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

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.

The activity of DPD was measured using [4-(14)C]thymine followed by separation of substrate and products with reversed-phase HPLC with on-line detection of the radioactivity.

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 (<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 initially detected in PBM cells, containing substantial amounts of myeloid cells, from a patient suffering from 5-fluorouracil toxicity. However, after the patient 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 range of DPD activities of obligate heterozygotes overlaps the range of DPD activities of controls.

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.