OPMD: Stopping a Long Protein from Shortening a Life

Article Highlights:
  • This article is part of a 2009 "In Focus" report on oculopharyngeal muscular dystrophy (OPMD).
  • The root cause of OPMD is a remarkably small increase in the length of the protein PABPN1.
  • The extra length of PABPN1 seems to cause it to fold improperly and clump up, preventing its function in cells.
  • Researchers are trying to understand how PABPN1 is implicated in OPMD; why certain muscles are affected and why the disease seems to affect mainly older people.
  • Promising OPMD research strategies include: helping cells resist cell death; breaking up clumps to free PABPN1; increasing the level of normal-length PABPN1; and destroying abnormal PABPN1 genetic instructions or protein molecules.
  • Major drug companies are researching strategies to deal with protein clumping in other diseases, which ultimately may benefit OPMD research.
by Margaret Wahl on October 1, 2009 - 3:52pm

QUEST Vol. 16, No. 4

Seattle resident Ken Lang (see “Like a Frog”) says he knew a disease that impairs swallowing and speaking was a possibility for him, because his father, uncle and grandmother had been affected by such a disorder. “I first became symptomatic when I was about 51 or 52,” says Lang, now 62, “although I didn’t get an official diagnosis until about four years ago.” Now a writer and a dispatcher for a transportation company, Lang says his throat muscle problems gradually increased until he couldn’t even swallow liquids and was having trouble speaking. 

Carol Forde (see “Not Glad”), in Strum, Wis., also has a family history of insidious and specific muscle weakness, although earlier generations didn’t have a diagnosis. “I’m sure it goes back farther than my maternal grandmother, but all we knew was that my grandmother had really, really droopy eyelids, to the point where she would have to sit there with her elbow on the table and use her fingertips to hold her eyelids open,” says the 53-year-old technical writer.  

The disease Lang and Forde are describing is oculopharyngeal muscular dystrophy (OPMD), a form of MD that runs in families and has a preference for weakening the muscles of the upper eyelid and the throat. 

Until recently, not much more than that was known. But that’s changing. 

A slightly longer protein

The underlying problem in OPMD is a remarkably small increase in the length of a protein known as PABPN1 (polyadenylate binding protein 1).  

The increase in length is caused by a correspondingly small increase in the length of the gene for PABPN1, in the form of extra repeated sequences of the chemicals guanine, cytosine, guanine, or GCG. These repeated sequences are known as “GCG triplet repeats.” Each GCG triplet forms the instructions for a molecule of the amino acid alanine, a protein component. When there are extra GCG repeats, there are extra alanine molecules in the PABPN1 protein. 

Why a tiny increase in the length of a protein should wreak such havoc in cells, affect muscle fibers in particular, and affect certain muscles more than others, is largely unknown, and several hypotheses are being tested. 

Clumps in the nucleus

Normally, the PABPN1 gene, located on chromosome 14, contains six GCG repeats. Two PABPN1 genes are inherited by each person, one from the mother and one from the father. As few as eight, instead of six, GCG repeats, in one of the two genes, is enough to cause OPMD.

Illustration: Cell Nucleus
Evidence suggests that a normal function of the PABPN1 protein is to help attach a protective tail to pieces of RNA in cell nuclei. In OPMD, PABPN1 protein molecules that are slightly longer than normal, fold improperly, and form clumps. PABPN1, trapped in clumps, may fail to play its role in putting tails on RNA strands, leaving these strands vulnerable to destruction.
Illustration: OPMD-Affected Cell Nucleus
In OPMD, PABPN1 protein molecules that are slightly longer than normal, fold improperly, and form clumps. PABPN1, trapped in clumps, may fail to play its role in putting tails on RNA strands, leaving these strands vulnerable to destruction.

The molecular nature of OPMD “bears a superficial resemblance” to other so-called “triplet repeat” genetic diseases, such as myotonic dystrophy, says University of Rochester (N.Y.) neurologist Charles Thornton. “But in fact, whether there is really any overlap between the two in terms of what goes wrong, is unclear.” 

