What is Hyperhomocysteinemia?

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  • Written By: Caitlin Kenney
  • Edited By: Bronwyn Harris
  • Last Modified Date: 05 March 2020
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Hyperhomocysteinemia is a medical condition in which the total plasma homocysteine concentration is abnormally elevated. This disorder is associated with several vascular, neurological, and skeletal problems including stroke, heart attack, clotting, mental retardation, eye abnormalities, and osteoporosis, amongst many other complications. Hyperhomocysteinemia can be caused by the presence of one or two inherited mutated genes that cause a failure to break down homocysteine, a naturally occurring chemical in the body. Elevated homocysteine can also occur due to excessive exercise, a diet too low in folate, vitamin B6 or vitamin B12, kidney disease, or in reaction to medication.

Hyperhomocysteinemia increases the risk of clotting, or thrombosis, and leads to various serious complications, such as stroke or heart attack. Homocysteine is normally produced within the body for various necessary functions within cells, but it is meant to be quickly broken down again by enzymes. Enzymes are proteins within the body that have the capacity to react with and cut up other chemicals. The enzymes involved in breaking down homocysteine are called and methionine synthase (MS), methylenetetrahydrofolate reductase (MTHFR), and cystathionine beta-synthase (CBS).


Methionine synthase needs vitamin B12, a vitamin found in animal products, to carry out its functions, so diet and other disorders that affect B12 can lead to the MS form of hyperhomocysteinemia. Likewise, if MTHFR or CBS cannot be formed, it can also lead to elevated homocysteine in the blood. Some people, however, are born with mutated genes that prevent the CBS and MTHFR enzymes from forming properly.

The rare disorder affecting CBS is called homocystinuria, or CBS deficiency. This disorder often shows no symptoms at birth, though nearly a quarter of all those with CBS deficiency die by the age of thirty. Symptoms include seizure, mental retardation, lanky build, knock-knees, eye abnormalities, clotting, homocysteine in the urine, and hardening of the arteries. This disease is caused by a recessive gene, so a person needs to get the gene from both parents to develop the full-blown disorder. Someone with only one gene, however, may experience mild symptoms.

A person with one gene mutation for MTHFR may also suffer from mild increases of homocysteine, but a person with two mutations, or a homozygous mutation, will develop a much more troublesome hyperhomocysteinemia. The manifestations of this disorder are similar to a CBS deficiency and include stroke, heart attack, and other thrombotic complications at an usually early age, lengthy bones, eye abnormalities, mental retardation, seizures, fatty liver, and osteoporosis. Some have also proposed a relationship between Alzheimer’s and hyperhomocysteinemia.

Treatment for hyperhomocysteinemia includes vitamin supplements, a diet low in protein, and anticoagulants. These treatments are not always effective, but can, in some cases, help to lengthen the lifespan of someone with profoundly elevated homocysteine. For those with more situational rises in homocysteine, a diet change can be quite effective.


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