Fifteen years of genetic research in Parkinson's disease (PD) have led to the identification of several monogenic forms of the disorder and of numerous genetic risk factors increasing the risk to develop PD. Monogenic forms, caused by a single mutation in a dominantly or recessively inherited gene, are well-established, albeit relatively rare types of PD. They collectively account for about 30% of the familial and 3%-5% of the sporadic cases. In this article, we will summarize the current knowledge and understanding of the molecular genetics of PD. In brief, we will review familial forms of PD, basic genetic principles of inheritance (and their exceptions in PD), followed by current methods for the identification of PD genes and risk factors, and implications for genetic testing.

Calcifications in the basal ganglia are a common incidental finding and are sometimes inherited as an autosomal dominant trait (idiopathic basal ganglia calcification (IBGC)). Recently, mutations in the PDGFRB gene coding for the platelet-derived growth factor receptor β (PDGF-Rβ) were linked to IBGC. Here we identify six families of different ancestry with nonsense and missense mutations in the gene encoding PDGF-B, the main ligand for PDGF-Rβ. We also show that mice carrying hypomorphic Pdgfb alleles develop brain calcifications that show age-related expansion. The occurrence of these calcium depositions depends on the loss of endothelial PDGF-B and correlates with the degree of pericyte and blood-brain barrier deficiency. Thus, our data present a clear link between Pdgfb mutations and brain calcifications in mice, as well as between PDGFB mutations and IBGC in humans.

Next-generation sequencing has identified mutations in the PRRT2 (proline-rich transmembrane protein 2) gene as the leading cause for a wide and yet evolving spectrum of paroxysmal diseases. PRRT2 mutations are found in the majority of patients with benign familial infantile epilepsy, infantile convulsions and choreoathetosis and paroxysmal kinesigenic dyskinesia, confirming a common disease spectrum that had previously been suggested based on gene linkage analyses and shared clinical features. Beyond these clinical entities, PRRT2 mutations have been described in other childhood-onset movement disorders, different forms of seizures, headache disorders, and intellectual disability. PRRT2 encodes a protein that is expressed in the central nervous system and is thought to be involved in the modulation of synaptic neurotransmitter release. The vast majority of mutations lead to a truncated protein or no protein at all and thus to a haploinsufficient state. The subsequent reduction of PRRT2 protein may lead to altered synaptic neurotransmitter release and dysregulated neuronal excitability in various regions of the brain, resulting in paroxysmal movement disorders and seizure phenotypes. In this review, we examine the genetics and neurobiology of PRRT2 and summarize the evolving clinical and molecular spectrum of PRRT2-associated diseases. Through a comprehensive review of 1444 published cases, we provide a detailed assessment of the demographics, disease characteristics and genetic findings of patients with PRRT2 mutations. Benign familial infantile epilepsy (41.7%; n = 602), paroxysmal kinesigenic dyskinesia (38.7%; n = 560) and infantile convulsions and choreoathetosis (14.3%; n = 206) constitute the vast majority of PRRT2-associated diseases, leaving 76 patients (5.3%) with a different primary diagnosis. A positive family history is present in 89.1% of patients; and PRRT2 mutations are familial in 87.1% of reported cases. Seventy-three different disease-associated PRRT2 mutations (35 truncating, 22 missense, three extension mutations, six putative splice site changes, and seven changes that lead to a complete PRRT2 deletion) have been described to date, with the c.649dupC frameshift mutation accounting for the majority of cases (78.5%). Expanding the genetic landscape, 15 patients with biallelic PRRT2 mutations and six patients with 16p11.2 microdeletions and a paroxysmal kinesigenic dyskinesia phenotype have been reported. Probing the phenotypic boundaries of PRRT2-associated disorders, several movement, seizure and headache disorders have been linked to PRRT2 mutations in a subset of patients. Of these, hemiplegic migraine emerges as a novel PRRT2-associated phenotype. With this comprehensive review of PRRT2-associated diseases, we hope to provide a scientific resource for informing future research, both in laboratory models and in clinical studies.

This first comprehensive MDSGene review is devoted to the 3 autosomal recessive Parkinson's disease forms: PARK-Parkin, PARK-PINK1, and PARK-DJ1. It followed MDSGene's standardized data extraction protocol and screened a total of 3652 citations and is based on fully curated phenotypic and genotypic data on >1100 patients with recessively inherited PD because of 221 different disease-causing mutations in Parkin, PINK1, or DJ1. All these data are also available in an easily searchable online database (www.mdsgene.org), which also provides descriptive summary statistics on phenotypic and genetic data. Despite the high degree of missingness of phenotypic features and unsystematic reporting of genotype data in the original literature, the present review recapitulates many of the previously described findings including early onset (median age at onset of ∼30 years for carriers of at least 2 mutations in any of the 3 genes) of an overall clinically typical form of PD with excellent treatment response, dystonia and dyskinesia being relatively common and cognitive decline relatively uncommon. However, when comparing actual data with common expert knowledge in previously published reviews, we detected several discrepancies. We conclude that systematic reporting of phenotypes is a pressing need in light of increasingly available molecular genetic testing and the emergence of first gene-specific therapies entering clinical trials. © 2018 International Parkinson and Movement Disorder Society.

BackgroundAdenylyl cyclase 5 (ADCY5) mutations is associated with heterogenous syndromes: familial dyskinesia and facial myokymia; paroxysmal chorea and dystonia; autosomal‐dominant chorea and dystonia; and benign hereditary chorea. We provide detailed clinical data on 7 patients from six new kindreds with mutations in the ADCY5 gene, in order to expand and define the phenotypic spectrum of ADCY5 mutations.MethodsIn 5 of the 7 patients, followed over a period of 9 to 32 years, ADCY5 was sequenced by Sanger sequencing. The other 2 unrelated patients participated in studies for undiagnosed pediatric hyperkinetic movement disorders and underwent whole‐exome sequencing.ResultsFive patients had the previously reported p.R418W ADCY5 mutation; we also identified two novel mutations at p.R418G and p.R418Q. All patients presented with motor milestone delay, infantile‐onset action‐induced generalized choreoathetosis, dystonia, or myoclonus, with episodic exacerbations during drowsiness being a characteristic feature. Axial hypotonia, impaired upward saccades, and intellectual disability were variable features. The p.R418G and p.R418Q mutation patients had a milder phenotype. Six of seven patients had mild functional gain with clonazepam or clobazam. One patient had bilateral globus pallidal DBS at the age of 33 with marked reduction in dyskinesia, which resulted in mild functional improvement.ConclusionWe further delineate the clinical features of ADCY5 gene mutations and illustrate its wide phenotypic expression. We describe mild improvement after treatment with clonazepam, clobazam, and bilateral pallidal DBS. ADCY5‐associated dyskinesia may be under‐recognized, and its diagnosis has important prognostic, genetic, and therapeutic implications. © 2016 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society