Abstract

Homocystinuria can be caused by severe deficiency in either methylenetetrahydrofolate reductase (MTHFR) or methionine synthase reductase (MTRR). Patients present with neurological symptoms, some of which include developmental delays, mental retardation, motor abnormalities and brain atrophy. Biochemically patients have reduced enzyme activity, hyperhomocysteinemia and decreased levels of serum folic acid, methionine and S-adenosylmethionine (SAM). Mouse models for genetic deficiencies in MTHFR and MTRR have been developed to investigate the in vivo effects of these deficiencies. Young BALB/c Mthfr-/- mice have elevated levels of plasma homocysteine as well as decreased methylation and SAM levels in brain tissue. Mtrr is expressed in the developing neural tube and Mtrr expression is increased in brain tissue of MTRR-deficient mice. The first two objectives of this thesis will examine the brain function of MTHFR and MTRR in adult male mice. Adult C57BL/6 Mthfr-/- male mice have elevated plasma homocysteine levels, impaired memory function along with changes in hippocampus, including morphology, global DNA hypomethylation, altered choline metabolites and increased levels of choline acetyltransferase (ChAT) and glucocorticoid receptor (GR). In the cerebellum of Mthfr-/- mice we confirmed morphological changes and increased apoptosis previously described in young BALB/c Mthfr-/- mice. Additionally, we identified changes in function, decreased volume and alterations in choline metabolites in the cerebellum. Adult mice deficient in MTRR have mildly elevated plasma homocysteine levels, short-term memory impairments, gait and affective behavior changes along with reduced methylation in hippocampus and cerebellum and reduced hippocampal volume. Changes in choline metabolism in hippocampus, cerebellum, liver and plasma were also identified.Maternal contributions of MTHFR, folic acid and choline during fetal and early neonatal brain development are essential. The last two objectives of this thesis will investigate the effects of maternal deficiencies in MTHFR, folic acid or choline on brain function and structure of 3-week-old Mthfr+/+ offspring. Preliminary data suggests that maternal deficiencies in MTHFR can lead to impaired short-term memory and motor function in Mthfr+/+ offspring. Elevated maternal plasma homocysteine and increased apoptosis in cerebellum and hippocampus of Mthfr+/+ offspring may be responsible for these behavioural changes. Maternal folic acid (FADD) or choline (ChDD) deficient diets result in impaired short-term memory in Mthfr+/+ offspring. Maternal ChDD results in impaired motor function, whereas maternal FADD changes affective behaviors in Mthfr+/+ offspring. Both maternal FADD and ChDD result in increased apoptosis, alterations in choline metabolites and increased ChAT protein levels in the cerebellum and hippocampus of Mthfr+/+ offspring.These behavioural, morphological and biochemical results suggest that MTHFR, MTRR and maternal folate and choline are critical for brain development, maturation…