The Role of Vitamin B12 in Fasting Hyperhomocysteinemia and Its Interaction with the Homozygous C677T Mutation of the Methylenetetrahydrofolate Reductase (MTHFR) Gene A Case-control Study of Patients with Early-onset Thrombotic Events
Armando D’Angelo (1) , Antonio Coppola (2) , Pasquale Madonna (2) , Isabella Fermo (3) , Anna Pagano (2) , Giuseppina Mazzola (1) , Laura Galli (4) , Anna Maria Cerbone (2)
From the (1) Coagulation Service and Thrombosis Research Unit, (3) Department of Laboratory Medicine, (4) Epidemiology Unit, I.R. C.C.S. H. S. Raffaele, Milan, Italy, (2) Department of Clinical and Experimental Medicine, University of Naples "Federico I
Summary Total fasting plasma homocysteine (tHcy), homozygosity for theC677T mutation of the methylenetetrahydrofolate reductase (MTHFR)gene and for the A2756G mutation of the methionine synthase (MS)gene, vitamin B12 and folate plasma levels were evaluated in 170 consecutivepatients (89 M, 81 F; mean age 41 ± 12 yrs) with documentedearly-onset thrombosis (89 venous, 69 arterial, 12 both; mean age atfirst episode 36 ± 11 yrs), and in 182 age- and sex-matched healthycontrol subjects. Moderate hyperhomocysteinemia (HHcy, tHcy19.5 M in men and 15 M in women) was detected in 45 patients(26.5%) and in 18 controls (9.9%, Mantel-Haenszel OR and 95% C.I.after stratification for arterial or venous thrombosis: 3.25, 1.78-5.91).The 677TT MTHFR genotype was not significantly more prevalent inpatients (27.6%) than in controls (21.4%, RR = 1.42; 0.84-2.41), andmarkedly contributed to HHcy (Mantel-Haenszel RR after stratificationfor case/control status: 8.29, 4.61-14.9). The 2756GG MS genotype,observed in 4 patients (2.4%) and 8 controls (4.4%), was not associatedto HHcy. tHcy was negatively correlated to folate and vitamin B12 levels,with better correlation found in subjects with the 677TT mutation(r = -0.42 and -0.25) than with the 677CC or CT MTHFR genotype(r = -0.37 and -0.11). However, folate was similar in patients andcontrols and vitamin B12 was higher in patients (460 ± 206 vs. 408 ±185 pg/ml, p = 0.011). In a generalized linear model, 44% of the variationin tHcy levels was explained by folate and vitamin B12 levels, theMTHFR genotype, gender, and by the interaction of the MTHFR genotypewith folate (p 0.028); the interactions of vitamin B12 with theMTHFR genotype, gender and patient/control status also significantlycontributed to the variation in tHcy levels (p 0.028). A 4-week administrationof 5-methyltetrahydrofolate (15 mg/day) markedly lowered plasma tHcy in 24 patients with MTHFR 677TT genotype, but theresponse to treatment correlated with vitamin B levels (p = 0.023).Subjects carrying the MTHFR 677TT genotype have higher folate andvitamin B12 requirements irrespective of the A2756G polymorphism ofthe MS gene. Yet unidentified abnormalities of MS or of any of theenzymes participating in the synthesis of methylated vitamin B12 mayplay an important role in the phenotypic expression of moderate hyperhomocysteinemia.