Scientists studying the genomes of nearly 5,000 people have pinpointed a genetic variant tied to an increased risk for stroke, and have also uncovered new details about an important metabolic pathway that plays a major role in several common diseases. Together, their findings may provide new clues to underlying genetic and biochemical influences in the development of stroke and cardiovascular disease, and may also help lead to new treatment strategies.
“Our findings have the potential to identify new targets in the prevention and treatment of stroke, cardiovascular disease and many other common diseases,” said Stephen R. Williams, Ph.D., a postdoctoral fellow at the University of Virginia Cardiovascular Research Center and the University of Virginia Center for Public Health Genomics, Charlottesville.
Dr. Williams, Michele Sale, Ph.D., associate professor of medicine, Brad Worrall, M.D., professor of neurology and public health sciences, all at the University of Virginia, and their team reported their findings March 20, 2014 in PLoS Genetics. The investigators were supported by the National Human Genome Research Institute (NHGRI) Genomics and Randomized Trials Network (GARNET) program.
Stroke is the fourth leading cause of death and a major cause of adult disability in this country, yet its underlying genetics have been difficult to understand. Numerous genetic and environmental factors can contribute to a person having a stroke. “Our goals were to break down the risk factors for stroke,” Dr. Williams said.
The researchers focused on one particular biochemical pathway called the folate one-carbon metabolism (FOCM) pathway. They knew that abnormally high blood levels of the amino acid homocysteine are associated with an increased risk of common diseases such as stroke, cardiovascular disease and dementia. Homocysteine is a breakdown product of methionine, which is part of the FOCM pathway. The same pathway can affect many important cellular processes, including the methylation of proteins, DNA and RNA. DNA methylation is a mechanism that cells use to control which genes are turned on and off, and when.
But clinical trials of homocysteine-lowering therapies have not prevented disease, and the genetics underlying high homocysteine levels – and methionine metabolism gone awry – are not well defined.
Dr. Williams and his colleagues conducted genome-wide association studies of participants from two large long-term projects: the Vitamin Intervention for Stroke Prevention (VISP), a trial looking at ways to prevent a second ischemic stroke, and the Framingham Heart Study (FHS), which has followed the cardiovascular health and disease in a general population for decades. They also measured methionine metabolism – the ability to convert methionine to homocysteine – in both groups. In all, they studied 2,100 VISP participants and 2,710 FHS subjects.
In a genome-wide association study, researchers scan the genome to identify specific genomic variants associated with a disease. In this case, the scientists were trying to identify variants associated with a trait – the ability to metabolize methionine into homocysteine.
Investigators identified variants in five genes in the FOCM pathway that were associated with differences in a person’s ability to convert methionine to homocysteine. They found that among the five genes, one – the ALDH1L1 gene – was also strongly associated with stroke in the Framingham study. When the gene is not working properly, it has been associated with a breakdown in a normal cellular process called programmed cell death, and cancer cell survival.
They also made important discoveries about the methionine-homocysteine process. “GNMT produces a protein that converts methionine to homocysteine. Of the five genes that we identified, it was the one most significantly associated with this process,” Dr. Williams said. “The analyses suggest that differences in GNMT are the major drivers behind the differences in methionine metabolism in humans.”
“It’s striking that the genes are in the same pathway, so we know that the genomic variants affecting that pathway contribute to the variability in disease and risk that we’re seeing,” he said. “We may have found how genetic information controls the regulation of GNMT.”
The group determined that the five genes accounted for 6 percent of the difference in individuals’ ability to process methionine into homocysteine among those in the VISP trial. The genes also accounted for 13 percent of the difference in those participants in the FHS, a remarkable result given the complex nature of methionine metabolism and its impact on cerebrovascular risk. In many complex diseases, genomic variants often account for less than 5 percent of such differences.
“This is a great example of the kinds of successful research efforts coming out of the GARNET program,” said program director Ebony Madden, Ph.D. “GARNET scientists aim to identify variants that affect treatment response by doing association studies in randomized trials. These results show that variants in genes are associated with the differences in homocysteine levels in individuals.”
The association of the ALDH1L1 gene variant with stroke is just one example of how the findings may potentially lead to new prevention efforts, and help develop new targets for treating stroke and heart disease, Dr. Williams said.
“As genome sequencing becomes more widespread, clinicians may be able to determine if a person’s risk for abnormally high levels of homocysteine is elevated,” he said. “Changes could be made to an individual’s diet because of a greater risk for stroke and cardiovascular disease.”
The investigators plan to study the other four genes in the pathway to try to better understand their potential roles in stroke and cardiovascular disease risk.
In addition to NHGRI, the research was supported by funds from the National Heart, Lung and Blood Institute, the National Institute of Neurological Disorders and Stroke, the National Institute on Aging and the Robert Dawson Evans Endowment of the Department of Medicine at Boston University School of Medicine.
