Lung Function and Death from Chronic Obstructive Pulmonary Disease in ₁-Antitrypsin MZ Heterozygotes

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The summary below is from the full report titled "Change in Lung Function and Morbidity from Chronic Obstructive Pulmonary Disease in ₁-Antitrypsin MZ Heterozygotes: A Longitudinal Study of the General Population." It is in the 19 February 2002 issue of Annals of Internal Medicine (volume 136, pages 270-279). The authors are M Dahl, A Tybjærg-Hansen, P Lange, J Vestbo, BG Nordestgaard.

What is the problem and what is known about it so far?

Chronic obstructive pulmonary disease (COPD) is a type of lung disease that is usually caused by cigarette smoking. The lungs of people with this disease are progressively destroyed by the irritation caused by cigarette smoke and by infection (bronchitis) that often accompanies smoking. Normal people can usually limit the destructive effects of this irritation because the body manufactures a protein that calms the inflammation caused by irritation from cigarette smoke. This protein (₁-antitrypsin) is produced because of the genes that we inherit from our parents. Each parent contributes one copy of the gene to make ₁-antitrypsin. Very few people have a total lack of ₁-antitrypsin (called ₁-antitrypsin deficiency)—meaning that both parents have contributed a defective gene. The totally deficient person is known as a homozygote and is very likely to develop COPD and die at an early age. Many more people have inherited the defective gene from only one parent (heterozygotes). Physicians have been uncertain whether heterozygotes are more likely to develop COPD than are other persons in the normal population.

Why did the researchers do this particular study?

To compare heterozygotes with normal people to find out if they had more COPD, more hospitalizations for COPD, and more deaths from COPD than those who have two copies of the normal gene for ₁-antitrypsin.

Who was studied?

9187 people, aged 20 years or older, who were part of the Copenhagen City Heart Study in Denmark.

How was the study done?

Exams were done three times (1976–1978, 1981–1983, and 1991–1994). Participants completed a questionnaire about smoking habits, underwent breathing tests, and had blood drawn during the third exam to see whether they had one copy of the normal ₁-antitrypsin gene or two copies.

What did the researchers find?

451 participants (4.9%) had only one copy of the gene for ₁-antitrypsin (heterozygotes), and they produced 31% less ₁-antitrypsin than normal individuals. Heterozygotes had a slightly faster decline in lung function than genetically normal people. Nineteen percent of heterozygotes had COPD, while 15% of genetically normal participants had COPD. Hospitalization or death from COPD occurred 1.5 times more frequently in heterozygotes than in those with two copies of the normal gene.

What were the limitations of the study?

Approximately one third of the participants had only one breathing test, and their data could not be used to calculate the rate of decline in lung function over time. Blood to test genetic makeup was drawn only during the third examination; because people with more severe lung disease may have dropped out of the study before the third examination, the researchers may not have recorded the genetic makeup of many of these participants.

What are the implications of the study?

People with only one copy of the gene for ₁-antitrypsin have an increased risk for developing COPD and are more likely to die of COPD than genetically normal people. Because heterozygotes are much more numerous than are homozygotes, the number of heterozygotes with COPD is about the same as the number of homozygotes.