Andrea Carmine Belin

Parkinson’s disease: A genetic perspective

Parkinson’s disease (PD) is a common neurodegenerative disorder in the aging population, affecting more than 1% over the age of 65 years. Certain rare forms of the disease are monogenic, representing 5–10% of PD patients, but there is increasing evidence that multiple genetic risk factors are important also for common forms of PD. To date, 13 genetic loci, PARK1‐13, have been suggested for rare forms of PD such as autosomal dominant and autosomal recessive PD. At six of these loci, genes have been identified and reported by several groups to carry mutations that are linked to affected family members. Genes in which mutations have been linked to familial PD have also been shown to be candidate genes for idiopathic forms of PD, as those same genes may also carry other mutations that merely increase the risk. Four of the PARK genes, SNCA at PARK1, UCH‐L1 at PARK5, PINK1 at PARK6 and LRRK2 at PARK8, have been implicated in sporadic PD. There are indeed multiple genetic risk factors that combine in different ways to increase or decrease risk, and several of these need to be identified in order to begin unwinding the causative pathways leading to the different forms of PD. In this review, we present the molecular genetics of PD that are understood today, to help explain the pathways leading to neurodegeneration.

Lack of replication of thirteen single-nucleotide polymorphisms implicated in Parkinson’s disease: a large-scale international study

BACKGROUND A genome-wide association study identified 13 single-nucleotide polymorphisms (SNPs) significantly associated with Parkinsons disease. Small-scale replication studies were largely non-confirmatory, but a meta-analysis that included data from the original study could not exclude all SNP associations, leaving relevance of several markers uncertain. METHODS Investigators from three Michael J Fox Foundation for Parkinsons Research-funded genetics consortia-comprising 14 teams-contributed DNA samples from 5526 patients with Parkinsons disease and 6682 controls, which were genotyped for the 13 SNPs. Most (88%) participants were of white, non-Hispanic descent. We assessed log-additive genetic effects using fixed and random effects models stratified by team and ethnic origin, and tested for heterogeneity across strata. A meta-analysis was undertaken that incorporated data from the original genome-wide study as well as subsequent replication studies. FINDINGS In fixed and random-effects models no associations with any of the 13 SNPs were identified (odds ratios 0.89 to 1.09). Heterogeneity between studies and between ethnic groups was low for all SNPs. Subgroup analyses by age at study entry, ethnic origin, sex, and family history did not show any consistent associations. In our meta-analysis, no SNP showed significant association (summary odds ratios 0.95 to 1.08); there was little heterogeneity except for SNP rs7520966. INTERPRETATION Our results do not lend support to the finding that the 13 SNPs reported in the original genome-wide association study are genetic susceptibility factors for Parkinsons disease.

Developmental regulation of leucine-rich repeat kinase 1 and 2 expression in the brain and other rodent and human organs: Implications for Parkinson’s disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) constitute the most common known cause of Parkinsons disease (PD), accounting for both familial and sporadic forms of the disease. We analyzed the tempo-spatial activity of leucine-rich repeat kinase 1 (LRRK1) and LRRK2 at the cellular level in human and rat tissues including development and aging. Lrrk2 mRNA is expressed in adult rat striatum, hippocampus, cerebral cortex, sensory and sympathetic ganglia, lung, spleen and kidney. In the developing rat striatum, Lrrk2 transcription is first observed at postnatal day (P) 8 followed by increasing mRNA levels during the following 3 weeks, as revealed by quantitative in situ hybridization, after which levels remain up to 24 months of age. The time-course of postnatal development of Lrrk2 activity in striatum thus closely mirrors the postnatal development of the dopamine innervation of striatum. Lrrk2 mRNA is seen in P1 rat lung, heart, and kidney, whereas Lrrk1 is found in many areas of the P1 rat. Lrrk1 is present in adult rat brain, adrenal gland, liver, lung, spleen and kidney and also in embryonic brain, with declining gene activity after birth. LRRK1 and LRRK2 are active in the adult human cortex cerebri, hippocampus and LRRK2, but not LRRK1, in striatum. Transcription of both genes is also seen in the young human thymus and LRRK2 is active in tubular parts of the adult human kidney. Our findings suggest that the two paralogous genes have partly complementary expression patterns in the brain, as well as in certain peripheral organs including lymphatic tissues. While the strong presence of Lrrk2 message in striatum is intriguing in relation to PD, the many other neuronal and non-neuronal sites of Lrrk2 activity also needs to be taken into account in deciphering possible pathogenic pathways.

