Neural tube defects and folate pathway genes : Family-based association tests of gene-gene and gene-environment interactions

Of 1,000 births worldwide, in one embryo the neural tube will fail to close properly 28 days after conception, resulting in some form of neural tube defect (NTD). Failed closure at the cranial end, known as anencephaly, is a lethal condition, whereas failed closure at the caudal end usually results in a myelomeningocele. NTDs are the most common debilitating birth defect. Familial studies indicate a significant genetic component to NTDs, with a 40-fold increase in risk in first-degree relatives (Elwood et al. 1992). Myriad environmental exposures have been implicated in the development of NTDs; most notably, a significant decrease in risk can be achieved by maternal folic acid supplementation before conception. The mechanism by which dietary folate supplementation prevents NTDs is poorly understood (MRC Vitamin Study Research Group 1991). Folic acid derivatives are essential for the synthesis of DNA, cell division, tissue growth, and DNA methylation (Morrison et al. 1998). Methylation enables proper gene expression and chromosome structure maintenance, both of which are critical in the developing embryo (Razin and Kantor 2005). The folate and methionine cycles are linked by the conversion of homocysteine to methionine (Figure 1). In the absence of food frequency data, maternal vitamin supplementation can also serve as a proxy for overall health because of the positive correlation between supplement intake, diet, and a healthy lifestyle (Slesinski et al. 1996). Vitamin supplementation is an important cofactor to consider when studying nutritionally related genes. Figure 1 The folate and methionine cycles highlighting the 11 genes included in this study. Substrates are shown in rectangular boxes; enzymes are shown in ellipses. Adapted from Nijhout et al. (2004) and Reed et al. (2004). Substrate abbreviations: AdoHcy, S ... Animal models demonstrate that periconceptional folate supplementation protects against congenital defects in the face, neural tube, and conotruncal region of the heart. Low folate could directly limit its availability to cells or indirectly disrupt methionine metabolism, thereby increasing homocysteine in the maternal serum (Rosenquist and Finnell 2001). Either mechanism implicates folate receptor and methionine–homocysteine regulatory genes. Folate enters cells by folate receptor 1 [FOLR1; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"NM_016725","term_id":"262331571","term_text":"NM_016725"}}NM_016725 (http://www.ncbi.nih.gov/GenBank)] and folate receptor 2 (FOLR2; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"NM_000803","term_id":"166064049","term_text":"NM_000803"}}NM_000803) or carrier-mediated internalization by solute carrier family 19 member 1(SLC19A1; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"U15939","term_id":"1222522","term_text":"U15939"}}U15939), also known as reduced folate carrier protein 1. Transcobalamin II (TCN2; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"NM_000355","term_id":"296080702","term_text":"NM_000355"}}NM_000355) imports vitamin B12, cobalamin, a cofactor for another folate enzyme, 5-methyltetrahydrofolate-homocys-teine methyltransferase (MTR; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"NM_000254","term_id":"169790922","term_text":"NM_000254"}}NM_000254).The reactions within the folate metabolism cycle can be very complex, with methylenetetrahydrofolate dehydrogenase 1 (MTHFD1; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"J04031","term_id":"187464","term_text":"J04031"}}J04031), serine hydroxymethyl-tranferase 1 (SHMT1; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"NM_004169","term_id":"528881072","term_text":"NM_004169"}}NM_004169), and 5,10-methylenetetrahy-drofolate reductase (MTHFR ; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"NM_005957","term_id":"260898771","term_text":"NM_005957"}}NM_005957) being widely studied in the NTD literature. MTHFR rs1801133 is the most frequently investigated polymorphism in NTDs with conflicting results in different populations: Dutch and Irish populations associate the TT allele with risk (Shields et al. 1999; van der Put et al. 1995), whereas a protective effect is seen in Italians (De Marco et al. 2002) and other populations have no evidence of association (Gonzalez-Herrera et al. 2002; Revilla et al. 2003; Stegmann et al. 1999). This polymorphism also has a confirmed role heart disease (Frosst et al. 1995). Homocysteine can accumulate from low dietary folate, cobalamin, and/or genetic factors (Morrison et al. 1998; Ramsbottom et al. 1997) and is elevated in some NTD mothers (Mills et al. 1995; Steegers-Theunissen et al. 1994). Homocysteine itself may be teratogenic (Rosenquist et al. 1996) or impair substrates for methylation reactions (Essien and Wannberg 1993). Enzymes that degrade homocysteine regulate homocysteine levels; for example, MTR converts homocysteine to methionine and folate to tetrahydrofolate (Trembath et al. 1999). 5-Methyltetrahydrofolate-homocysteine methyltransferase reductase (MTRR; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"AF025794","term_id":"2981302","term_text":"AF025794"}}AF025794) maintains MTR in its active state. Betaine-homocysteine methyltransferase (BHMT; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"BC012616","term_id":"15214971","term_text":"BC012616"}}BC012616) remethylates homocysteine to methionine with a betaine cofactor (Morin et al. 2003). Cystathionine-betasynthase (CBS; GenBank accession no. {"type":"entrez-nucleotide","attrs":{"text":"NM_000071","term_id":"209862802","term_text":"NM_000071"}}NM_000071) controls homocysteine levels by degrading homocysteine into cystathionine (Morrison et al. 1998). Detecting moderate effects of multiple folate genes will be particularly difficult if they are interactive or additive with environmental impacts (Morrison et al. 1998). This complex pathway has several known metabolic interactions, such as MTRR maintaining MTR in an active state. Previous studies found an association of MTHFR and MTRR (Gueant-Rodriguez et al. 2003; Wilson et al. 1999) plus CBS and the MTHFR thermolabile variant with NTDs (Afman et al. 2003; Ramsbottom et al. 1997; Speer et al. 1999). Thus, genes involved in folate metabolism are compelling candidates for NTDs, from both a genetic and an environmental perspective.