Osteotoxicology
An advocate for the study of the environment’s role in bone diseases, Edward Puzas chases his primary and persistent culprit, lead.
By Michael Wentzel
J. Edward Puzas, Ph.D.
In one way, it all started with a summer student who needed something to do.
The student was assigned to the lab of J. Edward Puzas, Ph.D. (MS ’73, PhD ’76), the Donald and Mary Clark Professor of Orthopaedics who instructed the student to apply low levels of lead acetate to osteoblasts in culture and describe the result.
“The lead had toxic effects and I thought: ‘I ought to look into this,’ ” Puzas recalled.
That “look” has translated into an extensive investigation of the impact of lead in bone and the role of lead in bone diseases, in particular, osteoporosis. For at least a decade, Puzas has been a leading voice—sometimes the lone voice—warning of lead’s toxic impact on bone cells and tying lead to osteoporosis.
Puzas now has taken a major step in his drive, as he says, “to crack the case on lead exposure.”
“Does exposure to lead predispose someone to osteoporosis? The answer is yes,” he said. “The lead burden in bone does affect the optimal functioning of bone cells. As many as 15 percent of the cases of osteoporosis could be lead induced. That’s millions of people. And now we think we understand the molecular mechanism that chronically keeps the bone mass low in people exposed to lead.”
Puzas accuses a naturally occurring protein known as sclerostin as lead’s co-conspirator in osteoporosis. Normally a potent inhibitor of bone formation, sclerostin plays a part in the body’s system of checks and balances on bone formation. But lead deposits in bone appear to boost sclerostin, disrupting a balanced system with harmful effect.
“We still have to make the case for lead and osteoporosis time after time,” said Puzas, a past president of the Orthopaedic Research Society and the United States Bone and Joint Decade. “The bone crowd has not jumped on board. Physicians don’t think to check bones for lead. They don’t factor in lead in their differential diagnosis. People in toxicology know and understand what we’re doing. We keep working at this. It can take a number of years to change a mindset.”
Few would challenge a statement that lead is bad for the brain, liver and kidney. Studies have shown lead can damage the nervous system, dull cognition and slow the development of children.
Ten years ago, in an article in the journal Current Opinion in Orthopaedics, Puzas officially presented his case that lead’s toxic effects in the skeleton should be added to the list:
“Our hypothesis is that lead adversely affects the function of growth plate chondrocytes, osteoblasts, and osteoclasts. The effects on osteoblasts are likely to be greater than on osteoclasts, which contributes to a small but chronic imbalance in skeletal remodeling. Taken together, the effects on growing bone and adult remodeling could predispose an exposed individual (most likely a female) to a greater chance of osteoporotic fractures . . . If we accept the notion that the deleterious effects of lead on bone cells are a continuum related to its concentration and that its effects are superimposed on an already described disease of aging, it might be difficult to identify the specific effect of lead on fractures. Nevertheless, considering the very large number of people affected by osteoporosis and its huge health care costs, a clearer picture of the role of lead in bone metabolism is warranted.”
The article only suggested where the research would take Puzas.
Linking lead, a gene and bone loss
In 2002, Puzas talked publicly about an increased risk of osteoporosis caused by exposure to lead for those in the baby boom generation, especially women. In 2003, he received a four-year, $9-million grant from the National Institute of Environmental Health Sciences to investigate whether lead could cause osteoporosis and also to determine if lead skewed the results of bone density tests.
The DXA scan is the major diagnostic tool used to measure bone density and diagnose osteoporosis. Puzas said lead makes a person’s bone mass look denser than it actually is. His findings estimated that a DXA scan overestimated density by 4 to 11 percent—enough to indicate a healthy diagnosis when, in reality, the person could have osteoporosis. The DXA scan has been reengineered in recent years so a lead-induced overestimate is not nearly as severe. But using other technology, Puzas has measured the amount of lead in bone and found that the higher the lead concentration, the higher the incidence of osteoporosis.
“The idea that a lot of lead concentrated in bones could give you an artificially high bone density got the NIH to take a good look at what we were doing,” Puzas said. “We all have known for years that the human skeleton is a repository for lead and that the lead had a long life in the bones. Most people thought the storage of lead in the bone was benign, but we were showing the opposite is true.”
A gene called SOST encodes for a protein known as sclerostin. A very potent inhibitor of bone formation, sclerostin represses osteoblast function and number and can introduce cellular pathways that cause cell death.
Mutations in the SOST gene lead to a serious disease known as sclerostosis that is characterized by thick bones, entrapment of peripheral nerves, deafness, nerve palsies and intracranial pressure.