PITX3 polymorphism is associated with early onset Parkinson's disease

PITX3 is a transcription factor of importance for the differentiation and survival of midbrain dopaminergic neurons, the gene of which is disrupted in a putative mouse model for Parkinsons disease (PD). The A-allele of a HapMap tagging SNP (rs4919621) that was genotyped in a population of 361 PD patients, 69 of which had early onset, and in 333 controls, was significantly more common in PD patients with an early age of onset when compared either to controls (p=0.002) or to PD patients with late onset (p=0.001). In contrast, a previous finding suggesting a SNP (rs3758549) in the putative promoter region of the PITX3 gene to be associated with PD could not be replicated.

Association study of two genetic variants in mitochondrial transcription factor A (TFAM) in Alzheimer's and Parkinson's disease

Mitochondrial (mt) dysfunction has been implicated in Alzheimers (AD) and Parkinsons disease (PD). Mitochondrial transcription factor A (TFAM) is needed for mtDNA maintenance, regulating mtDNA copy number and is absolutely required for transcriptional initiation at mtDNA promoters. Two genetic variants in TFAM have been reported to be associated with AD in a Caucasian case-control material collected from Germany, Switzerland and Italy. One of these variants was reported to show a tendency for association with AD in a pooled Scottish and Swedish case-control material and the other variant was reported to be associated with AD in a recent meta-analysis. We investigated these two genetic variants, rs1937 and rs2306604, in an AD and a PD case-control material, both from Sweden and found significant genotypic as well as allelic association to marker rs2306604 in the AD case-control material (P=0.05 and P=0.03, respectively), where the A-allele appears to increase risk for developing AD. No association was observed for marker rs1937. We did not find any association in the PD case-control material for either of the two markers. The distribution of the two-locus haplotype frequencies (based on rs1937 and rs2306604) did not differ significantly between affected individuals and controls in the two sample sets. However, the global P-value for haplotypic association testing indicated borderline association in the AD sample set. Our data suggests that the rs2306604 A-allele could be a moderate risk factor for AD, which is supported by the recent meta-analysis.

Leucine-rich repeat kinase 2 (LRRK2) mutations in a Swedish parkinson cohort and a healthy nonagenarian

Specific variants of Leucine‐rich repeat kinase 2 (LRRK2) have been shown to associate with Parkinsons disease (PD). Several mutations have been found in PD populations from different parts of the world. We investigated the occurrence of three mutations (R1441G/C/H, G2019S, and I2020T) in our Swedish case–control material and identified four carriers of the G2019S mutation in 284 PD cases and 1 95‐year‐old carrier in 305 controls. The other two variants were absent in our material. We conclude that the LRRK2 G2019S mutation constitutes a significant factor for PD in the Swedish population and that it is not completely penetrant.

Association of a polymorphism in the ABCB1 gene with Parkinson's disease.

The ATP-binding cassette, sub-family B, member 1 (ABCB1) gene encoding the protein P-glycoprotein (P-gp) has been implicated in the pathophysiology of Parkinsons disease (PD) due to its role in regulating transport of endogenous molecules and exogenous toxins. In the present study, we analyzed the ABCB1 single nucleotide polymorphisms (SNPs) 1236C/T (exon 12), 2677G/T/A (exon 21) and 3435C/T (exon 26) in 288 Swedish PD patients and 313 control subjects and found a significant association of SNP 1236C/T with disease (p=0.0159; chi(2)=8.28), whereas the distributions of wild-type and mutated alleles were similar for 2677G/T/A and 3435C/T in patients and controls. Haplotype analysis revealed significant association of the 1236C-2677G haplotype with PD (p=0.026; chi(2)=4.955) and a trend towards association with disease of the 1236C-2677G-3435C haplotype (p=0.072; chi(2)=3.229). Altered ABCB1 and/or P-pg expression was recently shown in PD patients, and impaired drug efflux across barriers such as the gastrointestinal and nasal mucosal linings or the blood-brain barrier, might result in accumulation of drugs and/or endogenous molecules in toxic amounts, possibly contributing to disease. ABCB1 polymorphisms thus constitute an example of how genetic predisposition and environmental influences may combine to increase risk of PD.

