Methylenetetrahydrofolate reductase (MTHFR) plays a major role in folate 
metabolism. Disturbed function of the enzyme results in hyperhomocysteinemia and 
causes severe vascular and neurological disorders and developmental delay. Five 
patients suspected of having non-classical homocystinuria due to MTHFR 
deficiency were examined with respect to their symptoms, MTHFR enzyme activity 
and genotypes of the MTHFR gene. All patients presented symptoms of severe 
central nervous system disease. Two patients died, at the ages of 15 months and 
14 years. One patient is currently 32 years old, and is being treated with 
betaine and folinic acid. The other two patients, with an early diagnosis and a 
severe course of the disease, are currently improving under treatment. MTHFR 
enzyme activity in the fibroblasts of four of the patients was practically 
undetectable. We found four novel mutations, three of which were missense 
changes c.664G> T (p.V218L), c.1316T> C (p.F435S) and c.1733T> G (p.V574G), and 
the fourth was the 1-bp deletion c.1780delC (p.L590CfsX72). We also found the 
previously reported nonsense mutation c.1420G> T (p.E470X). All the patients 
were homozygous. Molecular modelling of the double mutant allele (p.V218L; 
p.A222V) revealed that affinity for FAD was not affected in this mutant. For the 
p.E470X mutation, the evidence pointed to nonsense-mediated mRNA decay. In 
general, genotype-phenotype analysis predicts milder outcomes for patients with 
missense changes than for those in which mutations led to severe alterations of 
the MTHFR protein.