Identification of two novel sequence variants affecting thiopurine methyltransferase enzyme activity.
The polymorphic enzyme thiopurine methyltransferase (TPMT) is involved in the methylation of thiopurines. On comparing the phenotype with the genotype in Swedish patients with inflammatory bowel disease and healthy individuals, we found two discordant cases with low TPMT enzyme activity (0.3 and 0.4 U/ml packed red blood cells (pRBC). Genotyping by pyrosequencing revealed that they carried the nucleotide substitutions 460G>A and 719A>G, giving two possible genotypes (TPMT*1/*3A or TPMT*3B/*3C). DNA sequencing of exon III to X was performed in the patients and their parents. We identified an A>G transition in the start codon (exon III, 1A>G, Met>Val, TPMT*14) in one of the patients and her father (6.3 U/ml pRBC). The mother in this family carried the 460G>A and 719A>G nucleotide substitutions (TPMT*1/*3A; 5.0 U/ml pRBC). In the second family, sequencing revealed a G>A transition in the acceptor splice site in intron VII/exon VIII (IVS7 -1G>A, TPMT*15) in the patient and his mother (6.9 U/ml pRBC). His father was genotyped as TPMT*1/*3A (6.0 U/ml pRBC). Hence, we report the identification of two novel sequence variants, present in highly conserved nucleotide positions of the human TPMT gene, resulting in a loss of enzyme activity.
Lindqvist
Malin
M
Department of Medicine and Care, Division of Clinical Pharmacology, Faculty of Health Sciences, Linköping University, SE-58185 Linköping, Sweden. malin.lindqvist@imv.liu.se
Haglund
Sofie
S
Almer
Sven
S
Peterson
Curt
C
Taipalensu
Jan
J
Hertervig
Erik
E
Lyrenäs
Ebbe
E
Söderkvist
Peter
P
D001483:Base Sequence; D016022:Case-Control Studies; D017931:DNA Primers; D005260:Female; D005838:Genotype; D006801:Humans; D015212:Inflammatory Bowel Diseases / Q000201:enzymology / Q000235:genetics; D008297:Male; D008745:Methylation; D008780:Methyltransferases / Q000235:genetics / Q000378:metabolism*; D010641:Phenotype
D017931:DNA Primers; D008780:Methyltransferases; C022745:thiopurine methyltransferase