Role for ATXN1, ATXN2, and HTT intermediate repeats in frontotemporal dementia and Alzheimer's disease

Huntingtin CAG repeats in neuropathologically confirmed tauopathies: Novel insights

Previous studies have suggested a relationship between the number of CAG triplet repeats in the HTT gene and neurodegenerative diseases not related to Huntington's disease (HD). This study seeks to investigate whether the number of CAG repeats of HTT is associated with the risk of developing certain tauopathies and its influence as a modulator of the clinical and neuropathological phenotype. Additionally, it aims to evaluate the potential of polyglutamine staining as a neuropathological screening. We genotyped the HTT gene CAG repeat number and APOE-ℰ isoforms in a cohort of patients with neuropathological diagnoses of tauopathies (n=588), including 34 corticobasal degeneration (CBD), 98 progressive supranuclear palsy (PSP) and 456 Alzheimer's disease (AD). Furthermore, we genotyped a control group of 1070 patients, of whom 44 were neuropathologic controls. We identified significant differences in the number of patients with pathological HTT expansions in the CBD group (2.7%) and PSP group (3.2%) compared to control subjects (0.2%). A significant increase in the size of the HTT CAG repeats was found in the AD compared to the control group, influenced by the presence of the Apoliprotein E (APOE)-ℰ4 isoform. Post-mortem assessments uncovered tauopathy pathology with positive polyglutamine aggregates, with a slight predominance in the neostriatum for PSP and CBD cases and somewhat greater limbic involvement in the AD case. Our results indicated a link between HTT CAG repeat expansion with other non-HD pathology, suggesting they could share common neurodegenerative pathways. These findings support that genetic or histological screening for HTT repeat expansions should be considered in tauopathies.

Selective microRNA expression of exosomes from retinal pigment epithelial cells by oxidative stress

The function of exosomal miRNAs (miRs) in retinal degeneration is largely unclear. We were aimed to investigate the functions of exosomes as well as their miRs derived from retinal pigment epithelial (RPE) cells following exposure to oxidative stress (OS). After the OS by lipopolysaccharide and rotenone on RPE cells, interleukin-1 beta (IL-1β), Interleukin-6 (IL-6), Tumor Necrosis Factor-alpha (TNF-α) were upregulated, along with the decreased mitochondrial membrane potential and upregulated oxidative damage marker 8-OH-dG in RPE cells. RPE-derived exosomes were then isolated, identified, injected into the subretinal space in mice. After subretinal injection, RPE-exosomes after OS not only induced higher ROS level and apoptotic retinal cells, but also elevated IL-1β, IL-6 alongside TNF-α expressions among retina/RPE/choroidal complex. Next, miRs inside the exosomes were sequenced by the next generation sequencing (NGS) technology. NGS revealed that certain miRs were abundant in exosomes, while others were selectively kept by RPE cells. Further, downregulated miRs, like miR-125b-5p, miR-125a-5p, alongside miR-128-3p, and upregulated miR, such as miR-7-5p were validated byRT-qPCR. Finally, Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were used to find the possible target genes of those selective exosomal miRs. Our results proved that the RPE-derived exosomes after OS selectively express certain miRs, providing novel insights into the pathogenesis of age-related macular degeneration (AMD) in future.

Somatic CAG repeat instability in intermediate alleles of the HTT gene and its potential association with a clinical phenotype

Huntington disease (HD) is a neurodegenerative disorder caused by ≥36 CAGs in the HTT gene. Intermediate alleles (IAs) (27-35 CAGs) are not considered HD-causing, but their potential association with neurocognitive symptoms remains controversial. As HTT somatic CAG expansion influences HD onset, we hypothesised that IAs are somatically unstable, and that somatic CAG expansion may drive phenotypic presentation in some IA carriers. We quantified HTT somatic CAG expansions by MiSeq sequencing in the blood DNA of 164 HD subjects and 191 IA (symptomatic and control) carriers, and in the brain DNA of a symptomatic 33 CAG carrier. We also performed genotype-phenotype analysis. The phenotype of symptomatic IA carriers was characterised by motor (85%), cognitive (27%) and/or behavioural (29%) signs, with a late (58.7 ± 18.6 years), but not CAG-dependent, age at onset. IAs displayed somatic expansion that were CAG and age-dependent in blood DNA, with 0.4% and 0.01% of DNA molecules expanding by CAG and year, respectively. Somatic expansions of +1 and +2 CAGs were detected in the brain of the individual with 33 CAGs, with the highest expansion frequency in the putamen (10.3%) and the lowest in the cerebellum (4.8%). Somatic expansion in blood DNA was not different in symptomatic vs. control IA carriers. In conclusion, we show that HTT IAs are somatically unstable, but we found no association with HD-like phenotypes. It is plausible, however, that some IAs, close to the HD pathological threshold and with a predisposing genetic background, could manifest with neurocognitive symptoms.

