Plasma Folate, Vitamin B12, and Total Homocysteine and Homozygosity for the C677T Mutation of the 5,10-Methylene Tetrahydrofolate Reductase Gene in Patients with Alzheimer’s Dementia

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Abstract
Background: Elevated total plasma homocysteine (tHcy) levels are considered a risk factor for cerebrovascular disease and may also play an important role in the pathogenesis of Alzheimer’s disease (AD). High values of plasma tHcy and low levels of vitamin B12 and folate are frequently present in AD patients. Moreover, the homozygous mutation (C677T) of the methylene tetrahydrofolate reductase (MTHFR) gene, related to a thermolabile type of the encoded enzyme, causes hyperhomocysteinemia by reducing the 5-methyltetrahydrofolate availability. Objective: The aim of the study was to investigate plasma levels of folate, vitamin B12 and tHcy in patients with AD. These values were also related to the severity and the duration of the disease and to the possible role of the MTHFR genotype (C677T). Method: Plasma tHcy levels, homozygosity for the C677T mutation of the MTHFR gene, and folate and vitamin B12 plasma levels were evaluated in 74 patients with AD (45 men, 29 women, mean age 68 years) and in 74 healthy matched controls (42 men, 32 women, mean age 68 years). Results: AD patients had higher mean (± SD) plasma levels of tHcy (20.9 ± 15 µmol/l compared to 11.8 ± 5 µmol/l, p < 0.001) and lower mean plasma folate (5.7 ± 2.1 ng/ml compared to 8.5 ± 3.2 ng/ml, p < 0.001) and vitamin B12 (491 ± 144 pmol/l compared to 780 ± 211 pmol/l, p < 0.001) concentrations. Homozygosity for the C677T mutation of the MTHFR gene had a similar prevalence among controls (18%) and AD patients (20%). Homozygous AD patients (n = 15) had higher plasma tHcy values than nonhomozygotes, in spite of similar mean plasma folate and vitamin B12 levels. This difference in plasma tHcy levels was not observed in controls. Patients with levels of plasma tHcy above and of plasma folate below the normal limits were more frequent in the homozygous AD group. The duration of the disease correlated with plasma levels of tHcy (r = +0.832, p < 0.001), plasma folate (r = –0.580, p < 0.05), and vitamin B12 (r = –0.460, p < 0.05). However, when all the data were corrected for age, serum creatinine levels, and duration of the disease, mean plasma tHcy, folate, and vitamin B12 levels were not statistically different between controls and AD patients. Conclusions: Our data suggest that rather than a risk factor for AD, hyperhomocysteinemia is related to its progression and increasing severity. This might be particularly relevant in homozygotes for the C677T mutation of the MTHFR gene and supports the possible need for continuous supplements in this setting.