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Although genetic mechanisms have long been recognized as central in the causation of cancer, the practical importance of genetic and familial aspects has only now been widely appreciated. In the past, clinical genetics services were involved only in rare Mendelian cancer genetic disorders in which the risks were clear and intervention or surveillance was possible. It has recently become clear that in some of the more common cancers, notably breast, ovarian and colon cancer, there is a Mendelian subset in which the genetic risks are also extremely high. This subset is considerably more common than the combined incidence of the rarer, single-gene familial cancer syndromes.
The discovery of mutations and specific genes conferring a high risk of colorectal, breast or ovarian cancer means that comparable genetic approaches are now potentially applicable to this subset of common tumours. It is increasingly possible to distinguish family members who carry such genes from those who do not. As a result, surveillance and management resources can be directed towards family members who carry the predisposing gene, and those who do not carry such a gene can avoid the often uncomfortable diagnostic procedures and the possibility of surgical intervention.
A further influence on service demand has been intense media publicity and the high scientific profile of recent discoveries in breast cancer. Concerns are inevitably raised by such stories, and not just in families in which the genetic risk is high.
Abnormalities presumed to occur during foetal life have been associated with schizophrenia. Polydactyly is a developmental abnormality but no previous association has been reported between Polydactyly and functional psychotic illness.
Individuals with both Polydactyly and a functional psychosis were ascertained during a study of familial schizophrenia.
Five such individuals were ascertained in the course of assessing 234 individuals with familial psychosis, giving a rate of Polydactyly in the sample of around 10 times the general population rate.
This study provides preliminary evidence that Polydactyly is over-represented in individuals with familial schizophrenia and related psychotic illnesses.
Predictive genetic testing for Huntington's disease has been available in Cardiff since 1987 using linked genetic markers, and since 1993 using direct mutation testing, which can also be used as a diagnostic test. During this period there have been numerous referrals that have required liaison with psychiatric services at all stages of the testing programme.
A series of cases was selected to highlight issues from both genetic prediction and diagnosis that are relevant to psychiatric practice and have arisen during the testing programme.
Issues have been raised concerning competence to consent to testing in the context of psychotic illness; depression and suicidal ideation in test candidates; requests for testing from third parties such as psychiatrists, social services and the courts; and testing of children.
As genetic testing becomes possible for more disorders the lessons learned from Huntington's disease will provide valuable guidelines for counselling.
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