On the Right Track - Dystrophin Research
In the nursery, an infant's eyes trace the circuit of a moth. Already the child's hands are grasping at things, and he has begun incessantly rehearsing the sounds that will eventually blossom into language. The youngster's progress appears on schedule. A chromosomal glitch, however, will soon make itself evident. This boy was born with a disease known as Duchenne Muscular Dystrophy (DMD). The root of this ailment is a defect in a complex human gene known as dystrophin, and is a focus of research for IIT's Nick Menhart.
The challenges of studying dystrophin, the largest human gene, are formidable. "This gene by itself is .1 percent of the total genetic material," Menhart explains. "So it's 300 times larger than your average gene." In healthy individuals, the dystrophin gene codes for a protein of the same name, one vitally important to muscle cells.
"Most cells just sit there," Menhart says. "They don't change shape." Muscle cells are different. "If you think of a piece of sheet metal bent back and forth, eventually it will break due to metal fatigue. This is what happens to muscle cells when they lack [the protein] dystrophin," Menhart adds.
Duchenne's is one of many so-called X-linked recessive gene diseases. Should a child inherit a defective X chromosome from his mother--one in which the dystrophin gene is damaged--his body will fail to produce the dystrophin protein, and the result is DMD. But DMD is a peculiar genetic disease in that about 50 percent of cases are not inherited. Rather, they are the result of new mutations specific to the individual. Because the gene is so large, thousands of underlying defects are possible, making genetic treatment especially vexing.
Duchenne is the most common form of muscular dystrophy, striking 1 in 3,500 boys. It is also one of the most pitiless. The first signs occur between two and six years of age. The calves of the child's legs may appear oddly muscular and enlarged. Soon, the boy is walking with a peculiar waddling gait. By the time he is in his teens, the child's confinement to a wheelchair will be permanent. Most stricken with DMD die by their mid-20s. Few survive past age 30.
At the microscopic level, the trouble begins when the language needed to make the muscle protein is mistranslated from the dystrophin gene. Nucleotides--which act like lettered beads on a charm bracelet--combine to form three letter sequences, known as codons. With Duchenne, such nucleotides may by spuriously added or deleted, corrupting the codon 'words' and making them illegible. The result of these genetic misspellings is known as a frameshift mutation.
"It's like a train derailment," Menhart explains. The "track" in question are segments of DNA on the gene known as exons. These critical pieces combine to form the proper recipe for the dystrophin protein. If the body misreads the code, synthesis of the protein cannot proceed properly. As Menhart points out, "It doesn't really matter where the train tracks are broken. If I don't get all the way to the end, I don't get the protein."
And what if there were a way to skip over a defective segment of track and continue the journey, producing a slightly altered or abbreviated, but nevertheless functional dystrophin protein? Such technology is now being attempted in human trials. It is considered among the most promising approaches to the treatment of Duchenne's.
As Menhart explains, "If they can skip some of these defective exons, that person would be cured and start making his own dystrophin again, minus the little defective piece. We can get to the end of the track, and we're fine."
If the theory sounds straightforward, the practice of treating Duchenne by exon skipping is frustratingly complex. Many intricacies of the gene and the affected exons have not been satisfactorily studied. Menhart insists that current attempts to re-engage the train without a more thorough understanding of the segments of track is a strategy largely relying on luck. "Here's the problem: nobody knows what the effects are of putting this thing back together," he says.
Hence, efforts to compensate for the track derailment--to ferry passengers by bus around the accident site, as it were--are usually unsuccessful. The bus driver has no idea where to drop off the passengers. As Menhart puts it, "Sometimes they can see the station and walk to it, sometimes they are way off and they just wander around in a bad part of town until they are mugged."
So Menhart and IIT students are trying to fill in this deficit by researching the detailed structure of the dystrophin gene. "What we're doing is studying all the little pieces, to see which ones go together and how they can work if you remove them. Nobody knows which exons to skip--how to get the train back on track. That's what we're doing."
Those with Duchenne have considerable cause for hope, according to Menhart: "If we can learn more about the structural consequences of exon skipping, I would be hopeful for a treatment within a decade, maybe sooner."
Reprinted from IIT Magazine, Fall 2007