Michael Jackson, the moonwalking pop star whose health problems have often shared the spotlight with him, is reportedly wracked with severe emphysema and potentially deadly internal bleeding.

According to Ian Halperin, an investigative journalist who is writing an unauthorized biography of the singer, Jackson, 50, has been fighting the genetically inherited disorder alpha-1 antitrypsin deficiency for several years.

Last year he was seen in a wheelchair near his home in Las Vegas. And earlier this month, photographers snapped a shot of him outside a doctor's office with his face hidden beneath a mask and a fedora.

If Jackson has alpha-1 antitrypsin deficiency, it means he cannot protect his lungs from his body's own defenses against bacteria. The disease eventually leads to difficulty breathing, and some forms of the illness can affect the liver and skin. Halperin told the British newspaper Sunday Express that Jackson needs a lung transplant to survive but that he may be too physically frail to endure such an operation. The biographer did not describe his sources in the article but said that the singer can barely speak and has lost 95 percent of the vision in his left eye.

Jackson's public health troubles began soon after the release of his chart-topping album Thriller in the early 1980s when the 5' 11" singer reportedly weighed just 105 pounds, and some speculated that he was suffering from anorexia. He was later diagnosed with vitiligo—which results in a loss of skin pigmentation—and the potentially lethal autoimmune disease lupus in which the body's immune system gets out of whack and attacks healthy tissue. In 2005, when he was tried and cleared in California of child molestation charges, he became dependent on morphine and the painkiller Demerol, according to his attorneys.

The star's older brother Jermaine told Fox News that the singer is not doing well, but on Monday a Jackson rep issued a statement dismissing the alpha-1 antitrypsin deficiency reports as "total fabrication."

"Mr. Jackson is in fine health," the statement said, "and finalizing negotiations with a major entertainment company and television network for both a world tour and a series of specials and appearances."

To find out more about the condition, we spoke with James Stoller, a pulmonary critical care doctor at the Cleveland Clinic who has studied alpha-1 antitrypsin deficiency for more than 20 years.

An edited transcript of the interview follows.

What is alpha-1 antitrypsin deficiency?

It is a genetic defect in the production of a protective protein called alpha-1 antitrypsin, which is made in the liver and circulates in the bloodstream. This protein primarily protects the lungs against an enzyme known as neutrophil elastase, which our body uses to break down bacterial cell walls, but it also has the collateral damage of breaking down elastin, the support protein of the lung. This results in the development of emphysema. Some individuals with a genetic disposition for the disease go through life and never develop emphysema; others develop early-onset severe emphysema in their 40s and 50s. The disease is exacerbated by smoking and exposure to other noxious inhaled stimuli that lead to inflammation of the lungs.

How common is it, and how is it diagnosed?

It is very much under-recognized. The best estimate in the U.S. is there are probably about 100,000 severely affected Americans. If one looks at carriers of the disease, that probably affects 3 percent of Americans. It's quite common—one of the most common genetic variants in the U. S.

It can be diagnosed by checking blood at birth, but that has not been the usual practice here. It is usually diagnosed because one presents manifestations of the disease, most commonly emphysema and liver disease, or because one has a family member who is affected.

If confirmed, does Michael Jackson's case sound serious?

It's hard to know. I'm not aware of any association with eye disease, at least any direct link. The gastrointestinal bleeding may be unrelated to alpha-1 antitrypsin deficiency, but to the extent that the deficiency is associated with liver and lung disease, one could develop stomach bleeding.

If he needs a lung transplant on the basis of alpha-1 antitrypsin deficiency, it would bespeak of a fairly advanced degree of emphysema. Such transplants are regrettably not very rare. Of all lung transplants listed in the database of the International Society for Heart and Lung Transplantation, 8 to 11 percent are performed on the basis of emphysema for alpha-1 antitrypsin deficiency.

What type of medications would Jackson have been taking to treat it?

The treatment of emphysema related to alpha-1 includes all the usual medications: medicines to open up the airways, broncodilators; preventive strategies like influenza vaccine, pneumonia vaccine; occasionally oxygen when the individual's oxygen level is specifically low enough to justify using oxygen pulmonary rehab; sometimes inhaled cortical steroids [to reduce inflammation]. There are also some specific therapies for alpha-1 antitrypsin deficiency, including so-called augmentation therapy. This involves the weekly or monthly intravenous infusion of purified human alpha-1 antitrypsin, which causes the levels in the blood to rise above the protective threshold. The best available studies suggest that this medication can slow the rate of decline of lung function.