DPYD genotype should be extended to rare variants: report on two cases of phenotype / genotype discrepancy.

The enzyme dihydropyrimidine dehydrogenase (DPD) is the primary catabolic pathway of fluoropyrimidines including 5 fluorouracil (5FU) and capecitabine. Cases of lethal toxicity have been reported in cancer patients with complete DPD deficiency receiving standard dose of 5FU or capecitabine. DPD is encoded by the pharmacogene DPYD in which more than 200 variants have been identified. Different approaches have been developed for screening DPD-deficiency, including DPYD genotyping and phenotyping. Plasma uracil ([U]) and dihydrouracil ([UH₂]) concentrations are routinely used as surrogate markers for systemic DPD activity: [U] ≥ 16 ng/ml and < 150 ng/ml, and [U] ≥ 150 ng/mL indicate partial and complete DPD deficient phenotype, respectively, while values of 5 or 10 for [UH2]/([U] ratio are often cited. Four clinically relevant DPYD defective variants (DPYD*13, DPYD*2A, p.Asp949Val and haplotype B3), are targeted in genetic testing via PCR. In practice, pretreatment [U], alone or combined with these 4 recommended DPYD alleles guides individual dosage selection, though this approach has limitations. This is illustrated by two cases showing discrepancy between DPD deficient phenotype and normal standard genotype. In these two cases, DPYD exome sequencing with Next Generation Sequencing identified rare inactive variants, establishing concordance between phenotype and genotype. In patient 1, [U] levels of 21.1 and 25.5 ng/mL, indicated partial deficiency though the targeted genotype was normal and 5FU dose was adjusted based on the phenotype. In patient 2, [U] levels of 16.2 and 15.2 ng/mL were near the 16 ng/ml threshold. With a normal genotype, he as considered non-deficient as targeted genotype was normal and the standard dose was administered. These two cases underscore the need to pair DPD phenotyping with whole DPYD gene sequencing, due to the frequent discrepancies between these pharmacogenetic tools, the burden of rare variants and ethnic differences in variant frequencies.