University President Anthony Monaco was at bat last Friday at the Experimental College's ongoing lecture series, "A Taste of Tufts: A Sampling of Faculty Research." His lecture, entitled "Genes for Walking, Genes for Talking: 25 years of Human Molecular Genetics," focused mainly on his studies involving Duchenne Muscular Dystrophy and specific speech impairments. During the presentation, he gave a summary of his involvement with genes that have been linked to these disorders.
Duchenne Muscular Dystrophy (DMD) is a genetic disease that involves a rapidly worsening form of muscular weakness. It has a recessive X−linked inheritance pattern, meaning that the genes that code for it are located on the sex chromosomes in humans. Therefore, since males only have a single copy of the gene, due to their Y chromosome, they are much more likely to suffer from this debilitating disorder. DMD affects 1 in 3,500 live male births, making it one of the most frequently occuring "rare" inherited disorders. Patients present symptoms at 3 to 5 years of age, showing decreasing motor skills and extreme muscle weakness, and they are generally confined to a wheelchair by the age of 12.
When Monaco started his research, very few human genes had been isolated. He said that geneticists were able to identify some defective proteins and backtrack from there, but only with certain diseases.
Essentially, genes are distinct portions of deoxyribonucleic acid (DNA) located on chromosomes, which make ribonucleic acid (RNA), which in turn make proteins. By identifying faulty proteins, it is possible to laboriously backtrack and pinpoint exactly where on the chromosome the gene lies.
However, not everything on the original genome is expressed; a splicing process separates exons from introns, expressing only the exons in RNA and discarding the coding from the introns. The only thing that was known about the gene that codes for DMD when Monaco began his research in 1982 was that it was located on the short arm of the X−chromosome, on band Xp21.
"I was a Ph.D. student in the Program in Neuroscience at Harvard and was introduced to Louis Kunkel through his seminar in our Neuroscience of Disease course in my first year," Monaco said. "He presented a genetic approach to understanding Duchenne Muscular Dystrophy. I was so inspired by his proposal that I asked him to take me on as his student on the project the next day."
A group in Toronto approached the gene by examining the DNA of the rare cases of women who suffered from DMD to look for translocations that would have caused a dissection of the DMD gene. During translocation, chromosomes exchange pieces with each other, and the zones that are split are therefore rendered inactive. Kunkel and Monaco tried a different approach, which relied on deletions of chromosome material in the band Xp21 in male patients.
Eventually, Monaco and Kunkel were able to identify the dystrophin gene, which codes for an important component of muscles that is missing underneath the muscle cell membrane in patients suffering from DMD. The gene proved to be massive, consisting of 79 exons spanning at least 2,300 kilobases, making it the largest known gene. In fact, it comprises 0.1 percent of the human genome.
"The identification of the gene responsible for Duchenne Muscular Dystrophy with Louis Kunkel was my most rewarding research discovery because at that time, nobody had ever identified a gene for a human inherited disorder using genetics alone," Monaco said. "Therefore, a number of intellectual and practical hurdles had to be overcome in order to find and clone disease genes to which Lou and I contributed in this project. These solutions influenced the field of human molecular genetics for many years until the completion of the Human Genome Project in 2003."
While working as a student researcher, Monaco also served as Kunkel's lab representative in the Neuromuscular Program clinic at Children's Hospital Boston. Once a week, he would visit the clinic and meet the patients and their families and hear firsthand was it was like to battle against a disease that would eventually take their lives.
"To say the least, that experience in the clinic was one of the most rewarding aspects of my Ph.D project and a real motivator to finding the gene," Monaco said. "After we identified the gene, we had hundreds of letters from families with boys with muscular dystrophy expressing their gratitude and hope."
There are myriad applications of Monaco's research, and this discovery has the power to impact tens of thousands of people. Knowing which gene is implicated in DMD allows for 80 percent accuracy in prenatal screening, which would allow mothers to screen their fetuses for the debilitating disease. Perhaps even more importantly, current clinical trials are working with a molecular mechanism that Kunkel and Monaco hypothesized in 1988, which could alleviate the symptoms of Duchenne patients.
"Clinical trials have been successful in trying to force the dystrophin gene with Duchenne deletions to skip an additional exon to put the reading frame back in place, therefore allowing a Becker−type protein to be made … they now need to scale it up in larger trials on more patients," Monaco said.
Becker Muscular Dystrophy is a milder form of muscular dystrophy, which includes some muscle weakness and occasional heart issues and progresses more slowly than DMD. In contrast to those with DMD, patients with Becker Muscular Dystrophy can lead a fairly full life.
Monaco pointed out that the idea of "deletions" can be explained by the sentence "the sky was blue," using 3−letter reading frames. If a single letter were deleted, the sentence would now read "hes kyw asb lue," which makes no sense at all. However, in contrast, if the whole first word were deleted, the sentence would read, "sky was blue," which is slightly cropped but still comprehensible.
This is the difference between Duchenne and Becker; in Duchenne, the message coming from the genome is completely garbled, while in Becker it is just slightly abbreviated. According to Monaco, by forcing more deletions, the reading frames can be reestablished, therefore reducing the severity of the disease.
In his lecture, Monaco also briefly covered his research with a specific speech disorder, which is linked with the FOXP2 gene. The gene is expressed in areas of the brain that are important in motor planning and sequencing of events, both of which are related to speech.
"The FOXP2 gene was found to be mutated in rare cases of patients with a severe speech and language disorder which was inherited in an autosomal dominant fashion," Monaco said. "It is not mutated in more common forms of specific language impairments that represent 5 percent of school−age children. However, we did find that FOXP2 regulated a target gene called Contactin Associated Protein 2, which is associated with common forms of specific language impairments and also with autism. Therefore, it shows how FOXP2 gave us an entry point into the pathway of genes that are involved in the brain pathways for speech and language," he said.
Interestingly, Monaco said, chimps' FOXP2 genes are more similar to those of mice than those of humans, although humans and chimps share close to 98 percent of the same genes. This suggests the human−specific changes in FOXP2 probably occurred largely in the last 200,000 years, which is consistent with the changes in the human language in that same time frame.
Although in the last 15 years Monaco has switched his main focus to more common disorders, he still takes pride in the advances he helped to bring about in the study of genetics.
"My favorite moments in my research were those times when we had been searching for a disease gene for years and got the first sequence of it," Monaco said. "At that moment we knew that we had opened up a whole new field of knowledge based on our discovery that will increase our understanding of basic biology and hopefully provide insights for better diagnosis and treatments for patients."
