1 Characterization of allelic variants of the TPMT gene (TPMT) responsible for changes in TPMT activity, and elucidation of the mechanism by which these alleles act, are required because of the clinical importance of this polymorphism for patients receiving thiopurine drugs. 2 We de®ned the mutational and allelic spectrum of TPMT in a group of 191 Europeans. Using PCR ± SSCP, we screened for mutation the entire coding sequence, the exon-intron boundaries, the promoter region and the 3'-¯anking region of the gene. Six mutations were detected throughout the ten exons and seven TPMT alleles were characterized. Four of them, TPMT*2, *3A, *3C and *7, harbouring the known mutations, G238C, G460A, A719G or T681G, were nonfunctional and accounted for 0.5, 5.7, 0.8 and 0.3% of the allele totality, respectively. 3 Within the promoter region, six alleles corresponding to a variable number of tandem repeats (VNTR), were identi®ed. VNTR*V4 and *V5a which harbour four or ®ve repeats of a 17 ± 18 bp unit, were the most frequent (55% and 34%, respectively). The other VNTR alleles, having from ®ve to eight repeats, were rarer. 4 The TPMT phenotype was correctly predicted by genotyping for 87% of individuals. A clear negative correlation between the total number of repeats from both alleles and the TPMT activity level was observed, indicating that VNTRs contribute to interindividual variations of TPMT activity. Therefore, additional analysis of the promoter region of TPMT can improve the phenotype prediction rate by genotyping.

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Aims To identify the molecular basis for a low CYP1A2 metabolic status, as determined by a caffeine phenotyping test, in a 71-year-old, nonsmoking, Caucasian woman who presented with very high clozapine concentrations despite being administered a standard dose of the drug. Methods The nucleotide sequence of the 7 exons, exon-intron boundaries and 5 ¢ -flanking region of the CYP1A2 gene was analysed by direct sequencing. Results Only one heterozygous point mutation was identified in the donor splice site of intron 6 (3534 G > A) of CYP1A2 . This mutation could cause abnormal RNA splicing and therefore lead to a truncated nonfunctional enzyme. No other carrier of this mutation was identified in a population of 100 unrelated healthy Caucasians. Conclusions This is the first report of a splice-site mutation affecting the CYP1A2 gene. This polymorphism is a likely explanation for the low CYP1A2 activity associated with high clozapine concentrations in this patient.

The genetic polymorphism affecting the CYP3A5 enzyme is responsible for interindividual and interethnic variability in the metabolism of CYP3A5 substrates. The full extent of the CYP3A5 genetic polymorphism was analysed in French Caucasian, Gabonese and Tunisian populations using a polymerase chain reaction-single strand conformational polymorphism (PCR-SSCP) strategy. In the three populations, eight, 17 and ten single nucleotide polymorphisms (SNPs), respectively, were identified, among which nine correspond to rare new mutations. Also identified were 16 alleles including eight new allelic variants. Significant differences were observed in the distribution of these alleles. Particularly, the frequency of the CYP3A5*3C null allele in French Caucasians (81.3%) and in Tunisians (80.0%) is higher than in the Gabonese population (12.5%) (p < 0.001). Considering the CYP3A5 genotypes of the tested individuals, only 10.4% of French Caucasians and 30.0% of Tunisians were identified as CYP3A5 expressors. In contrast, 90.0% of Gabonese subjects appear to express the CYP3A5 protein.

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