Research in the Gene and Environment Interaction Section is focused on defining changes in the genes that underlie inherited susceptibilities to common diseases such as cancer and birth defects. Changes in folate and vitamin B12 metabolism are associated with tumor formation, birth defects and cognitive decline. Folate and vitamin B12 genes are also involved in the methylation of DNA and normal brain function. We are searching for genetic variants in genes related to folate, methionine and homocysteine metabolism. Individuals affected with spina bifida (one form of neural tube defects) will be tested for these variants. Variants found at higher frequency in individuals with disease will help us identify genes associated with risk. The field of neural tube defects has historically focused on evaluating variation in genes in the folate/vitamin B12 metaboli pathway. As genotyping technology has increasingly offered more information at lower cost, the next logical step in this research is to screen the entire genome for additional genes associated with NTDs. This type of experiment requires a very large sample size. Although we have one of the worlds largest samples of NTDs that are available for genetic research, our sample is too small to carry out a genome wide association study (GWAS). In collaboration with Anne Molloy, Trinity College Dublin, we have organized an international collaboration with the goal of pooling samples for a GWAS. Over ten groups have joined this collaborative effort. The total number of samples collected by all groups exceeds 5,000. We have obtained external funding to coordinate this study and collect the samples at a central location. We have collected DNA samples from these collaborators, and in the past year we have continued recruitment of additional investigator participation in replication studies. Other groups have measured the impact of genetic variants on the level of vitamin B12 in blood. We have measured vitamin B12 in the blood of over 5,000 individuals using standard methods. We then subjected these samples to assays that resolve circulating vitamin B12 into pools that correspond to its two major carrier proteins; transcobalamin, the bioactive carrier that is taken up by all cells, or haptocorrin, which is taken up by the liver for eventual recirculation or elimination. We recently published our work describing our insight that the variant repeatedly reported to influence circulating vitamin B12 is actually influencing the subset of vitamin B12 bound to haptocorrin (Velkova 2017). In a previous study of this cohort (Molloy 2016), we published that the common genetic variant most associated with a circulating marker of vitamin B12 deficiency was in a gene unrelated to vitamin B12 transport or metabolism. These findings could be relevant to clinical measures of vitamin B12 testing where it is known that individuals on either side of the normal range are given false positive and negative results. It may be possible to use individual genetic variation to refine the interpretation of clinical testing. In addition to vitamin B12, we have recently published on genetic influence on other circulating metabolites related to the folate one-carbon metabolic pathway. Such variants may be part of the normal population variation and still, in combination with other genetic and environmental factors, contribute to disease states. We have collaborated to publish on genetic variants influencing formate (Brosnan 2018), glycine (OReilly 2018) and folate and homocysteine (Shane in press). We are also using animal models to understand the biology of genes involved in vitamin B12 metabolism. We have developed strains of zebrafish and mice in which we have disrupted vitamin B12 transport genes. In our zebrafish model, we targeted the only known circulatory carrier of vitamin B12. Although these fish should not be able to deliver vitamin B12 to their cells and tissue, they appear to develop normally. This led to a search for an alternate vitamin B12 transport protein in zebrafish. A bioinformatics approach revealed two coding regions that are highly similar to the known carrier protein. We have shown that in an artificial system these partial proteins can be expressed and bind vitamin B12 with affinities comparable to known carrier proteins. These proteins may have a biological role in vitamin B12 transport in these fish. This work is has been peer reviewed and is being revised for publication. Our other model of vitamin B12 deficiency is in mice, where we have targeted the cellular receptor for vitamin B12 uptake. These animals appear to mimic a number of aspects of vitamin B12 deficiency in humans, especially when placed on a diet lacking vitamin B12. First, these mice exhibit the metabolic hallmarks of vitamin B12 deficiency observed in humans (elevated circulating homocysteine and methylmalonic acid). They are also prone to developing anemia as they age, which can be temporarily rescued with an injection of vitamin B12. Last, we have been investigating female-specific infertility in these mice. These dams appear to ovulate normally, and we have shown their embryos can develop for a few days but implantation is generally unsuccessful. Maternal injections of vitamin B12 restore the ability of these dams to sustain a pregnancy. This work was recently accepted for publication (Bernard in press). Future work is needed to determine whether vitamin B12 deficiency in the offspring also contributes to the apparent infertility of their dams. Future work with this animal model may include exploring the impact of vitamin B12 deficiency on neurological function (gene expression in the brain, ability to sense heat, balance and anxious behaviours), cell division (e.g., melanocyte activity as it relates to hair color), and retinal health. The literature contains a variety of strength of evidence of the impact of vitamin B12 on these aspects of human health, and our mouse model provides a way to more fully interrogate these processes. Over the past year we have carried out studies aimed to find the genetic bases of metabolic traits related to folate, vitamin B12 and one carbon metabolism.
Showing the most recent 10 out of 65 publications
