CDU

The congenital Marfan syndrome afflicts some 200,000 people in the U.S. Worldwide, the incidence is one in 10,000, or about 700,000 of the planet's population. That syndrome, said molecular geneticist Francisco Ramirez, chief scientific officer at the Hospital for Special Surgery (New York), "is an autosomal dominant disorder of connective tissue caused by mutations in fibrillin-1 protein (encoded by FBN1 in humans and Fbn1 in mice) a matrix component of extracellular microfibrils."

Ramirez, one of three senior authors of an article in the March 2003 issue of Nature Genetics, said a subgroup of people with Marfan syndrome "have emphysema-like distal airspace enlargement, which may result in spontaneous lung rupture (pneumothorax)." To investigate the pathogenesis of genetically imposed emphysema, he said he and the other researchers involved in the project analyzed the lung phenotype of mice deficient in fibrillin-1.

"The major finding in this article," Ramirez said, "is that the loss of the fibrillin protein not only affects the integrity of the body's connective tissue but also alters the signaling events that are required for the normal formation and function of the connective tissue." That is a new concept, he said, "because if you look at the dissecting aortic aneurysm in Marfan, you will only impute that defect to a weakness of the connective tissue. But in reality, the loss of function of fibrillin also triggers other events, which are cellular in nature, and not mechanical like an aneurysm."

Ramirez said that this finding "opens up opportunities for therapies that are directed not only to reinforcing the infrastructure of the tissue but in blocking the secondary events associated with the malfunction of the cells." He said a crucial development in recent years has been the characterization through mouse models of "novel antigenic sequellae in Marfan syndrome, which we have demonstrated also occur in humans."

Currently, people diagnosed with Marfan syndrome are followed closely by physicians because of their high risk of life-threatening vascular complications. Treatments lower blood pressure or otherwise protect the integrity of the aorta, which carries blood away from the heart, but there is no cure other than surgery. "Once Marfan is diagnosed," Ramirez said, "the medical course of the disease is for the surgeon to monitor the progress of the aneurysm, and once it reaches a critical size, to intervene surgically." So, he said, "90% of the patients undergo surgery."

What would be nice to get out of such scientific investigation, Ramirez said, are "smart new ways to delay the aneurysm, because we do recognize there are similar components that participate in and exacerbate Marfan's progression." He said that the mouse model developed in the mid-1990s "is a great advance, because now we can try these therapies first in mice and see whether or not we can delay or correct the pathology."

In addition to the aorta, Ramirez and his co-authors reported in Cell Genetics that the lung is another affected organ. "The lung was always thought to be an emphysema-type phenotype," he said, "associated with inflammation." But as the researchers demonstrated, "the defect is a developmental effect, which occurs without inflammation." He said it "comes early on in gestation during the development of the lung. That is why the indications there are morphogenetic signals, which are regulated by the extracellular matrix." Ramirez added: "You end up with a form that looks like emphysema, which is destruction of the lung surface of Marfan syndrome pathology."

He said the researchers' first in vivo experiment "was to eliminate the fibrillin gene from the mouse genome." Believing that the animals' lung abnormality was due to a TGF-beta mis-activation, they injected neutralizing antibodies against the TGF-beta into late-gestation pregnant mice "so that the embryo had this cytokine knocked out throughout the end of the pregnancy." And, Ramirez added, "when we repeated this, we rescued the lung abnormality by what amounted to embryogenic therapy. Blocking TGF-beta just before birth with antibodies prevented lung problems in the offspring."

He said that "cytokines, of which TGF-beta is a typical example, are signals the cells use to communicate from one tissue to another. The signals are transported throughout the extracellular matrix, but they are also stored within that matrix, and remodel it for the use of the enzyme that the cells secrete."

Ramirez said his group's findings "show that the transport and sequestration of the signal in the matrix is not a passive process but is involved in the structural components such as the fibrillin." In the tissue of a knockout mouse that has no fibrillin, he said, the communications between cells are altered, and as a result the lung doesn't form properly.

He said the researchers' "ongoing idea" is to "nail down the connection between an abnormal cellular matrix and altered signaling. And then, particularly in the vascular system, what is important is to prove it in the aorta, because that is where people with Marfan syndrome die."