An unborn infant who has inherited osteogenesis imperfectacongenita is likely to come into the world with numerous skeletalfractures, including skull as well as limbs. This severe brittle bonedisease is rare but can be rapidly lethal.

An estimated 10 thousand to 20 thousand children who have survivedthe bone-violating traumas of gestation and childbirth face frequentfractures of their fragile bones from trivial stresses that can add up toextreme crippling and death, with no available therapy.

Osteopathic surgeons can now replace local defects of bone andcartilage (usually sports-inflicted) by injecting stromal (connective orframework) cells into joints or bones. (See BioWorld Today, Oct. 6,1994, p. 1.) But the idea of correcting the whole-body skeletal labilityof brittle bone disease remains visionary.

Now a report in today's Proceedings of the National Academy ofSciences has switched on a light at the end of this tunnel. The article,which previews potential therapy for osteogenesis imperfecta, andother bone and muscle diseases, is titled: "Cultured adherent cellsfrom marrow can serve as long-lasting precursor cells for bone,cartilage and lung in irradiated mice."

Its senior author is Darwin Prockop, who heads biochemistry andmolecular biology at Thomas Jefferson University, in Philadelphia,and directs Jefferson's Institute of Molecular Medicine. Prockop isalso on the scientific advisory boards of Genetic Therapy Inc., ofGaithersburg, Md., and of FibroGen Inc., of Sunnyvale, Calif.

If he can overcome a number of outstanding "ifs" in the design andapproval of a multicenter human trial, Prockop hopes to move frommice to brittle-bone children "in less than a year."

Plastic-Hugging Cells Firm Up Fragile Bones

His point of departure is to focus on precursor cells in the bonemarrow that give rise to bone and cartilage cells in the body. "There'sbeen evidence for 20 years," Prockop told BioWorld Today, "that ifyou put bone marrow in a culture dish, less than 1 percent of its cellswill adhere to the plastic of the plate. Nobody knows why, but theystick very tightly."

What's more, he explained, "those cells that stick are nothomogeneous, but a mixture of cells. After resting for two or three orfour days, they suddenly begin to divide very rapidly. Then, if youchange the culture conditions, they will begin to make small amountsof tissue that look like colonies of bone and cartilage."

These are the raw materials that the surgeons inject into traumatizedskeletal sites to reform bones or joints. Prockop and his co-authorsinjected the adherent cells into the bloodstream of transgenic mice,which deposited new cells skeleton-wide.

Not all at once, though. "We did the experiment," Prockop said,"because we had a tag in the form of a transgene in our transgenicmice." This marker consisted of the human gene for a type ofcollagen that causes brittle bone disease in mice. So we took adherentcells from these transgenic donor mice, grew them in culture, andinjected them into an irradiated recipient mouse.

"For the first week or so," he continued, "we couldn't find theinjected precursor cells. Their number was so small that we couldn'tsee them, even with very sensitive PCR amplification assays."

"At five months, we found that our injection had replaced 3 to 10percent of the cells in a number of the recipient's tissues. Not onlymarrow and spleen, which we expected, but to our surprise, in boneand cartilage."

Whether in mouse or person, a bone-marrow transplant _ usuallydone to treat cancer _ requires irradiating the recipient's body, orinjecting a cytotoxic compound, to clear out space for the incomingdonor cells. But Prockop isn't at all sure that this drastic kill-all-cellsprep is necessary for his procedure, which doesn't involvemalignancy.

"Two recent reports," he observed, "say you may not need that X-rayradiation. What they're saying, simply, is that if you inject 100 timesmore cells than are normally done in mice or dogs, you don't needthat marrow ablation." He noted that these findings "are not widelyaccepted in the field yet, because people have not had the chance toreproduce the experiments."

He points out the "broad implications for future application of whatwe have observed: If you don't need to do the marrow ablation, thenobviously you can treat people with diseases that aren't life-threatening."

He and his team are now "moving to develop a protocol to helpchildren with brittle bone disease. "Our first clinical trials," he said,"would be cellular treatment rather than gene therapy. The ideawould be to find a matched donor, grow his or her adherent cells inculture, and inject them in a child with severe osteogenesisimperfecta. We would probably have to subject the recipient patientto X-ray or chemotherapy to make room for the new precursor cells."

Prockop's hope is to "replace something like 30 percent of the cellsin that child, and thereby strengthen the bone."

His long-range ideal is "to take cells from the patient rather than adonor, fix the defective collagen gene, then put it back." Even furtherdown the pike, he foresees "moving on to osteoporosis andosteoarthritis," which affect millions.

Prockop concluded: "We're setting up collaborations with leadingcenters for bone marrow transplantation in the world, and they arevery optimistic, based on our data." The leading center in the world,he said, is St. Jude's Children's Research Hospital in Memphis,Tenn.

Its head of bone marrow transplantation is pediatric immunologistMalcolm Brenner. He told BioWorld, "When we heard of Prockop'swork, we were very excited because it was direct confirmation thatmarrow indeed was a source of mesenchymal [primitive] as well ashemopoietic stem cells, potentially for cartilage, muscle and so on.So marrow transplantation could be extended to treat a whole newrange of conditions."

Brenner added, "We hope that we'll be collaborating with histreatment of patients, institutional review board permitting."

Eventually, Brenner foresees the approach working on treatment ofmuscle diseases, such as Duchenne's muscular dystrophy. "But tobegin with," he said, "it will open up the treatment of hereditarydiseases of bone and cartilage."

He also envisages perhaps replacing irradiation or chemical ablationof bone marrow with monoclonal antibodies directed at progenitorcells. n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.