Cerebellar α-synuclein levels are decreased in Parkinson’s disease and do not correlate with SNCA polymorphisms associated with disease in a Swedish material

Alterations of brain and plasma α‐synuclein levels and SNCA gene variability have been implicated in the pathogenesis of Parkinsons disease (PD). We therefore measured α‐synuclein protein levels in postmortem PD and control cerebellum tissue using Western blot and investigated whether the levels correlated to SNCA genotype. We found markedly decreased α‐synuclein levels in PD patients (n=16) compared to gender‐and age‐matched controls (n=14; P= 0.004) normalized to α‐tubulin. We also performed an association study of the noncoding polymorphisms rs2737029 (A/G) and rs356204 (A/G) (intron 4), and of rs356219 (T/C) (34′‐region) of SNCAin a Swedish PD case‐control material. Using a two‐sided χ test, we found significant association of rs2737029 (P= 0.003; χ2 =9.07) and rs356204 (P=0.048; χ2 =3.91) with disease, strengthening the involvement of SNCA polymorphisms in sporadic PD. Stratification of the human postmortem brain material by genotype of the three investigated polymorphisms, did not indicate any influence of genotype on α‐synuclein protein levels when comparing PD with controls. Taken together, our findings demonstrate that the investigated Parkinson patients have markedly reduced levels of α‐synuclein in cerebellum, and that this reduction is general, rather then correlated to the investigated polymorphisms, although two of the polymorphisms also associated with disease in a Swedish material.—Westerlund, M., Belin, A. C., Anvret, A., Håkansson, A., Nissbrandt, H., Lind, C., Sydow, O., Olson, L., and Galter, D. Cerebellar α‐synuclein levels are decreased in Parkinsons disease and do not correlate with SNCA polymorphisms associated with disease in a Swedish material. FASEB J. 22, 3509–3514 (2008)

MAGI1 copy number variation in bipolar affective disorder and schizophrenia.

BACKGROUND Bipolar affective disorder (BPAD) and schizophrenia (SZ) are devastating psychiatric disorders that each affect about 1% of the population worldwide. Identification of new drug targets is an important step toward better treatment of these poorly understood diseases. METHODS Genome-wide copy number variation (CNV) was assessed and variants were ranked by co-occurrence with disease in 48 BPAD families. Additional support for involvement of the highest-ranking CNV from the family-based analysis in psychiatric disease was obtained through analysis of 4084 samples with BPAD, SZ, or schizoaffective disorder. Finally, a pooled analysis of in-house and published datasets was carried out including 10,925 cases with BPAD, SZ, or schizoaffective disorder and 16,747 controls. RESULTS In the family-based analysis, an approximately 200 kilobase (kb) deletion in the first intron of the MAGI1 gene was identified that segregated with BPAD in a pedigree (six out of six affected individuals; parametric logarithm of the odds score = 1.14). In the pooled analysis, seven additional insertions or deletions over 100 kb were identified in MAGI1 in cases, while only two such CNV events were identified in the same gene in controls (p = .023; Fishers exact test). Because earlier work had identified a CNV in the close relative MAGI2 in SZ, the study was extended to include MAGI2. In the pooled analysis of MAGI2, two large deletions were found in cases, and two duplications were detected in controls. CONCLUSIONS Results presented herein provide further evidence for a role of MAGI1 and MAGI2 in BPAD and SZ etiology.

Expression of multi-drug resistance 1 mRNA in human and rodent tissues: reduced levels in Parkinson patients.

The membrane transporter multi-drug resistance 1 (MDR1, P-gp) regulates the bioavailability of endogenous and exogenous compounds and has been implicated in disorders such as Parkinson’s disease, cancer, epilepsy, human immunodeficiency virus disease, and inflammatory bowel disease. To promote further understanding of the role of MDR1 in disease, we have characterized cellular MDR1 mRNA expression in post-mortem human and fresh-frozen Sprague-Dawley rat tissues by using radioactive oligonucleotide probe in situ hybridization. We report MDR1 mRNA in human and rat endothelial cells of small vessels in the brain and pia mater. Mdr1 mRNA is also expressed in the blood vessel walls of rat sensory dorsal root and sympathetic ganglia. In peripheral tissues, we have observed MDR1 mRNA in human and rat liver and renal tubules and in human adrenal cortex and the epithelial lining of rat intestine. In female and male reproductive tissues of rat, strong gene activity has been found in steroid-hormone-synthesizing cells. Quantification of MDR1 mRNA in human striatum has revealed reduced levels in Parkinson patients compared with control individuals. The high expression of MDR1 mRNA in blood vessels of the nervous system, in tissues involved in absorption and excretion, and in tissues forming barriers to the environment support the physiological role of MDR1 as a regulator of intracellular levels of endogenous and exogenous compounds.