The synthesis of alpha1-antitrypsin is controlled by a pair of genes at the proteinase inhibitor (Pi) locus on chromosome 14. The genes are inherited as co-dominant alleles, meaning that products of both genes can be found in the circulation. Many abnormal variants have been very well characterized; they result from point mutations in the gene and most commonly have one or two amino acid substitutions when compared to the normal protein. Some of these changes result in little (or rarely, no) alpha1-antitrypsin in the circulation, and can lead to the clinical condition of alpha1-antitrypsin deficiency. Inheritance of two abnormal alleles is required to cause severe alpha1-antitrypsin deficiency (Alpha-1). However, since the products of each alpha1-antitrypsin gene are expressed in plasma, inheritance of even abnormal gene can cause health risks (for example, see Pi MZ below).
Brothers and sisters of deficient individuals have a 25% chance of also having the condition. Children of deficient individuals can be expected to be heterozygotes for the deficiency. These children have only a small risk of having severe Alpha-1, and this risk is present only if the partner of the deficient individual has one or more abnormal alpha1-antitrypsin genes. As explained below, heterozygotes have less severe,but definite, health risks. Phenotyping or genotyping is necessary to reliably detect heterozygotes, since alpha1-antitrypsin levels of normals and heterozygotes overlap to some extent. This is further explained in our description of "Phenotyping" and "Genotyping".
Most of the alpha1-antitrypsin in the body is synthesized in the liver and is secreted into the bloodstream to reach its target organs (particularly the lungs). Thus, in testing for the presence of the disease it is appropriate to measure the concentration of alpha1-antitrypsin in the blood. Further analyses may then be appropriate to extend the understanding of, and to confirm, any abnormalities that are found in the initial test. This is further explained in our description of The Testing Process. The complete testing process will identify any variant type(s) of alpha1-antitrypsin that are found.
Heterozygotes and individuals with several different unusual mutations usually do not have "normal" circulating levels of alpha1-antitrypsin. The clinical implications of these test results are less certain than the implications of severe Alpha-1 and may vary from patient to patient. This issue is discussed below, and in more detail in A Review for Physicians.
Alpha1-antitrypsin is an acute phase protein. This means that in times of stress, such as infections, other acute diseases, or after surgery, alpha1-antitrypsin levels in the blood increase. Oral contraceptive therapy also increases alpha1-antitrypsin levels in the blood. While this issue should be kept in mind, it is important to point out that individuals with severe Alpha-1 have only a minimal acute phase response of their alpha1-antitrypsin, and they never have normal alpha1-antitrypsin concentrations in the blood. Thus, the diagnosis of severe Alpha-1 is never masked by an acute phase response.
At Alpha1Center, alpha1-antitrypsin concentrations are expressed in micromoles per liter (µM). Our normal reference interval is 23.7 to 41.7 µM. Individuals with severe Alpha-1 have concentrations <11 µM, and individuals with mild or intermediate deficiency have concentrations between 11 and 20 µM. For a table of the laboratory reference ranges as related to phenotype see Laboratory Reference Ranges.
Some other laboratories express their results in units of milligrams per deciliter (mg/dL). To convert µM units to mg/dL, it is necessary to multiply by 5.2.
More than 500 different genetic variants of alpha1-antitrypsin are now recognized, though many of these are quite rare. The most common variants will be discussed below. The remainder of these rare mutations are beyond the scope of this review, but Alpha1Center can provide more information about specific variants upon request.
The normal M alleles: The normal M alleles represent by far the largest group of alpha1-antitrypsin alleles. They result in normal amounts, and therefore normal functionality of protein in the blood. The M1, M2, and M3 alleles differ only subtly from one another, and the differences are not clinically important.
