Phosphatidylethanolamine N-methyltransferase (PEMT) gene polymorphisms and risk of spina bifida

Recently, our group demonstrated a reduction in risk of neural tube defects (NTDs) in infants of mothers who had elevated intakes of dietary choline [Shaw et al., 2004]. We hypothesized that disturbance in choline metabolism that are secondary to either a choline deficiency, or choline pathway genes mutations, may represent risk factors for NTDs. Evidence to further support this hypothesis is derived from in vitro studies. These studies showed that inhibition of choline uptake and metabolism during murine neurulation result in growth retardation and developmental defects, including an increase in the prevalence of NTDs in mouse embryos [Fisher et al., 2001, 2002]. Choline or its derivatives, such as acetylcholine, betaine, phosphocholine, phosphatidylcholine (PtdCho), and sphingomyelin, are nutrients critical to many cellular processes. These include the maintenance of the structural integrity of cell membranes, phospholipid biosynthesis, cholinergic neurotransmission, transmembrane signaling, and lipid-cholesterol transport and metabolism. Choline is also one of the major sources of methyl groups in the diet and has essential roles in methyl-metabolism [Zeisel and Blusztajn, 1994]. The phosphatidylethanolamine N-methyltransferase (PEMT, EC 2.1.1.17) pathway contributes approximately one-third of the synthesis of PtdCho, which is the most abundant mammalian phospholipids [Walkey et al., 1999; Reo et al., 2002]. In the absence of dietary supplementation, the PEMT pathway provides the only de novo biosynthesis of choline. This reaction requires S-adenosylmethionine (SAM) as the methyl donor [Bremer and Greenberg, 1961], generating S-adenosylhomocysteine (SAH), which is subsequently hydrolyzed yielding adenosine and homocysteine (Hcy) [Finkelstein, 1998]. In mice, PEMT expression enhances not only plasma Hcy levels, but also Hcy secretion from hepatocytes [Schneider and Vance, 1978]. Hyperhomocysteinemia has been identified as a risk factor for NTDs [Rosenquist and Finnell, 2001]. Knowing the regulatory role of PEMT gene in choline metabolism and ultimately in determining Hcy levels, we examined two non-synonymous SNPs, rs7946 (Met212Val) and rs897453 (Val95Ile). We investigated whether PEMT gene variants would disturb PEMT function, rendering affected individuals more susceptible to spina bifida due to their increased dietary requirements for choline and the dysregulation of Hcy homeostasis. Our analytic strategy investigated potential spina bifida risks associated with PEMT gene polymorphisms in a population-based case-control study.