Meinsma et al. (1995) determined the molecular basis of dihydropyrimidine dehydrogenase deficiency (274270) by studying the phenotype and genotype of a family of a patient with no DPD activity. Fibroblast mRNAs from the patient and 4 family members were subjected to RT-PCR using primers generated from the human DPYD cDNA sequence. In the patient, DPYD mRNA was found to lack a segment of 165 nucleotides as a result of exon skipping. In the parents and 1 sib, DPYD mRNA was found to be heterozygous for the deletion, while a brother had only normal transcript. The deficient patient had no detectable DPD protein. The precise nature of the presumed splicing defect was not identified. The homozygous proband, who was first described by van Gennip et al. (1994), had been admitted to hospital at the age of 25 months with bilateral microphthalmia, coloboma of the iris and choroid, nystagmus, and a gradually increasing psychomotor retardation. No growth retardation or other neurologic abnormalities were detected. All other members of the pedigree were healthy and showed no ocular abnormalities.
Wei et al. (1996) identified heterozygosity for the 165-nucleotide deletion in a British a cancer patient with partial DPD deficiency and severe toxicity after 5-fluorouracil treatment (see 274270). They found that a G-to-A transition within the 5-prime splice site of intron 14 resulted in exon skipping and an inactive DPYD allele.
Independently, Vreken et al. (1996) showed that the 165-nucleotide deletion in the mature DPD mRNA was due to a G-to-A transition in the invariant GT dinucleotide splice donor site downstream of the skipped exon. The same mutation was identified in another, unrelated, Dutch patient. Because this mutation destroyed a unique MaeII restriction site, rapid screening was possible using restriction enzyme cleavage of the amplified genomic region encompassing this mutation. Analysis of 50 controls revealed no individuals heterozygous for the mutation. Although both patients had psychomotor retardation, the other features were different. The second patient presented with convulsions and had no ocular manifestations and no microcephaly.
Van Kuilenburg et al. (1997) described another patient with severe 5-fluorouracil-related toxicity and heterozygosity for the same G-to-A splice site mutation with a deletion of 165 basepairs. Hyperpigmentation and cardiotoxicity were observed as side effects of 5-fluorouracil in this patient. The enzyme activity in leukocytes of the patient proved to be in the heterozygous range. The patients came from Denmark, Finland, and the Netherlands. The authors stated that the G-to-A mutation had been found in 8 of 11 patients with complete deficiency of the enzyme.
Vreken et al. (1998) stated that the deletion of exon 14 due to the G-to-A mutation had been identified in 22 of 42 alleles from patients with complete DPD deficiency.
Van Kuilenburg et al. (1999) found that the IVS14+1G-A mutation, leading to deletion of exon 14 of the DPYD gene, accounted for 23 of 44 (52%) DPD deficiency alleles.
