Parkinson disease in twins - An etiologic study

Context The cause of Parkinson disease (PD) is unknown. Genetic linkages ha ve been identified in families with PD, but whether most PD is inherited ha s not been determined, Objective To assess genetic inheritance of PD by studying monozygotic (MZ) and dizygotic (DZ) twin pairs. Design Twin study comparing concordance rates of PD in MZ and DZ twin pairs . Setting and Participants A total of 19 842 white male twins enrolled in the National Academy of Sciences/National Research Council World War II Vetera n Twins Registry were screened for PD and standard diagnostic criteria for PD were applied. Zygosity was determined by polymerase chain reaction or qu estionnaire. Main Outcome Measure Parkinson disease concordance in twin pairs, stratifie d by zygosity and age at diagnosis. Results Of 268 twins with suspected parkinsonism and 250 presumed unaffecte d twin brothers, 193 twins with PD were identified (concordance-adjusted pr evalence, 8.67/1000). In 71 MZ and 90 DZ pairs with complete diagnoses, pai rwise concordance was similar (0.129 overall, 0.155 MZ, 0.111 DZ; relative risk, 1.39; 95% confidence interval, 0.63-3.1). In 16 pairs with diagnosis at or before age 50 years in at least 1 twin, MZ concordance was 1.0 (4 pai rs), and DZ was 0.167 (relative risk, 6.0; 95% confidence interval, 1.69-21 .26). Conclusions The similarity in concordance overall indicates that genetic fa ctors do not play a major role in causing typical PD, No genetic component is evident when the disease begins after age 50 years. However, genetic fac tors appear to be important when disease begins at or before age 50 years.