Investigation of differential gene expression in Parkinson's disease patients: A whole transcriptome approach

Borrageiro, Genevie (2016-12)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH SUMMARY: Parkinson’s disease (PD) is the most common neurodegenerative movement disorder and is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta. Dopaminergic neuronal loss results in motor symptoms such as resting tremor, bradykinesia, rigidity and postural instability. Although several PD-causing genes have been identified, the process(es) that lead to progressive neuronal loss is poorly understood. Therefore, PD research efforts have turned to genome-wide next generation sequencing technologies to provide clues to the etiology. The focus of this study was to investigate the entire transcriptome in South African patients with PD (particularly in the under-studied mixed ancestry population) to identify biological pathways that may shed light on the pathobiology of PD. A total of 40 study participants (20 patients and 20 controls) were recruited for the study. The PD patients were recruited from the Movement Disorders clinic at Tygerberg Hospital in Cape Town, South Africa and had to meet the UK Parkinson’s disease Society Brain Bank Diagnostic Criteria for PD. All individuals were from the South African mixed ancestry ethnic group which is a unique admixture of Khoisan, Black, Caucasian, and Asian populations. Copy number variation (CNV) detection in known PD genes (SNCA, PARK2 (Parkin), UCHL1, PINK1, PARK7, LRRK2, GCH1 and ATP13A2 and two point mutations (SNCA A30P and LRRK2 G2019S) was determined using the Multiplex Ligation-dependent Probe Amplification (MLPA) technique. Total RNA was extracted from whole blood samples and RNA-Sequencing was performed at NXT-Dx (Gent, Belgium) on the Illumina HiSeq® 4000. Bioinformatic analysis was performed using Partek® Flow® software. DAVID and Ingenuity Pathway Analysis (IPA) were used for enrichment analysis and to identify possible biological pathways involved. One candidate gene of interest was selected for further verification by quantitative real-time PCR (qPCR). MLPA analysis revealed a heterozygous exon 2 deletion in PARK2 in one patient however a second mutation in this gene was not identified. Therefore, all patients potentially have idiopathic PD and were analysed as one group for RNA-Seq. All samples produced good quality reads, as determined using FastQC (scores > 39) and on average 95.3% of the transcripts generated for each sample could be aligned to the reference genome (hg19). Bioinformatics analysis resulted in a candidate gene list of 132 differentially expressed genes. Analysis of these genes using IPA identified five significant dysregulated canonical pathways which included regulation of eIF4 and p70S6K signaling, EIF2 signaling, LPS/IL-1 mediated inhibition of RXR function, xenobiotic metabolism signaling and maturity onset diabetes of young (MODY) signaling which contribute to PD pathogenesis. A possible link between CEBPA, PGC-1α and PD was also highlighted as both genes were in the prioritized candidate list and IPA gene network. CEBPA has been shown to interact with known PD genes and PGC-1 is a transcriptional regulator of mitochondrial biogenesis a key pathway linked to PD. CEBPA was prioritized for replication studies using qPCR and was found to be significantly down-regulated in patients which contrasted with the RNA-Seq results. The present study revealed candidate genes, CEBPA and PGC-1α which could potentially be involved in the development of PD and a possible link to diabetes through mitochondrial mechanisms. We speculate that the increased PGC-1α levels observed is in response to loss of mitochondria, which leads to increased levels of reactive oxygen species that ultimately result in death of dopaminergic neurons. Our study illustrates that the use of RNA-Seq in combination with IPA is a powerful approach that may reveal candidate genes and biological pathways involved in PD. A better understanding of the molecular mechanisms underlying PD is critical for development of therapeutic modalities in order to prevent, stop or reverse dopaminergic neuronal loss in PD patients.

