THYMIDINE PHOSPHORYLASE ASSAY
Mitochondrial NeuroGastroIntestinal Encephalomyopathy (MNGIE) is a devastating autosomal recessive disease caused by mutations in the thymidine phosphorylase (TP) gene. MNGIE is characterized clinically by 1) gastrointestinal dysmotility; 2) cachexia; 3) ptosis (droopy eyelids) or ophthalmoparesis (impaired eye movements), or both; 4) peripheral neuropathy; 5) leuko-encephalopathy (abnormal brain white matter) evident on brain MRI scans; and 6) biochemical or histological evidence of abnormal mitochondria. All MNGIE patients have had severe decreases of TP enzyme activity in buffy coat (<10% of the normal control mean); therefore, measurement of buffy coat thymidine phosphorylase enzyme activity is diagnostic of MNGIE. The diagnosis can be confirmed by the presence of elevated plasma levels of thymidine (patients >3 μmol/L, normal <0.05) and deoxyuridine (patients >5 μmol/L, normal <0.05) and by detection of mutations in the gene encoding TP.
INDICATIONS FOR TESTING
Molecular confirmation of clinically suspected cases of MNGIE
Presymptomatic testing for individuals with positive family history of the condition
The enzymatic assay is based on the measurement of the rate of thymine formation from thymidine in the presence of TP at an alkaline pH. Buffy coat homogenate is incubated with thymidine in Tris-arsenate buffer and the rate of formation of thymine is measured by following changes in optical density at a wavelength of 300-nm. Plasma thymidine and deoxyuridine are measured by a gradient-elution reversed phase HPLC method with ultraviolet detection. The DNA analysis involves sequencing the entire coding region of the thymidine phosphorylase gene, located on chromosome 22q13.32.
SPECIMEN COLLECTION AND SHIPPING
Draw two 7.0 mL yellow-top tubes (ACD) of whole blood and invert several times to mix. Heparinized blood, serum, or plasma is NOT ACCEPTED. Specimen collected must be shipped at ambient temperature and arrive in our laboratory within 24-48 hours of collection. Do not refrigerate or freeze specimens. A completed requisition form with reason for referral and a signed consent form must accompany the specimen. For more information, please contact our laboratory.
The blood should be sent at room temperature by overnight carrier to the address below:
Dr. Ali Naini
Laboratory of Melocular Neurogenetics
Columbia University Medical Center
Department of Pathology
630 West 168th Street, P&S 17th Floor, Room 401
New York, NY 10032
82657 and 83912