The Alzheimer's Knowledge Base: A Knowledge Graph for Alzheimer Disease Research

BACKGROUND

As global populations age and become susceptible to neurodegenerative illnesses, new therapies for Alzheimer disease (AD) are urgently needed. Existing data resources for drug discovery and repurposing fail to capture relationships central to the disease's etiology and response to drugs.

OBJECTIVE

We designed the Alzheimer's Knowledge Base (AlzKB) to alleviate this need by providing a comprehensive knowledge representation of AD etiology and candidate therapeutics.

METHODS

We designed the AlzKB as a large, heterogeneous graph knowledge base assembled using 22 diverse external data sources describing biological and pharmaceutical entities at different levels of organization (eg, chemicals, genes, anatomy, and diseases). AlzKB uses a Web Ontology Language 2 ontology to enforce semantic consistency and allow for ontological inference. We provide a public version of AlzKB and allow users to run and modify local versions of the knowledge base.

RESULTS

AlzKB is freely available on the web and currently contains 118,902 entities with 1,309,527 relationships between those entities. To demonstrate its value, we used graph data science and machine learning to (1) propose new therapeutic targets based on similarities of AD to Parkinson disease and (2) repurpose existing drugs that may treat AD. For each use case, AlzKB recovers known therapeutic associations while proposing biologically plausible new ones.

CONCLUSIONS

AlzKB is a new, publicly available knowledge resource that enables researchers to discover complex translational associations for AD drug discovery. Through 2 use cases, we show that it is a valuable tool for proposing novel therapeutic hypotheses based on public biomedical knowledge.

Intermediate Repeat Expansion in the ATXN2 Gene as a Risk Factor in the ALS and FTD Spanish Population

Intermediate CAG expansions in the gene ataxin-2 (ATXN2) are a known risk factor for ALS, but little is known about their role in FTD risk. Moreover, their contribution to the risk and phenotype of patients might vary in populations with different genetic backgrounds. The aim of this study was to assess the relationship of intermediate CAG expansions in ATXN2 with the risk and phenotype of ALS and FTD in the Spanish population. Repeat-primed PCR was performed in 620 ALS and 137 FTD patients in three referral centers in Spain to determine the exact number of CAG repeats. In our cohort, ≥27 CAG repeats in ATXN2 were associated with a higher risk of developing ALS (odds ratio [OR] = 2.666 [1.471-4.882]; p = 0.0013) but not FTD (odds ratio [OR] = 1.446 [0.558-3.574]; p = 0.44). Moreover, ALS patients with ≥27 CAG repeats in ATXN2 showed a shorter survival rate compared to those with <27 repeats (hazard ratio [HR] 1.74 [1.18, 2.56], p = 0.005), more frequent limb onset (odds ratio [OR] = 2.34 [1.093-4.936]; p = 0.028) and a family history of ALS (odds ratio [OR] = 2.538 [1.375-4.634]; p = 0.002). Intermediate CAG expansions of ≥27 repeats in ATXN2 are associated with ALS risk but not with FTD in the Spanish population. ALS patients carrying an intermediate expansion in ATXN2 show more frequent limb onset but a worse prognosis than those without expansions. In patients carrying C9orf72 expansions, the intermediate ATXN2 expansion might increase the penetrance and modify the phenotype.

