Biochem. J. (2014) 461, 205–212 (Printed in Great Britain) doi:10.1042/BJ20131568 205 Mouse model for deficiency of methionine synthase reductase exhibits short-term memory impairment and disturbances in brain choline metabolism Nafisa M. JADAVJI* 1 , Renata H. BAHOUS* 1 , Liyuan DENG*, Olga MALYSHEVA†, Marilyn GRAND’MAISON‡, Barry J. BEDELL‡, Marie A. CAUDILL† and Rima ROZEN* 2 *Departments of Human Genetics and Pediatrics, McGill University, Montreal Children’s Hospital site of the McGill University Health Centre Research Institute, 4060 Ste. Catherine St. West, Montreal, Quebec, Canada, H3Z 2Z3 †Division of Nutritional Sciences, Cornell University, 241 Kinzelberg Hall, Ithaca, NY 14853 6300, U.S.A. ‡Department of Neurology and Neurosurgery, McGill University, Montreal Neurological Institute, 3801 University Street, Montreal, Quebec, Canada, H3A 2B4 Hyperhomocysteinaemia can contribute to cognitive impairment and brain atrophy. MTRR (methionine synthase reductase) activates methionine synthase, which catalyses homocysteine remethylation to methionine. Severe MTRR deficiency results in homocystinuria with cognitive and motor impairments. An MTRR polymorphism may influence homocysteine levels and reproductive outcomes. The goal of the present study was to determine whether mild hyperhomocysteinaemia affects neurological function in a mouse model with Mtrr deficiency. Mtrr + / + , Mtrr + /gt and Mtrr gt/gt mice (3 months old) were assessed for short-term memory, brain volumes and hippocampal morphology. We also measured DNA methylation, apoptosis, neurogenesis, choline metabolites and expression of ChAT (choline acetyltransferase) and AChE (acetylcholinesterase) in the hippocampus. Mtrr gt/gt mice exhibited short-term memory impair- ment on two tasks. They had global DNA hypomethylation and decreased choline, betaine and acetylcholine levels. Expression of ChAT and AChE was increased and decreased respectively. At 3 weeks of age, they showed increased neurogenesis. In the cerebellum, mutant mice had DNA hypomethylation, decreased choline and increased expression of ChAT. Our work demonstrates that mild hyperhomocysteinaemia is associated with memory impairment. We propose a mechanism whereby a deficiency in methionine synthesis leads to hypomethylation and compensatory disturbances in choline metabolism in the hippocampus. This disturbance affects the levels of acetylcholine, a critical neurotransmitter in learning and memory. Key words: acetylcholine, DNA methylation, folate, hippocam- pus, homocysteine, memory. INTRODUCTION Elevated levels of plasma homocysteine have been associated with vascular disease [1] and impaired cognitive function [2]. An increased risk for the development of AD (Alzheimer’s disease) and dementia has also been suggested although the mechanisms are not fully understood [3,4]. The most common causes of hyperhomocysteinaemia are genetic and nutritional disturbances in folate metabolism. Folate derivatives maintain homocysteine at non-toxic levels, via the donation of a carbon group from methyltetrahydrofolate [synthesized by MTHFR (methylenetetrahydrofolate reductase)] for homocysteine remethylation to methionine. This reaction, catalysed by MTR (methionine synthase), is ubiquitous. Optimal activity of MTR requires vitamin B 12 and MTRR (methionine synthase reductase; E.C. 2.1.1.135) for reductive reactivation of the cobalamin moiety of the vitamin cofactor. Methionine is required for the generation of SAM (S-adenosylmethionine), the primary methyl donor in numerous methylation reactions including DNA methylation and phospholipid biosynthesis [5]. An alternate source of carbon donors for homocysteine remethylation is betaine, via the action of BHMT (betaine homocysteine methyltransferase); this enzyme is present primarily in the liver and kidney [6]. Choline is an important nutrient in methyl metabolism as the metabolic source of betaine, and is obtained via CHDH (choline dehydrogenase). It is also critical for the synthesis of acetylcholine, a major neurotransmitter, by ChAT (choline acetyltransferase) [6]. Acetylcholine is believed to play a role in memory [7]. Reduced acetylcholine levels are present in the hippocampus of patients with dementia [8] and AD [9]. The critical role of folate-dependent methionine synthesis in brain function is well illustrated by the inborn error of metabolism, homocystinuria, which can be caused by a deficiency of MTRR or MTR, or of other enzymes with functions involved in folate, cobalamin or homocysteine metabolism. Homocystinuric patients with deleterious mutations in MTRR exhibit developmental delay, brain atrophy, nystagmus, seizures, neuropathy and dementia [10]. A milder disturbance of MTRR function is due to a polymorphism, c.66A→G (p.I22M), which ranges in homozygosity frequencies from 20 to 35 % in the Caucasian Abbreviations: AChE, acetylcholinesterase; AD, Alzheimer’s disease; BHMT, betaine homocysteine methyltransferase; CBS, cystathionine-β- synthase; ChAT, choline acetyltransferase; CHDH, choline dehydrogenase; DG, dentate gyrus; DI, discrimination index; GAPDH, glyceraldehyde-3- phosphate dehydrogenase; HRP, horseradish peroxidase; HSD, honestly significant difference; MTHFR, methylenetetrahydrofolate reductase; MTR, methionine synthase; MTRR, methionine synthase reductase; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PEMT, phosphatidylethanolamine methyltransferase; SAM, S-adenosylmethionine; SI, substantia innominata; TUNEL, terminal deoxynucleotidyltransferase-mediated dUTP nick-end labelling. 1 These authors contributed equally to this work. 2 To whom correspondence should be addressed (email rima.rozen@mcgill.ca). c  The Authors Journal compilation c  2014 Biochemical Society Biochemical Journal www.biochemj.org