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Genetic tests for disease

Genetic Tests for Disease Chapter

Genetic diseases are due to a change at the genetic level, be this a mutation, addition or deletion of a certain gene. A variety of genetic tests are used to screen and diagnose these diseases so that preventative measures can be put in place (diet, medical treatment etc.).

This chapter looks at when is best to test, how the tests work and the ethical implications involved.

When to Test?

When to test and specific diseases diagram

When to Test?

  • Pre-conception – This is when the parents are screened prior to conception to check to see if they are carriers of certain genetic diseases, and their likelyhood of having a diseased child. e.g. Sickle Cell disease is an Autosomal recessive disease, so if both parents are carriers then there is a chance that they could have a diseased child. Parents can then progress with conception based on the test results.

 

  • Pre-implantation – This commonly occurs when couples undergo in vitro fertilization (IVF) and is where embryos are screened for disease before implantation into the womb. This allows healthy embryos to be implanted and diseased ones not to be.

     

    • Foetus – If pre-implantation is not an option then genetic testing can be performed on the foetus. In the case of chromosomal abnormalities e.g. Down’s syndrome a combination of blood tests and ultrasound is used to diagnose a genetic disease. With chromosomal diseases these tests are performed in the first trimester so that couples can decide on termination or not.

     

    • Newborn – Some metabolic genetic diseases such as Phenlyketonuria are not identifiable until after birth. At birth a ‘heel prick’ is done to perform a Guthrie test that looks for the enzyme involved in breaking down the amino acid phenylalanine. With this disease symptoms can be prevented by a strict diet with no phenylalanine.

     

      • Pre-symptoms – Some diseases are best screened before symptoms are observed. In the inherited form of breast cancer (BRCA1) testing can be done so that the individual is regularly checked for the disease, as individuals with this mutation have a higher risk of developing the disease.

       

        • Diagnostic – Some genetic diseases are screened for to aid a clinician in a diagnosis and to provide appropriate treatment. This is commonly seen in the neurodegenerative Huntingdon’s disease. Genetic testing is used to test for the disease and can be done earlier (pre-symptoms) to aid treatment.

         

        How Genetic Tests work?

        Direct Testing

        Most direct testing is done using the Polymerase Chain Reaction (PCR) and consists of a direct sample (blood, cheek cells, hair, semen etc.). Specific primers are added to the sample to replicate a specific gene to look for a mutation.

        The most important thing in PCR testing is to make sure that no contamination occurs to affect the results. Therefore strict procedures are in place to avoid contamination of samples.

         

        PCR consists of 3 stages:

        Denaturation, Annealing and Extension.

        • Denaturation – Once the Sample and Primers have been added the samples are heated so that the DNA double strands detach and become single strands.

         

          • Annealing – Once the DNA has separated into single strands the sample is cooled so that the specific primers can bind to the single stranded DNA.

           

            • Extension – The enzyme DNA polymerase then adds nucleotides to the 3’ end where the primer has annealed to give a copy of a specific gene.

             

            This process is then repeated so that multiple copies of the gene are made.

             

            PCR Flow diagram

            How Genetic Tests work?

            Gene tracking

            This is used in family studies to discover whether or not the patient in discussion has inherited the high-risk chromosome from a parent. 

            Gene tracking diagrams are done to establish the risk using the following key:

             

             

            Key for Gene tracking

            How Genetic Tests work?

            Gene Tracking

            Example:

            In the diagram The X-linked disease Duchenne Muscular Dystrophy. Patient (1) wants to know the likelihood of her giving birth to a child with the disease as her brother, Patient (2) has the disease and she is a carrier. From the diagram Patient (1) has a 1 in 4 chance (25%) of giving birth to a diseased offspring and a 1 in 4 chance (25%) of giving birth to a carrier female.

             

            Example of Genetic Tracking

            How Genetic Tests work?

            Specific Screening – Guthrie Test

            In the UK all babies are tested after birth for Phenylketonuria. A ‘heel prick’ is done to obtain a drop of blood on a piece of card (Guthrie card). The Phenylalanine level is measured using this drop of dried blood using either chromatography or the ‘bacterial growth test’.

            If a baby has a higher level of phenylalanine than the threshold they are then called back for a definitive diagnostic test.

             

            Ethical Implications

            "Ethics is a term referring to moral principles that are good for an individual. Ethics is about right from wrong, good from bad and rights and responsibilities."

             

            • The majority of people agree upon genetic testing for diseases that are preventable and treatable. However, diseases with no known cure or preventative measures are less agreed on and present ethical issues.

             

            • If an individual is diagnosed later in life with a genetic disease, after conceiving children, there are ethical issues involved in testing them for the disease. E.g. some diseases like Huntingdon’s disease have a 50% chance of being inherited due to them being autosomal dominant. If a child is told he will inherit the disease this could lead to the child needing counseling etc.

             

            • If genetic testing is done on the foetus or embryos then termination by some can be seen as an ethical issue.  Also if genetic testing is done on a newborn the baby itself does not give informed consent, this is done by the parents.

             

             

            Criteria for screening

            • Disease must be high incidence
            • Test inexpensive
            • Voluntary
            • Anonymous
            • Confidential information
            • Patients should not be pressurized

            Ethical Implications

            Requirements for Genetic Testing

            References

            STRACHAN T, READ AP. (1999) Human Molecular Genetics, Wiley-Liss, New York.

            TURNPENNY P, ELLARD S. (2007) Emery’s Elements of Medical Genetics. 13th ed, Elsevier, USA.

            NORRGARD K. (2008) Ethics of Genetic Testing: Medical Insurance and Genetic Discrimination, Scitable by Nature Education (access via http://www.nature.com/scitable/topicpage/ethics-of-genetic-testing-medical-insurance-and-651)

             

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