Pygmy paradox prompts a short answer.

Pygmy paradox prompts a short answer

African Pygmies and some other diminutive individuals may be short on specialized cell receptors that interact with a hormone essential for normal growth, two new studies suggest. The reports offer tantalizing clues to the cause of this type of dwarfism. If confirmed, they may point the way to new forms of treatment for certain kinds of very short stature.

African Pygmies have long puzzled scientists studying growth disorders because these people have normal amounts of growth hormone but rarely grow taller than 4 feet 10 inches. U.S. physicians see a similar disorder in children who have normal growth-hormone levels yet lag far behind their peers in height. Two separate research teams now present evidence hinting that the trouble lies with the receptor that provides a docking site for human growth hormone circulating in the bloodstream.

Gerhard Baumann and Melissa A. Shaw of the Northwestern University Medical School in Chicago and Thomas J. Merimee of the University of Florida in Gainesville studied blood samples from 20 Pygmies living in Zaire's Ituri Forest. Because it is difficult to count the cell receptors directly, Baumann's group developed a method that detects complex molecules consisting of growth hormone attached to a binding protein, which is probably a fragment of a cell receptor that has broken off during the docking process, Merimee says. The research team found that Pygmies, compared with controls, have half the amount of this protein-hormone complex in their blood plasma. Control subjects in the study, reported in the June 29 NEW ENGLAND JOURNAL OF MEDICINE, were five non-Pygmy black Africans and seven U.S. whites, all of normal height.

The low levels of the protein-hormone complex suggest Pygmies have a shortage of growth-hormone receptors, Merimee says. If so, Pygmies would have trouble responding to their normal levels of growth hormone because many of the molecules couldn't find docking sites on the cell. That theory fits well with the observation that Pygmies grow slowly during childhood and show seriously stunted growth as teenagers, when a full supply of receptors is most crucial. Scientists have speculated that teenagers normally produce extra growth-hormone receptors during this time of rapid growth.

Some U.S. children appear to have the same disorder, scientists reported June 23 at the annual meeting of the Endocrine Society in Seattle. Jane L. Lynch and James R. Gavin III of the University of Oklahoma Health Sciences Center in Oklahoma City studied 26 white children who were more than two standard deviations shorter than their peers. A subset of these children had normal growth-hormone levels but showed "striking" deficits in the protein-hormone complex that Baumann's team measured, Gavin says. Like their African counterparts, such children reach adult heights well under 5 feet. These preliminary findings suggest the children have a receptor deficiency that prevents the growth hormone from doing its job, the Oklahoma researchers conclude.

Scientists are just beginning to unravel the complicated defects involved in such disorders. They have not ruled out the possibility that affected individuals have normal numbers of receptors that are flawed. In either case, both studies suggest that the cause of the growth disorder lies in the genes. Circumstantial evidence points to a genetic defect causing faulty production of growth-hormone receptors, Baumann says.

Gavin thinks the new findings should prompt researchers to pursue new therapies aimed at bypassing receptor problems. For example, scientists are beginning to consider clinical trials that would give patients a growth-promoting substance normally formed in cells after growth hormone interacts with the receptor.