A systematic review and integrative approach to decode the common molecular link between levodopa response and Parkinson’s disease
Levodopa-induced dyskinesias (LID) present a common but elusive complication of levodopa therapy in Parkinson’s disease (PD). In order to identify genetic factors associated with LID, 352 (213 males) levodopa-treated Israeli PD patients were genotyped for 34 polymorphisms within three candidate genes affecting dopaminergic activity and synaptic plasticity: dopamine transporter gene (DAT1 or SLC6A3) [14 single nucleotide polymorphisms (SNPs) and 40-bp variable number tandem repeat (VNTR)], DRD2 [11 SNPs and dinucleotide CA short tandem repeat (STR)], and BDNF (7 SNPs). A comparison of patients with and without LID was performed by applying a time-oriented approach, with survival analyses evaluating LID development hazard rate over time [Cox proportional hazards and accelerated failure time (AFT) lognormal models]. Overall, 192 (54.5 %) participants developed LID, with a mean latency of 5.0 (±4.5) years. After adjusting for gender, age at PD onset, duration of symptoms prior to levodopa exposure, and multiple testing correction, one SNP in SLC6A3 (with 81 % genotyping success) was significantly associated with LID latency: the C allele of the rs393795 extended the time to LID onset, time ratio = 4.96 (95 % CI, 2.3–10.9; p = 4.1 × 10−5). This finding should be validated in larger, ethnically diverse PD populations, and the biological mechanism should be explored.