By Dean Haycock
Special To BioWorld Today
For many casual biology-watchers, genetic disorders are associated with straightforward mutations. This limited view might be promoted by the frequency with which the press announces the discovery of genes for various diseases.
Sickle cell anemia is a textbook example of this type of mutation-disease link. The substitution of a single amino acid in hemoglobin molecule alters its the structure and function enough to distort red blood cells, causing them to assume a sickle shape instead of their normal circular shape. This change severely impairs their ability to travel through small blood vessels. Thus, a single amino acid substitution leads to repeated episodes of severe pain.
Other genetic disorders are much harder to explain. Huntington¿s disease, myotonic dystrophy, fragile X syndrome, Friedreich ataxia and spinocerebellar ataxia (SCA) are not caused by a simple amino-acid substitution. Instead, they are associated with triplet repeats. That is, they are caused by a type of DNA stuttering, in which the three letters or bases of the DNA code are repeated an unusually high number of times.
Coding Can Lead To Malfunctioning Proteins
In some instances, this extra, unhelpful coding information can lead to aberrant, malfunctioning proteins produced by the gene containing the repeat triplets. In other cases, the repeats may disrupt the expression of the gene in which they are found. In either case, the result is serious for individuals who inherit these repeating DNA motifs.
¿All of these disorders are complex and may involve more than one mechanism,¿ Lubov Timchenko, assistant professor, Baylor College of Medicine, in Houston, told BioWorld Today.
A paper in the April issue of Nature Genetics offers a reminder that there are inherited diseases that represent even more challenging mechanisms to explain. In their paper titled ¿An untranslated CTG expansion causes a novel form of spinocerebellar ataxia (SCA8),¿ assistant professors Michael Koob and Laura Ranum of the Department of Neurology and the Institute of Human Genetics, at the University of Minnesota, in Minneapolis, and their co-authors report a new form of the triplet repeat disorder SCA.
Until now, seven forms of the inherited neurodegenerative disease SCA had been described. Most are characterized by abnormally long repeats of the DNA bases CAG. The repeated motif in SCA8, however, consists of the DNA bases CTG. And, unlike SCA1 through 7, the expanded repeats in cases of SCA8 are inherited mostly from mothers. Myotonic dystrophy was the only known disease to be caused by a CTG expansion, until this report.
¿Myotonic dystrophy was sort of out on its own for quite a while, having CTG outside of a coding frame,¿ Koob said. ¿His is the second example of that type of motif. It will probably have implications in understanding what is pathological about a repeat like that.¿
Ranum said, ¿One of the things that really surprised us was that we found something so different using a different method to find it. We used a method [rapid cloning] that we published about a year ago that allows us to isolate a gene directly from an individual¿s DNA sample. It allows us to pull out long repeats where 3 of the 4 letters of the genetic code are repeated extra times.¿
Koob noted that, ¿rather than coming at it from the traditional standpoint, we did the genetics afterwards.¿
Other techniques rely on very large families and a strategy called positional cloning, Ranum said. ¿What we have developed is like a magnet,¿ she said. ¿We know we are looking for a triplet repeat. If you use the analogy of looking for a needle in a haystack, we have a big magnet that will pull out all the metal objects and leave the hay.¿
After pulling out the unexpected repeat, they had to determine whether or not it had anything to do with the disease. They found more individuals from one large family who had the same large expansion of repeat bases. Then, they looked at the general population and showed it was not a common repeat. Looking at the affected family more carefully, the researchers found that some members had larger repeats than others. When the repeats were small, there was no evidence of disease.
¿If it had nothing to do with the repeats, you wouldn¿t find that correlation,¿ Koob said.
Many Opportunities For Drug Development
In the previously described SCAs, the expanded repeats are presumed to interfere with the genes in which they appear. In SCA8, as the newly described SCA is called, the repeats do not appear to be associated with a mutation affecting any known gene. The researchers, however, detected RNA transcripts containing the CTG repeats in particularly high levels in the brain. This is consistent with a neurodegenerative disease, but the transcripts do not appear to encode a protein. One explanation offered by the authors is that this novel triplet repeat produces an abnormal RNA that somehow interferes with the functions of a gene that remains to be identified.
No other antisense RNAs have been implicated in SCA, according to the authors. Koob cautioned, however, that ¿strictly speaking, we can¿t say that RNA has anything to do with the disease at this point. I would be surprised if that was actually the case, but we haven¿t actually proven that the RNA is involved.¿
Based on her independent research, Timchenko had suggested that the pathology behind myotonic dystrophy may be at the level of RNA. ¿The similar location and the same sequence of triplicate expansion suggests that the molecular mechanism for myotonic dystrophy and SCA8 are similar,¿ Timchenko said. ¿Obviously, it provides a lot of opportunity for drug development, because, I guess, it will be much easier to develop drugs if the mechanism for the disease is at the RNA level.¿