Click on the image above to see the sclerostin diagram larger
“With the mutations, the gene is not effective and that stops the production of sclerostin,” Puzas said. “If the sclerostin is not there, you make a lot of bone. It takes the brakes off bone formation. This is a rare genetic disease but it points to the importance of the gene. We all have it so we don’t overproduce bone. Lead elevates or up-regulates sclerostin by tenfold or more, which dramatically down-regulates bone formation. Bone is always being eaten away and formed. Lead depresses the formation process. It gets thrown way out of whack. Resorption exceeds formation and you end up losing bone more rapidly—which is the case in osteoporosis. It looks to us that the lead effect is mediated through a regulation of this sclerostin gene.”
Puzas has plans for a series of experiments to further demonstrate the sclerostin connection and define the cellular pathways through which it works. A therapeutic agent that blocks the action of sclerostin is in testing and could be an effective treatment for lead-induced osteoporosis.
“By blocking the inhibitor, you stimulate bone formation,” Puzas said. “If lead is working through sclerostin, this agent would be a perfect treatment. But we have to know a lot more. The more we learn about pathways that control these cells, the more therapeutic options appear. If you can manipulate this process, even if there is no lead present, you can increase the bone formation rate. If it is the mechanism through which lead decreases bone formation rate, then that is your therapeutic target. The key to this is linking a pervasive toxin to a serious bone disease. That’s not been done before and that’s why I’m still working on this.”
Seeking a familiar villain
With a rich history of research, the Department of Orthopaedics routinely ranks as one of the top departments in funding in the county.
“The department has been somewhat ahead of the translational wave. Our basic scientists always aimed at a clinical problem or pathology,” said Puzas, who also is the School of Medicine and Dentistry’s senior associate dean for basic research.
“Whenever we do experiments, we are always thinking about how to take what we find and apply it to cure a problem in humans,” he said. “We are attacking the community’s problem of lead exposure at a very fundamental cellular level. But our goal is to take this work from test tubes and make it something of value to the health of people in the community. That is what translational science is and the Department of Orthopaedics has done that a lot. We always have tried to take what basic science we have and apply it to a clinician’s problem. We’re not just cloning another gene or checking enzymes for the sake of checking enzymes. Actually, the whole School of Medicine has that approach, much more so than some other schools.”
Enthusiasm for research from the department’s clinical faculty has taken off in recent years, Puzas said. Four years ago, the department had three projects that required approval by the Institutional Review Board. It now has more than 130.
Puzas himself is involved in research on teriparatide, or Forteo, a drug that can boost the body’s bone stem cell production to the point that an elderly person’s bones appear to have the ability to heal at the rate typically seen in younger adults. Patients, who had been confined to wheelchairs and who took the drug, were able to walk again because their broken bones healed.
He also is investigating the fundamental cycle in which osteoclasts remove bone and osteoblasts restore the bone.
“When the osteoclasts take away the bone, a resorption laucuna or pit is created in the bone,” Puzas said. “At some later time, osteoblasts find the pit and settle on the surface and fill in the pit. Osteoporosis occurs when cells don’t quite fill in the pit and you end up with a deficit of bone. Millions of pits are being formed over decades of time. When bone is replaced, it is replaced on the lacuna at the surface. That told me that something unique called the osteoblasts to this area of bone. We think we have found the signal. It is a molecule that osteoclasts leave behind. The osteoblasts use it to find their way. They only make bone where this molecule is. This is a unique discovery. Through this mechanism, the skeleton tells the cells where to make bone.
“If I could take that signal and use it to tell the cells that I want them to make bone around an implant—for example, if I want to anchor that implant solidly in bone as in a hip replacement—I can use the same natural signal that the body uses on the implant. I would have a tremendous way to fix that implant in place. There is a lot of basic science but I can see some therapeutic uses too.”
Puzas also works with the Department of Environmental Medicine on research projects, part of an effort to create a field he calls “osteotoxicology.”
“There are just a handful of people in the country who deal with the toxicology of bones,” he said. “It is critically important that we study how the environment affects all of our tissues, but bone has to be included.”
One project focuses on the effects of cigarette smoke on bone.
“The failure rate in healing fractures and spine fusions is really unacceptable. Why is that?” Puzas said. “We have been looking at a receptor called AHR, the aryl hydrocarbon receptor, which binds polycyclic aromatic hydrocarbons, the dangerous toxins in cigarette smoke. This receptor translocates to the nucleus where it does all the bad things to bone. This receptor is a key mediator of the toxic effects of the smoke and hydrocarbons. This research looks promising. It is another example of where researchers in the field of skeletal biology have joined forces with researchers in toxicology.”
Puzas also has expanded his work on osteoporosis in projects with toxicology researchers. In this case, the problem is arthritis and once again his suspected culprit is lead.
“Lead was legislated out of our environment in the 1970s but it is still here in many places. It does not degrade,” Puzas said. “In cities like Rochester, in the older or poorer neighborhoods, we see a lot of lead in the homes. Mexico still has leaded gasoline, as do some places in Europe. People are still being exposed to lead. Lead is still settling in the bone.”
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