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.

Neurological disorders include a wide variety of mostly multifactorial diseases related to the development, survival, and function of the neuron cells. Single-nucleotide polymorphisms (SNPs) have been extensively studied in neurological disorders, and in a number of instances have been reproducibly linked to disease as risk factors. The RIT2 gene has been recently shown to be associated with a number of neurological disorders, such as Parkinson's disease (PD) and autism. In the study reported here, we investigated the association of the rs12456492 and rs16976358 SNPs of the RIT2 gene with PD, essential tremor (ET), autism, schizophrenia (SCZ), and bipolar disorder (BPD; total of 2290 patients), and 1000 controls, by using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Significant association was observed between rs12456492 and two disorders, PD and ET, whereas rs16976358 was found to be associated with autism, SCZ, and BPD. Our findings are indicative of differential association between the RIT2 SNPs and different neurological disorders.

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Evolutionary analyses of the critical core promoter interval support a selective advantage for expanding the length of certain short tandem repeats (STRs) in humans. We recently reported genome-wide data on human core promoter STRs that are "exceptionally long" (≥6-repeats). Near the top of the list, the neuron-specific gene, RIT2, contains one of the longest GA-STRs at 11-repeats. In the present study, we analyzed the evolutionary implications of this STR across species. We also studied this STR in a sample of 2,143 Iranian human subjects that encompassed a number of neuropsychiatric disorders and controls. We report that this GA repeat is functional and different lengths of the repeat result in significant alteration in gene expression activity. The 11-repeat allele was human specific and the sole allele detected in 110 unrelated Iranian individuals randomly selected and sequenced from our control pool. Remarkably, homozygosity for a 5-repeat allele was detected in a consanguineous, hospitalized case of schizophrenia, which significantly decreased gene expression activity (p < 5 × 10). The frequency of the 5-repeat allele in the Iranian population was calculated at <0.0001, putting this allele in the deleterious mutations category based on allele frequency. The 5-repeat allele is annotated in the Ensembl database in the heterozygous status (5/11) in one of four indigenous hunter-gatherer men sequenced from southern Africa (BUSHMAN KB1: rs113265205). The present findings indicate for the first time, selective advantage for a human-specific allele at an STR locus, and a phenomenon in which genotypes and alleles at the extreme length of STRs occur with disease only. This is a pilot study that warrants large-scale sequencing of the RIT2 core promoter STR in diseases and characteristics that are linked to the brain function.