Null variants: A variety of different null variants, such as deletions, transpositions, and creation of premature stop codons, have been described in the alpha1-antitrypsin gene. These mutations result in no alpha1-antitrypsin being secreted into the blood. These are not detected in the usual type of genotyping tests and are silent in protein phenotyping tests because null alleles do not direct synthesis of any protein that could be detected by isoelectric focusing. A very small number of Pi null null individuals, who have no alpha1-antitypsin in their blood, have been identified. These individuals are thought to be at very high risk for early development of lung disease. Much more commonly, null alleles are found in combination with other alpha1-antitrypsin alleles. In such individuals, null alleles are confounders for correct diagnosis of a patient's alpha1-antitrypsin status unless measurement of alpha1-antitrypsin concentration is the first test performed to alert the laboratory to the necessity for further testing. Null alleles are suggested by discrepancies between the measured alpha1-antitrypsin concentration and the concentration that would be expected for the patient's known phenotype and genotype. When a null variant is suggested by the initial testing procedures, Alpha1Center analyzes the sequence of the coding regions of the α1-antitrypsin DNA to identify and characterize the possible null variant. Without this powerful test, null alleles cannot be conclusively identified by laboratory testing alone.
The Z variant: The most prevalent type of severe Alpha-1 by far is classified as phenotype Pi Z. In these individuals, isoelectric focusing reveals only an abnormally migrating Pi Z type alpha1-antitrypsin. These individuals may be either Pi ZZ homozygotes or Pi Znull heterozygotes, since a null allele may be present that produces no alpha1-antitrypsin in the circulation. Genotyping is necessary to distinguish between these two possibilities, although family studies of the pattern of inheritance of low alpha1-antitrypsin levels may also be helpful. By protocol, Alpha1Center will always perform genotyping for samples showing this abnormality before we report the test results, in order to distinguish between Pi ZZ homozygotes and Pi Znull heterozygotes.
The Z variant has two amino acid substitutions when compared to the most prevalent normal type of alpha1-antitrypsin. It is subtly abnormal as an inhibitor of leukocyte elastase. However, the most striking abnormality in affected individuals is that circulating levels of the protein are only 10%-15% of normal. When livers of these individuals are examined, the hepatocytes contain an abnormal accumulation of alpha1-antitrypsin. The Z variant is synthesized normally, but is secreted to a much smaller extent and secreted abnormally slowly by both hepatocytes and monocytes. One of the amino acid substitutions (Glu342 to Lys342) results in misfolding of the alpha1-antitrypsin in the endoplasmic reticulum. The abnormal folding causes the alpha1-antitrypsin to aggregate by loop-sheet polymerization, leading to intracellular accumulation and subsequently intracellular degradation of the abnormal protein. Loop-sheet polymerization occurs when the reactive center loop of one molecule becomes inserted into an opening in the A sheet of another molecule.
Individuals with phenotype Pi Z, whether they are Pi ZZ homozygotes or Pi Znull heterozygotes, have a substantially increased risk of both lung and liver disease. These individuals should be counseled regarding the transmission of the Zvariant and/or the null mutation to future generations.
The S variant: The S variant has a single amino acid substitution (Glu264 to Val264) when compared with the most prevalent normal type of alpha1-antitrypsin. The S mutation is not associated with intracellular accumulation of the protein, and the S protein inhibition of elastase is only minimally abnormal. The amounts of the S protein that reach the circulation are slightly lower than normal, due to intracellular degradation of the alpha1-antitrypsin before it is secreted. The S allele is slightly more prevalent than the Z allele among US Caucasians, and it is much more prevalent in the Iberian Peninsula and neighboring countries.
As is the case for the Z variant discussed above, individuals with the Pi S phenotype may be either Pi SS homozygotes or Pi Snull heterozygotes. By protocol, Alpha1Center will always perform genotyping to make the important distinction between these two possibilities before reporting the test results.
Pi SS homozygotes disease have 2 S alleles. They have mildly reduced, and occasionally normal, levels of alpha1-antitrypsin in their plasma. There is no reason to suspect that they would have an increased risk of liver disease. Not enough cases have been studied to allow a determination of their risk for lung disease. Clinical judgment must apply to the management of these individuals. It is advisable to encourage testing of available family members.
