The average human kidney faces daunting duty, filtering more than 70 liters of fluid each day through internal structures called glomerular tufts. That filtering raises the local concentration of protease enzymes that can slowly digest a critical component of the tufts -- the glomerular basement membrane -- requiring that the membrane be highly resistant to the action of these enzymes. In most people, the membrane is tough enough to stand up to these conditions -- but not so in people with Alport syndrome, in whom the integrity of membrane is gradually lost.
The scientists discovered that, in these people, a genetic mutation known to be associated with the disease prevents a vital transformation process from occuring in two foundational collagen proteins. In healthy individuals, the two collagen proteins that form the fetal glomerular basement membranes switch during normal development to become three different -- and much stronger -- collagens that are then used to build adult kidney membranes. In people with Alport syndrome, however, this developmental transformation cannot occur. And, over time, protease enzymes break down the two early-stage collagens left behind in the disrupted process, thereby destroying the critical membrane.
"It's as though these people inherited a bad set of tires," explains Eric G. Neilson, MD, C. Mahlon Kline Professor of Medicine and senior author on the study. "The collagen proteins in their kidney membranes wear out sooner than in normal people."
The rugged conditions within the kidney are probably the reason for the switch-over between sets of collagens during development, according to the investigators.
"The environment in which filtration occurs in the kidney is one in which there are many protease enzymes present," Neilson says. "It's our hypothesis that evolution provided several new collagen proteins to protect the kidney from wearing out too quickly."
Alport syndrome affects about 1 in 10,000 children born in the United States, although its frequency in some regions is higher. In Utah, for example, the rate climbs to 1 in 5,000 births.
The five proteins analyzed in the study, all members of a group known as Type IV collagens, are referred to as alpha-1, -2, -3, -4, and -5 collagens. Alpha-1 and -2 collagens are abundant in many parts of the body in addition to the fetal kidney, while the more specialized alpha-3, -4, and -5 collagens that appear in the glomerular filtering unit of the adult kidney are less common.
The mutation known to be responsible for about 80 percent of Alport syndrome cases disables production of the alpha-5 collagen; the affected gene, COL4-alpha-5, is located on the X chromosome, so that it affects many more males than females and is much more severe in males. In addition to its appearance in the kidney, alpha-5 is also present in the ear, the eye, and the skin, and individuals with the mutant gene are often deaf.
Interestingly, even though the genetic flaw affects only one of the three collagens involved in construction of the glomerular basement membrane of the adult kidney, all three are absent in the kidney membranes of people with Alport syndrome. Although the reason for this is not known, Neilson notes that alpha-3, -4, and -5 collagen chains are heavily cross-linked in the normal membrane, which is one of the reasons for the membrane's strength and ability to resist enzyme digestion. The absence of all three when only one is genetically flawed might be the result of a related transcriptional or self-assembly problem, Neilson suggests.
"These chains probably assemble in a coordinated way," Neilson says. "It may be that certain chains need to pair in certain ways, so that if they don't have the appropriate partner, they never get used."
The lead author on the paper is Penn research associate and instructor Raghuram Kalluri, PhD. Coauthors on the study are Charles F. Shield III, MD, at St. Francis Regional Medical Center, Wichita, Kansas; and Parvin Todd, PhD, and Billy G. Hudson, PhD, at the University of Kansas Medical Center, Kansas City. The work was supported by a series of grants from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) at the National Institutes of Health (NIH).
The University of Pennsylvania Medical Center's sponsored research ranks fifth in the United States, based on grant support from the National Institutes of Health, the primary funder of biomedical research in the nation -- $149 million in federal fiscal year 1996. In addition, for the second consecutive year, the institution posted the highest growth rate in its research activity -- 9.1 percent -- of the top ten U.S. academic medical centers during the same period.
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