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vitamins, specifically folate, in the prevention of NTDs. Coverage of other risk factors for NTDs is beyond the scope of this report.

Genetic Evidence. An assessment of heritability for common forms of NTDs has been put at 60 percent (Emery, 1986). Data from demographic, family, and mouse model studies have prompted a search for candidate genes that predispose individuals to an NTD. A defect in enzymes involved in homocysteine metabolism is suggested by altered folate, vitamin B12, homocysteine, and methylmalonate values in mothers of infants with NTDs (Mills et al., 1995; Steegers-Theunissen et al., 1994); the prevention of some human NTDs by folate administration; and the prevention of NTDs in some rodent models by methionine (Essien, 1992; Vanaerts et al., 1994). These enzymes are 5,10-methylenetetrahydrofolate reductase (MTHFR), cystathionine β-synthase, and methionine synthase (Figure 8-3). Interestingly, families with homocystinuria caused by severe mutations in genes for each of these enzymes do not exhibit NTDs

FIGURE 8-3 Major pathways depicting involvement of vitamin B12 and folate in homocysteine metabolism.



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