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Huntingtons Disease: Eradicating a devastating disease

General | June 18, 2015 | Author: The Super Pharmacist

general

Huntingtons Disease: Eradicating a devastating disease

Huntingtons disease (HD) is a neurodegenerative genetic disorder that affects movement, cognition and behaviour. It is a condition that gets progressively worse over time and currently has no cure. Life expectancy after the disorder has manifested is usually between 15-20 years (1).

Genes, chromosomes and the development of HD

HD is caused by a faulty gene found on chromosome number four. A parent with this gene has one good copy and one bad copy, and their child will inherit one of these genes. As such, there is a 50% chance that children of such parents will eventually develop HD themselves. Additionally, there is also a 50% chance that child will pass the faulty gene onto any other children that they may have in the future. This pattern of genetic inheritance is known as ‘autosomal dominant’ (2).

Are there any possibilities for HD to be eradicated?

HD is a genetic disorder and can, therefore, technically be eradicated. However, there are a wide range of clinical, psychological and ethical considerations that must be considered in its prevention and diagnosis. There are two options available at present to determine if individuals in families at high risk of HD are also carrying the faulty gene that will determine whether or not they are carrying the disease: Preimplantation genetic diagnosis (PGD) and Prenatal Testing (PT). 

Preimplantation genetic diagnosis (PGD)

It is considered a safe reproductive option for HD carriers and couples at risk of transmitting HD (3) and involves testing embryos produced using in vitro fertilization (IVF). A small number of cells (typically 1-2) are extracted from an embryo and tested for genetic abnormalities. Once the prospective parents are aware of whether or not the embryo has the faulty HD gene, they can then make a decision on whether or not to make a decision to have the embryo implanted. If they chose not to implant an embryo that is carrying the genetic defect, this can prevent future generations within their own family from inheriting HD.

There are different types of PGD testing that prevent parents from having to reveal their own parental genotype, meaning that they can have the test without finding out further information regarding their own probability of developing HD. This is often referred to as non-disclosure or exclusion testing. The exclusion test process involves comparing DNA embryos with that of both parents and grandparents to examine family history and associated risk of developing HD. In non-disclosure tests, those responsible for the testing procedure will only implant embryos that do not have the HD faulty gene. This way, parents who know they may have the risk of developing HD but choose not to find out for themselves know that they do not also pass this risk onto their own children (4). If parents do decide to find out their probability of developing HD with a pre-symptomatic test, this is often done with the aid of genetic counselling that can provide information, care and support in both the pre- and post-testing phase. In some cases, the physical blood test will be accompanied by a psychological or psychiatric assessment, as well as the specific services of the genetic counsellor. At present, guidelines in most countries suggest a minimum of three counselling sessions prior to the test (5). Uptake of pre-symptomatic testing from those at risk of developing HD is often very low, with the main reason cited to be the lack of treatment for the disease(6).

Prenatal Testing (PT) is the second option available to parents who want to find out regarding any potential risk of their child developing HD is  of an embryo or foetus in the womb. This test requires foetal genetic material that is obtained through a process known as chorionic villus sampling early on in the pregnancy (7).

If the pregnancy has progressed to between 14-18 weeks and parents decided they want to know if their child has a risk of HD, an amniocentesis can be carried out (a small sample of amniotic fluid is taken from the amniotic sac surrounding the foetus). However, this is less likely to be carried out than a chorionic villus sample, primarily because it is more invasive and carries a greater risk of miscarriage (8). The amniocentesis process can also be paired with non-disclosure testing to avoid disclosure of parental risk of HD development. Again, parents are also counselled through the process due to the potentially difficult decisions they may have to make. These include the possibilities of having a child who has the identified gene, or other difficult decisions that may be made, including termination of pregnancy (9).

Testing for HD via prenatal testing remains relatively low: parents (or potential parents) have usually already exercised a decision to have a child regardless of the risk of developing HD, or they have avoided testing or knowing about potential risk in the hope that future developments will result in an identified cure for HD. For parents who have decided against the process of having children due to a risk of passing on the faulty gene, some may consider non-disclosure testing and IVF treatment, or formal adoption procedures.

Predictive Testing and Medical Ethics

The ethical principle which underpins HD testing is the principle of autonomy and a person’s right to decide what they consider to be best. Many parents and individuals taking part in the testing process for children who have already been born do not do so because they consider that the only ethical justification for doing so is if an advantage can be clearly demonstrated to the child. As there is no known cure for the disease, numbers of parents screening at this stage is very low. For parents undergoing pre-birth tests (particular tests that may result in the termination of pregnancy), personal ethics and religious beliefs will play a central role in any decision making process. It is not uncommon for adoption agencies to request testing, as adoptive parents have a right to know if a child may have an inherited a genetic disorder such as HD. However, this must be balanced against concerns that such a diagnosis could have significant negative effects on the child’s outcomes (10). The ethical dilemmas highlight the importance of proper genetic counselling for all couples considering either of the tests available for the future prediction of HD.

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References

1. Pfister EL, Zamore PD. (2009). Huntington's disease: silencing a brutal killer. Exp Neurol 220:226-229

2. Agostinho LA, Dos Santos SR, Alvarenga RM, Paiva CL. (2013). A systematic review of the intergenerational aspects and the diverse genetic profiles of Huntington’s disease. Genet Mol Res 13:12(2):1974-81

3. Van Rij MC, Rademaeker MD, Moutou C, Dreesen JCFM et al. (2012). Preimplantation genetic diagnosis (PGD) for Huntington’s Disease: the experience of three European centres. Eur J Hum Genet 20(4):368-375

4. Stern HJ, Harton GL, Sisson ME, Jones SL, Fallon LA et al. (2002). Non-disclosing preimplantation genetic diagnosis for Huntington disease. Prenatal Diagnosis 22(6):503-507

5. Smith JA, Michie S, Stephenson M, Quarrell O. (2002). Risk perception and decision-making processes in candidates for genetic testing for Huntington’s Disease: An interpretative phenomenological analysis. Jour Health Psych 7(2): 131-44

6. Walker FO. (2007). Huntington’s Disease. Lancet 369(9557):218-28

7. Adam S, Wiggins S, Whyte P et al. (1993). Five year study of prenatal testing of Huntington’s disease: demands, attitudes, and psychological assessment. J Med Genet 30(7):549-56

8. Akolekar R, Beta J, Picciarelli G, Ogilvie C, D’Antonio F. (2015). Procedure-related risk of miscarriage following amniocentesis and chorionic villus sampling: a systematic review and meta-analysis.

9. Young ID. (1984). Ethical dilemmas in clinical genetics. J Med Ethics 10(2):73-6

10. Morris M, Tyler A, Harper PA. (1998). Adoption and genetic prediction for Huntington’s disease. Lancet 2:1069-1070

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