RNA-binding deficient TDP-43 drives cognitive decline in a mouse model of TDP-43 proteinopathy

TDP-43 proteinopathies including frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative disorders characterized by aggregation and mislocalization of the nucleic acid-binding protein TDP-43 and subsequent neuronal dysfunction. Here, we developed endogenous models of sporadic TDP-43 proteinopathy based on the principle that disease-associated TDP-43 acetylation at lysine 145 (K145) alters TDP-43 conformation, impairs RNA-binding capacity, and induces downstream mis-regulation of target genes. Expression of acetylation-mimic TDP-43K145Q resulted in stress-induced nuclear TDP-43 foci and loss of TDP-43 function in primary mouse and human-induced pluripotent stem cell (hiPSC)-derived cortical neurons. Mice harboring the TDP-43K145Q mutation recapitulated key hallmarks of FTLD, including progressive TDP-43 phosphorylation and insolubility, TDP-43 mis-localization, transcriptomic and splicing alterations, and cognitive dysfunction. Our study supports a model in which TDP-43 acetylation drives neuronal dysfunction and cognitive decline through aberrant splicing and transcription of critical genes that regulate synaptic plasticity and stress response signaling. The neurodegenerative cascade initiated by TDP-43 acetylation recapitulates many aspects of human FTLD and provides a new paradigm to further interrogate TDP-43 proteinopathies.

A spatially defined human Notch receptor interaction network reveals Notch intracellular storage and Ataxin-2-mediated fast recycling

The Notch signaling pathway controls cell growth, differentiation, and fate decisions. Dysregulation of Notch signaling has been linked to various human diseases. Notch receptor resides in multiple cellular compartments, and its translocation plays a central role in pathway activation. However, the spatial regulation of Notch receptor functions remains largely elusive. Using TurboID-based proximity labeling followed by affinity purification and mass spectrometry, we establish a spatially defined human Notch receptor interaction network. Notch receptors interact with different proteins in distinct subcellular compartments to perform specific cellular functions. This spatially defined interaction network also reveals that a large fraction of NOTCH is stored at the endoplasmic reticulum (ER)-Golgi intermediate compartment and recruits Ataxin-2-dependent recycling machinery for rapid recycling, Notch signaling activation, and leukemogenesis. Our work provides insights into dynamic Notch receptor complexes with exquisite spatial resolution, which will help in elucidating the detailed regulation of Notch receptors and highlight potential therapeutic targets for Notch-related pathogenesis.

Short tandem repeat expansions in sporadic amyotrophic lateral sclerosis and frontotemporal dementia

Pathogenic short tandem repeat (STR) expansions cause over 20 neurodegenerative diseases. To determine the contribution of STRs in sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), we used ExpansionHunter, REviewer, and polymerase chain reaction validation to assess 21 neurodegenerative disease-associated STRs in whole-genome sequencing data from 608 patients with sporadic ALS, 68 patients with sporadic FTD, and 4703 matched controls. We also propose a data-derived outlier detection method for defining allele thresholds in rare STRs. Excluding C9orf72 repeat expansions, 17.6% of clinically diagnosed ALS and FTD cases had at least one expanded STR allele reported to be pathogenic or intermediate for another neurodegenerative disease. We identified and validated 162 disease-relevant STR expansions in C9orf72 (ALS/FTD), ATXN1 [spinal cerebellar ataxia type 1 (SCA1)], ATXN2 (SCA2), ATXN8 (SCA8), TBP (SCA17), HTT (Huntington's disease), DMPK [myotonic dystrophy type 1 (DM1)], CNBP (DM2), and FMR1 (fragile-X disorders). Our findings suggest clinical and pathological pleiotropy of neurodegenerative disease genes and highlight their importance in ALS and FTD.

Native functions of short tandem repeats

Over a third of the human genome is comprised of repetitive sequences, including more than a million short tandem repeats (STRs). While studies of the pathologic consequences of repeat expansions that cause syndromic human diseases are extensive, the potential native functions of STRs are often ignored. Here, we summarize a growing body of research into the normal biological functions for repetitive elements across the genome, with a particular focus on the roles of STRs in regulating gene expression. We propose reconceptualizing the pathogenic consequences of repeat expansions as aberrancies in normal gene regulation. From this altered viewpoint, we predict that future work will reveal broader roles for STRs in neuronal function and as risk alleles for more common human neurological diseases.

