Phenotyping and genotyping for the dihydropyrimidine dehydrogenase test in Italy: a precise diagnostic strategy to detect rare variants and improve drug administration

Introduction

Dihydropyrimidine dehydrogenase (DPD) is the enzyme implicated in the catabolism of the fluoropyrimidines (FP), a class of chemotherapeutics used to treat many cancers. The DPYD gene is known to have a huge number of variants potentially associated with DPD deficiency. Therefore, phenotypic and genotypic characterization of DPD is fundamental for cancer patients before undergoing treatment with FP. The AIOM (Italian Association of Medical Oncology) requires genetic analysis, with the purpose to identify a panel of 5 single nucleotide polymorphisms associated with 5-fluorouracil (5-FU)-induced toxicity, while the biochemical test, which measures uracil levels to predict the residual activity of DPD, is only recommended and not mandatory.

Methods

Single nucleotide polymorphisms were analyzed by real-time polymerase chain reaction (PCR) from peripheral blood. Uracil and dihydrouracil were quantified using an ultra-performance liquid chromatography/tandem mass spectrometry system in plasma obtained from patients before 5-FU treatment. Sanger sequencing was performed for the DPYD gene.

Results

Here, we describe the case of a 70-year-old Caucasian male patient with pancreatic cancer for whom real-time PCR highlighted a heterozygous pathogenic variant c.1905 + 1G > A associated with a 50 % reduction of DPD enzymatic activity. This finding was not confirmed by the biochemical test which revealed a complete absence of DPD activity. By performing Sanger sequencing, we highlighted the concomitant presence of a likely pathogenic variant c.2622 + 1G > A, not compatible with the administration of 5-FU.

Conclusion

Thus far, guidelines provide a limited panel for the identification of pathogenic or likely pathogenic variants of the DPYD gene, therefore we encourage the mandatory use of biochemical tests as a precise diagnostic strategy to detect rare variants and improve drug administration.