was assessed by the same method. The patients had detectable plasma thymidine (>3 µmol/L) but it was undetectable in the healthy controls. The patient's TP-specific activity decreased to less than 5% relative to the controls (14±4 nmol/h/mg vs. 525±165 nmol/h/mg, P<0.05). A diagnostic algorithm for the definitive diagnosis of MNGIE is suggestible based on the results of this study which relies on the measurement of plasma thymidine, TP-specific activity in leukocytes, or both. In this study, we set up a sensitive and rapid assay for the evaluation of TP-specific activity by using RP-HPLC in Iran. In addition, we established reference values for TP-specific activity and plasma thymidine in the Iranian patients.