TR-FRET-Based Immunoassay to Measure Ataxin-2 as a Target Engagement Marker in Spinocerebellar Ataxia Type 2

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominantly inherited neurodegenerative disease, which belongs to the trinucleotide repeat disease group with a CAG repeat expansion in exon 1 of the ATXN2 gene resulting in an ataxin-2 protein with an expanded polyglutamine (polyQ)-stretch. The disease is late manifesting leading to early death. Today, therapeutic interventions to cure the disease or even to decelerate disease progression are not available yet. Furthermore, primary readout parameter for disease progression and therapeutic intervention studies are limited. Thus, there is an urgent need for quantifiable molecular biomarkers such as ataxin-2 becoming even more important due to numerous potential protein-lowering therapeutic intervention strategies. The aim of this study was to establish a sensitive technique to measure the amount of soluble polyQ-expanded ataxin-2 in human biofluids to evaluate ataxin-2 protein levels as prognostic and/or therapeutic biomarker in SCA2. Time-resolved fluorescence energy transfer (TR-FRET) was used to establish a polyQ-expanded ataxin-2-specific immunoassay. Two different ataxin-2 antibodies and two different polyQ-binding antibodies were validated in three different concentrations and tested in cellular and animal tissue as well as in human cell lines, comparing different buffer conditions to evaluate the best assay conditions. We established a TR-FRET-based immunoassay for soluble polyQ-expanded ataxin-2 and validated measurements in human cell lines including iPSC-derived cortical neurons. Additionally, our immunoassay was sensitive enough to monitor small ataxin-2 expression changes by siRNA or starvation treatment. We successfully established the first sensitive ataxin-2 immunoassay to measure specifically soluble polyQ-expanded ataxin-2 in human biomaterials.

Psychiatric-Like Impairments in Mouse Models of Spinocerebellar Ataxias

Many patients with spinocerebellar ataxia (SCA) suffer from diverse neuropsychiatric issues, including memory impairments, apathy, depression, or anxiety. These neuropsychiatric aspects contribute per se to the reduced quality of life and worse prognosis. However, the extent to which SCA-related neuropathology directly contributes to these issues remains largely unclear. Behavioral profiling of various SCA mouse models can bring new insight into this question. This paper aims to synthesize recent findings from behavioral studies of SCA patients and mouse models. The role of SCA neuropathology for shaping psychiatric-like impairments may be exemplified in mouse models of SCA1. These mice evince robust cognitive impairments which are shaped by both the cerebellar as well as out-of-cerebellar pathology. Although emotional-related alternations are also present, they seem to be less robust and more affected by the specific distribution and character of the neuropathology. For example, cerebellar-specific pathology seems to provoke behavioral disinhibition, leading to seemingly decreased anxiety, whereas complex SCA1 neuropathology induces anxiety-like phenotype. In SCA1 mice with complex neuropathology, some of the psychiatric-like impairments are present even before marked cerebellar degeneration and ataxia and correlate with hippocampal atrophy. Similarly, complete or partial deletion of the implicated gene (Atxn1) leads to cognitive dysfunction and anxiety-like behavior, respectively, without apparent ataxia and cerebellar degeneration. Altogether, these findings collectively suggest that the neuropsychiatric issues have a biological basis partially independent of the cerebellum. As some neuropsychiatric issues may stem from weakening the function of the implicated gene, therapeutic reduction of its expression by molecular approaches may not necessarily mitigate the neuropsychiatric issues.

Intermediate repeat expansions of TBP and STUB1: Genetic modifier or pure digenic inheritance in spinocerebellar ataxias?

PURPOSE

CAG/CAA repeat expansions in TBP_>49 are responsible for spinocerebellar ataxia (SCA) type 17 (SCA17). We previously detected cosegregation of STUB1 variants causing SCA48 with intermediate alleles of TBP in 2 families. This cosegregation questions the existence of SCA48 as a monogenic disease.

METHODS

We systematically sequenced TBP repeats in 34 probands of dominant ataxia families with STUB1 variants. In addition, we searched for pathogenic STUB1 variants in probands with expanded alleles of TBP_>49 (n = 2) or intermediate alleles of TBP_≥40 (n = 47).

RESULTS

STUB1 variants were found in half of the TBP_40-49 cohort. Mirroring this finding, TBP_40-49 alleles were detected in 40% of STUB1 probands. The longer the TBP repeat length, the more likely the occurrence of cognitive impairment (P = .0129) and the faster the disease progression until death (P = .0003). Importantly, 13 STUB1 probands presenting with the full SCA48 clinical phenotype had normal TBP_37-39 alleles, excluding digenic inheritance as the sole mode.

