Increased 5-Methylcytosine and Decreased 5-Hydroxymethylcytosine Levels are Associated with Reduced Striatal A2AR Levels in Huntington’s Disease

Abstract

Adenosine A2A receptor (A2AR) is a G-protein-coupled receptor highly expressed in basal ganglia. Its expression levels are severely reduced in Huntington's disease (HD), and several pharmacological therapies have shown its implication in this neurodegenerative disorder. The main goal of this study was to gain insight into the molecular mechanisms that regulate A2AR gene (ADORA2A) expression in HD. Based on previous data reported by our group, we measured the methylcytosine (5mC) and hydroxymethylcytosine (5hmC) content in the 5'UTR region of ADORA2A in the putamen of HD patients and in the striatum of R6/1 and R6/2 mice at late stages of the disease. In this genomic region, 5mC and 5hmC remained unchanged in both mice strains, although low striatal A2AR levels were associated with reduced 5mC levels in 30-week-old R6/1 mice and reduced 5hmC levels in 12-week-old R6/2 mice in exon m2. In order to analyze when this mechanism appears during the progression of the disease, a time course for A2AR protein levels was carried out in R6/1 mice striatum (8, 12, and 20 weeks of age). A2AR levels were reduced from 12 weeks of age onwards, and this downregulation was concomitant with reduced 5hmC levels in the 5'UTR region of ADORA2A. Interestingly, increased 5mC levels and reduced 5hmC were found in the 5'UTR region of ADORA2A in the putamen of HD patients with respect to age-matched controls. Therefore, an altered DNA methylation pattern in ADORA2A seems to play a role in the pathologically decreased A2AR expression levels found in HD.

The Role of Adenosine Tone and Adenosine Receptors in Huntington's Disease

“…152,160,[164][165][166] Animal and cell models of HD have been fundamental in identifying the molecular mechanisms by which mHtt causes a reduction in the expression of A 2A R. Most of these models clearly show A 2A R downregulation at the protein and transcription level, but there are conflicting results when tested in the H46, YAC72, and Tg51 transgenic models. 153,164,[167][168][169][170][171] Aggregated mHtt causes changes in gene expression profiles leading to specific protein-protein interactions with several transcription factors, [172][173][174][175] while mHtt does not change gene expression relating to cytoskeleton proteins, enzymes of metabolism, mitochondrial proteins, and caspases. 164,176 Chiang et al found that mHtt expression significantly reduced the transcript levels of endogenous A 2A R in PC12 cells and striatal neurons in culture.…”

Implication of Epigenetic Modifications in Neurodegenerative Disorders: Traces and Imprints

“…The genetic components responsible for their development have already been discussed above. Clinical studies aimed at treatment of Alzheimer's have affirmed a possible approach wherein in vitro or dietary administration of the components required for production of SAM like folate, homocysteine and B -group vitamins can be utilized to restore methylation levels of PSEN1, protract brain atrophy and minimize oxidative stress (Figure 3) [76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94]. In addition, several HDAC inhibitors have been developed for the therapeutic strategies targeted for Alzheimer's.…”

Epigenetic Mechanisms of Neurodegeneration in Huntington's Disease

“…Altered DNA methylation has recently been found in HD patients and HD transgenic mice [44][45][46]. Interestingly, Ng et al [44] showed that the promoter regions of the Ap-1, Sox2, Pax6, and Nes genes are highly methylated, and that expression levels of these genes are significantly reduced.…”

“…It has been shown that the adenosine A (2A) receptor (A2AR) is markedly reduced in HD and it has been implicated as a potential therapeutic target, but regulation of the A2AR gene (ADORA2A) expression is not known in HD [45]. A recent study shows that while the reduction of A2AR is correlated with increased levels of 5-methylcytosine in the 5′-UTR region of ADORA2A gene in the striatum of HD patients, A2AR down-regulation is correlated with a reduction of 5-hydroxymethylcytosine in the 5′-UTR region of the ADORA2A gene in HD transgenic mice [46].…”