Genetic analysis of transcription factors in dopaminergic neuronal development in Parkinson's disease

BACKGROUND

Genetic variants of dopaminergic transcription factor-encoding genes are suggested to be Parkinson's disease (PD) risk factors; however, no comprehensive analyses of these genes in patients with PD have been undertaken. Therefore, we aimed to genetically analyze 16 dopaminergic transcription factor genes in Chinese patients with PD.

METHODS

Whole-exome sequencing (WES) was performed using a Chinese cohort comprising 1917 unrelated patients with familial or sporadic early-onset PD and 1652 controls. Additionally, whole-genome sequencing (WGS) was performed using another Chinese cohort comprising 1962 unrelated patients with sporadic late-onset PD and 1279 controls.

RESULTS

We detected 308 rare and 208 rare protein-altering variants in the WES and WGS cohorts, respectively. Gene-based association analyses of rare variants suggested that MSX1 is enriched in sporadic late-onset PD. However, the significance did not pass the Bonferroni correction. Meanwhile, 72 and 1730 common variants were found in the WES and WGS cohorts, respectively. Unfortunately, single-variant logistic association analyses did not identify significant associations between common variants and PD.

CONCLUSIONS

Variants of 16 typical dopaminergic transcription factors might not be major genetic risk factors for PD in Chinese patients. However, we highlight the complexity of PD and the need for extensive research elucidating its etiology.

The Crucial Roles of Pitx3 in Midbrain Dopaminergic Neuron Development and Parkinson's Disease-Associated Neurodegeneration

The degeneration of midbrain dopaminergic (mDA) neurons, particularly in the substantia nigra pars compacta (SNc), is one of the most prominent pathological hallmarks of Parkinson's disease (PD). To uncover the pathogenic mechanisms of mDA neuronal death during PD may provide therapeutic targets to prevent mDA neuronal loss and slow down the disease's progression. Paired-like homeodomain transcription factor 3 (Pitx3) is selectively expressed in the mDA neurons as early as embryonic day 11.5 and plays a critical role in mDA neuron terminal differentiation and subset specification. Moreover, Pitx3-deficient mice exhibit some canonical PD-related features, including the profound loss of SNc mDA neurons, a dramatic decrease in striatal dopamine (DA) levels, and motor abnormalities. However, the precise role of Pitx3 in progressive PD and how this gene contributes to mDA neuronal specification during early stages remains unclear. In this review, we updated the latest findings on Pitx3 by summarizing the crosstalk between Pitx3 and its associated transcription factors in mDA neuron development. We further explored the potential benefits of Pitx3 as a therapeutic target for PD in the future. To better understand the transcriptional network of Pitx3 in mDA neuron development may provide insights into Pitx3-related clinical drug-targeting research and therapeutic approaches.

Developmental pathways linked to the vulnerability of adult midbrain dopaminergic neurons to neurodegeneration

The degeneration of dopaminergic and other neurons in the aging brain is considered a process starting well beyond the infantile and juvenile period. In contrast to other dopamine-associated neuropsychiatric disorders, such as schizophrenia and drug addiction, typically diagnosed during adolescence or young adulthood and, thus, thought to be rooted in the developing brain, Parkinson's Disease (PD) is rarely viewed as such. However, evidences have accumulated suggesting that several factors might contribute to an increased vulnerability to death of the dopaminergic neurons at an already very early (developmental) phase in life. Despite the remarkable ability of the brain to compensate such dopamine deficits, the early loss or dysfunction of these neurons might predispose an individual to suffer from PD because the critical threshold of dopamine function will be reached much earlier in life, even if the time-course and strength of naturally occurring and age-dependent dopaminergic cell death is not markedly altered in this individual. Several signaling and transcriptional pathways required for the proper embryonic development of the midbrain dopaminergic neurons, which are the most affected in PD, either continue to be active in the adult mammalian midbrain or are reactivated at the transition to adulthood and under neurotoxic conditions. The persistent activity of these pathways often has neuroprotective functions in adult midbrain dopaminergic neurons, whereas the reactivation of silenced pathways under pathological conditions can promote the survival and even regeneration of these neurons in the lesioned or aging brain. This article summarizes our current knowledge about signaling and transcription factors involved in midbrain dopaminergic neuron development, whose reduced gene dosage or signaling activity are implicated in a lower survival rate of these neurons in the postnatal or aging brain. It also discusses the evidences supporting the neuroprotection of the midbrain dopaminergic system after the external supply or ectopic expression of some of these secreted and nuclear factors in the adult and aging brain. Altogether, the timely monitoring and/or correction of these signaling and transcriptional pathways might be a promising approach to a much earlier diagnosis and/or prevention of PD.

Genetics of cognitive dysfunction in Parkinson's disease

Presentation and progression of cognitive symptoms in Parkinson's disease are highly variable. PD is a genetically complex disorder with multiple genetic risk factors and understanding the role that genes play in cognitive outcomes is important for patient counseling and treatment. Currently, there are seven well-described genes that increase the risk for PD, with variable levels of penetrance: SNCA, LRRK2, VPS35, PRKN, PINK1, DJ1 and GBA. In addition, large, genome-wide association studies have identified multiple loci in our DNA which increase PD risk. In this chapter, we summarize what is currently known about each of the seven strongly-associated PD genes and select PD risk variants, including PITX3, TMEM106B, SNCA Rep1, APOɛ4, COMT and MAPT H1/H1, along with their respective relationships to cognition.

Can Biomarkers Help the Early Diagnosis of Parkinson's Disease?