CONCLUSION

We show that intermediate TBP_40-49 alleles act as disease modifiers of SCA48 rather than a STUB1/TBP digenic model. This distinction from what has been proposed before has crucial consequences for genetic counseling in SCA48.

Rapid and comprehensive diagnostic method for repeat expansion diseases using nanopore sequencing

We developed a diagnostic method for repeat expansion diseases using a long-read sequencer to improve currently available, low throughput diagnostic methods. We employed the real-time target enrichment system of the nanopore GridION sequencer using the adaptive sampling option, in which software-based target assignment is available without prior sample enrichment, and built an analysis pipeline that prioritized the disease-causing loci. Twenty-two patients with various neurological and neuromuscular diseases, including 12 with genetically diagnosed repeat expansion diseases and 10 manifesting cerebellar ataxia, but without genetic diagnosis, were analyzed. We first sequenced the 12 molecularly diagnosed patients and accurately confirmed expanded repeats in all with uniform depth of coverage across the loci. Next, we applied our method and a conventional method to 10 molecularly undiagnosed patients. Our method corrected inaccurate diagnoses of two patients by the conventional method. Our method is superior to conventional diagnostic methods in terms of speed, accuracy, and comprehensiveness.

Exploring the phenotype of Italian patients with ALS with intermediate ATXN2 polyQ repeats

OBJECTIVE

To detect the clinical characteristics of patients with amyotrophic lateral sclerosis (ALS) carrying an intermediate ATXN2 polyQ number of repeats in a large population-based series of Italian patients with ALS.

METHODS

The study population includes 1330 patients with ALS identified through the Piemonte and Valle d'Aosta Register for ALS, diagnosed between 2007 and 2019 and not carrying C9orf72, SOD1, TARDBP and FUS mutations. Controls were 1274 age, sex and geographically matched Italian subjects, identified through patients' general practitioners.

RESULTS

We found 42 cases and 4 controls with≥31 polyQ repeats, corresponding to an estimated OR of 10.4 (95% CI 3.3 to 29.0). Patients with≥31 polyQ repeats (ATXN2+) compared with those without repeat expansion (ATXN2-) had more frequently a spinal onset (p=0.05), a shorter diagnostic delay (p=0.004), a faster rate of ALSFRS-R progression (p=0.004) and King's progression (p=0.004), and comorbid frontotemporal dementia (7 (28.0%) vs 121 (13.4%), p=0.037). ATXN2+ patients had a 1-year shorter survival (ATXN2+ patients 1.82 years, 95% CI 1.08 to 2.51; ATXN2- 2.84 years, 95% CI 1.67 to 5.58, p=0.0001). ATXN2 polyQ intermediate repeats was independently related to a worse outcome in Cox multivariable analysis (p=0.006).

CONCLUSIONS

In our population-based cohort, ATXN2+ patients with ALS have a distinctive phenotype, characterised by a more rapid disease course and a shorter survival. In addition, ATXN2+ patients have a more severe impairment of cognitive functions. These findings have relevant implications on clinical practice, including the possibility of refining the individual prognostic prediction and improving the design of ALS clinical trials, in particular as regards as those targeted explicitly to ATXN2.

Huntingtin gene intermediate alleles influence the progression from Subjective Cognitive Decline to Mild Cognitive Impairment: a 14-year follow-up study

BACKGROUND

Huntingtin (HTT) is a gene containing a key region of CAG repeats. HTT alleles containing from 27 to 35 CAG repeats are termed as intermediate alleles (IAs). We aim to assess the effect of IAs on progression of cognitive impairment in patients with subjective cognitive decline (SCD).

METHODS

We included 106 patients with SCD. All the patients underwent neuropsychological assessments and blood sample collections at baseline. Patients were followed-up for a median time of 13.75 (IQR=8.17) years. We genotyped APOE and HTT at the end of the follow-up.

