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Advances in genetics have probably had more practical applications in the field of the inherited neurological disorders than in any other area of medicine. A remarkably high number of serious neurological disorders follow single-gene Mendelian inheritance, in both childhood and adult life. Genetic testing now forms an important and integral part of both neurological and clinical genetics practice and has greatly enhanced both diagnostic precision and the options associated with genetic counseling. This chapter focuses on the use and implications of testing, especially in relation to genetic counseling. Risk estimations in relation to carrier testing for both autosomal and X-linked recessive disorders may, in fact, be quite complex and may need the overall family structure to be considered. The chapter also presents some of the numerous issues that need to be considered in relation to presymptomatic testing.
To study the frequency, clinical features and clinical genetics of familial Parkinson’s disease (PD).
Family history for PD and tremors was studied in 100 consecutive PD cases. Spouses served as controls. Clinical features were compared between personally verified familial and sporadic PD cases, from the same consecutive clinical series. Clinical genetic analysis was performed in a larger group of non-consecutive multicase PD families.
Family history for PD was positive in 24% of consecutive PD cases and in 6% of spouse controls (p < 0.001). When family history for isolated tremor is also considered, the number of positive cases rises to 43% compared with 9% in controls (p < 0.001). Nine of the consecutive cases had at least one living affected relative, for a total of 20 familial PD cases. These familial cases showed an earlier onset age when compared with sporadic ones from the same consecutive series. Within 22 non-consecutive PD families with at least two living and personally examined PD cases (total 52 PD cases), the crude segregation ratios were similar for parents and siblings and the lifetime cumulative risks approached 0.4 in siblings and tended to be comparable, but at later ages, in parents. Ancestral relatives were all unilaterally distributed. In some families, anticipation of onset age in new generations was observed.
The frequency of positive family history for PD and for PD and tremor is higher among PD cases than controls. Familial and sporadic PD only differ in onset age. The clinical genetic analyses support autosomal dominant inheritance with strongly age-related penetrance as most likely in familial PD.
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