Our examination of a population-based sample of case and control participants of the LIBSCP revealed that breast cancer risk was reduced 24% among women with a high dietary intake of choline and was increased 30% among women homozygous for the minor allele of PEMT rs12325817, a gene of choline metabolism. We also noted a significant interaction between betaine intake and the PEMT rs7946 polymorphism. To the best of our knowledge, this is the first study on choline metabolism and breast cancer. The population-based study design, in which cases encompassed a broad range of ages and were drawn from a defined geographic area, yields results that are more generalizable than a series of cases from a narrow age range or from a single institution. In addition, the relatively large sample size allows multiple risk factors to be taken into consideration in studying associations, with the ability to conduct stratified analyses and adjustment in multivariate models.
Choline is an essential nutrient required for one-carbon metabolism, structural integrity and signaling functions of cell membranes, and neurotransmitter synthesis (7
). Cancers usually exhibit a significantly altered choline phospholipid metabolite profile that is characterized by elevations of phosphocholine and total choline-containing compounds compared with those in normal tissues (26
). Although animal studies have implied a causal relationship between choline deficiency and carcinogenesis (28
), its role in human carcinogenesis is not clear.
In a case-control study conducted by the March of Dimes in Berkley, California, low dietary intake of choline in pregnant women significantly increased neural tube and oral cleft defects in babies (30
). This population-based study (~800 subjects) complements smaller clinical studies in which 60 or so subjects developed organ dysfunction when deprived of choline (32
). The lack of additional studies may be due in part to the fact that a food composition database for choline and betaine has not been available until recently (9
). The general intake level of choline of the U.S. population is not known, and only adequate intakes of ~550 mg/day for men and ~425 mg/day for women have been estimated (33
). Two recent epidemiological studies examined choline and betaine intake at the population level. One is the Nurses’ Health Study, in which the median intakes of total choline and of betaine were 323 and 189 mg/day, respectively (12
). The other is the European Prospective Investigation into Cancer and Nutrition (EPIC) study, in which the mean choline intake was 300 mg/day and mean betaine intake was 214 mg/day (34
We previously reported on folate-mediated one-carbon metabolism in relation to breast cancer risk in this population (5
). The pathways of choline and one-carbon metabolism intersect at the formation of methionine from homocysteine, which makes choline and folate metabolism closely interrelated. Folate mediates the generation of methionine from homocysteine using 5-methyltetrahydrofolate, which is derived de novo
from the one-carbon pool; alternatively, a methyl group from choline, via
the intermediate betaine, can be used for the methylation of homocysteine. SAM is formed from methionine catalyzed by methionine adenosyltransferase (). Our current analysis suggests an inverse association between choline intake and breast cancer risk. Lower choline availability may tilt the SAM pool balance and consequently induce aberrant DNA methylation. Animal studies have suggested that choline deficiency has cancer-initiating and cancer-promoting activities, given the fact that choline-deficient rats had a higher incidence of spontaneous hepatocarcinoma and were more sensitive to carcinogens (28
). Aberrant DNA methylation may be the underlying mechanism, as rats with a choline-deficient diet have hypomethylation of CpG sites of c
-myc along with overexpression of this gene (29
). Results from the Nurses’ Health Study showed that higher choline intake was associated with an elevated risk of colorectal adenoma (12
), and the positive association persisted after adjustment for multiple dietary factors and when assessed for different sizes and sites of the adenoma. The different relation between choline intake and disease outcome could be due to the different etiology of breast cancer and colorectal adenoma.
We found that one SNP (rs12325817) of PEMT
was associated with an increased breast cancer risk in our study population. PEMT is responsible for endogenous biosynthesis of the choline moiety and this activity is increased by estrogen treatment (25
). The rs12325817 (−774G>C) SNP resides in the promoter region of PEMT
, which was proposed to have an estrogen response element. Thus, this SNP may alter estrogen responsiveness of the promoter (15
). This mechanism was supported by our observation that a stronger PEMT-
breast cancer relationship was observed among HRT users.
We also observed that carriers of the minor T allele of the CHDH
rs12676 (+432G>T) SNP had a modest increase in breast cancer risk. CHDH converts choline to betaine aldehyde, which is then oxidized to betaine, a methyl donor for homocysteine. A previous study showed that this SNP was associated with increased susceptibility to choline deficiency (15
). This SNP produces an amino acid substitution that could alter the enzyme activity and consequently affect the methyl moiety availability for the methylation reaction. However, we should interpret this finding with caution as the genotype distribution in our control group was not in agreement with HWE, even though we have excluded genotyping error as an underlying cause.
Although neither PEMT rs7946 nor betaine intake was independently related to breast cancer risk in this study population, a significant gene-environmental interaction was found. Women carrying the variant allele with low betaine intake have an ~2-fold increased risk. The underlying mechanism is not clear. If this finding is substantiated, it implies that the effect of genotype could be modified by lifestyle such as dietary intake. The genetic effect is only obvious when the betaine intake is suboptimal (low). However, such an observation could be a chance finding. We should interpret the result with caution, and replication from other large studies is warranted.
We did not measure choline or betaine levels in peripheral blood. Such measurements, although biologically relevant, may only reflect short-term dietary intake and do not accurately reflect long-term exposure (35
). For this reason estimates based on a FFQ were considered to be the best estimates of dietary intake of choline and betaine. A potential limitation of the Block FFQ used in our study is its lack of complete assessment of commonly consumed products that are choline-rich. The current nutrient databases for choline and betaine are limited but suitable for FFQ diet intake analyses. One study showed that a 3-day food record underestimated choline intake by 25% compared with direct measurements from food intake (36
). It is likely that FFQ estimates were similarly low. However, the ranking of intakes was unlikely to change between methods of assessment as the Block FFQ was shown to be a valid and reliable dietary assessment tool for estimating usual food intake and ranking individuals into categories of intake of micronutrients (37
). Future studies should incorporate newly developed and commonly consumed choline-rich products in the FFQs or other dietary history assessment tools to enhance coverage.
In summary, we found that dietary choline intake was inversely associated with breast cancer risk. Two SNPs of PEMT and CHDH were also associated with breast cancer risk. Results from our study indicate that choline metabolism may play an important role in breast cancer etiology, and further investigation is needed to clarify the underlining mechanisms.