Evaluation of PINK1 variants in Indian Parkinson's disease patients

Abstract

Mutations in PINK1 have been identified in familial and sporadic cases of early onset Parkinson's disease (PD). To determine the contribution of PINK1 variants in Indian PD patients, the gene was screened in 250 patients and 205 ethnically matched controls by polymerase chain reaction, single-stranded conformation polymorphism and DNA sequencing. Two potentially pathogenic variants (Arg246Gln & Arg276Gln) were detected in the heterozygous state in 5 patients; none of the patients carried homozygous or compound heterozygous mutations. In addition, 13 other variants were identified, including a known polymorphism (Ala340Thr), a few synonymous or intronic changes, none of which are likely to be pathogenic. Unlike the Chinese population, the Ala340Thr variant did not show any association with PD in Indian population. Six single nucleotide polymorphisms (SNPs) selected from dbSNP were genotyped in 531 normal, healthy individuals representing different ethnic groups of India. Most of the SNP markers were observed to be highly heterozygous among Indians, which could be used for segregation analysis of PINK1 alleles in familial PD cases.

Introduction

Parkinson's disease (PD) is the second most common neurodegenerative disorder [1]. The prevalence of PD varies significantly around the world [2]. There is evidence that genetic factors also play a major role in the etiology of some cases of PD. Approximately 5–10% of PD patients have a familial pattern of inheritance. To date twelve chromosomal loci (PARK1- PARK13; PARK1 and 4 are same) and nine underlying genes, including PTEN induced kinase 1 (PINK1) gene, have been implicated in PD [3].

PINK1 is a 581 amino acid protein, containing an N-terminal mitochondrial targeting sequence (MTS) and a conserved serine/threonine kinase domain (amino acids, 156–509) [4]. It is ubiquitously expressed in the human brain, and is considered to be neuroprotective as illustrated in PINK1 knock out drosophila and mammalian cellular models [5]. Protein phosphorylation is likely to play an important role in PD pathogenesis, and PINK1 may participate either by directly phosphorylating or binding key cellular proteins governing neuronal survival [5]. It has been reported that in PD, PINK1 protein localizes to 5–10% of the Lewy bodies in the brain [6]. PINK1 mutations account for early onset PD (EOPD) approximately 1–7% in Caucasians [7], [8], [9], [10], [11]; about 8.9% in Japanese [12] and 3.7% in patients of Chinese origin [13], but no study has been reported on Indian patients.

The coding sequence of PINK1 is spread over eight exons encompassing ∼18.8 kb of genomic DNA. Among the variant alleles of the gene, a common polymorphism (Ala340Thr) was examined for its association with PD and Thr340 allele was proposed to pose a risk factor among Chinese but not in Finnish & Caucasians [14], [15], [16], [17]. The difference in the reported observation could be due to overall genomic variation among the population groups studied. This study was undertaken to examine contribution of PINK1 variants in Indian PD patients, and to our knowledge it is the first report on molecular genetic studies on Indian PD using PINK1 as a candidate gene.