Does dietary folic acid supplementation in mouse NTD models affect neural tube development or gamete preference at fertilization?

Abstract

BackgroundNeural tube defects (NTDs) are the second most common birth defect in humans. Dietary folic acid (FA) supplementation effectively and safely reduces the incidence of these often debilitating congenital anomalies. FA plays an established role in folate and homocysteine metabolism, but the means by which it suppresses occurrence of NTDs is not understood. In addition, many cases remain resistant to the beneficial effects of folic acid supplementation. To better understand the molecular, biochemical and developmental mechanisms by which FA exerts its effect on NTDs, characterized mouse models are needed that have a defined genetic basis and known response to dietary supplementation.ResultsWe examined the effect of FA supplementation, at 5-fold the level in the control diet, on the NTD and vertebral phenotypes in Apob tm1Unc and Vangl2 Lp mice, hereafter referred to as Apob and Lp respectively. The FA supplemented diet did not reduce the incidence or severity of NTDs in Apob or Lp mutant homozygotes or the loop-tail phenotype in Lp mutant heterozygotes, suggesting that mice with these mutant alleles are resistant to FA supplementation. Folic acid supplementation also did not affect the rate of resorptions or the size of litters, but instead skewed the embryonic genotype distribution in favor of wild-type alleles.ConclusionSimilar genotypic biases have been reported for several NTD models, but were interpreted as diet-induced increases in the incidence and severity of NTDs that led to increased embryonic lethality. Absence of differences in resorption rates and litter sizes argue against induced embryonic lethality. We suggest an alternative interpretation, namely that FA supplementation led to strongly skewed allelic inheritance, perhaps from disturbances in polyamine metabolism that biases fertilization in favor of wild-type gametes.

Parent-of-origin effects of A1CF and AGO2 on testicular germ-cell tumors, testicular abnormalities, and fertilization bias

“…The χ 2 contingency and goodness-of-fit tests were used as appropriate to test relations between TAs, TGCTs, genotype, and paternal phenotype. Previously described methods were used to test for departures from Mendelian expectations of genotype segregation among intercross progeny (129). In all cases, the significance threshold was set at 0.05.…”

Do gametes woo? Evidence for non-random unions at fertilization

“…These results raise a provocative question, namely does folate correct a developmental defect in the neural tube, or do other explanations apply such as reducing the incidence of cases by biasing fertilization away from at-risk genotypes? 74 Interestingly, for most models, the percentage of affected m/m was similar in the test and control protocols, suggesting that supplemental folic acid did not reduce the proportion of affecyed mutant homozygotes. 75,76 In humans where NTD genetics is not as clearly understood as in mouse models, with genetic heterogeneity a significant issue and isolated cases common, distinguishing a protective effect versus biased fertilization would be difficult.…”

“…During studies to identify mouse NTD models that are resistant to the benefits of folic acid treatments, two examples of fertilization bias were found. 74 Reanalysis of published reports then revealed three additional NTD models that show biased fertilization in response to folic acid treatment ( Table 2). In several cases (Apob, Lrp6, Vangl2), significant deviations are found in the high folic acid (10 ppm; parts per million) test, but in other cases (L3P, Zic2) deviations were found in the low (2 ppm) test, with similar litter sizes for each mutant on the two diet protocols (suppl .…”

“…49,59,182,183 Genetic anomalies together with dietary, hormonal and other environmental influences may induce germline dysfunctions that manifest as conventional genetic effects, unconventional epigenetic inheritance, and now preference for particular gamete combinations at fertilization. 49,59,74 Perspectives Given this evidence, how might biased fertilization work? We can identify essential elements and conditions, but we can be less certain about the ways that these genes affect fertilization.Two components are needed for bias, one in females, the other in males; neither alone is sufficient.…”

Network correlation analysis revealed potential new mechanisms for neural tube defects beyond folic acid

“…In the present study, we downloaded the microarray data GSE51285, which contained the RNA expression profiles from livers of five groups of NTD mutants and their corresponding wildtype (WT) controls. Among the NTD mutants, Pax3 Sp/Sp (Wlodarczyk, Tang, Triplett, Aleman, & Finnell, 2006) and Lrp6 Cd/Cd (Carter, Ulrich, Oofuji, Williams, & Ross, 1999) were reported to be FA-responsive mutants, while Grhl3 ct/c t (Greene & Copp, 1997), Apob tm1Un c (Homanics et al, 1995) and Vangl2 Lp/Lp (Nakouzi & Nadeau, 2014) mutants were considered to be FA-resistant. The differentially expressed genes (DEGs) between NTDs heterozygous and WT mice, as well as the DEGs between FA-responsive and FA-resistant mutants were evaluated.…”