Received on August 21, 2003
Accepted on October 21, 2003

Evidence-based Laboratory Medicine and Test Utilization

Definitive Diagnosis of Mitochondrial Neurogastrointestinal Encephalomyopathy by Biochemical Assays

¹ Department of Neurology, Columbia University College of Physicians and Surgeons, New York, NY and Centre d’Investigacions en Bioquimica i Biologia Molecular, Hospital Universitari Vall d’Hebron, Barcelona, Spain
² Department of Neurology, Columbia University College of Physicians and Surgeons, New York, NY and Division of Molecular Neurogenetics, National Neurological Institute "Carlo Besta", Milan, Italy
³ Department of Neurology, Columbia University College of Physicians and Surgeons, New York, NY
⁴ Department of Neurology, Columbia University College of Physicians and Surgeons, New York, NY and Department of Neuromuscular Research, National Institute of Neuroscience, National Center of Neurology and Psychiatry, Tokyo, Japan

^* To whom correspondence should be addressed. E-mail: mh29{at}columbia.edu.

Background: Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is caused by mutations in the gene encoding thymidine phosphorylase (TP). The clinical manifestations of MNGIE are recognizable and homogeneous, but in the early stages, the disease is often misdiagnosed. This study assesses the reliability of biochemical assays to diagnose MNGIE.

Methods: We studied 180 patients with clinical features suggestive of MNGIE, 14 asymptomatic TP mutation carriers, and 20 controls. TP enzyme activity in the buffy coat was determined by a fixed-time method, and the plasma nucleosides thymidine (dThd) and deoxyuridine (dUrd) were assessed by a gradient-elution reversed phase HPLC method. TP was sequenced through standard procedures in patients who met the clinical criteria for MNGIE.

Results: Twenty-five of the 180 patients fulfilled the clinical criteria for MNGIE and had homozygous or compound heterozygous TP mutations. All had drastically decreased TP activity [mean (SD), 10 (15) nmol thymine formed · h^-1 · (mg protein)^-1 vs 634 (217) nmol thymine formed · h^-1 · (mg protein)^-1 for the controls]. Relative to the control mean, TP activities were reduced to 35% in mutation carriers and 65% in MNGIE-like patients. All 25 MNGIE patients had detectable plasma dThd [8.6 (3.4) µmol/L] and dUrd [14.2 (4.4) µmol/L]. Controls, carriers, and MNGIE-like patients showed no detectable plasma dThd and dUrd.

Conclusions: We propose a diagnostic algorithm based on the determination of plasma dThd and dUrd, TP activity in buffy coat, or both to make a definitive diagnosis of MNGIE. Increased concentrations of dThd (>3 µmol/L) and dUrd (>5 µmol/L) in plasma or a decrease in buffy coat TP activity to 8% relative to controls is sufficient to diagnose MNGIE.© 2004 American Association for Clinical Chemistry.