Pitfalls in the Diagnosis of Patients with a Partial Dihydropyrimidine Dehydrogenase Deficiency

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Abstract
Background: Dihydropyrimidine dehydrogenase (DPD) catalyzes the degradation of thymine, uracil, and the chemotherapeutic drug 5-fluorouracil. To identify patients suffering from complete or partial DPD deficiency and to identify pitfalls that can preclude the proper diagnosis of patients with partial DPD deficiency, a sensitive and accurate assay is necessary. Methods: The activity of DPD was measured using [4-14C]thymine followed by separation of substrate and products with reversed-phase HPLC with on-line detection of the radioactivity. Results: Complete baseline separation of radiolabeled thymine and all degradation products was achieved within 15 min. The detection limit for dihydrothymine was 0.4 pmol. In lymphocytes, the DPD activity deviated from linearity at low protein concentrations (Conclusions: The low activity of DPD measured in PBM cells containing myeloid cells or that measured at a low protein concentration in the assay mixture is not indicative of heterozygosity for a mutant DPD allele. Although fibroblasts are suitable to establish a complete deficiency of DPD, unambiguous detection of heterozygotes is not possible.