is currently no cure, sufferers can be helped by a number of measures, such as physiotherapy to help keep the lungs clear, and replacement therapies for the defective digestive enzymes. Cystic fibrosis is also one of the frontline illnesses in modern medical research aimed at curing the condition by correcting the genetic cause of the disease. To understand what this means, we need to know a little more about genes and how a malfunction of their normal operation can help in understanding the underlying causes of many important diseases. In fact the basis of genetics is quite simple, and logical, so that anybody can grasp the essential details.
One way of looking at genes is to regard each gene as a very long word written in a code we call DNA. The code itself is made up from an alphabet of just four letters. These letters are chemicals known as nucleotides, containing the nucleic acids guanine, adenine, cytosine and thymine, which are conveniently referred to using the letters G, A, C and T. It might appear a very limited alphabet but if you imagine the many different permutations of just those four letters that are possible in a word that is anything from hundreds to thousands of letters long, you appreciate how the DNA code offers virtually an unlimited variety of words. The 20,000 human genes are grouped together into 46 chromosomes – following the word analogy, the chromosomes might be seen as 46 chapters, which make up the book of our nuclear genome. In the formation of eggs and sperm inside the human ovaries and testes, the gene CFTR must be copied. Each of these germ cells will then contribute a single copy of CFTR to the offspring, so that every baby will be born with one gene from the father and another from the mother.
If, during the copying process, an error is made, so that the spelling of CFTR is defective, the code will be altered. This is what we mean by a mutation. But if you think it through, a mutation such as this will only affect one of the two copies of CFTR. Thus if the baby gets one defective copy and one normal copy, the normal copy might still be enough to prevent disease.
Here we turn to another strand of the synthesis – Mendelian genetics. In Mendel’s day, naturalists assumed that heredity arose through a process of blending of the parental characters, which was adopted by Darwin as the basis for hereditary change in his evolutionary theory. Mendel, the abbot of an Augustinian monastery in Czechoslovakia, happened to be a farmer’s son, and he studied the effects of cross-fertilising different varieties of peas, which he grew in the monastery’s vegetable garden. When, for example, he took the pollen from yellow peas and used it to fertilise the female parts of the flowers of green peas, the offspring were not a yellowish green, as one might have expected if parental characteristics blended. Instead they were all yellow. Even more intriguingly, when Mendel crossbred this new all-yellow generation, the next generation reverted to a mixture of yellow and green, like the original parents. Even stranger still, the ratio of yellow to green in the new generation was not equal: there were three times as many yellow as green peas. By analysing his results, Mendel realised that the inheritance of pea colour could not be based on blending, but rather some discrete factors must be responsible for the two different colours. He had discovered that the coding of heredity comes in small packages, which we inherit from either parents and which we now call genes. But this was not all that Mendel had discovered. What was the meaning of the curious ratios he had observed in the colour experiments?
In fact what he had discovered was that when the offspring inherited two different variations of a gene, sometimes one of the two variations dominated over the other. In the case of the peas, the gene for yellow was dominant. Thus when he blended green and yellow, the offspring, although some only had a single gene for yellow, all appeared yellow. When he further crossbred generations that had one yellow and one green gene, on the law of averages the offspring had a one-in-four chance of having two yellow genes, a two-in-four chance of having one yellow and one green, and a one-in-four chance of having two green genes. Not only does this explain Mendel’s findings, it also proves helpful when we go back to consider the genetics of cystic fibrosis.
Medical geneticists have indeed confirmed that when a child inherits one normal copy of the gene CFTR from one parent and a mutated version of the gene from the other parent, the coding for the normal copy dominates over that of the mutated gene. From the coding perspective, the mutated gene is essentially passive in the presence of the second normal gene. And this, in turn, implies that only if he or she inherits a mutated gene from both parents will a child suffer from cystic fibrosis. In medical genetics, this is known as a recessive pattern of inheritance. From this level of understanding, we see that there are two aspects of the recessive inheritance of cystic fibrosis that make it particularly amenable to gene therapy. The disease is the result of a malfunction of a single gene, CFTR. Moreover, the two defective copies of the CFTR in the sufferer’s chromosomes are passive and can be ignored. All that the sufferer needs to correct the condition is the introduction of a single copy of the normal CFTR gene.
I have no doubt that, in time, it will become possible to correct the genetic cause of cystic fibrosis through the introduction of a single copy of CFTR into the chromosomes of sufferers, though there will be problems, both ethical and technical, to be overcome before we reach this stage. For the moment, scientists have restricted their efforts to gene therapy directed exclusively at stem cells within the lungs, which, to date, have had a limited success.
Other single gene disorders may be the result of dominantly inherited mutations, for example achondroplasia, which causes a profound shortening of the limbs, leading to a common form of dwarfism, and Huntington’s disease, which causes jerky involuntary movements of the body and limbs and a decline in mental abilities. When a mutation affects a gene on the sex chromosomes, the genetics becomes a little more complex. For example, haemophilia, which causes excessive bleeding through defects in the blood-clotting factor VIII, is a recessive condition arising from mutations of a gene carried on the X chromosome. But since males only have a single X chromosome, inherited from their mothers, the single copy of the recessive gene will still give rise to the disease. This is why females, who have two X chromosomes, one inherited from each of the parents, rarely suffer from haemophilia – they would need both copies of the gene to be mutated before haemophilia could manifest. Thus we see that haemophilia is not only sex-linked, it is also a Mendelian recessive condition. Other mutations affecting genes on the sex chromosomes can be dominant, for example the condition known as Vitamin-D resistant rickets, so that a mutated gene on just a single X chromosome will cause the disease in either sex.
To date, geneticists have found causative mutations for more than 5,000 single-gene disorders. Other mutations can change the number of chromosomes, as in Down’s syndrome, where the individual has an additional copy of chromosome 21, or delete, duplicate, fragment, or otherwise damage the structure of chromosomes, giving rise to a variety of medical conditions. While specific gene therapy is at an early stage in the treatment of such conditions, a number of approaches to family screening, advice and prevention are already established and available to assist families known to have an increased risk of mutation and hereditary disease.
The medical approach includes prevention, through genetic counselling, public education about the risks of increasing maternal age, avoidance of risk factors such as radiation of the germ cells and foetus, caution over drug and chemical exposure, such as thalidomide, and vaccination against the rubella virus, which is known to damage the developing foetus. Newer genetic measures, such as in vitro fertilisation of the sperm and egg, followed by genetic screening of the resultant foetus when it is at the stage of a ball of cells, can be offered to high-risk families. Known as pre-implantation genetic diagnosis, or PGD, this may be helpful in a variety of diseases, including sex-linked disorders, single gene defects and chromosomal disorders. The potentially amenable sex-linked disorders include haemophilia, fragile X syndrome, most of the neuromuscular disorders (currently there are more than 900 recognised neuromuscular dystrophies) and hundreds of other diseases. Indeed, the potentially amenable single gene defects also include cystic fibrosis, Tay-Sachs disease, sickle-cell anaemia and Huntington’s disease.
As a general rule, we can see that a genetic abnormality is more likely to respond to PGD if it is predictable, because the genetic inheritance is known, and if its effects can be
