Associations between polymorphisms within the thymidylate synthase gene and spina bifida

Polymorphisms within the thymidylate synthase (TS) gene that influence enzyme activity may affect plasma folate levels and, indirectly, plasma homocysteine concentrations. We investigated whether TS polymorphisms contribute to spina bifida (SB) risk, given that a reduction in the risk of SB has been linked to folate metabolism.

Genomic DNA was extracted from newborn-screening blood spots obtained from case infants with SB, and randomly selected, nonmalformed control infants. Genotype frequencies of two polymorphisms in the TS gene-a 28-bp tandem repeat in the promoter enhancer region (TSER) and a 6-bp deletion in the 3'UTR-were determined by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) methods. Additionally, all seven exons of the TS gene were sequenced to identify variations within the coding region of the gene.

We found that the TSER 2/2 homozygous genotype was associated with a slightly increased risk for SB infants (odds ratio [OR] = 1.4 [0.8-2.4], p = 0.1). When the cohort was divided into separate ethnic groups, this risk increased by 4-fold with the TSER 2/2 homozygous genotype (OR = 4.0 [1.8-8.8], p = 0.001), and by 3-fold with the 3'UTR +/+ homozygous genotype (OR = 3.6 [1.3-10.1], p = 0.02) in non-Hispanic white cases. The combined TSER,3'UTR (2/2,+/+) genotype showed a more than 4-fold increased risk for SB within this specific ethnic group (OR = 4.7 [1.1-19.8], p = 0.04).

This study is the first to evaluate how TS polymorphisms contribute to the risk of SB. The current findings indicate that polymorphisms in the untranslated regions of the TS gene are associated with 4-fold or more increased risks of SB in non-Hispanic whites, but not in Hispanic whites, African-Americans, or Asian-Americans.