Vitamin B12 (cobalamin, Cbl) deficiency in humans produces megaloblastic anemia and neuropathologic disorders. Cbl requirement for the methionine synthase enzyme in the recycling of folate results in defective DNA synthesis and the anemia. The neurological disorders result from demyelinating changes in the peripheral nerves, the spinal cord and changes yet to be defined in the brain. The metabolic events leading to the structural and functional alterations and the genes involved are not known. Cellular uptake of Cbl in all cell types is mediated by transcobalamin (TC), a Cbl binding protein in the circulation, secreted by the vascular endothelium and a ubiquitous transcobalamin receptor (TCblR/CD320) that specifically binds TC saturated with Cbl and mediates cellular uptake. The TCblR gene knockout mouse provides a model to study the effects of Cbl deficiency in an animal model to define the metabolic basis for the structural and functional alterations of the nervous system seen in Cbl deficiency. The TCblR gene knockout is not lethal to the fetus and the homozygous mice appear to develop and breed normally. The liver and kidney stores of Cbl in these animals are also normal. However, from birth to adulthood, the B12 stores in the brain decrease with time to very low levels. Some decrease is also observed in the spleen and bone marrow. These preliminary observations are important to establishing a mouse model of B12 deficiency to evaluate the hematologic and neuropathologic changes in Cbl deficiency. Therefore, the objectives of this proposal are to define the role of Cbl in the metabolic and functional integration of the brain in a mouse model and to study the hematologic and neuropathologic consequences of Cbl deficiency. Towards these objectives, we will determine the distribution of Cbl and induction of B12 deficiency in the TCblR knockout mice, metabolic and structural changes in the brain and what functional deficits are produced by these changes. We will also evaluate the expression of specific genes and gene products contributing the pathology. This study will aim to identify possible mechanisms and pathways affected in Cbl deficiency that include decreased DNA synthesis, disruptions in gene expression due to methylation changes, damage due to oxidative stress, changes in neurogenesis, neural degeneration and apoptosis in the brain. The proposed project is designed to understand the role of Cbl in brain function. The key hypotheses being examined are: 1. That Cbl plays a pivotal role in maintaining the integrity of the CNS and PNS and 2. The structural and functional alterations in Cbl deficiency are due to interruptions in pathways connecting Cbl pathways with pathways effecting myelin synthesis. The approach outlined in this proposal is designed to identify metabolic pathways connecting Cbl pathways with myelin synthesis pathways, the genes involved in regulating these pathways and to examine structural and functional changes in the CNS due to Cbl deficiency that produce behavioral abnormalities.
Vitamin B12 deficiency causes elevated homocysteine, methylmalonic acid and altered methylation contributing to neurological deficits. The receptor knockout mouse developed provides a novel model to study the effects of B12 deficiency, pathways involved and how these are disrupted in B12 deficiency. This is of great public health relevance in light of known inborn errors of B12 metabolism that produce severe neurological deficits and functional deficits associated with the ageing brain.
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