Pitfalls in the diagnosis of patients with a partial dihydropyrimidine dehydrogenase deficiency

Background: Dihydropyrimidine dehydrogenase (DPD) catalyzes the degradation of thymine, uracil, and the chemotherapeutic drug 5-fluorouracil. To ident ify patients suffering from complete or partial DPD deficiency and to ident ify pitfalls that can preclude the proper diagnosis of patients with partia l DPD deficiency, a sensitive and accurate assay is necessary. Methods: The activity of DPD was measured using [4-C-14]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 degra dation products was achieved within 15 min. The detection limit for dihydro thymine was 0.4 pmol. In lymphocytes, the DPD activity deviated from linear ity at low protein concentrations (<0.2 g/L). Profoundly decreased activity of DPD was detected in the peripheral blood mononuclear cells (PBM cells) of two tumor patients when measured at low protein concentrations. Low DPD activity comparable to that observed in obligate heterozygotes was initiall y detected in PBM cells, containing substantial amounts of myeloid cells, f rom a patient suffering from 5-fluorouracil toxicity. However, after the pa tient experienced full clinical recovery, normal DPD activity was observed in the PBM cells. No significant differences in DPD activity were observed between exponentially growing fibroblasts and those at confluence. The rang e of DPD activities of obligate heterozygotes overlaps the range of DPD act ivities of controls. Conclusions: The low activity of DPD measured in PBM cells containing myelo id cells or that measured at a low protein concentration in the assay mixtu re is not indicative of heterozygosity for a mutant DPD allele. Although fi broblasts are suitable to establish a complete deficiency of DPD, unambiguo us detection of heterozygotes is not possible. (C) 2000 American Association for Clinical Chemistry.