Thornton co-directs the MDA clinic at the University of Rochester Medical Center, has had MDA support to study OPMD, and serves on MDA’s Medical Advisory Committee. “The size of the repeated sequence is much shorter than it is in other such diseases,” Thornton says, “and it doesn’t show changeability from one generation to the next, which is dramatic in some of these other diseases.” 

On the other hand, the very small increase in the length of the gene causes the slightly longer protein to behave in a “different fashion” from normal, Thornton notes. “You can see that it forms clumps in the cell nucleus. But whether those clumps are closely connected with why people have muscle problems is still unclear.” 

The abnormal clumps (also known as “aggregates” or “inclusions”) are in the nuclei of muscle cells that carry the OPMD genetic abnormality. The extra length of the PABPN1 protein seems to cause it to fold improperly and clump up. 

“The aggregates themselves could be toxic in some way,” says Anita Corbett, a molecular biologist at Emory University in Atlanta. However, she says, “as a full explanation for the disease, that’s losing favor.” 

In fact, some recent studies have suggested that the clumping may actually protect cells that contain expanded PABPN1 protein molecules, at least up to a point, although that remains controversial. 

A loss of PABPN1

Corbett says there’s evidence to support the theory that depletion of PABPN1, because it’s “tied up in the aggregates,” also is a factor in the disease. She notes that normal-size, as well as extra-long, PABPN1 molecules, get trapped in the clumps, and that other proteins are trapped there, too. 

A loss of PABPN1 function could have widespread effects on cells, Corbett says, because the protein appears to play a role in keeping pieces of genetic material stable in what can be a harsh cellular environment. 

Pinning the tail on the RNA

“Cells are not a good environment for RNA,” Corbett says. (RNA is the genetic compound that’s made from DNA and from which protein molecules are then produced.) Therefore, cells protect RNA by attaching a protective tail to it, which serves as a buffer. “The tail keeps the RNA molecules from being chewed up,” Corbett says.

PABPN1, it’s been found, is among the compounds that help put a tail on RNA. The tails get shorter without PABPN1, Corbett says, although they’re not completely absent.

PABPN1’s role in RNA tail length and maintenance is pretty clear, at least in experiments in flies and mice. PABPN1 may also help RNA molecules move out of the nucleus into the main part of each cell, a necessary step before RNA can be used for protein synthesis, Corbett says, although that’s less certain.

A preference for certain muscles

Grace Pavlath, also a molecular biologist at Emory, received MDA support from 2007 through June 2009 to collaborate with Corbett on a study of cellular mechanisms underlying OPMD. She also serves on MDA’s Scientific Advisory Committee. 

Pavlath is particularly interested in why the clumping or loss of function of PABPN1 leads specifically to effects on muscle, since PABPN1 is found in virtually all the body’s cells.  

“In our MDA-funded work, we’ve been examining whether loss of PABPN1 affects myogenesis [muscle making],” she says. “We’ve taken cells from mice and used genetic technologies to eliminate PABPN1 to see whether that affects myogenesis. We’ve found that if you eliminate some of the PABPN1 in muscle cells, it causes problems for the functioning of these cells. We don’t get rid of 100 percent of the PABPN1, but we significantly lower the level of PABPN1 that’s normally present, and we clearly have problems with muscle-specific function,” she says.  

Those experiments show that losing some of PABPN1’s function probably plays a role in OPMD, notes Pavlath. Clumps may be important, she says, “but we don’t think they’re the only thing that goes wrong in this disease.”  

As for why the muscles of the throat and eyelids are particularly affected in OPMD, Pavlath says, “There are a number of reasons why it may be those muscles. Embryologically, the head and neck muscles develop differently from others. They utilize different genetic pathways, for instance, and they have different physiologic properties from other muscles.  

“It’s possible that these particular muscles are more dependent on PABPN1 function than others and that muscle in general is more dependent on it than other tissues, such as the liver or skin.” 

Late onset

In addition to its selective effects on certain muscle groups, OPMD also selectively affects older people, posing yet another set of research questions. “Muscles age,” Pavlath says, “and various properties of muscles change with age. It’s not known if different muscles age at a different rate, but some muscles may age differently. We get wise with age, but it doesn’t do great things to our organs.” 

Troubling symptoms 

“I would say the most troublesome aspect of OPMD is the difficulty with swallowing,” says Thornton. 

