Presymptomatic and antenatal testing

Since a direct gene test has become available it has been possible to provide an unequivocal diagnosis in patients with inherited forms of the disease. This has also led to the possibility of performing presymptomatic testing of unaffected but at-risk family members, as well as antenatal testing. (100) Because of the effect of codon 129 genotype on the age of onset of disease associated with some mutations it is possible to determine within a family whether a carrier of a mutation will have an early or late onset of disease. Most of the mutations appear to be fully penetrant; however, experience with some is extremely limited. In families with the E200K mutation there are examples of elderly unaffected gene carriers who appear to have escaped the disease.

Genetic counselling in prion disease resembles that of Huntington's disease in many respects and those protocols established for Huntington's disease can be adapted for prion disease counselling. PrP gene analysis may have very important consequences for family members other than the individual tested, and it is preferable to have discussed all the issues with the whole family before testing commences. Following the identification of a mutation the family should be referred for genetic counselling. Testing of asymptomatic individuals should only follow adequate counselling of individuals and will require their full informed consent.

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