Profound dihydropyrimidine dehydrogenase deficiency resulting from a novel compound heterozygote genotype

A familial approach was used to elucidate the genetic determinants of profound and partial dihydropyrimidine dehydrogenase (DPD; EC 1.3.1.2) deficiency in an Alabama family. In 1988, our laboratory diagnosed profound DPD deficiency in a breast cancer patient with grade IV toxicity after cyclophosphamide/methotrexate/5-fluorouracil chemotherapy (R. B. Diasio et aL, J. Clin. Investig., 81: 47-51, 1988). We now report the genetic analysis of archived genomic DNA that reveals that the proband was a compound heterozygote for two different mutations, one in each allele: (a) a G to A mutation in the GT 5′ splicing recognition sequence of intron 14, which results in a 165-bp deletion (corresponding to exon 14) in the DPD mRNA (DPYD*2A); and (b) a T1679G mutation (now designated DPYD*13), which results in a I560S substitution. Sequence analysis revealed segregation of both mutations with the son and the daughter each inheriting one mutation. Phenotype analysis (DPD enzyme activity) confirmed that both children were partially DPD deficient. Plasma uracil and DPD mRNA levels were found to be within normal limits in both children. We conclude that profound DPD deficiency in the proband resulted from a combination of two mutations (one mutation in each allele) and that heterozygosity for either mutation results in partial DPD deficiency. Lastly, we identified two variant alleles reported previously as being associated with DPD enzyme deficiency [T85C resulting in a C29R substitution (DPYD*9A) and A496G (M166V) in a family member with normal DPD enzyme activity]. These data suggest that both variant alleles are unrelated to DPD deficiency and emphasize the need to perform detailed familial genotypic and phenotypic analysis while characterizing this pharmacogenetic syndrome.