VOL: 102, ISSUE: 47, PAGE NO: 25
Emma Wilkinson, MA, BSc, is acting assistant clinical editor, Nursing TimesGenetic screening took another step forward recently when the first babies were born in the UK using a technique called preimplantation genetic haplotyping, in which doctors screen embryos for genetic markers associated with a disease. The healthy twins' parents are both carriers of cystic fibrosis and already have one child with the disease.
Genetic screening took another step forward recently when the first babies were born in the UK using a technique called preimplantation genetic haplotyping, in which doctors screen embryos for genetic markers associated with a disease. The healthy twins' parents are both carriers of cystic fibrosis and already have one child with the disease.
Genetic testing for diseases is becoming more common in the UK as evidenced by a the case of twin sisters who had their stomachs removed after inheriting a defective gene that had caused fatal stomach cancer in three family members.
Genetics services in the NHS were first established 50 years ago but our knowledge of the genetic basis of disease has increased dramatically in recent years. The human genome was sequenced in 2003 and in a white paper published in the same year, the government outlined a vision for the NHS to be a world leader in genetics-based healthcare and set aside £50m funding over three years to improve diagnosis and testing facilities and finance research projects (Department of Health, 2003).
Genetic information can be obtained through family history and biochemical tests but 'genetic testing' generally refers to the analysis of the structure of DNA (cytogenetic testing) or changes within the DNA sequence itself (molecular testing). Genetic tests are used both before and after the appearance of symptoms, for example to ascertain if someone has Huntington's disease or to confirm a diagnosis of muscular dystrophy (Parliamentary Office of Science and Technology, 2004).
There are 22 regional genetics centres in the UK. DNA molecular tests are offered for about 300 rare single-gene disorders on the NHS. However, about 6,000 single-gene disorders have been identified. Single-gene disorders, such as cystic fibrosis, affect between 1-5% of the population and cost the NHS and social services about £2bn a year. There are also genetic tests for familial risk of certain disorders. For example, the BRCA1 gene can increase a lifetime risk of breast cancer by 35-80%.
In the future it is likely that genetic tests will be developed to help predict someone's lifetime risk of developing common diseases such as diabetes and heart disease.
Genetic tests can also be performed to find out if someone is a carrier even if they do not have symptoms of the condition, for example sickle cell disease.
Potential parents can be screened for carrier status to assess the risk of them passing on a genetic condition to a child, for example Fragile X and thalassaemia (Parliamentary Office of Science and Technology, 2004).
Antenatal screening is routinely undertaken to test for Down's syndrome. It is also possible to screen for cystic fibrosis during pregnancy but this is not routinely carried out in the UK. Antenatal screening can also be carried out for other conditions where there is a strong family history, such as spina bifida or congenital heart defect.
The national newborn screening programme tests babies for phenylketonuria, congenital hypothyroidism (both easily treatable) and more recently cystic fibrosis and sickle cell and thalassaemia. All four of these tests are done through the heel blood spot test in the first week of life (www.newbornscreening-bloodspot.org.uk).
Some of the most recent developments in genetic screening have been in preimplantation genetic diagnosis (PGD). PGD is used to screen embryos to select those without the defective gene.
Preimplantation genetic haplotyping, as used in the case cited earlier, identifies a 'fingerprint' of genetic markers inherited along with the condition, which means a wider range of disorders can potentially be screened for. PGD is licensed for around 50 conditions including ectodermal dysplasia and Li Fraumeni syndrome.
In 2004 the procedure was also licensed for selecting embryos free of BRCA1 and BRCA2 genes, which increase the risk of breast and ovarian cancer and the HNPCC gene, which increases the risk of colon cancer (www.hfea.org.uk).
Only 5-10% of cancers are thought to be caused by an inherited altered gene and many people worry unnecessarily that they have a high risk of cancer because of their family history. However, individuals may be at increased risk if two or more close blood relatives on the same side of the family develop the same type of cancer, if cancers are occurring at young ages, or if a close relative has had two different types of cancer (www.cancerbackup.org).
Genetic tests for mutations that may increase the risk of cancer are generally only carried out in those who have a very high risk of breast, ovarian or bowel cancer on the basis of family history. NICE recommends genetic counselling and testing for those patients with a 10-year risk of greater than 8% (for women aged 40-49 years) or a lifetime risk of 30% or greater of developing breast cancer as worked out from a detailed family history (NICE, 2006). High risk also includes a 20% or greater chance of a faulty BRCA1, BRCA2 or TP53 gene in the family.
People with a strong family history of bowel cancer (for example a first-degree relative diagnosed before 45 or two first-degree relatives diagnosed at any age) may be referred to a specialist genetics service. Familial adenomatous polyposis and hereditary non-polyposis colorectal cancer are both inherited conditions that increase a patient's risk of developing bowel cancer.
A trial is currently under way in Scotland to test every patient who develops bowel cancer under the age of 55 to see if they have the faulty HNPCC gene, with the ultimate goal of screening relatives of those who test positive (www.cancerhelp.org.uk).
Counselling and referral
People may need to see a genetic counsellor if they are newly diagnosed with a genetic disorder, if they are planning a pregnancy or if they are worried they may have a disorder that runs in the family. They are generally referred by their GP or hospital consultant. The patient may also need to see a clinical geneticist to obtain a proper diagnosis.
Genetic counsellors (who often have a nursing background) help patients to understand the nature of the disease and what it means in practical terms, what options there might be for prevention and testing, the risks of recurrence and the implications for other family members (Janson-Smith, 2003).
In the genetics white paper, the government proposed funding for 50 additional genetic counsellors over the next five years (DH, 2003).
Tests for genetic disorders may lead to difficult decisions. For example, women with a familial risk of breast cancer may be offered the choice of a double mastectomy. Or patients may receive a diagnosis for a disease for which there is no treatment.
Decisions around genetic testing and screening in people planning to start a family may be particularly difficult. The Human Genetics Commission recommends that midwives and other health professionals emphasise that prenatal screening for any condition is voluntary.
Parents of foetuses diagnosed with a serious condition need to receive appropriate counselling and support. Couples contemplating PGD also need counselling to ensure they understand the implications of their decisions, as it is a long and complicated process and pregnancy rates are low (HGC, 2006).
The range of genetic screening and testing continues to expand both for those seeking to reduce the risk or eliminate the inheritance of a condition, and for those who have an increased risk of a disease and wish to receive any available treatment. Genetic counsellors are available to help people understand their condition and make informed decisions about testing and treatment.
Types of genetic disorders
Single-gene disorders are inherited disorders caused by the mutation of a single gene. Examples include cystic fibrosis and sickle cell disease.
Chromosomal disorders are caused by an excess or deficiency of genes contained in a chromosome. For example, Down's syndrome is the result of an extra copy of chromosome 21.
X-linked disorders are caused by a gene on the X chromosome. Most of these disorders occur in boys, for example Duchenne muscular dystrophy.
Multifactorial diseases are caused by a combination of variations in more than one gene and environmental factors. Most cancers and coronary heart disease are multifactorial.