Common Genetic Variant Association with Altered HLA Expression, Synergy with Pyrethroid Exposure, and Risk for Parkinson's Disease: An Observational and Case-Control Study

Abstract

Background/objectives: The common non-coding single nucleotide polymorphism (SNP) rs3129882 in HLA-DRA is associated with risk for idiopathic Parkinson's disease (PD). The location of the SNP in the major histocompatibility complex class II (MHC-II) locus implicates regulation of antigen presentation as a potential mechanism by which immune responses link genetic susceptibility to environmental factors in conferring lifetime risk for PD.

Methods: For immunophenotyping, blood cells from 81 subjects were analyzed by qRT-PCR and flow cytometry. A case-control study was performed on a separate cohort of 962 subjects to determine association of pesticide exposure and the SNP with risk of PD.

Results: Homozygosity for G at this SNP was associated with heightened baseline expression and inducibility of MHC class II molecules in B cells and monocytes from peripheral blood of healthy controls and PD patients. In addition, exposure to a commonly used class of insecticide, pyrethroids, synergized with the risk conferred by this SNP (OR = 2.48, p = 0.007), thereby identifying a novel gene-environment interaction that promotes risk for PD via alterations in immune responses.

Conclusions: In sum, these novel findings suggest that the MHC-II locus may increase susceptibility to PD through presentation of pathogenic, immunodominant antigens and/or a shift toward a more pro-inflammatory CD4+ T cell response in response to specific environmental exposures, such as pyrethroid exposure through genetic or epigenetic mechanisms that modulate MHC-II gene expression.

Figure 1

The high-risk rs3129882 GG genotype is associated with an increased level of major histocompatibility complex class II (MHC-II) expression in B cells and monocytes and with increased inducibility of surface HLA-DQ expression. (a) Frequency of HLA-DR/DQ double positive, (b) average level of HLA-DR expression, and (c) average level of HLA-DQ expression in B cells and monocytes was determined by flow cytometry staining of total peripheral blood mononuclear cells. One-tailed Student’s t-test between high-risk and low-risk allele groups was used to test for significance. *P<0.05. HLA-DR median fluorescence intensity (MFI): CTRL AA versus CTRL GG B cells t(33)=2.28, P<0.05; monocytes t(34)=2.14, P<0.05. HLA-DQ MFI CTRL AA versus CTRL GG B cells t(28) =1.76, P<0.05; PD AA versus PD GG t(35)=1.82, P<0.05. Surface MHC-II expression in paramagnetically, positively sorted monocytes stimulated with various concentrations of interferon-γ (IFN-γ) was measured by flow cytometry staining to measure increase in (d) frequency of HLA-DR/DQ double-positive cells, (e) level of HLA-DR expression, and (f) level of HLA-DQ expression. Two-way analysis of variance was performed to test for significance between GG and AA groups. ***P<0.001. (d) PD GG versus PD AA F(1,190)=11.97, P<0.001. (f) CTRL GG versus CTRL AA F(1, 163)=10.39, P<0.001. Inset for DQ MFI (f) panel shows sorted monocytes from PD patients stimulated with 1,000 U/ml IFN-γ. One-tailed t-test was performed for significance. t(31)=1.52, P=0.05. PD, Parkinson’s disease.

Figure 2

The high-risk rs3129882 GG genotype is associated with increased baseline expression and inducibility of major histocompatibility complex class II (MHC-II) messenger RNA (mRNA). RNA was isolated from paramagnetically, positively sorted (a) B cells and (b) monocytes. MHC-II mRNA expression was quantified relative to 18S rRNA with RT-PCR.*P<0.05, **P<0.01, ****P<0.0001. B-cell CTRL GG versus CTRL AA HLA-DRA t(35)=2.01, P<0.05; HLA-DRB1 t(34)=2.04, P<0.05; HLA-DQB1 t(33)=3.28, P<0.01. B-cell PD GG versus PD AA HLA-DRB1 t(25)=1.59, P=0.06; HLA-DQB1 t(33)=1.64, P=0.06. Monocytes CTRL GG versus CTRL AA HLA-DRB1 t(32) =1.90, P<0.05; HLA-DQB1 t(35)=2.08, P<0.01; PD GG versus PD AA HLA-DRB1 t(26)=2.24, P<0.05; HLA-DQB1 t(30)=2.28, P<0.05. (c) Fold change in MHC-II expression with or without 100 U/ml interferon-γ stimulation in paramagnetically, positively sorted monocytes was measured by RT-PCR after normalization to 18S rRNA levels. One-tailed t-test was performed as indicated. ****P<0.0001 **P<0.01, *P<0.05. CTRL AA versus GG HLA-DRA t(33)=1.77 P<0.05; HLA-DQA1 t(13)=1.80, P<0.05; PD AA versus PD GG HLA-DRA t(36)=1.76, P<0.01; HLA-DRB1 t(29)=1.65, P<0.01; HLA-DQA1 t(24)=1.82, P<0.01; HLA-DQB1 t(30)=2.53, P<0.0001. PD, Parkinson’s disease.

Figure 3

Model depicting the association of the rs3129882 single-nucleotide polymorphism (SNP) with altered major histocompatibility complex class II (MHC-II) expression on antigen-presenting cells and the potential for skewing the adaptive immune response and the predicted effects on vulnerable neuronal populations affected in Parkinson’s disease (PD). Our data suggest that the rs3129882 SNP is linked to a genetic or epigenetic element that increases messenger RNA expression of MHC-II and ultimately surface expression of MHC-II. Increased surface expression of MHC-II can influence CD4+ T-cell activation and differentiation leading to a heightened proinflammatory state that hastens neuronal dysfunction and death and predisposed individuals to PD. Two individuals exposed to the same environmental stimulus, one with the low-risk SNP (a) and the other with the high-risk SNP (b), are likely to respond differently immunologically because of underlying genetic and epigenetic mechanism(s) that influence MHC-II expression.