1.1.5 Late-onset single-gene disorders

An individual might know that a late-onset disease such as Huntington's disease (HD) is present in their immediate family and that they might have inherited the disease gene(s). The problems of genetic testing for HD revolve around the fact that it is pre-symptomatic.

One dilemma is the long delay between testing positive and developing the clinical symptoms of the disorder in middle age. Is it better not to know and live in hope, or as one victim cried ‘get it over, I'm so tir
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1.1.4 Genetic testing of adults

Huntington's disease is a good example of a late-onset disorder because it is fatal, non-treatable, relatively frequent and has a strong genetic element that can be tested for. There are others that fall into a similar category, i.e. mainly relate to a single gene, such as adult polycystic kidney disease. The issues surrounding late-onset multifactorial diseases, such as diabetes and breast cancer, will be dealt with separately. To date, relatively few diseases that fall into both these categ
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1.1.3 Genetic testing of children

Within clinical genetic services, a difference has grown up between the testing of children and the testing of adults. Sometimes the genetic testing of children is relatively uncontroversial. For example, the genetic test may simply be to confirm a medical diagnosis that has been made on clinical grounds. So a three-year-old with low weight, blocked lungs and poor digestion may be given a genetic test to see whether they have CF or not.

There are other cases where a test is used predict
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1.1.2 Pre-natal diagnosis

The type of genetic testing that the majority of us are most likely to come across is still pre-natal diagnosis (PND). This involves testing a fetus during pregnancy, to see whether it is likely to suffer from a number of different disorders — some genetic, some not. While recent developments allow tests for certain multifactorial genetic diseases (such as spina bifida), pre-natal diagnosis has been available since the 1960s to test for Down's syndrome.

Most cases of Down's syn
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1.1.1 Genetic counselling

In the UK and many other countries, genetic testing is provided by the National Health Service (NHS) or its equivalent, only after patients have undergone genetic counselling. This is defined as the provision of information and advice about inherited disorders, and includes helping people to:

  • Understand medical facts;

  • Appreciate the way in which inheritance contributes to the disease in question;

  • Understand the
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Learning outcomes

After studying this unit you should understand:

  • something of the role of a genetic counsellor and its non-directiveness

  • the difference between pre-natal diagnosis, childhood testing and adult testing and give some examples of diseases that may be tested for

  • the ethical and moral difficulties involved in making decisions on whether or not to carry out such tests


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Introduction

This unit looks at three different uses of genetic testing: pre-natal diagnosis, childhood testing and adult testing. Such tests provide genetic information in the form of a predictive diagnosis, and as such are described as predictive tests. Pre-natal diagnosis uses techniques such as amniocentesis to test fetuses in the womb. For example, it is commonly offered to women over 35 to test for Down's syndrome. Childhood testing involves testing children for genetic diseases that may not
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Acknowledgements

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2.5 Summary

When we read about genetics and the future of medicine, we should also think about genetics and the future of health services that have to deliver medical care. The advent of predictive medicine, based on more detailed DNA profiling of individual genotypes using technologies like gene chips, rather than screening for one gene at a time, may shift the relationship between doctor and patient. People will be seeking advice on how to manage their susceptibilities or genetic risks, rather than loo
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2.4 Current UK provision

One way of describing the organizational shift that the advent of predictive medicine would demand is to suggest that genetics would become a general, rather than a specialist service. But it is much easier to say that than to explain how it will happen. For all the publicity about genes, genomes and genetic information, medical genetics is a very small part of current health services.

In the UK, an indication that a patient or a family has a genetic problem will lead to a referral to a
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2.3 Scaling up

They may look at dozens of alleles, and involve thousands of people, but existing screening programmes have been concerned with individual genes. But the technologies now being developed will soon permit the recording of hundreds of genes at a time. So-called gene chips combine the skills of microchip designers with DNA sequence information to offer rapid, easy-to-read results for an individual covering hundreds of genetic variants. A gene chip is a thin slice of glass about the
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2.2.4 Longer-term considerations

Something else to ponder is the effect that screening might have on the longer-term incidence of disease and (not the same thing) on the incidence of gene variants linked to disease. Sometimes, the impact on a disease can be dramatic. Take thalassaemia, a haemoglobin disorder similar to sickle cell disease, in which premature destruction of haemoglobin-containing red blood cells leads to anaemia. It is relatively common in some Mediterranean countries. Like sickle cell disease, it is understo
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2.2.3 Cystic fibrosis

A different model for the genetic tests of the future is screening for cystic fibrosis (CF). This is a DNA-based test, which became possible after the gene involved in CF was identified in 1989. CF is a recessive disease, and it should be easy to test to see if prospective parents carry a mutated allele. A simple mouthwash yields enough cells for DNA extraction. If both partners are carriers, they can consider further counselling before conception, and/or pre-natal testing of any potentially
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2.2.1 Phenylketonuria

The classic example of population screening is testing new-born babies for phenylketonuria (PKU). Individuals with PKU fail to make a protein, a certain enzyme, and develop mental retardation. The absence of the enzyme results in both an accumulation of phenylalanine, which causes the mental retardation, and a deficiency of tyrosine in the body, as shown in Author(s): The Open University

2.2 Population screening for genetic disease: the precedents

Knowing about particular genes, or their effects, also permits screening – the search in a population for persons with certain genotypes that are associated with a particular disease. Thus the test may be offered to one and all. Until now, screening programmes have focused on one gene at a time, or one disease at a time, in cases where a mutated gene poses serious health problems and something can be done for those who are found to carry the mutation. What that something is varies with the
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Learning outcomes

After studying this unit you should understand:

  • some of the ways in which genetic knowledge could affect medical practice, in particular in relation to predictive medicine

  • how populations are screened for conditions such as phenylketonuria and whether screening could be used for carriers of recessive genetic disorders such as cystic fibrosis

  • how gene chips may be used to screen for large numbers of genes at once, making it possible to predict the li
    Author(s): The Open University

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Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

Figures

Figure 1 Science Photo Library;

Figure 8a This photograph has been provided by Railway Technical Research Institute in Japan;

Figure 22 Proceedings of the Royal Society A248 464. The Royal Society;

F
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5.2 Neural ageing: article 2

In the article presented here by Esteban (2004) entitled ‘Living with the enemy: a physiological role for theβ-amyloid peptide’, Trends in Neurosciences, 27, pp. 1–3, the author introduces us to a very important molecule implicated in the aetiology of Alzheimer's disease. However, the β-am
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5.2.2 Calorie restriction

Since the publication of Osborne, Mendel and Ferry's paper (Science, 1917, Vol 45, pp. 294–5) calorie restriction has been the most reliable method of extending the lifespan of laboratory animals. These results have been confirmed by many researchers and have been extended to a variety of verteb
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