Pi Snull heterozygotes deserve special attention, and are discussed below.
Pi Znull: Pi Znull individuals have one allele for the Z variant and one null allele. These individuals have severe Alpha-1, and their clinical manifestations are similar to those of Pi ZZ homozygotes. An important difference, however, is that they can pass on a potentially "hidden" null allele to future generations. Thus, it is important to identify null alleles when they are present. These individuals should be counseled regarding the transmission of the Zvariant and/or the null mutation to future generations.
Pi Snull: Pi Snull individuals have one allele for the S variant and one null allele. The alpha1-antitrypsin concentration in their blood averages about 1/3 of normal, and tends to be right on the borderline of the 11µM concentration that defines severe Alpha-1. Pi Snull heterozygotes should not have an increased risk of liver disease, and none has been reported. Relatively few of these individuals have been identified, and there has been no clinical research to evaluate whether or not Pi Snull heterozygotes have an increased risk of lung disease. However, because of the severity of the reduction of alpha1-antitrypsin concentration in their blood, these individuals deserve special consideration regarding their probable substantially increased risk of lung disease. These individuals should be counseled regarding the transmission of the S variant and null mutation to future generations.
Pi SZ: Pi SZ individuals have one allele for the S variant and one allele for the Z variant. They have alpha1-antitrypsin levels that range from approximately 1/3 to 1/2 of normal. Pi SZ heterozygotes are more common than Pi Z (severely deficient) individuals in American population. The livers of Pi SZ heterozygotes show mild accumulation of alpha1 antitrypsin. Liver disease risk has not been studied adequately, but it is probably similar or identical to that of the Pi MZ heterozygotes discussed below. A meta-analysis of clinical research studies has found that, when compared with Pi M controls, Pi SZ individuals have a significantly increased risk for the development of chronic obstructive pulmonary disease. These individuals should be counseled regarding the transmission of the S and Z variants to future generations.
Pi MZ: Pi MZ individuals have one normal allele and one allele for the Z variant. They usually have decreased levels of alpha1-antitrypsin in their circulation. However, they are capable of mounting an acute phase response, and their levels can fall within the normal range (particularly if they are ill or taking oral contraceptives). The livers of Pi MZ heterozygotes show mild intracellular accumulation of the protein. Clinical studies have shown a significantly increased risk of obstructive airways disease leading to a diagnosis of chronic obstructive pulmonary disease in Pi MZ individuals who are current or ex-smokers. An excess risk of lung disease in lifelong non-smoking Pi MZ individuals has not been proven. There is a minimally increased risk of clinically significant liver disease in Pi MZ heterozygotes. These individuals should be counseled about the risk of transmission of the Z variant to future generations.
Pi Mnull: Pi Mnull individuals have one normal allele and one null (non-expressing) allele. They usually have moderately decreased levels of alpha1-antitrypsin in their circulation. Relatively few of these individuals have been identified, and there has been no clinical research to evaluate whether or not Pi Mnull heterozygotes have an increased risk of lung disease. In the absence of specific clinical research, however, there is reason for concern about their risk of lung disease if they are current or ex-smokers. Because the alpha1-antitrypsin concentration in the plasma of Pi Mnull individuals is less, on average, than that for Pi MZ individuals, Pi Mnull individuals should be considered to be have at least as much risk as Pi MZ individuals for the development of obstructive airways disease leading to a diagnosis of chronic obstructive pulmonary disease. These individuals should be counseled about the risk of transmission of the null allele to future generations.
Pi MS: Pi MS individuals have one normal allele and one allele for the S variant. They have nearly normal, and occasionally normal, levels of alpha1-antitrypsin. Pi MS heterozygotes should not have an increased risk of liver disease, and none has been reported. In smokers, there is minimal, if any, excess risk of lung disease in Pi MS heterozygotes.