RESULTS

Eleven out of 106 patients (10.38% [95%C.I.=4.57-16.18]) were carriers of IA (IA⁺ ). During the follow-up, 44 patients (41.51% [95%C.I.=32.13-50.89]) progressed to MCI (p-SCD), while 62 patients (58.49% [95% C.I.=49.11-67.87]) did not (np-SCD). Rate of progression to MCI was associated with IAs, age at baseline, and APOE ɛ4. We dichotomized age at baseline (<60 = younger patients [YP], >60 = older patients [OP]) and classified patients into four groups: YP/IAs^- , YP/IAs⁺ , OP/IAs^-  and OP/IAs⁺ . OP/IAs⁺  had a higher proportion of progression from SCD to MCI (85.71% [95%C.I.=59.79-100]) as compared to YP/IAs^-  (28.57% [95%C.I.=13.60-43.54], χ² =15.25, p<0.001) and OP/IAs^-  (45.00% [95%C.I.=32.41-57.59], χ² =7.903, p=0.005). We classified patients according to APOE and IA as: ɛ4^- /IA^- , ɛ4^- /IA⁺ , ɛ4⁺ /IA^- , ɛ4⁺ /IA⁺ . Proportion of progression in ɛ4⁺ /IA⁺  group (100%) was higher as compared to ɛ4^- /IA^-  (33.33% [95%C.I.=21.96-44.71], χ² =14.43, p <0.001) and ɛ4⁺ /IA^-  (55.56% [95%C.I.=36.81-74.30], χ² =4.60, p=0.032).

CONCLUSIONS

IAs interact with age and APOE ɛ4 increasing the risk of progression to MCI in SCD patients.

Advancing genomic technologies and clinical awareness accelerates discovery of disease-associated tandem repeat sequences

Expansions of gene-specific DNA tandem repeats (TRs), first described in 1991 as a disease-causing mutation in humans, are now known to cause >60 phenotypes, not just disease, and not only in humans. TRs are a common form of genetic variation with biological consequences, observed, so far, in humans, dogs, plants, oysters, and yeast. Repeat diseases show atypical clinical features, genetic anticipation, and multiple and partially penetrant phenotypes among family members. Discovery of disease-causing repeat expansion loci accelerated through technological advances in DNA sequencing and computational analyses. Between 2019 and 2021, 17 new disease-causing TR expansions were reported, totaling 63 TR loci (>69 diseases), with a likelihood of more discoveries, and in more organisms. Recent and historical lessons reveal that properly assessed clinical presentations, coupled with genetic and biological awareness, can guide discovery of disease-causing unstable TRs. We highlight critical but underrecognized aspects of TR mutations. Repeat motifs may not be present in current reference genomes but will be in forthcoming gapless long-read references. Repeat motif size can be a single nucleotide to kilobases/unit. At a given locus, repeat motif sequence purity can vary with consequence. Pathogenic repeats can be "insertions" within nonpathogenic TRs. Expansions, contractions, and somatic length variations of TRs can have clinical/biological consequences. TR instabilities occur in humans and other organisms. TRs can be epigenetically modified and/or chromosomal fragile sites. We discuss the expanding field of disease-associated TR instabilities, highlighting prospects, clinical and genetic clues, tools, and challenges for further discoveries of disease-causing TR instabilities and understanding their biological and pathological impacts-a vista that is about to expand.

CAG Repeats Within the Non-pathological Range in the HTT Gene Influence Personality Traits in Patients With Subjective Cognitive Decline: A 13-Year Follow-Up Study

OBJECTIVE

HTT is a gene containing a key region of CAG repeats. When expanded beyond 39 repeats, Huntington disease (HD) develops. HTT genes with <35 repeats are not associated with HD. The biological function of CAG repeat expansion below the non-pathological threshold is not well understood. In fact higher number of repeats in HTT confer advantageous changes in brain structure and general intelligence, but several studies focused on establishing the association between CAG expansions and susceptibility to psychiatric disturbances and to other neurodegenerative disease than HD. We hypothesized that HTT CAG repeat length below the pathological threshold might influence mood and personality traits in a longitudinal sample of individuals with Subjective Cognitive Decline.

METHODS

We included 54 patients with SCD. All patients underwent an extensive neuropsychological battery at baseline, APOE genotyping and analysis of HTT alleles. We used the Big Five Factors Questionnaire (BFFQ) and Hamilton Depression Rating Scale (HDRS), respectively, to assess personality traits of patients and depression at baseline. Patients who did not progress to Mild Cognitive Impairment (MCI) had at least 5-year follow-up time.

