Severe methylenetetrahydrofolate reductase (MTHFR) deficiency is an inborn error 
of folate metabolism, and is inherited as an autosomal recessive trait. MTHFR is 
a key enzyme in folate-dependent remethylation of homocysteine, and reduces 
5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate. Patients with this 
severe enzymatic deficiency are biochemically characterised by homocystinuria 
and hypomethioninaemia, and may suffer from neurological abnormalities, mental 
retardation and premature vascular disease. Here we report the molecular basis 
of severe MTHFR deficiency in four unrelated families from Turkish/Greek 
ancestry. By use of reverse-transcriptase (RT)-PCR, subsequently followed by 
direct sequencing analysis, we were able to identify four novel mutations in the 
MTHFR gene: two missense (983A-->G; 1027T-->G) and two nonsense (1084C-->T; 
1711C-->T) mutations. Furthermore, a splice variant containing a premature 
termination codon, was observed in one patient, probably as a secondary effect 
of the 1027T-->G missense mutation. The ongoing identification and 
characterisation of mutations in the MTHFR gene will provide further insight 
into the heterogeneity of the clinical phenotype in severe MTHFR deficiency.