Pathogenic Huntingtin Repeat Expansions in Patients with Frontotemporal Dementia and Amyotrophic Lateral Sclerosis

Motoneuron Diseases

Motoneuron diseases (MNDs) represent a heterogeneous group of progressive paralytic disorders, mainly characterized by the loss of upper (corticospinal) motoneurons, lower (spinal) motoneurons or, often both. MNDs can occur from birth to adulthood and have a highly variable clinical presentation, even within gene-positive forms, suggesting the existence of environmental and genetic modifiers. A combination of cell autonomous and non-cell autonomous mechanisms contributes to motoneuron degeneration in MNDs, suggesting multifactorial pathogenic processes.

N-alpha-acetylation of Huntingtin protein increases its propensity to aggregate

Huntington’s disease (HD) is a neurodegenerative disorder caused by a poly-CAG expansion in the first exon of the HTT gene, resulting in an extended poly-glutamine tract in the N-terminal domain of the Huntingtin (Htt) protein product. Proteolytic fragments of the poly-glutamine–containing N-terminal domain form intranuclear aggregates that are correlated with HD. Post-translational modification of Htt has been shown to alter its function and aggregation properties. However, the effect of N-terminal Htt acetylation has not yet been considered. Here, we developed a bacterial system to produce unmodified or N-terminally acetylated and aggregation-inducible Htt protein. We used this system together with biochemical, biophysical, and imaging studies to confirm that the Htt N-terminus is an in vitro substrate for the NatA N-terminal acetyltransferase and show that N-terminal acetylation promotes aggregation. These studies represent the first link between N-terminal acetylation and the promotion of a neurodegenerative disease and implicates NatA-mediated Htt acetylation as a new potential therapeutic target in HD.

Neurofilament Light Chain and Intermediate HTT Alleles as Combined Biomarkers in Italian ALS Patients

Objective

To study the possible implication of the two biomarkers, intermediate alleles (IAs) of the Huntingtin (HTT) gene and neurofilament light chain (NfL) levels in plasma, in amyotrophic lateral sclerosis (ALS) patients.

Methods

We analyzed IAs in a cohort of 106 Italian ALS patients and measured the plasma NfL levels in 20% of the patients of the cohort. We correlated the two biomarkers with clinical phenotypes.

Results

Intermediate alleles were present in 7.5% of the patients of our cohort, a frequency higher than that reported in general population. Plasma NfL levels increased with age at onset (p < 0.05). Patients with bulbar onset (BO) had higher plasma NfL concentration (CI −0.61 to −0.06, p = 0.02) and a later age at onset of the disease (CI −24.78 to −4.93, p = 0.006) with respect to the spinal onset (SO) form. Additionally, two of the patients, with IAs and plasma NfL concentration lower with respect to normal alleles’ carriers, presented an age at onset higher than the mean of the entire cohort.

Conclusion

According to our findings, plasma NfL and IAs of HTT gene may represent potential biomarkers in ALS, providing evidence of a possible implication in clinical phenotype.

Molecular mechanisms underlying nucleotide repeat expansion disorders

The human genome contains over one million short tandem repeats. Expansion of a subset of these repeat tracts underlies over fifty human disorders, including common genetic causes of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (C9orf72), polyglutamine-associated ataxias and Huntington disease, myotonic dystrophy, and intellectual disability disorders such as Fragile X syndrome. In this Review, we discuss the four major mechanisms by which expansion of short tandem repeats causes disease: loss of function through transcription repression, RNA-mediated gain of function through gelation and sequestration of RNA-binding proteins, gain of function of canonically translated repeat-harbouring proteins, and repeat-associated non-AUG translation of toxic repeat peptides. Somatic repeat instability amplifies these mechanisms and influences both disease age of onset and tissue specificity of pathogenic features. We focus on the crosstalk between these disease mechanisms, and argue that they often synergize to drive pathogenesis. We also discuss the emerging native functions of repeat elements and how their dynamics might contribute to disease at a larger scale than currently appreciated. Lastly, we propose that lynchpins tying these disease mechanisms and native functions together offer promising therapeutic targets with potential shared applications across this class of human disorders.

