By David N. Leff

One day in the late 1960s, a sharp-eyed worker at a major mouse-breeding facility noticed that certain members of an inbred colony had ears and tails that were strikingly lighter in color than those of their littermates.

The spontaneous mutation that gave these animals such pale ears and caudal appendages also affected more systemic and vital parts of their anatomy. The mutants were dubbed ep mice, denoting pale ears. Only now are they recognized as fairly faithful animal models of human Hermansky-Pudlak syndrome.

A decade earlier in Czechoslovakia, two Czech physicians, Hermansky and Pudlak by name, reported in the journal Blood (1959: 14:162-169) on "albinism associated with hemorrhagic diathesis and unusual pigmented reticular cells in the bone marrow: Report of two cases with histochemical studies." Throughout their lives, both had suffered prolonged bleeding episodes following tooth extraction and bruises.

The two, a man and a woman, both 33 years of age, became the first two victims of Hermansky-Pudlak syndrome (HPS) to be described in the medical literature. That number has by now grown to 50 plus much larger populations in various parts of the world.

The most prevalent, and most studied, individuals with this rare but significant inherited malady occur in Puerto Rico, where the incidence of HPS is 1 in 1,800. The island's population is close to 4 million, plus 2.7 million on the mainland U.S.

Given its autosomal, recessive inheritance pattern, the statistics suggest that one in 21 members of that population is a carrier for the disease, which occurs when both parents carry the HPS gene.

That gene, on the long arm of human chromosome 10, and the largely homologous ep mouse gene have at last been identified, as reported in today's Proceedings of the National Academy of Sciences (PNAS), dated Aug. 19, 1997. The paper bears the title: "The mouse pale ear (ep) mutation is the homologue of human Hermansky-Pudlak syndrome." Its lead author is molecular biologist John Gardener, whose research at the Fox Chase Cancer Center, in Philadelphia, focuses on mouse genetics.

Genomically, the mouse and human genes are 83 percent identical. The proteins they encode share 89 percent similarity.

Three Clues To The Syndrome

HPS packs a pathological triple whammy, which distinguishes it from other diseases of washed-out pigmentation. In fact, the triad of symptoms serve as diagnostic markers of the syndrome:

Accompanying the colorless or pink irises of the eyeball, marking the absence of pigmentary melanosomes, are diminished visual acuity and spasmodic fluttering movements (nystagmus) of the eyes, along with strabismus (squinting).

Far more serious is the depletion of so-called dense bodies in blood platelets, a bleeding disorder that superficially resembles hemophilia.

The third leg of the HPS triad is the build-up of a waxy substance called ceroid in the lysosomes of lungs, kidneys and colon. The main cause of post-neonatal death in HPS is pulmonary fibrosis, caused by this ceroid accumulation.

All three of these abnormalities, Gardener pointed out, have in common defects in intracellular organelles--melanosomes in the eye, dense bodies in platelets, lysosomes in ceroid glut.

"Dense bodies," he explained, "are intracellular organelles inside the platelet. They contain certain molecules that are important in signaling and in the clotting cascade. Platelets are a key factor in the cascade, and without the dense bodies they don't function properly. The platelets are filled with these small organelles, which, when they're released promote clotting and aggregation of platelets. Without them, poor clotting and a tendency to bleeding result.

"Therefore," he pointed out, "the patients bruise easily, have frequent nose bleeds, and when they cut, they tend to bleed longer.

"The interesting thing about this syndrome," Gardener told BioWorld Today, "is that it seems to affect several different cell types. Platelets are generated from megakaryocytes in bone marrow mainly. They are multinucleated cells that bud off platelets from their periphery.

"So the HPS gene is expressed in megakaryocytes, because platelets are obviously affected by defects in this gene. It also suggests that this gene functions in the genesis of lysosomes, which target ceroid.

"This HPS/ep mouse/human gene," Gardener went on, "is one of the first of this class of genes that has been cloned. Right now, we're very interested in its cell biology: How does this gene function in the macromolecular biogenesis of these cellular organelles--melanosomes, platelet dense bodies and lysosomes. That's the $64,000 question."

Closing In On Intracellular Organelles

Seeking answers, he and his co-authors "are busy making antibodies, and trying to figure out ways to investigate the function of this gene, and its resulting protein, in the cells that are affected. That way, we hope to gain clues into the biogenesis of these specialized organelles."

The Fox Chase team also hopes to gain other clues:

"Now that we have the gene cloned," Gardener observed, it will be possible to do several things:

"First, genetic screening of those populations that are highly affected, and identify those individuals that are carrying the mutation--the Puerto Rican population in particular. For example, if an individual is a member of a pedigree or a family with a history of HPS, and they're interested in whether they are at a carrier state, we'll be able to determine that now by a genetic screening test.

"Prenatal diagnosis would be a future option, though I certainly wouldn't say that at this stage we're ready to offer that," Gardener said.

"We're a long way away from being able to suggest a therapy," he added.

He cited "one more interesting tidbit that we did pick out from the molecular biology of the protein. There's a structural motif present in the predicted protein sequence, called the di-leucine repeat motif.

"And we know for a fact that the Puerto Rican mutation results in the disruption of that motif. Its discovery is probably the most important biological result to come out of our work.

"It will likely have important implications, either for production or for understanding the biosynthetic mechanisms involved in lysosome or melanosomal proteins. And those proteins," Gardener concluded, "are going to be an area of intense biotechnology interest, I believe, in a couple of years." *