The search for genetic polymorphisms in the homocysteine/folate pathway that contribute to the etiology of human neural tube defects

Abstract

In this paper, we trace the history of current research into the genetic and biochemical mechanisms that underlie folate-preventable neural tube defects (NTDs). The inspired suggestion by Smithells that common vitamins might prevent NTDs ignited a decade of biochemical investigations-first exploring the nutritional and metabolic factors related to NTDs, then onto the hunt for NTD genes. Although NTDs were known to have a strong genetic component, the concept of common genetic variance being linked to disease risk was relatively novel in 1995, when the first folate-related polymorphism associated with NTDs was discovered. The realization that more genes must be involved started a rush to find polymorphic needles in genetic haystacks. Early efforts entailed the intellectually challenging and time-consuming task of identifying and analyzing candidate single nucleotide polymorphisms (SNPs) in folate pathway genes. Luckily, human genome research has developed rapidly, and the search for the genetic factors that contribute to the etiology of human NTDs has evolved to mirror the increased level of knowledge and data available on the human genome. Large-scale candidate gene analysis and genome-wide association studies are now readily available. With the technical hurdles removed, the remaining challenge is to gather a sample large enough to uncover the polymorphisms that contribute to NTD risk. In some respects the real work is beginning. Although moving forward is exciting, it is humbling that the most important result-prevention of NTDs by maternal folic acid supplementation-was achieved years ago, the direct result of Smithells' groundbreaking studies.

Figure 1.

Pathways of one-carbon metabolism. Folate cofactors are used in the distribution of one-carbon units across two distinct metabolic cycles, one relating to the de novo synthesis of DNA and the other involved in providing methyl groups for at least 40 different methyltransferase reactions. In the DNA synthesis cycle, formyl derivatives of tetrahydrofolate (THF) are used in de novo purine biosynthesis. In de novo pyrimidine biosynthesis, 5,10-methyleneTHF is required in the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). Thus folate cofactors play an essential role in cellular proliferation. The THF cofactor pool also provides a continuous supply of methyl groups for biologic methylation reactions via the conversion of homocysteine to methionine, using 5-methylTHF as the methyl donor, with vitamin B₁₂-dependent methionine synthase (MS) transferring the methyl group to homocysteine. (In liver and kidney homocysteine is also remethylated to methionine through an alternative folate-independent pathway involving betaine homocysteine methyltransferase.) Methionine is then converted to S-adenosylmethionine (SAM), the cosubstrate of all methyltransferase enzymes. This cycle is completed by the regeneration of homocysteine from S-adenosylhomocysteine (SAH), the coproduct of the methylation process. Methylenetetrahydrofolate reductase (MTHFR) catalyzes the irreversible conversion of 5,10-methyleneTHF to 5-methylTHF, thereby committing one-carbon units to methylation reactions and away from DNA synthesis. Mitochondrial and cytosolic folate-linked metabolism of serine and formate generate the majority of one-carbon groups for all of these processes. In addition to all of the enzymes involved in folate pathways, the reduced folate carrier (RFC), folate receptors (FRs), proton coupled folate transporter (PCFT), and mitochondrial folate transporter (MFT) play key roles in maintaining folate concentrations within the cell.