Studies of twins may be used to estimate the genetic contribution to the pathogene-sis of a neurodegenerative condition. If a genetic component is present, concordance will be greater in monozygotic twins than in dizygotic twins. If a disorder is exclusively genetic in origin and the diagnosis is not compounded by age-associated pen-etrance or stochastic or environmental factors, monozygotic concordance may be close to 100%. However, PD is a heterogeneous, age-associated disease with reduced penetrance. Even within families in whom parkinsonism is caused by a single-gene defect, the age at onset and the progression and severity of symptoms in affected carriers may vary widely. Thus, even large studies of hundreds of twin pairs are likely to be underpowered to exclude a genetic component (18).
Although earlier twin studies in PD were inconclusive (19-21), Tanner et al. (22) demonstrated the presence of genetic factors in the pathogenesis of PD when disease begins at or before age 50 years. They studied twins enrolled in the twin registry of the National Academy of Science and the National Research Council World War II Veteran Twins Registry. No genetic component was evident when the onset of symptoms occurred after age 50 years. A more recent study using the Swedish Twin Registry, conducted by Wirdefeldt et al. (23), demonstrated low concordance rates in twins, whether monozygotic or dizygotic. However, both of these studies were largely based on limited, cross-sectional clinical observations, and diagnostic accuracy might be improved by longitudinal evaluations (24).
Positron emission tomography (PET) studies with [18F]6-fluorodopa ([18F]6-FD) may help circumvent the need for extended follow-up. Indeed, reduced striatal uptake of [18F]6-FD has been demonstrated in some clinically asymptomatic co-twins (25). Using longitudinal evaluation with measurement of [18F]6-FD, Piccini et al. (26) demonstrated 75% concordance rate in monozygotic twins versus 22% in dizygotic twins. However, these results must be interpreted cautiously. At best, [18F]6-FD uptake is only a surrogate of PD and only 18 monozygotic and 16 dizygotic twins were studied.
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