Researchers have recently shown that they can use this information to treat Fragile X syndrome, at least in genetically engineered animals. Mice which lack the Fragile X protein have problems with their spatial memory, and with their social interactions. A mouse that can’t find its way around and doesn’t know how to react to its fellow mice is a rodent that won’t last long. Researchers used these mice and applied genetic techniques to dial down the expression of one of the key messenger RNAs that would normally be controlled by the Fragile X protein. When they did this, the scientists detected marked improvements in the animals. Spatial memory was better and the mice behaved appropriately around other mice. They were also less susceptible to seizures than the standard Fragile X mouse models.
These symptomatic improvements were consistent with underlying changes that the scientists detected in the brains of the animals.{321} Neurons in normal brains have little mushroom-shaped spines that are characteristic of strong, mature connections. The neurons of humans and mice with Fragile X syndrome have fewer of these, and a larger number of long, spindly, immature connections. After the genetic treatment, there were more mushrooms and fewer noodles.
The most exciting aspect of this was that it suggested it could be possible to improve neuronal function even after symptoms had developed. We can’t use the genetic approach in humans but these data imply that it is worth trying to find drugs that will have a similar effect, as a potential means of treating Fragile X patients. This syndrome is the commonest inherited form of mental retardation so the benefits of developing a treatment could be dramatic both for individuals and for society.
As we saw at the start of this book, expansions in a three-base sequence at the other end of a gene can also cause a human genetic disease. The best-known example is myotonic dystrophy, which is caused by expansion of a CTG repeat in the untranslated region at the end of a gene. Repeats of 35 units or above are associated with disease, and the larger the repeat, the more severe the symptoms.{322}
Myotonic dystrophy is an example of a gain-of-function mutation. The main effect of the expansion in the Fragile X gene is to stop production of its messenger RNA. But this isn’t the case in myotonic dystrophy. The mutant version of the myotonic dystrophy gene is switched on, resulting in messenger RNA molecules with large expansions at the end of the molecule. It’s these multiple copies of CUG in the messenger RNA (remember that T is replaced by U in RNA) that cause the symptoms. If we turn back to Figure 2.6 (see page 23), we can see in outline how this happens. The expanded repeats act like a molecular sponge, soaking up particular proteins that are able to bind to them.
Junk DNA plays a remarkable role in myotonic dystrophy, as shown in Figure 16.4. The CTG expansion in the junk untranslated region binds abnormally large quantities of a key protein.[53] This protein is normally involved in removing the junk DNA that is found between amino acid-coding regions when DNA is first copied into RNA. Because so much of the protein is sequestered onto the expanded myotonic dystrophy untranslated repeat, it can’t carry out its normal function very well. Consequently, lots of RNA molecules from different genes aren’t properly regulated.
Figure 16.4 The excess binding of proteins to the expanded myotonic dystrophy repeat in the messenger RNA sequesters the proteins away from other RNA molecules that they should also be controlling. The other messenger RNAs are no longer properly processed, and this disrupts production of the proteins that they should be used to produce.
This titration of the binding protein, which occurs in any tissues where both it and the myotonic dystrophy gene are expressed, plays a large role in explaining why the disease can present so differently in different patients. Instead of being all-or-nothing, varying proportions of the binding protein may be ‘left over’ to regulate its target genes. The proportion will depend on the size of the expansion and the relative amounts of myotonic dystrophy messenger RNA and binding protein in a cell.{323}
It is worth looking in a bit more detail at the proteins that are ultimately affected by these deficits (proteins A, B and C in Figure 16.4). The best-validated ones are the insulin receptor,{324} a heart protein{325} and a protein in skeletal muscle that transports chloride ions across membranes.{326} Insulin is required to maintain muscle mass. If the muscle cells don’t express enough of the receptor that binds insulin, they will start to waste away. The heart protein is one that we know is important for the correct electrical properties of the heart.{327} Transport of chloride ions across skeletal muscle membranes is an important stage in the cycles of muscle contraction and relaxation. So, the defects in the processing of the messenger RNAs coding for these proteins are consistent with some of the major symptoms in myotonic dystrophy, i.e. muscle wasting, sudden cardiac death because of fatal abnormalities in heart rhythm, and the difficulty in relaxing a muscle after it has contracted.
Myotonic dystrophy is a great example of the importance of junk DNA in human health and disease. Although the mutation lies in the messenger RNA produced from a protein-coding gene, the mutation has little if any effect on the protein itself. Instead, the mutated RNA region is itself the pathological agent, and it causes disease by altering how the junk regions of other messenger RNAs are processed.
Say ‘AAAAAAAAA’
The untranslated regions at the end of protein-coding messenger RNAs have a number of functions in normal circumstances. One of the most important involves a process that affects all messenger RNA molecules. ‘Naked’ messenger RNA molecules can be broken down in a cell very quickly, via a process that probably evolved to help us get rid of certain types of viruses rapidly. In order to stop this happening, and to make sure the messenger RNA molecules linger long enough to be translated into protein, the messenger molecules are modified very soon after production. Essentially, lots of A bases are added to the end of the messenger RNA, by a process that is outlined in Figure 16.5. There are usually about 250 A bases on the end of a mammalian messenger RNA. They are important for stability and also for making sure that the messenger RNA is exported out of the nucleus where it is made and into the ribosomes where it is translated into protein.
Figure 16.5 A sequence in the untranslated region at the end of a messenger RNA attracts an enzyme (shown by the scissors) that binds at a specific site and then cuts the molecule a little further along. Lots of A bases are added to the cut end of the messenger RNA molecule, even though these were not coded for in the original DNA sequence.
There is a critical motif in the untranslated region at the end of the messenger RNA. This is shown by the triangle in Figure 16.5 and is called the polyadenylation signal (the A base is adenosine, so adding lots of A bases is called polyadenylation). This is a sequence of six bases (AAUAAA) within the junk of the untranslated region. It acts as a signal for a messenger RNA-processing enzyme. The enzyme recognises the six-base motif, and cuts the messenger RNA a little distance away, usually ten to 30 bases further downstream. Once the messenger RNA has been cut in this way, another enzyme can add the multiple A bases.[54]
54
This is known as a non-templated change because there is no underlying DNA template for these A bases in the genome.