RESULTS

In the whole sample, CAG repeat number in the shorter HTT allele was inversely correlated with conscientiousness (Pearson = -0.364, p = 0.007). There was no correlation between HDRS and CAG repeats. During the follow-up, 14 patients [25.93% (95% C.I. = 14.24-37.61)] progressed to MCI (MCI⁺) and 40 [74.07% (95% C.I. = 62.39-85.76)] did not (MCI^-). When we performed the same analysis in the MCI⁺ group we found that: CAG repeat length on the shorter allele was inversely correlated with energy (Pearson = 0.639, p = 0.014) and conscientiousness (Pearson = -0.695, p = 0.006). CAG repeat length on the longer allele was inversely correlated with conscientiousness (Pearson = -0.901, p < 0.001) and directly correlated with emotional stability (Pearson = 0.639, p = 0.014). These associations were confirmed also by multivariate analysis. We found no correlations between BFFQ parameters and CAG repeats in the MCI^- group.

DISCUSSION

Personality traits and CAG repeat length in the intermediate range have been associated with progression of cognitive decline and neuropathological findings consistent with AD. We showed that CAG repeat lengths in the HTT gene within the non-pathological range influence personality traits.

Natural selection at the RASGEF1C (GGC) repeat in human and divergent genotypes in late-onset neurocognitive disorder

Expression dysregulation of the neuron-specific gene, RASGEF1C (RasGEF Domain Family Member 1C), occurs in late-onset neurocognitive disorders (NCDs), such as Alzheimer's disease. This gene contains a (GGC)13, spanning its core promoter and 5' untranslated region (RASGEF1C-201 ENST00000361132.9). Here we sequenced the (GGC)-repeat in a sample of human subjects (N = 269), consisting of late-onset NCDs (N = 115) and controls (N = 154). We also studied the status of this STR across various primate and non-primate species based on Ensembl 103. The 6-repeat allele was the predominant allele in the controls (frequency = 0.85) and NCD patients (frequency = 0.78). The NCD genotype compartment consisted of an excess of genotypes that lacked the 6-repeat (divergent genotypes) (Mid-P exact = 0.004). A number of those genotypes were not detected in the control group (Mid-P exact = 0.007). The RASGEF1C (GGC)-repeat expanded beyond 2-repeats specifically in primates, and was at maximum length in human. We conclude that there is natural selection for the 6-repeat allele of the RASGEF1C (GGC)-repeat in human, and significant divergence from that allele in late-onset NCDs. STR alleles that are predominantly abundant and genotypes that deviate from those alleles are underappreciated features, which may have deep evolutionary and pathological consequences.

Toxicity of pathogenic ataxin-2 in Drosophila shows dependence on a pure CAG repeat sequence

Spinocerebellar ataxia type 2 is a polyglutamine (polyQ) disease associated with an expanded polyQ domain within the protein product of the ATXN2 gene. Interestingly, polyQ repeat expansions in ATXN2 are also associated with amyotrophic lateral sclerosis (ALS) and parkinsonism depending upon the length of the polyQ repeat expansion. The sequence encoding the polyQ repeat also varies with disease presentation: a pure CAG repeat is associated with SCA2, whereas the CAG repeat in ALS and parkinsonism is typically interrupted with the glutamine encoding CAA codon. Here, we asked if the purity of the CAG sequence encoding the polyQ repeat in ATXN2 could impact the toxicity of the ataxin-2 protein in vivo in Drosophila. We found that ataxin-2 encoded by a pure CAG repeat conferred toxicity in the retina and nervous system, whereas ataxin-2 encoded by a CAA-interrupted repeat or CAA-only repeat failed to confer toxicity, despite expression of the protein at similar levels. Furthermore, the CAG-encoded ataxin-2 protein aggregated in the fly eye, while ataxin-2 encoded by either a CAA/G or CAA repeat remained diffuse. The toxicity of the CAG-encoded ataxin-2 protein was also sensitive to the translation factor eIF4H, a known modifier of the toxic GGGGCC repeat in flies. These data indicate that ataxin-2 encoded by a pure CAG versus interrupted CAA/G polyQ repeat domain is associated with differential toxicity, indicating that mechanisms associated with the purity of the sequence of the polyQ domain contribute to disease.

Ataxin-2 gene: a powerful modulator of neurological disorders

PURPOSE OF REVIEW

To provide an update on the role of Ataxin-2 gene (ATXN2) in health and neurological diseases.

