This study aimed to elucidate the genetic causes underlying early-onset parkinsonism (EOP) in a consanguineous Iranian family. To attain this, homozygosity mapping and whole-exome sequencing were performed. As a result, a homozygous mutation (c.773G>A; p.Arg258Gln) lying within the NH2-terminal Sac1-like inositol phosphatase domain of polyphosphoinositide phosphatase synaptojanin 1 (SYNJ1), which has been implicated in the regulation of endocytic traffic at synapses, was identified as the disease-segregating mutation. This mutation impaired the phosphatase activity SYNJ1 against its Sac1 domain substrates in vitro. We concluded that the SYNJ1 mutation identified here is responsible for the EOP phenotype seen in our patients probably due to deficiencies in its phosphatase activity and consequent impairment of its synaptic functions. Our finding not only opens new avenues of investigation in the synaptic dysfunction mechanisms associated with parkinsonism, but also suggests phosphoinositide metabolism as a novel therapeutic target for parkinsonism.

Robust SNP genotyping technologies and data analysis programs have encouraged researchers in recent years to use SNPs for linkage studies. Platforms used to date have been 10 K chip arrays, but the possible value of interrogating SNPs at higher densities has been considered. Here, we present a genome-wide linkage analysis by means of a 500 K SNP platform. The analysis was done on a large pedigree affected with Parkinsonian-pyramidal syndrome (PPS), and the results showed linkage to chromosome 22. Sequencing of candidate genes revealed a disease-associated homozygous variation (R378G) in FBXO7. FBXO7 codes for a member of the F-box family of proteins, all of which may have a role in the ubiquitin-proteosome protein-degradation pathway. This pathway has been implicated in various neurodegenerative diseases, and identification of FBXO7 as the causative gene of PPS is expected to shed new light on its role. The performance of the array was assessed and systematic analysis of effects of SNP density reduction was performed with the real experimental data. Our results suggest that linkage in our pedigree may have been missed had we used chips containing less than 100,000 SNPs across the genome.

In this study, the role of known Parkinson's disease (PD) genes was examined in families with autosomal recessive (AR) parkinsonism to assist with the differential diagnosis of PD. Some families without mutations in known genes were also subject to whole genome sequencing with the objective to identify novel parkinsonism-related genes. Families were selected from 4000 clinical files of patients with PD or parkinsonism. AR inheritance pattern, consanguinity, and a minimum of two affected individuals per family were used as inclusion criteria. For disease gene/mutation identification, multiplex ligation-dependent probe amplification, quantitative PCR, linkage, and Sanger and whole genome sequencing assays were carried out. A total of 116 patients (50 families) were examined. Fifty-four patients (46.55%; 22 families) were found to carry pathogenic mutations in known genes while a novel gene, not previously associated with parkinsonism, was found mutated in a single family (2 patients). Pathogenic mutations, including missense, nonsense, frameshift, and exon rearrangements, were found in Parkin, PINK1, DJ-1, SYNJ1, and VAC14 genes. In conclusion, variable phenotypic expressivity was seen across all families.