CCG polymorphisms in the huntingtin gene have no effect on the pathogenesis of patients with Huntington's disease in mainland Chinese families

E-motif formed by extrahelical cytosine bases in DNA homoduplexes of trinucleotide and hexanucleotide repeats

Atypical DNA secondary structures play an important role in expandable trinucleotide repeat (TR) and hexanucleotide repeat (HR) diseases. The cytosine mismatches in C-rich homoduplexes and hairpin stems are weakly bonded; experiments show that for certain sequences these may flip out of the helix core, forming an unusual structure termed an ‘e-motif’. We have performed molecular dynamics simulations of C-rich TR and HR DNA homoduplexes in order to characterize the conformations, stability and dynamics of formation of the e-motif, where the mismatched cytosines symmetrically flip out in the minor groove, pointing their base moieties towards the 5′-direction in each strand. TRs have two non-equivalent reading frames, (GCC)_n and (CCG)_n; while HRs have three: (CCCGGC)_n, (CGGCCC)_n, (CCCCGG)_n. We define three types of pseudo basepair steps related to the mismatches and show that the e-motif is only stable in (GCC)_n and (CCCGGC)_n homoduplexes due to the favorable stacking of pseudo GpC steps (whose nature depends on whether TRs or HRs are involved) and the formation of hydrogen bonds between the mismatched cytosine at position i and the cytosine (TRs) or guanine (HRs) at position i − 2 along the same strand. We also characterize the extended e-motif, where all mismatched cytosines are extruded, their extra-helical stacking additionally stabilizing the homoduplexes.

Genetic modifiers of age-at-onset in polyglutamine diseases

Polyglutamine (polyQ) diseases are a group of clinically and genetically heterogeneous neurodegenerative diseases. Expansion size correlates with age-at-onset (AO) and severity, and shows a critical threshold for each polyQ disease. Although an expanded CAG tract is sufficient to trigger disease, not all variation in AO is explained by (CAG)_n length, which suggests the contribution of other modifying factors. Methods used to identify genetic modifiers in polyQ diseases have progressed from candidate genes to unbiased genome-wide searches. Inconsistency of results from candidate-genes studies are partly explained by sample size, study design and variable population frequency of "polymorphisms"; a genome-wide search may help elucidating more precise disease mechanisms underlying specific interaction networks. We review known genetic modifiers for polyQ diseases, and discuss developing strategies to find modulation, from common variants to networks disclosing small cumulative effects of key genes and modifying pathways. This may lead to a better understanding of genotype-phenotype correlation and the proposal of new potential targets for therapeutical interventions.

KEAP1-modifying small molecule reveals muted NRF2 signaling responses in neural stem cells from Huntington's disease patients

The activity of the transcription factor nuclear factor-erythroid 2 p45-derived factor 2 (NRF2) is orchestrated and amplified through enhanced transcription of antioxidant and antiinflammatory target genes. The present study has characterized a triazole-containing inducer of NRF2 and elucidated the mechanism by which this molecule activates NRF2 signaling. In a highly selective manner, the compound covalently modifies a critical stress-sensor cysteine (C151) of the E3 ligase substrate adaptor protein Kelch-like ECH-associated protein 1 (KEAP1), the primary negative regulator of NRF2. We further used this inducer to probe the functional consequences of selective activation of NRF2 signaling in Huntington's disease (HD) mouse and human model systems. Surprisingly, we discovered a muted NRF2 activation response in human HD neural stem cells, which was restored by genetic correction of the disease-causing mutation. In contrast, selective activation of NRF2 signaling potently repressed the release of the proinflammatory cytokine IL-6 in primary mouse HD and WT microglia and astrocytes. Moreover, in primary monocytes from HD patients and healthy subjects, NRF2 induction repressed expression of the proinflammatory cytokines IL-1, IL-6, IL-8, and TNFα. Together, our results demonstrate a multifaceted protective potential of NRF2 signaling in key cell types relevant to HD pathology.

Development of Research on Huntington Disease in China

Huntington disease (HD) is a progressive autosomal dominantly inherited neurodegenerative disorder, characterized with the typical manifestations of involuntary movements, cognitive dysfunction, and psychiatric or behavioral disturbance. It results from an expansion in the number of CAG repeats in the first exon of the huntingtin (HTT) gene. In China, since the first case report in 1959, the knowledge of this disorder has been involving a lot, especially in the latest decade. In this review, we meta-analysis and summarize the research reports that were published by Chinese researchers since 1959, so that researchers whose native language were not Chinese can get a general idea of the research development of HD in China. Briefly, the research of HD in China can be broadly divided into three stages. Firstly, before 1993, there were scattered case reports of HD that were solely based on Clinical features and family history. Then, with the discovery of the HD gene in 1993, it became possible for the genetic confirmation of the reported cases that made the diagnosis more accurate and informative. In the last few years, Chinese researchers who were active in the HD research started to build their own database to study the clinical and genetic feature of this disorder and also collaborated a lot in this field. The progress outlined in this review indicates the beginning of an exciting new era in HD research in China.

