Alpha-1 Antitrypsin Health Article

Definition

Alpha-1 antitrypsin is one of the most common inherited diseases in the Caucasian population. The most common symptom is lung disease (emphysema). People with alpha-1 antitrypsin may also develop liver disease and/or liver cancer. The disease is caused by a deficiency in the protein alpha-1 antitrypsin, which is why the condition is sometimes called alpha-1 antitrypsin deficiency. Other names include anti-elastase, antitrypsin, and ATT. The development of lung disease is accelerated by harmful environmental exposures, such as smoking tobacco. Alpha-1 antitrypsin is inherited. The age of onset, rate of progression, and type of symptoms vary both between and within families.

Description

The protein alpha-1 antitrypsin is a protease inhibitor, which means that it inactivates other proteins called proteases. This is an important function, as proteases themselves disable proteins. In our bodies the levels of proteases and their inhibitors are balanced so that proteases can perform their functions but not over-perform, which leads to problems.

A protease called elastase is the most important target of alpha-1 antitrypsin. Elastase protects the lungs against bacteria and other foreign particles. However, if the action of elastase is not kept in check, elastase destroys lung tissue. Alpha-1 antitrypsin ensures that elastase is not overactive.

Individuals with alpha-1 antitrypsin have inadequate levels of the protein alpha-1 antitrypsin. Thus, certain proteases (especially in the lungs) are overactive, which leads to emphysema and sometimes to liver disease. Alpha-1 antitrypsin is made mostly in the liver.

Some alpha-1 antitrypsin proteins are abnormal in addition to being deficient. These abnormal proteins may not move from the liver to the blood stream correctly. The build-up of the proteins in the liver may lead to liver disease. Also, the abnormal proteins may not neutralize elastase as effectively. Thus, people with alpha-1 antitrypsin have fewer proteins; those they do have do not work as effectively.

Genetic profile

The genetics of alpha-1 antitrypsin are complicated. Scientists have identified many different forms of the gene that codes for the alpha-1 antitrypsin protein. This protein is often called Pi and the gene called PI, for protease inhibitor. One form of the gene, which scientists call Z, or PI Z, greatly reduced the amount of the active Pi protein. Because every person inherits one of each gene from his or her mother, and another copy of each gene from his or her father, everyone has two copies of every gene. People who have two copies of the PI Z gene have 85% less alpha-1 antitrypsin protein. These people have only 15% of the normal level of protein. The protein that they do have does not function as well as the normal protein. People who have one PI Z gene and one normal PI gene have about 60% of the normal level Pi protein. Other forms of the alpha-1 antitrypsin gene are associated with more or less severe deficiencies in protein.

Two other common forms of the Pi protein are called S and M. Pi M is the normal protein and PI M is the normal gene. The Pi M protein has many subtypes within the population, designated M1, M2, etc. A few abnormal alpha-1 antitrypsin genes also have unique names. The PI S gene is slightly abnormal, but not as abnormal as PI Z. Individuals with one PI S gene and one PI Z gene have approximately 38% functioning of the Pi protein (Pi SZ).

The inheritance of alpha-1 antitrypsin is autosomal recessive. This means that a person with alpha-1 antitrypsin has inherited one abnormal gene from each of his or her parents. The parents are most likely carriers, meaning they each have one normal gene and one abnormal gene. Two carriers have a one in four chance to have an affected child with each pregnancy. However, not all people with alpha-1 antitrypsin develop symptoms. Whether and when a person with two abnormal alpha-1 antitrypsin genes develops symptoms is related to the degree of harmful exposures, such as tobacco smoke. A person who is affected with alpha-1 antitrypsin is only at risk to have an affected child if the child's other parent is a carrier.

Although the inheritance of alpha-1 antitrypsin is autosomal recessive, the activity of the protein is equally determined by the gene inherited from either parent. For example, if a gene inherited from one parent codes for a protein with 100% activity, and the gene inherited from the other parent codes for a protein with 0% activity, the offspring would have 50% protein activity. The physical expression of the genes is autosomal recessive, but each gene has an equal effect on the protein activity—neither gene is dominant over the other gene. The gene for alpha-1 antitrypsin is on chromosome 14. More than 90 different forms of the gene have been identified.