A series of cases with Huntington-like phenotype and intermediate repeats in HTT

BACKGROUND

Intermediate Alleles (IAs) are expansions of CAG repeats in the HTT gene between 27 and 35 repeats which pathogenic meaning remains controversial. They are present in the general population but there is an increasing number of cases with Huntington-like phenotype reported.

METHODS

We reviewed the medical records of cases in our centre where the neurologist suspected Huntington's disease (HD) as one of the feasible diagnoses and genetic testing showed the number of CAG repeats was in the "intermediate range". We gathered the type of symptoms in all cases and the main neuroimaging findings when available.

RESULTS

We found 14 cases, 8 males and 6 females, with average age at onset at 64 years old. Most cases exhibited some type of extrapyramidal symptoms. Cognitive and/or behavioral symptoms were also present in most cases (being depression, anxiety and cognitive impairment the most frequent ones). In one case we found deposits of iron in the basal ganglia in the MRI, and in another case we found diffuse cortical hypometabolism with predominantly frontal bilateral involvement and bilateral focal deficit of both caudate and thalamus in the FDG-PET.

CONCLUSION

The clinical and neuroimaging findings of some cases with IA in this series are compatible with the clinical picture of HD but also with several other alternative diagnoses. Therefore we can not establish association between IA and HD. Larger series with more comprehensive diagnostic workout and neuropathological studies are needed to confirm or rule out whether IAs in the HTT gene may cause HD.

30 years of repeat expansion disorders: What have we learned and what are the remaining challenges?

Tandem repeats represent one of the most abundant class of variations in human genomes, which are polymorphic by nature and become highly unstable in a length-dependent manner. The expansion of repeat length across generations is a well-established process that results in human disorders mainly affecting the central nervous system. At least 50 disorders associated with expansion loci have been described to date, with half recognized only in the last ten years, as prior methodological difficulties limited their identification. These limitations still apply to the current widely used molecular diagnostic methods (exome or gene panels) and thus result in missed diagnosis detrimental to affected individuals and their families, especially for disorders that are very rare and/or clinically not recognizable. Most of these disorders have been identified through family-driven approaches and many others likely remain to be identified. The recent development of long-read technologies provides a unique opportunity to systematically investigate the contribution of tandem repeats and repeat expansions to the genetic architecture of human disorders. In this review, we summarize the current and most recent knowledge about the genetics of repeat expansion disorders and the diversity of their pathophysiological mechanisms and outline the perspectives of developing personalized treatments in the future.

Combating deleterious phase transitions in neurodegenerative disease

Protein aggregation is a hallmark of neurodegenerative diseases. However, the mechanism that induces pathogenic aggregation is not well understood. Recently, it has emerged that several of the pathological proteins found in an aggregated or mislocalized state in neurodegenerative diseases are also able to undergo liquid-liquid phase separation (LLPS) under physiological conditions. Although these phase transitions are likely important for various physiological functions, neurodegenerative disease-related mutations and conditions can alter the LLPS behavior of these proteins, which can elicit toxicity. Therefore, therapeutics that antagonize aberrant LLPS may be able to mitigate toxicity and aggregation that is ubiquitous in neurodegenerative disease. Here, we discuss the mechanisms by which aberrant protein phase transitions may contribute to neurodegenerative disease. We also outline potential therapeutic strategies to counter deleterious phases. State without borders: Membrane-less organelles and liquid-liquid phase transitions edited by Vladimir N Uversky.

Huntington's disease: lessons from prion disorders

Decades of research on the prion protein and its associated diseases have caused a paradigm shift in our understanding of infectious agents. More recent years have been marked by a surge of studies supporting the application of these findings to a broad array of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Here, we present evidence to suggest that Huntington's disease, a monogenic disorder of the central nervous system, shares features with prion disorders and that, it too, may be governed by similar mechanisms. We further posit that these similarities could suggest that, like other common neurodegenerative disorders, sporadic forms of Huntington's disease may exist.