Thornton routinely refers his OPMD patients to a speech and swallowing therapist (available through most MDA clinics) before referring them for surgery. He refers patients for surgery when swallowing becomes unsafe or very difficult. 

A close second in causing trouble for patients is probably difficulty with eyelid weakness, Thornton says. “At a point in time where the problem is severe enough to interfere with a person’s vision, or if people are unhappy with their appearance,” he refers them to an eyelid surgeon. “Most of our patients get this at some point,” he says. 

Another issue is fatigue. “Why it occurs I don’t understand very well,” Thornton says, “but fatigue is a big symptom with this disease. Muscles tire quickly with use, particularly the ones that show some weakness.” 

A part of OPMD that’s not universal but can be quite hard to deal with, Thornton notes, is limb weakness, particularly in the legs but sometimes in the arms. “It’s not a uniform aspect of the disease,” he says, “but many people have this. In some people, it can be aggressive — surprisingly so. We don’t understand why some people have this to a much greater extent than others.”  

As for activity and exercise, Thornton says, “We don’t know of information indicating that there’s much fragility of muscles in this condition, so that makes us less concerned about the possibility of doing damage to the muscles by working them hard. But even so, it’s sensible for people to tailor their activities and exercise for endurance, not power.”  

Genetic testing

DNA-based testing for OPMD using a blood sample has been available for several years. Your MDA clinic can steer you toward testing.

Next steps for scientists

Experiments in flies, worms, mice and cellular models of OPMD (all of which imperfectly mimic the human disease) have so far suggested some possible therapeutic avenues: helping cells resist a death spiral known as “apoptosis”; breaking up aggregates to free trapped proteins; raising the level of normal-length PABPN1 protein; and destroying abnormal PABPN1 genetic instructions or protein molecules. 

In 2005, experiments showed mice carrying mutated PABPN1 genes and demonstrating some of the signs of human OPMD responded to doxycycline, an antibiotic used to treat infections in humans. The researchers speculated that the drug could be exerting its effect by reducing clumping and by working against apoptosis. 

“There are some data showing that aggregates are the cause of the disease and that anti-aggregation therapies can help alleviate some of the effects,” Anita Corbett says. Whether the alleviation is the result of breaking up of the aggregates themselves or the release of necessary substances from their traps in these clumps, she says, isn’t known. 

Another idea, notes Grace Pavlath, is that breaking up aggregates may make cells “more resistant to death signals” in general. (On the down side, Pavlath cautions that anti-cell-death strategies have been known to cause malignancies.) 

Additional strategies also are being explored. If it becomes clear that loss of normal PABPN1 functions is central to OPMD, then increasing levels of this protein could be “potentially very important,” Pavlath says. She says one could envision various ways of doing that, such as putting in the protein itself or the gene for it, or by revving up protein production from the one normal copy of the gene that most patients have. (A few patients are unlucky enough to have inherited the genetic defect from both parents, and they tend to have severe OPMD.)

Targeting the extra repeats in the PABPN1 genetic instructions, Corbett notes, is yet another possible strategy, but a particularly difficult one. GCG repeats, in the PABPN1 gene and elsewhere, remain necessary to make alanine molecules, so targeting them would be problematic.

Getting rid of the abnormal PABPN1 protein itself, rather than the genetic instructions for it, could also be considered, she says, but the difference between the normal and the abnormal PABPN1 protein is small, so weapons of protein destruction are likely to affect both.

“No one has made a mouse that lacks PABPN1” so far, Corbett says. However, eliminating this protein in flies is lethal, leading her to think that PABPN1 is probably necessary.

Charles Thornton is equally cautious about targeting mutated PABPN1, either at the gene or protein level. OPMD patients have two copies of the PABPN1 gene, “one good and one bad. Getting at the bad without harming the good is a challenge.”

But, Thornton says, “the hopeful indication is evidence from several different research groups indicating that protein clumping, or aggregation, may be a fundamental part of what causes the malfunction in muscles. If this is true, that’s something that OPMD may share with other conditions, like Alzheimer’s disease, that are much more common and are the subject of huge efforts by the pharmaceutical industry. It might be possible to import strategies into OPMD from these other fields.”

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