The new genetics

There have been prodigious investments of money and effort in this area. Since the last edition of this book in 1996, the human genome project has come and gone with great fanfare. However, there has as yet been little benefit to psychiatric practice. For single-gene disorders, such as Huntington's disease, research at least has a clear starting point. There is now understanding of the genetic abnormality at the chemical level; efforts at 'gene therapy', however, have yet to bear fruit.

However, long before the advent of these new techniques, studies of inheritance patterns had made it clear that the hereditary component of the major psychiatric conditions is polygenic: that is, not due to one gene, but to small contributions from a number of genes. A recent review describes some of the various techniques being used to hunt this collection of needles in a haystack: 'coordinated pathway genotype analysis, genome-wide linkage scans . . . genome-wide association studies' (Aitchison et al., 2005).

Use is made of quantitative trait loci (QTLs), which are genetic markers associated with the characteristics in question. The task facing researchers is huge: 'Between 100 000 and 1 000 000 markers may be required for an exhaustive sweep of the genome for all QTLs contributing to a disorder' (Aitchison et al., 2005).

The localization of defective genes and identification of their DNA sequences and biochemical products may have various clinical applications, including the following:

• Diagnosis of affected or at-risk individuals, perhaps at a presymptomatic stage: that is, screening. There are obvious implications for ethics and insurance, however.

• Treatment possibilities, mainly theoretical at present as regards psychiatry, including:

- development of specific drugs

- 'gene therapy', where genes are introduced through viruses into affected individuals, as already being trialled in malignancies and in immune deficiency states.

The new techniques may bring marked clinical benefit, but they also raise ethical problems. There is a danger of patients' hopes being falsely raised, particularly by media stories about 'discovery of the gene for . . .', and doctors are already seeing possible negative effects.

Case example

A man consulted his GP in distress, having just found out that his divorced wife had Huntington's disease. He knew this disease is inherited, and that a test had recently become available to diagnose it in the presymptomatic stage. What, if anything, should he tell their 23-year-old daughter who lived with him, and who knew nothing of her mother's illness? The GP referred the man to a genetic counsellor. After prolonged discussion, in which it was explained that a test was available but preventive or symptomatic treatment was not, he decided against telling his daughter. He felt that the possibility of her finding out, through testing, that she was to develop such a disease in later life might blight her young adulthood. However, he felt that he would wish to review this decision should she later consider starting a family. He was left with the strain of carrying a secret.

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