Methylenetetrahydrofolate reductase (MTHFR) is a key enzyme in the regulation of 
plasma homocysteine levels. MTHFR deficiency, an autosomal recessive disorder, 
results in homocystinuria and hypomethioninaemia and presents with highly 
variable symptoms affecting many organs but predominantly the central nervous 
system. The common polymorphism of the MTHFR gene, c.677C>T, a known risk factor 
for elevated plasma homocysteine levels, occurs frequently in the caucasian 
population. In this study we investigated three subjects with moderate 
hyperhomocysteinaemia (total plasma homocysteine 72 micromol/L in case 1 and 90 
micromol/L in case 3, total non-protein-bound homocysteine 144-186 micromol/L in 
case 2) but different clinical presentation with no symptoms in case 1, muscle 
weakness at 17 years of age in case 2, and syncopes and cerebral convulsions at 
18 years of age in case 3. Each subject was compound heterozygous for the 
c.677C>T polymorphism and a novel mutation of the MTHFR gene (case 1: c.883G>A 
[p.D291N]; case 2: c.1552_c.1553delGA [p.E514fsX536]; case 3: c.616C>T 
[p.P202S]). Moderately decreased fibroblast MTHFR activity was associated with 
severely reduced affinity for NADPH and increased sensitivity to inhibition by 
S-adenosylmethionine (AdoMet) in case 2, and with mild FAD responsiveness in 
case 3. In case 1, fibroblast MTHFR activity was normal but the sensitivity to 
inhibition by AdoMet was slightly reduced. This study indicates that the 
sequence alteration c.677C>T combined with severe MTHFR mutations in compound 
heterozygous state may lead to moderate biochemical and clinical abnormalities 
exceeding those attributed to the c.677TT genotype and might require in addition 
to folate substitution further therapy to normalize homocysteine levels.