Risk of neural tube defect-affected pregnancy is associated with a block in maternal one-carbon metabolism at the level of N-5-methyltetrahydrofolate:homocysteine methyltransferase.
The disposition of whole blood mono-to hexaglutamyl methylfolate and plasma homocysteine (HCY) was used to evaluate potential lesion sites in one-carbon metabolism which could be responsible for neural tube defect(NTD)-affected pregnancies. An isocratic high-performance liquid chromatographic system (HPLC) with photodiode array detection was used to quantify and speciate whole-blood methylfolate into mono-, di-, tri-, tetra-, penta-, and hexaglutamate forms. This technique was also used with off-line radioassay to identify nonmethyl whole-blood folates. Isocratic HPLC with fluorescence detection was used to quantify SBDF derivatized homocysteine in plasma. The study investigated blood from 11 women who had experienced a previous NTD-affected pregnancy and 11 controls of similar age and social class. No subjects were pregnant. HCY levels were significantly higher in NTD subjects (P = 0.0486, 95% CI-2.799,0.001 using the Mann-Whitney test), as was the ratio of known intracellular (tri-to hexaglutamyl) methylfolate compared to extracellular (mono- and diglutamyl) methylfolate (P = 0.0062 95% CI-0.543, 3.862 using the Mann-Whitney test). Vitamin B12, red cell folate, circulating total methylfolate, and circulating mono-to hexaglutamyl methylfolates showed no difference between population groups. The disposition between individual and cumulative glutamate chain lengths of methylfolate showed significant trends which differed between population groups: (i) total blood methylfolate (Glu1-6) appears to be utilized by N-5-methyltetrahydrofolate:homocysteine methyltransferase (MS) in control blood but not NTD blood, where it appears to accumulate following a 45-min incubation; (ii) whole-blood hexaglutamyl methylfolate (5CH3-H4PteGlus) becomes a larger proportion of the total blood methylfolate in NTD than in control populations; and (iii) the intermediate glutamate chains of methylfolate (Glu1-5) remain relatively constant as 5CH3-H4PteGlu6 accumulates in NTD but appear to increase linearly with 5CH3-H4PteGlu6 in controls. The significant elevation of HCY in the NTD population is associated with the increasing proportion of 5CH3-H4PteGlu6 relative to the total methylfolate, since, when corrected for HCY level, the proportion of 5CH3-H4PteGlus to total methylfolate is similar in NTD and control populations. These trends are consistent with a defect at the level of vitamin B12 dependent MS which "traps" folate at the 5CH3-H4PteGlus level.
Lucock
M D
MD
University of Leeds Research School of Medicine, Centre for Reproduction, Growth and Development, Leeds General Infirmary, West Yorkshire, United Kingdom.
Wild
J
J
Lumb
C H
CH
Oliver
M
M
Kendall
R
R
Daskalakis
I
I
Schorah
C J
CJ
Levene
M I
MI
D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase / Q000097:blood / Q000172:deficiency* / Q000378:metabolism; D004912:Erythrocytes / Q000201:enzymology; D005260:Female; D005492:Folic Acid / Q000097:blood; D018698:Glutamic Acid / Q000097:blood; D006801:Humans; D008431:Maternal-Fetal Exchange; D009436:Neural Tube Defects / Q000201:enzymology* / Q000378:metabolism; D011247:Pregnancy; D011248:Pregnancy Complications / Q000201:enzymology* / Q000378:metabolism; D012307:Risk Factors
D018698:Glutamic Acid; D005492:Folic Acid; D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase