Getting a grip on genetic diseases

Image courtesy of alex-mit / iStockphoto

Sabine Hentze and Martina Muckenthaler tell Lucy Patterson about their work – detecting genetic diseases and counselling potentially affected patients. As part of the recent SET-routesw1 Insight Lecturesw2 series, two scientists from Heidelberg, Germany, talked about their work and experience in the field of genetic disease: Sabine Hentze, a medical doctor specialising in human genetics and genetic counselling, and Martina Muckenthaler, a professor at the Centre for Paediatric Medicine at Heidelberg University Hospital working on the hereditary iron storage disease haemochromatosis (see box).

Genetic diseases are caused by abnormalities in a person’s DNA. These can be as simple as a single nucleotide mutation in a single gene, or as complex as deletions and rearrangements of parts of or entire chromosomes.

Our DNA is constantly under attack by a range of factors that can cause mutations, such as naturally occurring radiation (for classroom experiments on radiation, see Peralta & Oliveira, 2009). In addition, copying the entire DNA sequence of a cell during each mitosis is not a fail-safe process. In response, each cell in our body keeps a fleet of DNA-repair enzymes that constantly patches up the damage. From time to time, however, mutations escape the repair process and become fixed in the DNA. If these mutations occur in important genes, they can cause serious diseases. As we have two copies of each gene (one from our mother and one from our father), mutations that damage only one copy of the gene do not necessarily cause immediate problems, as we still have one healthy copy.

Sabine Hentze (left) and Martina Muckenthaler (right) Images courtesy of EMBL Photolab

Indeed, we all carry between five and ten such recessively mutated genes without even realising it. Only dominant mutations will manifest as a disease when just one copy of the gene is damaged. Recessive mutations can become problematic, though, if both your mother and father happened to carry a mutant copy of the same gene: there is a risk that you could inherit both. This can cause diseases such as cystic fibrosis or sickle cell anaemia.

Chromosomal analysis of a female. One of the X chromosomes forms what is known as a ring chromosome, causing Turner syndrome. Click to enlarge Image courtesy of Sabine Hentze

When whole sections of chromosomes go missing or are switched around, or even entire chromosomes are duplicated or deleted, these aberrations are often lethal, so that the child dies before birth, or they lead to mental retardation and malformations, syndromes such as Down syndrome. It is also possible to inherit increased susceptibility to diseases. Certain mutations, although they do not actually cause diseases, dramatically increase a person’s risk of developing particular conditions: for instance, inherited mutations in the BRCA1 and BRCA2 genes lead to an increased risk of breast cancer.

It is part of Sabine’s job to investigate and diagnose these kinds of diseases in her patients: “A typical situation is one where parents introduce me to their child, saying ‘He has not developed properly for his age. Our neighbour’s son who is the same age is much taller, he’s already crawling. Mine can’t do that’.” There are a number of tools and tests available, but when it comes to making a diagnosis, she says: “First and foremost, at the centre are we, the doctors, whose job it is to look, listen, examine and assess.” After a thorough clinical examination, and armed with a detailed family history, Sabine may already be able to guess what might be the underlying cause of a patient’s disorder. The next step is to find out what kind of mutation lies at the heart of the disease.

Chromosome analysis was first used by French geneticist and paediatrician Jérôme Lejeune, who in 1959 determined that children suffering from Down syndrome had an extra copy of chromosome 21. Even today, there are many questions that can be more fully answered using this technique than with a genetic test: cells taken from a patient are cultured in the lab, then fixed, prepared and stained so that their chromosomes can be studied under the microscope.

Yet some diseases are caused by rearrangements or deletions too small to be seen by conventional chromosome analysis, so a technique called FISH (fluorescent in situ hybridisation) is used. Tiny, fluorescently labelled DNA probes are designed to hybridise, or stick, to a specific sequence in the region of the chromosome which is thought to be affected, and doctors check if and where they hybridise on a sample of the patient’s chromosomes. So when do we ultimately use a genetic test? “I use one if I strongly suspect a particular disease for which we know the gene and the possible mutation,” says Sabine.

FISH with probes that bind to specific sequences of DNA. Note the deletion in chromosome 22 in the sample on the right. This causes DiGeorge Syndrome, which is marked by the absence of thymus and parathyroid glands, and results in impaired immunity, short stature, and deformities of the face, heart and great vessels Image courtesy of John Crolla, Wellcome Images

Haemochromatosis, the disease Martina has specialised in, is not only the most common hereditary disease in the western world, but in 85-90% of cases in central Europe, is caused by one specific single mutation in the HFE gene (see box below). If patients seem to be suffering from the disease or are concerned that it runs in their family, it is possible to carry out a genetic test to look for HFE mutations. The patient’s DNA is isolated, usually from a blood sample, and the nucleotides of the HFE gene in which the most common point mutations occur are sequenced, and compared with the sequence found in healthy individuals.

When children suffer from an unspecific mental or physical disability, mutations in a range of genes on different chromosomes can be the cause. Here, chromosome-staining techniques don’t have a sufficiently high resolution to identify the defective gene – however, it would be an incredible amount of work to sequence all potentially affected genes individually. In these cases, geneticists like Sabine are starting to use a new technology: microarrays (for an in-depth explanation of microarrays and a suggestion of how to introduce them in the classroom, see Koutsos et al., 2009). Microarrays vastly speed up the process of genetic testing; as tens or hundreds of thousands of regions of the genome can be tested at the same time, it is possible to test for many disorders simultaneously. In the future, scientists hope that it might be possible to develop a microarray that can test for all genetic diseases and predispositions in one quick and simple test.

No parents would wish their child to be born with a genetic disease. Until recently, prenatal screening was the only option available to determine whether a baby would be born with a serious disorder. However, since the advent of in vitro fertilisation, it has become possible to examine the genetic makeup of an embryo before it is implanted into the womb, a technique called pre-implantation genetic diagnosis.

At present, many tests are available for well-known genetic diseases, allowing parents who are at risk of passing on a genetic disease to select a healthy embryo for implantation. In many respects this is great news, as it means that expectant parents do not have to go through the process of screening the embryo in the womb (amniocentesis, which itself carries a risk to the baby), waiting for the results, and taking the decision whether to terminate the pregnancy if the outcome is bad.

In vitro fertilisation Image courtesy of ktsimage / iStockphoto

The more we learn about the genetic basis of different diseases and traits, and the more sophisticated our screening methods become, the more we can screen for. Of course, no parent would want their child to suffer from a serious disease – but what about less severe disorders such as haemochromatosis, congenital deafness, or even short-sightedness? Who is to say that a child growing up with such a condition wouldn’t live as full a life as an otherwise healthy person? In essence, this issue raises the subject of what is normal. Where do we draw the line as to which genetically determined conditions or traits are acceptable and which are not? Furthermore, should parents ever be allowed to choose if their child is male or female, how tall or attractive or how intelligent they are?

Image courtesy of jgroup / iStockphoto

Another question raised by genetic testing is whether you would really want to know what your genes have in store for you. What if you were to find out that you were at high risk of developing a serious disease? You might be able to adjust your lifestyle to delay its onset or decrease the symptoms, but how would you feel if you knew your risk? And what if insurance companies or prospective employers were to have access to that information? What if there was a risk that you could pass this disease on to your children? Wouldn’t your partner want to know? Might it change the way he or she felt about you? For suggestions on how to stimulate discussions on such issues in the classroom, see Strieth et al. (2008).

It is at this point that the work of genetic counsellors like Sabine Hentze is really essential: “Besides my laboratory work, I spend much of my time on counselling patients, in other words on communication: what does this test result mean? What does it mean for me, for our child, for our family, for our future?”

And it is through the work of genetic counsellors that we have come to realise that perhaps one of the most important considerations in genetic testing is that people also have the right not to know.

References

Koutsos A, Manaia A, Willingale-Theune J (2009) Fishing for genes: DNA microarrays in the classroom. Science in School 12: 44-49. www.scienceinschool.org/2009/issue12/microarray

Peralta L, Oliveira C (2009) Radioactivity in the classroom. Science in School 12: 57-61. www.scienceinschool.org/2009/issue12/radioactivity

Strieth L et al. (2008) Meet the Gene Machine: stimulating bioethical discussions at school. Science in School 9: 34-38. www.scienceinschool.org/2008/issue9/genemachine

Web references

w1 – For more information about the SET-routes organisation, promoting women in science, see www.set-routes.org

w2 – The SET-routes Insight Lectures are a series of interactive scientific lectures for use in schools. Presented by exceptional women scientists, the lectures introduce the exciting world of science, engineering and technology (SET), covering fields as diverse as space science; climate change; genetic counselling; haemochromatosis and DNA chips; malaria; stem cells and regeneration; archaeology of the Universe; and cosmology. See: www.set-routes.org/lectures

Resources

Democs card games to debate the topics of pre-implantation diagnostics and over-the-counter genetic tests can be downloaded free here: www.neweconomics.org/gen/democs.aspx

For a Science in School article about Democs, see:

Smith K (2007) Democs: a conversation card activity for teaching science and citizenship. Science in School 4: 27-19. www.scienceinschool.org/2007/issue4/democs

The Genes are Us website offers short films and classroom activities about genetic diseases, see: www.genesareus.org

For an introduction to many of the most common genetic diseases, see the Genetic Disorders Library (http://learn.genetics.utah.edu/content/disorders/whataregd) section of Learn.Genetics (http://learn.genetics.utah.edu), the Genetic Science Learning Center from the University of Utah, USA.

To learn more about oxygen radicals in your body and how to counteract them, see: Farusi G (2009) Looking for antioxidant food. Science in School 13: 39-43. www.scienceinschool.org/2009/issue13/antioxidants

If this article has whet your appetite, you can find further reading on the most cutting-edge advances and opinion on genetic testing and personal genomics at Daniel MacArthur’s science blog, Genetic Future: http://scienceblogs.com/geneticfuture

If you found this article interesting and useful, you might like to browse all the medicine-related articles published in Science in School. See: www.scienceinschool.org/medicine

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Lucy Patterson finished her PhD at the University of Nottingham, UK, in 2005, and has since been working as a postdoctoral researcher, first in Oxford, UK, then in Freiburg and Cologne, Germany. During this time she has worked on answering several different questions in developmental biology, the study of how organisms grow and develop from a fertilised egg into a mature adult, using zebrafish embryos. She has a broad interest and enthusiasm for science, and is currently developing her own embryonic career as a science communicator.