The National Human Genome Research Institute is one of the 27 institutes and centers at the National Institutes of Health. The NHGRI Extramural Research Program supports grants for research and training and career development at sites nationwide.
Contact: Press Office – National Human Genome Research Institute/NIH
Source:National Human Genome Research Institute/NIH press release.
Although mammals cannot synthesize methionine, they can still use it in a variety of biochemical pathways:
There are two fates of homocysteine: it can be used to regenerate methionine, or to form cysteine.
Homocysteine can also be remethylated using glycine betaine (NNN-trimethyl glycine, TMG) to methionine via the enzyme betaine-homocysteine methyltransferase (E.C.188.8.131.52, BHMT). BHMT makes up to 1.5% of all the soluble protein of the liver, and recent evidence suggests that it may have a greater influence on methionine and homocysteine homeostasis than methionine synthase.
Reverse-transulfurylation pathway: conversion to cysteine
Homocysteine can be converted to cysteine.
- (5) Cystathionine-β-synthase (a PLP-dependent enzyme) combines homocysteine and serine to produce cystathionine. Instead of degrading cystathionine via cystathionine-β-lyase, as in the biosynthetic pathway, cystathionine is broken down to cysteine and α-ketobutyrate via (6) cystathionine-γ-lyase.
- (7) The enzyme α-ketoacid dehydrogenase converts α-ketobutyrate to propionyl-CoA, which is metabolized to succinyl-CoA in a three-step process (see propionyl-CoA for pathway).
The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. For methionine combined with cysteine, for adults 19 years and older, 19 mg/kg body weight/day.
|Egg, white, dried, powder, glucose reduced||3.204|
|Sesame seeds flour (low fat)||1.656|
|Egg, whole, dried||1.477|
|Cheese, Parmesan, shredded||1.114|
|Soy protein concentrate||0.814|
|Chicken, broilers or fryers, roasted||0.801|
|Fish, tuna, light, canned in water, drained solids||0.755|
|Beef, cured, dried||0.749|
|Beef, ground, 95% lean meat / 5% fat, raw||0.565|
|Pork, ground, 96% lean / 4% fat, raw||0.564|
|Beans, pinto, cooked||0.117|
|Rice, brown, medium-grain, cooked||0.052|
High levels of methionine can be found in eggs, sesame seeds, Brazil nuts, fish, meats and some other plant seeds; methionine is also found in cereal grains. Most fruits and vegetables contain very little of it. Most legumes are also low in methionine. However, it is the combination of methionine and lysine which is considered for completeness of a protein. Racemic methionine is sometimes added as an ingredient to pet foods.
There is scientific evidence that restricting methionine consumption can increase lifespans in fruit flies.
A 2005 study showed methionine restriction without energy restriction extends mouse lifespan.
A study published in Nature showed adding just the essential amino acid methionine to the diet of fruit fliesunder dietary restriction, including restriction of essential amino acids (EAAs), restored fertility without reducing the longer lifespans that are typical of dietary restriction, leading the researchers to determine that methionine “acts in combination with one or more other EAAs to shorten lifespan.” Restoring methionine to the diet of mice on a dietary restriction regimen blocks many acute benefits of dietary restriction, a process that may be mediated by increased production of hydrogen sulfide.
Several studies showed that methionine restriction also inhibits aging-related disease processes in mice and inhibits colon carcinogenesis in rats. In humans, methionine restriction through dietary modification could be achieved through a vegan diet. Veganism being a completely plant based diet is typically very low in methionine, however certain nuts and legumes may provide higher levels.
A 2009 study on rats showed “methionine supplementation in the diet specifically increases mitochondrial ROS production and mitochondrial DNA oxidative damage in rat liver mitochondria offering a plausible mechanism for its hepatotoxicity“.
However, since methionine is an essential amino acid, it cannot be entirely removed from animals’ diets without disease or death occurring over time. For example, rats fed a diet without methionine and choline developed steatohepatitis (fatty liver), anemia and lost two thirds of their body weight over 5 weeks. Administration of methionine ameliorated the pathological consequences of methionine deprivation. Short-term removal of only methionine from the diet can reverse diet-induced obesity and promotes insulin sensitivity in mice.
Methionine might also be essential to reversing damaging methylation of glucocorticoid receptors caused by repeated stress exposures, with implications for depression.
Loss of methionine has been linked to senile greying of hair. Its lack leads to a buildup of hydrogen peroxide in hair follicles, a reduction in tyrosinase effectiveness, and a gradual loss of hair color.
Methionine is an intermediate in the biosynthesis of cysteine, carnitine, taurine, lecithin, phosphatidylcholine, and other phospholipids. Improper conversion of methionine can lead to atherosclerosis.[