We thank Moran and colleagues for their comments regarding the data presented in our article on MNGIE due to mutations in the RRM2B gene.1 Unfortunately, there were typographical errors in the units used for thymidine levels that we failed to detect during the proofreading process. The plasma thymidine level in our patient was 100 nmol/L (not mmol/L), which was within the reference range and remarkably below the thymidine levels found in patients with MNGIE. In fact, the concentrations of thymidine in patients with MNGIE due to thymidine phosphorylase deficiency reach levels at least 60-fold higher than those of the controls and carriers (greater than 3 μmol/L).2,3 In our patient, thymidine was separated by high performance liquid chromatography and detected with tandem mass spectroscopy. Based on hundreds of samples tested by this method at the Diagnostic Laboratory at Baylor College of Medicine, Houston, Texas, the upper limit of thymidine levels in controls is 150 nmol/L (written communication, S. Qin, PhD, December 12, 2009). These small differences in the reference concentration probably reflect variations in separation and detection procedures as well as the detection limits of these methods.
Shaibani A, Shchelochkov OA, Zhang S, Katsonis P, Lichtarge O, Wong L, Shinawi M. Mitochondrial Neurogastrointestinal Encephalopathy Without Elevated Thymidine Levels—Reply. Arch Neurol. 2010;67(5):644-645. doi:10.1001/archneurol.2010.74