After 24 weeks, the boys who received placebo had got worse, as we would expect for this disease. They couldn’t walk as far as when they entered the trial. But the boys who received the drug could walk more than 30 metres further than when the trial started. The boys were tested again after 48 weeks. The placebo group had deteriorated even more. In the six-minute walking test, their performance was almost 25 metres less than when the trial started. The boys who had been treated could walk over eleven metres further than when the programme began.{357}
These data showed that, over time, even the boys who received the drug began to decline (look at the difference between 24 and 48 weeks), but this decline was dramatically slower than when the condition was running its normal course.
The results from this trial caused enormous excitement. Finally, it looked like there might be hope developing in the treatment of a previously intractable disorder. Even if the treatment didn’t cure the patients, it might significantly slow down the development of the irreversible symptoms. This was what everyone researching in the field, and the families of affected boys, had been working towards for decades. True, it wouldn’t work for all Duchenne sufferers, but between 10 and 15 per cent of patients were expected to be eligible for this approach, based on the kind of mutation in their dystrophin gene.
Just six months later, those hopes were in tatters. GlaxoSmithKline ran a larger trial and this time couldn’t find any significant difference between the treated and untreated groups.{358} The results from larger trials are more reliable than ones from smaller studies because they are less likely to be affected by odd patterns that look like a response but aren’t. GlaxoSmithKline had no doubts about its large trial, convinced that if there had been a genuine effect of the drug, it would have been detected. They handed the drug back to Prosensa and walked away. Prosensa is continuing with clinical studies, although its share price tanked after GlaxoSmithKline departed, reflecting concerns by analysts that this programme may be doomed.
There is another company that is also trying to exploit splicing patterns to leap over the troublesome region in the dystrophin gene in the same patient groups. This company is called Sarepta, and it is using a similar approach to treating the affected boys. Although the company remains very upbeat about its programme, the Food and Drug Administration has questioned whether its trials are large enough to give genuinely conclusive results. For example, one of the studies in which a dramatic difference between the untreated and treated groups was seen only contained twelve patients.
Investors in the companies are no doubt feeling a chill breeze, but it can’t begin to compare with what the families of affected boys must have gone through and be going through every day.
It would be tempting to look at the science in this chapter and decide that splicing is more trouble than it’s worth. It certainly seems to be an example of Sod’s law — if something can go wrong, it will. But the reality is that the same is true of almost every biological process. Billions of bases, thousands of genes, trillions of cells, billions of people. It’s a numbers game; nothing goes right every time. But the fact that this process of joining together split genes has been maintained through hundreds of millions of years of evolutionary history, using a highly conserved system, makes it pretty clear that the advantages of the sophistication, additional information content and sheer flexibility more than compensate for the off days.
18. Mini Can Be Mighty
Perhaps because we are quite large animals, we tend to be most impressed by other large animals. And that’s OK. After all, a big cat such as a jaguar is an impressive creature. We also tend to be impressed because the jaguar is a hunter, a top carnivore. An ant, by comparison, looks rather puny, even if it’s one of the Central and South America species of army ant. Sure, there is a certain gory charm in an insect with jaws so large and strong you can use them to hold the sides of a wound together. But it’s still difficult to be frightened by something we can squash with a small downward stomp of a hiking-booted foot.
But a colony of army ants, well that’s a different matter. A colony probably eats as much flesh as a jaguar does. If you saw a column of them heading your way you might be tempted to put on your boots and run like hell, rather than indulging in a cheery ant-stomping dance.
And so it is with our genome. There are thousands of examples of a particular type of very small junk nucleic acid.{359} Each one plays a role in fine-tuning gene expression, and individually their effects are subtle. But when we look at the totality of their impact, they are an impressive horde.
Welcome to the world of smallRNAs, the mighty army ants of our genome. As their name suggests these RNA molecules are little, typically just 20 to 23 bases in length. We can think of them as nudging molecules, which impart an additional fine-tuning process to control of gene expression.
Figure 18.1 shows how these smallRNAs are produced, and how they work. They are generated from double-stranded RNA molecules. They then bind to the untranslated regions at the ends of messenger RNAs, to create a new double-stranded RNA. The creation of this double-stranded structure, dependent on the interaction of one junk sequence with another, has one of two effects on the messenger RNA. It can target the messenger RNA for destruction, or it can make it difficult for the ribosomes to translate the messenger RNA sequence into proteins. The end result is essentially similar, a drop in the amount of protein generated from that specific messenger RNA.[61]{360}
Figure 18.1 Schematic describing how the cell creates two different classes of smallRNAs from longer RNA molecules. The two classes repress gene expression in different ways, as shown at the bottom of the illustration.
The smallRNAs that trigger the destruction of messenger RNA molecules have to be a perfect match for their targets. The ones that inhibit the translation of the messenger RNAs are much more promiscuous. They will bind to a messenger RNA even if only a seed sequence of six to eight consecutive bases matches the target. One of the consequences of this is that a single smallRNA may bind to more than one type of messenger RNA, and slow down its translation. Another potential consequence is that the relative levels of the different messenger RNAs in a cell will influence the extent to which each is controlled by a particular smallRNA. This means that any given smallRNA will have a different effect depending on which of its targets is being expressed in a cell, and the ratios of the target molecules.
61
The type of smallRNA that triggers degradation is called microRNA, or miRNA. The type that triggers poor translation is called small interfering RNA, or siRNA. In order to avoid excessive technical language, the term smallRNA will be used to describe both of these.