Predictive genetic testing in Huntington's disease.

Huntington's disease (HD) is an Autosomal Dominant Disease having a varying age at onset. There is no epidemiologic data on the prevalence of HD in India. However, among immigrants from the Indian subcontinent to Britain, there were 22 patients with HD in a population of 1.26 million, giving an age-adjusted prevalence rate of 1.75 per 100,000 population.[1] In a study of genetically confirmed HD patients from India, adult onset HD was found to be more common than Juvenile HD (below 20 years). HD commonly presents as chorea (88.5%) or with psychiatric symptoms (11.5%).[2]

The gene for HD is larger than normal. If an affected parent has one normal-sized copy of the gene and one larger-sized copy, there is a 50% chance of the child getting half of the large-sized genetic material. Thus, every pregnancy has a 50:50 chance or “1 in 2 chances” of developing this disease. If one has the faulty gene they will at some stage develop the disease. However, if one inherits the “good” gene, they will not develop the disease and will not pass the defective gene to their children. If one parent has one normal copy of the gene and another a large-sized copy, there is a 50:50 (or 50%) chance to get HD each time a child is born. Although the gene is present, the symptoms of HD appear in the 2nd or 3rd decade and sometimes, even later in life.

Genetic testing confirms the clinical diagnosis of HD and predicts if a family member is at risk and is thus likely to develop the disease in the future. Patients and others at risk have expanded CAG repeats. It is important to discuss the pros and cons with the family members or the concerned individual before predictive testing. It is essential to tell them that if testing shows that they carry expanded repeats they could develop neurologic or psychiatric symptoms at any age. It is a dilemma to test or not-to test as it involves ethical issues that an individual has to cope in which a test result will tell about his likelihood of developing a progressive neurologic disease that has no known treatment.

Early identification of diseases, such as HD, that appear later in life will help employing strategies to prevent their appearance or delay their progression. The former is dependent on individual preferences and coping styles and the latter depends on several complex interacting causes. The following reasons stress the utility of genetic testing.[3] The treatment of HD is mainly symptomatic.

The test is predictive

The disorder involved is serious

Standard screening or surveillance is unable to detect this illness

The illness is untreatable once the symptoms appear, and

Effective preventive or screening measures exist, but are too costly or difficult to recommend to the entire population.

In a disease, such as HD, clinicians must consider ethical questions associated with genetic testing as stated in the Hastings Center report:

Most experts who are speaking of this revolution are referring to a revolution of biotechnology and information science that would make medicine even more highly technological than it is today. While this may come to pass … genetics also has the potential paradoxically to reinforce humanistic, empathic, and communicative aspects of clinical care. Clinicians will conceivably become more aware of their patients’ values and more aware of the family, religious, and social structures within which their patients live their still very unpredictable lives. Genetics will revolutionize medicine. It will send it back to its roots; back to its future.[4]

It is problematic to do predictive genetic testing in family members who are at risk to develop HD or in people with late-onset chorea and do not have a positive family history. A decision to test or not-to test for HD is complex because it has no known treatment. A positive test suggests that an individual is at risk of developing HD in the future. Strong family ties among the Indians can be of great value.

Living with the possibility that one is at risk to develop a chronic illness that has no definite treatment is difficult. This is compounded by this disease being disabling and the existing treatments being effective in only controlling the symptoms. However, knowing that one does not carry the abnormal copy of the gene could be reassuring. A DNA test can now be carried out to give this information but it does not mean that it should be done as a matter of routine. The patient and families must carefully consider if doing the test is right for them. The person undergoing the test should be older than 18 years. The decision to undergo the test must be personal and family members, healthcare professionals, employers, or insurance companies must not persuade.

We compared responses toward predictive genetic testing for HD among at-risk family members, medical doctors, and laypeople in India. We found that 60% of the family members of Indian HD patients did not wish to tell the results of genetic tests to others. Medical doctors felt the decision to have a child if they had HD would depend on results of prenatal genetic testing.[5] Genetic testing for HD is best done when proper genetic counseling services are available.

Thus, choosing to undergo genetic testing for HD is personal and individuals who decide to undergo testing should have intensive pre-test counseling. Although knowledge about HD gene mutation helps some individuals with reproductive and career planning, other individuals at risk for HD could be concerned about the psychologic and potential discriminatory harms resulting from testing.