Progression of subtle motor signs in PINK1 mutation carriers with mild dopaminergic deficit

While homozygous mutations in the PINK1 gene cause recessively inherited early-onset Parkinson disease (PD), heterozygous mutations have been suggested as a susceptibility factor.

To evaluate this hypothesis, 4 homozygous PINK1 patients with PD and 10 asymptomatic carriers of a single heterozygous mutation from a large German family (family W) were included in this study. Clinical follow-up of the heterozygous mutation carriers 3 years after the initial visit included a detailed videotaped neurologic examination using the Unified Parkinson's Disease Rating Scale III protocol and smell and color discrimination testing. At follow-up, PET with 18-fluorodopa (FDOPA) of 13 family members was obtained in order to evaluate the clinical phenotype in light of nigostriatal dopaminergic functioning. The clinical and PET data were compared to those of healthy controls.

While there was mild worsening of clinical signs in previously affected heterozygous mutation carriers upon follow-up, 3 additional individuals had newly developed signs of possible PD. Hyposmia was found in 7 of the heterozygous mutation carriers, diminished color discrimination in 4. The homozygous mutation carriers who were all definitely affected with PD showed a severe, 60% decrease of caudate and putaminal FDOPA uptake; heterozygous offspring also had a significant 20% putaminal FDOPA uptake reduction compared to controls.

Our findings strengthen the hypothesis that heterozygous PINK1 mutations act as a susceptibility factor to develop at least subtle Parkinson disease motor and nonmotor signs, as supported by the finding of a reduced striatal dopaminergic FDOPA uptake not only in homozygous but also, albeit to a lesser extent, in heterozygous mutation carriers.