In agreement with clinical observations, imaging studies using (18F)-dopa positronemission tomography (PET) to quantitatively determine the amount of dopaminergic input from the SNpc to the striatum have suggested that PARKIN-positive cases resemble that of PARKIN-negative cases, although it was noted that a worse clinical outcome might be predicted by PET in PARKIN mutation cases than was actually observed (18). Likewise, no correlation between PET data and clinical presentation in PARKIN mutation cases was found, whereas cases without PARKIN mutations displayed a significant correlation (19). PET scanning performed over a number of years in a family with PARKIN mutations revealed a slower disease progression in PARKIN-positive cases versus cases without PARKIN mutations (20). Numerous studies suggest that carriers of a PARKIN mutation that are clinically asymptomatic have significantly reduced dopamine input to the striatum as compared with individuals with two normal PARKIN alleles (21-23). These observations favor the loss of function model of PARKIN pathogenesis and haploinsufficiency model of genetic inheritance. Taken together, PET scans in PARKIN-positive cases are similar to PARKIN-negative cases, illustrating the inability of PET to accurately predict PARKIN mutations. Further, PET studies demonstrate the phenotypic similarity between PD caused by PARKIN mutations and sporadic PARKIN-negative PD (24).
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