Parkinson's disease (PD) is a complex neurodegenerative disease with progressive loss of dopamine neurons. PD patients usually manifest a series of motor and non-motor symptoms. In order to provide better early diagnosis and subsequent disease-modifying therapies for PD patients, there is an urgent need to identify sensitive and specific biomarkers. Biomarkers can be divided into four categories: clinical, imaging, biochemical, and genetic. Ideal biomarkers not only improve our understanding of PD pathogenesis and progression, but also provide benefits for early risk evaluation and clinical diagnosis of PD. Although many efforts have been made and several biomarkers have been extensively investigated, few if any have been found useful for early diagnosis. Here, we summarize recent developments in the discovered biomarkers of PD and discuss their merits and limitations for the early diagnosis of PD.

Specificity of Pitx3-Dependent Gene Regulatory Networks in Subsets of Midbrain Dopamine Neurons

Dysfunction of midbrain dopaminergic (mDA) neurons is involved in Parkinson's disease (PD) and neuropsychiatric disorders. Pitx3 is expressed in mDA neuron subsets of the substantia nigra compacta (SNc) and of the ventral tegmental area (VTA) that are degeneration-sensitive in PD. The genetic network(s) and mode(s) of action of Pitx3 in these mDA neurons remain poorly characterized. We hypothesized that, given their distinct neuronal identities, Pitx3-expressing neurons of SNc and VTA should differ in their Pitx3-controlled gene expression networks and this may involve subset-specific co-regulators. Expression profiling of purified mDA neuronal subsets indicates that Pitx3 regulates different sets of genes in SNc and VTA, such as activating the expression of primary cilium gene products specifically in VTA neurons. Interaction network analysis pointed to the participation of differentially expressed Lhx/Lmo family members in the modulation of Pitx3 action in SNc and VTA mDA neurons. Conversely, global binding patterns of Pitx3 on genomic DNA of human dopaminergic cells revealed that Pitx3 is often co-recruited to regions that foster the formation of GATA-bHLH-BRN complexes, which usually involve Lmo co-regulatory proteins. We focused on Lmo3 for its preferential expression in SNc neurons and demonstrated that it functions as a transcriptional co-activator of Pitx3 by enhancing its activity on genomic regulatory elements. In summary, we defined the SN and VTA-specific programs of Pitx3-dependent gene expression and identified Lmo3 as a SN-specific co-regulator of Pitx3-dependent transcription.

Neuroprotective Transcription Factors in Animal Models of Parkinson Disease

A number of transcription factors, including En1/2, Foxa1/2, Lmx1a/b, Nurr1, Otx2, and Pitx3, with key roles in midbrain dopaminergic (mDA) neuron development, also regulate adult mDA neuron survival and physiology. Mouse models with targeted disruption of some of these genes display several features reminiscent of Parkinson disease (PD), in particular the selective and progressive loss of mDA neurons in the substantia nigra pars compacta (SNpc). The characterization of these animal models has provided valuable insights into various mechanisms of PD pathogenesis. Therefore, the dissection of the mechanisms and survival signalling pathways engaged by these transcription factors to protect mDA neuron from degeneration can suggest novel therapeutic strategies. The work on En1/2-mediated neuroprotection also highlights the potential of protein transduction technology for neuroprotective approaches in PD.

Clinical improvement of the aggressive neurobehavioral phenotype in a patient with a deletion of PITX3 and the absence of L-DOPA in the cerebrospinal fluid

The development of midbrain dopamine (DA) neurons is regulated by several transcription factors, including Nurr1, Wnt1, Lmx1a/1b, En1, En2, Foxa1, Foxa2, and Pitx3. PITX3 is an upstream co-activator of the TH (tyrosine hydroxylase) promoter. Pitx3(-/-) mice have a selective loss of dopaminergic neurons in the substantia nigra and ventral tegmental area, leading to the significantly reduced DA levels in the nigrostriatal pathway and in the dorsal striatum and manifest anomalous striatum-dependent cognitive impairment and neurobehavioral activity. Treatment with L-DOPA, dopamine, or dopamine receptor agonists in these mice reversed several of their sensorimotor impairments. Heterozygous missense mutations in PITX3 have been reported in patients with autosomal dominant congenital cataract and anterior segment (ocular) mesenchymal dysgenesis (ASMD) whereas homozygous missense mutations have been found in patients with microphthalmia and neurological impairment. Using a clinical oligonucleotide array comparative genomic hybridization (aCGH), we have identified an ∼317 kb hemizygous deletion in 10q24.32, involving PITX3 in a 17-year-old male with a Smith-Magenis syndrome-like phenotype, including mild intellectual impairment, sleep disturbance, hyperactivity, and aggressive and self-destructive behavior. Interestingly, no eye anomalies were found in our patient. Analysis of neurotransmitters in his cerebrospinal fluid revealed an absence of L-DOPA and significantly decreased levels of catecholamine metabolites. Importantly, L-DOPA treatment of our patient has led to mild mitigation of his aggressive behavior and mild improvement of his attention span, extended time periods of concentration, and better sleep.

PITX3 polymorphism is not associated with Parkinson's disease in a Chinese population

Several studies have indicated that three PITX3 single nucleotide polymorphisms (SNPs), rs2281983, rs4919621 and rs3758549, are likely to be associated with Parkinson's disease (PD) in Caucasians. Some studies also suggested an age-of-onset effect. We recently reported that allele and genotype frequencies did not differ between late-onset PD (LOPD) patients and controls for all three SNPs. To extend the analysis to early-onset PD (EOPD) patients, and to test whether an age-of-onset effect exists in Chinese, we genotyped these SNPs in 290 Chinese EOPD patients using a ligase detection reaction (LDR). For all three SNPs, allele and genotype frequencies did not differ between total PD patients and controls, between LOPD patients and controls, between EOPD patients and controls, or between LOPD and EOPD patients. Our results suggest that these PITX3 SNPs do not contribute to the risk of developing PD in EOPD or LOPD in Chinese.