AFRIKAANSE OPSOMMING: Parkinson siekte (PD) is die mees algemene neurodegeneratiewe beweging afwyking en word gekenmerk deur die verlies van dopaminergiese neurone in die substantia nigra pars compacta. Die verlies van hierdie neurone veroorsaak motoriese simptome soos rustende rillings, bradykinesia, rigiditeit en posturale onstabiliteit. Alhoewel ‘n klomp gene al geïdentifiseer was vir die oorsaak van PD is die proses(se) vat lei tot die verlies van neurone nog onduidelik. Vir die rede het navorsing studies begin fokus op genoom-wye ‘next generation sequencing’ tegnologie om die eitiologie van PD te verstaan. Die hoof doel van die navorsing was om die hele transkriptoom van Suid Afrikaanse patiente met PD te ondersoek om die biologiese paaie te identifiseer, met die gevolg dat meer inligting rakende die pathobiologie van PD ontbloot sal word. 'n Totaal van 40 deelnemers (20 pasiënte en 20 kontroles) is gebruik vir die studie. Die PD patiënte was gewerf van Tygerberg Hospitaal se beweging wanorde kliniek in Kaapstad, Suid-Afrika. Die patiënte moes voldoen aan die UK Parkinson’s Disease Society Brain Bank Diagnostic kriteria. Alle individue was van Suid-Afrika en het bestaan uit verskeie etniese groepe, onder andere Khoisan, wit, swart en Asiatiese populasies. Deur gebruik te maak van die Multiplex Afbinding-afhanklike Probe versterking (MLPA) tegniek was aantal kopie variasie (CVN) ondek in bekende PD gene (SNCA, PARK2 (Parkin), UCHL1, PINK1, PARK7, LRRK2, GCH1 en ATP13A2 en in twee puntmutasies (SNCA A30P en LRRK2 G2019S). Totale RNA was onttrek van heel bloedmonsters en RNS-volgordebepaling (RNS-Seq) was gedoen deur NXT-Dx (Gent, België) op die Illumina HiSeq® 4000. Bioinformatiese analise is uitgevoer met behulp van Partek® Flow sagteware. DAVID en Ingenuity pathway analysis (IPA®) is gebruik om die analise te verryk en om moontlike biologiese paaie betrokke te identifiseer. Een kandidaat geen was gekies vir verdere analise en verifikasie deur kwantitatiewe real-time PCR (qPCR) was voltooi. MLPA analise het in een van die patiёnte ‘n heterosigotiese exon 2 verlies in PARK2 onthul, maar ‘n tweede mutasie was nie in die geen geïdentifiseer nie. Dus het alle pasiënte potensieel idiopatiese PD en is ontleed as een groep vir RNS-Seq. Alle monsters het hoё kwaliteit ‘reads’ geproduseer en dit was bepaal deur FastQC (tellings > 39) en ‘n gemiddeld van 95.3% van die transkripsies wat gegenereer was vir elke monster was in lyn met die verwysing genoom (hg19). Bioinformatika analise het gelei tot 'n kandidaat geen lys van 132 differensieel uitgedrukte gene. Ontleding van hierdie gene met behulp van IPA® het vyf belangrike dysreguleёrde kanonieke paaie geïdentifiseer, wat regulering van eIF4 en p70S6K sein, EIF2 sein, LPS / IL-1 bemiddel inhibisie van RXR funksie, xenobiotiese metabolisme sein en maturity onset diabetes of young (MODY) sein insluit wat bydrae tot PD. A moontlike skakel tussen CEBPA, PCG-1 en PD was tot lig gebring siende dat beide gene in die geprioritiseerde kandidaat lys en in die “IPA” netwerk voorgekom het. Vorige studies het getoon dat CEBPA interaksie het met bekende PD gene, terwyl PCG-1 ‘n transkripsionele reguleerder van die mitochondriale biogenese padweë is, ‘n belangrike padweg in PD. 'n Moontlike verband tussen CEBPA, PGC-1α en PD is ook uitgelig. CEBPA was geprioritiseer vir verdere analise met die qPCR en resultate het gewys dat die geen aansienlik minder gereguleer is in pasiënte, wat in teenstelling is met die RNA-Seq resultate. Die huidige studie het die volgende kanidaat gene aan die lig gebring: CEBPA en PGC-1α wat albei potensieёl betrokke kan wees in die ontwikkeling van PD met ‘n moontlike skakel tot diabetes deur mitokondriale meganismes. Ons spekuleer dat mitokondriale verlies die waarneemende verhoogte vlakke van PGC-1α veroorsaak, en hierdie kan lei tot verhoogde vlakke van reaktiewe suurstof spesies wat uiteindelik die dood van dopaminergiese neurone veroorsaak. Ons studie dui aan dat die gebruik van RNA-Seq in kombinasie met IPA® 'n kragtige benadering is, wat kandidaat gene en biologiese padweë betrokke by PD kan onthul. 'n Beter begrip van die molekulêre meganismes onderliggend aan PD is van kritieke belang vir die ontwikkeling van nuwe therapie om die verlies van dopaminergiese neurone van PD patiënte te voorkom, te stop of om dit om te keer.

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