Huntington Disease in Asia

Objective:

The objective was to review the major differences of Huntington disease (HD) in Asian population from those in the Caucasian population.

Data Sources:

Data cited in this review were obtained from PubMed database and China National Knowledge Infrastructure (CNKI) from 1994 to 2014. All the papers were written in English or Chinese languages, with the terms of Asia/Asian, HD, genotype, epidemiology, phenotype, and treatment used for the literature search.

Study Selection:

From the PubMed database, we included the articles and reviews which contained the HD patients’ data from Asian countries. From the CNKI, we excluded the papers which were not original research. Due to the language's restrictions, those data published in other languages were not included.

Results:

In total, 50 papers were cited in this review, authors of which were from the mainland of China, Japan, India, Thailand, Taiwan (China), Korea, and western countries.

Conclusions:

The lower epidemiology in Asians can be partly explained by the less cytosine-adenine-guanine repeats, different haplotypes, and CCG polymorphisms. For the physicians, atypical clinical profiles such as the initial symptom of ataxia, movement abnormalities of Parkinsonism, dystonia, or tics need to be paid more attention to and suggest gene testing if necessary. Moreover, some pathogenesis studies may help progress some new advanced treatments. The clinicians in Asian especially in China should promote the usage of genetic testing and put more effects in rehabilitation, palliative care, and offer comfort of patients and their families. The unified HD rating scale also needs to be popularized in Asia to assist in evaluating the progression of HD.

Huntington's Disease: Relationship Between Phenotype and Genotype

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disease with the typical manifestations of involuntary movements, psychiatric and behavior disorders, and cognitive impairment. It is caused by the dynamic mutation in CAG triplet repeat number in exon 1 of huntingtin (HTT) gene. The symptoms of HD especially the age at onset are related to the genetic characteristics, both the CAG triplet repeat and the modified factors. Here, we reviewed the recent advancement on the genotype-phenotype relationship of HD, mainly focus on the characteristics of different expanded CAG repeat number, genetic modifiers, and CCG repeat number in the 3' end of CAG triplet repeat and their effects on the phenotype. We also reviewed the special forms of HD (juvenile HD, atypical onset HD, and homozygous HD) and their phenotype-genotype correlations. The review will aid clinicians to predict the onset age and disease course of HD, give the genetic counseling, and accelerate research into the HD mechanism.

Structural studies of CNG repeats

CNG repeats (where N denotes one of the four natural nucleotides) are abundant in the human genome. Their tendency to undergo expansion can lead to hereditary diseases known as TREDs (trinucleotide repeat expansion disorders). The toxic factor can be protein, if the abnormal gene is expressed, or the gene transcript, or both. The gene transcripts have attracted much attention in the biomedical community, but their molecular structures have only recently been investigated. Model RNA molecules comprising CNG repeats fold into long hairpins whose stems generally conform to an A-type helix, in which the non-canonical N-N pairs are flanked by C-G and G-C pairs. Each homobasic pair is accommodated in the helical context in a unique manner, with consequences for the local helical parameters, solvent structure, electrostatic potential and potential to interact with ligands. The detailed three-dimensional profiles of RNA CNG repeats can be used in screening of compound libraries for potential therapeutics and in structure-based drug design. Here is a brief survey of the CNG structures published to date.

Crystallographic characterization of CCG repeats

CCG repeats are highly over-represented in exons of the human genome. Usually they are located in the 5′ UTR but are also abundant in translated sequences. The CCG repeats are associated with three tri-nucleotide repeat disorders: Huntington’s disease, myotonic dystrophy type 1 and chromosome X-linked mental retardation (FRAXE). In this study, we present two crystal structures containing double-stranded CCG repeats: one of an RNA in the native form, and one containing LNA nucleotides. Both duplexes form A-helices but with strands slipped in the 5′ (native structure) or the 3′ direction (LNA-containing structure). As a result, one of two expected C-C pairs is eliminated from the duplex. Each of the three observed C-C pairs interacts differently, forming either one weak H-bond or none. LNA nucleotides have no apparent effect on the helical parameters but the base stacking is increased compared to the native duplex and the distribution of electrostatic potential in the major groove is changed. The CCG crystal structures explain the thermodynamic fragility of CCG runs and throw light on the observation that the MBNL1 protein recognises CCG runs, as well as CUG and CAG, but not the relatively stable CGG repeats.