Common genetic polymorphisms have been reported to influence human requirements for nutrients. For example, a common SNP in the methyltetrahydrofolate reductase gene increases dietary requirements for the vitamin folic acid (34
). However, these SNPs usually have very modest effects on nutrient needs. We recently reported that individuals who were carriers of the very common 5,10-methylenetetrahydrofolate dehydrogenase-1958A gene allele were more likely than noncarriers to develop signs of choline deficiency (35
). We now report that we have identified common genetic variations in the PEMT
genes that may be associated with developing organ dysfunction when choline is removed from the diet of humans. While a larger study is needed to confirm these findings, it does appear that these polymorphisms influence the susceptibility to developing organ dysfunction when fed a low choline diet, and thus they increase the dietary requirement for choline needed to sustain optimal health.
In particular, women with a common variant in the promoter region of the PEMT
gene rs12325817 (−774 G→C) were at significantly increased risk of developing organ dysfunction when dietary intake of choline is insufficient. PEMT activity is responsible for endogenous biosynthesis of choline moiety (1
), and this activity is increased by estrogen treatment (36
). We suggest that the promoter region of this gene is likely to have an estrogen response element (ERE). Indeed, the rs12325817 (−774 G→C) SNP is located within 50 bp of a putative ERE, which contains a perfect half-site consensus sequence (TGACC), but four of five bases differ in the other half-site (CGAAC vs. GGTCA). Given the sexually dimorphic effect of PEMT
rs12325817 (−774 G→C), it is possible that this SNP alters the estrogen responsiveness of the promoter. Studies are under way in our laboratory to confirm this. We suggest that premenopausal women who are heterozygous for the PEMT
rs12325817 (−774 G→C) C allele have sufficient estrogen to overcome the effects on estrogen-mediated transcription factor of the single allele, whereas post-menopausal women with lower estrogen levels are sensitive to the SNP. Men, with little estrogen, would be unaffected by an SNP that altered estrogen receptor complex binding.
The protective effect of the SNP in the CHDH gene (rs9001; 318 A→C) was more modest, and the frequency of this allele was relatively low (0.23). We observed a significant decrease in susceptibility to developing organ dysfunction on a low choline diet in all subjects, but a larger study would be needed to examine whether there are gender differences. More studies are also needed for the CHDH rs12676 (+432 G→T) SNP; first to confirm whether this SNP is associated with increased susceptibility to choline deficiency (we only achieved statistical significance in the subgroup of premenopausal women), then to explain this divergent response. It is not known whether these two SNPs have opposite effects on the activity of the CHDH enzyme.
The lack of effect of the SNP in exon 4 of the PEMT
gene (rs7946, +5465 G→A) was unexpected, because we previously reported that this is a loss of function SNP and that persons with the variant A allele have increased risk of nonalcoholic fatty liver disease (30
). Perhaps the modest decrease (30%) in activity of PEMT
associated with this SNP was overshadowed by compensatory induction of the enzyme that is associated with choline deficiency in males (37
) and by estrogen-mediated activation of PEMT in females. The BHMT SNP effect was not surprising because, although this SNP has been reported to be protective against the risk of cardiovascular disease (20
), the protein product of the gene variant did not differ in either catalytic activity or betaine binding when compared to the enzyme which did not contain the polymorphism (20
The SNPs we identified that increased susceptibility to developing organ dysfunction in humans fed low choline diets are likely to be of clinical importance. Humans fed intravenously (total parenteral nutrition) with solutions that deliver less choline than the adequate intake concentration often develop liver dysfunction that sometimes resolves when a choline source is added to their feeding solution (40
). We suggest that humans with the identified SNPs are the ones most likely to be susceptible to this complication of parenteral nutrition. These SNPs, combined with poor dietary intake of choline, could contribute to adverse outcomes during pregnancy, a time when choline demand is high (9
). As noted earlier, deficient maternal dietary intake of choline during pregnancy in humans was associated with a 4-fold increased risk of having a baby with a neural tube defect (9
). In rodent models, maternal dietary choline intake influenced brain development. More choline (~4× dietary levels) during days 11–17 of gestation in the rodent increased hippocampal progenitor cell proliferation (10
), decreased apoptosis in these cells (10
), enhanced long-term potentiation (LTP) in the offspring when they were adult animals (12
), and enhanced visuospatial and auditory memory by as much as 30% in the adult animals through out their lifetime (13
). Mothers fed choline-deficient diets during late pregnancy have offspring with diminished progenitor cell proliferation and increased apoptosis in fetal hippocampus (10
), insensitivity to LTP when they were adult animals (12
), and decremented visuospatial and auditory memory (13
). For these reasons, identification of common polymorphisms that increase dietary requirements for choline during pregnancy could enable us to identify women for whom we need to assure adequate dietary choline intake. Further work with a larger sample size is warranted to replicate these important findings and to explore the mechanisms involved.
In summary, we report for the first time that SNPs in the phosphatidylethanolamine N-methyltransferase (PEMT) and choline dehydrogenase (CHDH) genes are associated with altered susceptibility to developing organ dysfunction on a low choline diet, and they likely affect dietary requirements for the nutrient choline. These SNPs are extremely common, and their effects should be considered when setting dietary reference intake levels. Studies determining the prevalence of these genetic polymorphisms in human populations of diverse composition should be conducted to facilitate such recommendations. In addition, since the genes of interest have many more polymorphisms than we tested, we cannot rule out the possibility that unmeasured but causal genetic variation is in linkage disequilibrium with the SNPs we genotyped.