RECENT FINDINGS

There is a growing complexity emerging on the role of ATXN2 and its variants in association with SCA2 and several other neurological diseases. Polymorphisms and intermediate alleles in ATXN2 establish this gene as a powerful modulator of neurological diseases including lethal neurodegenerative conditions such as motor neuron disease, spinocerebellar ataxia 3 (SCA3), and peripheral nerve disease such as familial amyloidosis polyneuropathy. This role is in fact far wider than the previously described for polymorphism in the prion protein (PRNP) gene. Positive data from antisense oligo therapy in a murine model of SCA2 suggest that similar approaches may be feasible in humans SCA2 patients.

SUMMARY

ATXN2 is one of the few genes where a single gene causes several diseases and/or modifies several and disparate neurological disorders. Hence, understanding mutagenesis, genetic variants, and biological functions will help managing SCA2, and several human diseases connected with dysfunctional pathways in the brain, innate immunity, autophagy, cellular, lipid, and RNA metabolism.

The Effect of CAG Repeats within the Non-Pathological Range in the HTT Gene on Cognitive Functions in Patients with Subjective Cognitive Decline and Mild Cognitive Impairment

The Huntingtin gene (HTT) is within a class of genes containing a key region of CAG repeats. When expanded beyond 39 repeats, Huntington disease (HD) develops. Individuals with less than 35 repeats are not associated with HD. Increasing evidence has suggested that CAG repeats play a role in modulating brain development and brain function. However, very few studies have investigated the effect of CAG repeats in the non-pathological range on cognitive performances in non-demented individuals. In this study, we aimed to test how CAG repeats' length influences neuropsychological scores in patients with subjective cognitive decline (SCD) and mild cognitive impairment (MCI). We included 75 patients (46 SCD and 29 MCI). All patients underwent an extensive neuropsychological battery and analysis of HTT alleles to quantify the number of CAG repeats. Results: CAG repeat number was positively correlated with scores of tests assessing for executive function, visual-spatial ability, and memory in SCD patients, while in MCI patients, it was inversely correlated with scores of visual-spatial ability and premorbid intelligence. When we performed a multiple regression analysis, we found that these relationships still remained, also when adjusting for possible confounding factors. Interestingly, logarithmic models better described the associations between CAG repeats and neuropsychological scores. CAG repeats in the HTT gene within the non-pathological range influenced neuropsychological performances depending on global cognitive status. The logarithmic model suggested that the positive effect of CAG repeats in SCD patients decreases as the number of repeats grows.

Network analysis in aged C. elegans reveals candidate regulatory genes of ageing

Ageing is a biological process guided by genetic and environmental factors that ultimately lead to adverse outcomes for organismal lifespan and healthspan. Determination of molecular pathways that are affected with age and increase disease susceptibility is crucial. The gene expression profile of the ideal ageing model, namely the nematode Caenorhabditis elegans mapped with the microarray technology initially led to the identification of age-dependent gene expression alterations that characterize the nematode's ageing process. The list of differentially expressed genes was then utilized to construct a network of molecular interactions with their first neighbors/interactors using the interactions listed in the WormBase database. The subsequent network analysis resulted in the unbiased selection of 110 candidate genes, among which well-known ageing regulators appeared. More importantly, our approach revealed candidates that have never been linked to ageing before, thus suggesting promising potential targets/ageing regulators.

Genetic variation in APOE, GRN, and TP53 are phenotype modifiers in frontotemporal dementia

Frontotemporal dementia (FTD) is a clinical, genetic, and pathologic heterogeneous group of neurodegenerative diseases. In this study, we investigated the role of APOƐ4, rs5848 in GRN, and rs1042522 in TP53 gene as disease risk factors and/or phenotype modifiers in 440 FTD patients, including 175 C9orf72 expansion carriers. We found that the C9orf72 expansion carriers showing an earlier age at onset (p < 0.001). Among the clinical groups, the FTD-MND (motoneuron disease) showed the lowest survival (hazard ratio [HR] = 4.12), and the progressive nonfluent aphasia group showed the highest onset age (p = 0.03). In our cohort, the rs1042522 in TP53 was associated with disease onset (p = 0.02) and survival (HR = 1.73) and rs5848 GRN with a significantly shorter survival in CC homozygous patients (HR = 1.98). The frequency of APOƐ4 carriers was significantly increased in the C9orf72 noncarriers (p = 0.022). Although validation of our findings is necessary, our results suggest that TP53, GRN, and APOE genes may act as phenotype modifiers in FTD and should be considered in future clinical trials.