Phenotypic Discordance in Siblings with Identical Compound Heterozygous PARK2 Mutations.

mutations are the most common cause of early-onset Parkinson's disease. No genotype-phenotype correlation exists, and phenotypic variability is quite common. We report two siblings with confirmed identical compound heterozygous mutations in the gene manifesting strikingly different phenotypes. The older brother demonstrated marked parkinsonism by his mid-20's, whereas the younger brother developed exercise-induced dystonia in his mid-30's with no subsequent clinical progression, highlighting the clinical heterogeneity of the disease and implying the role of other genetic and/or environmental factors in disease progression. The younger sibling, despite his mild symptoms, had a clearly abnormal dopamine transporter (DaT)-SPECT scan. To our knowledge, this is the first such reported case of an abnormal DaT-SPECT scan in a patient with biallelic mutations who does not meet the clinical criteria for Parkinson's disease.