A Novel TFG Mutation in a Korean Family with α-Synucleinopathy and Amyotrophic Lateral Sclerosis

Cell type-specific gene therapy confers protection against motor neuron disease caused by a TFG variant

Inherited forms of motor neuron disease (MND), including hereditary spastic paraplegias (HSP), are associated with the death or dysfunction of nerve cells that control skeletal muscle activity. However, in some cases, the impacts of genetic variants underlying MND act in a non-cell autonomous manner, instead affecting the function of other cell types necessary for neuronal maintenance. Pathological mutations in TFG, which have been implicated in HSP, lead to axonopathy within the corticospinal tract, but it remains unclear whether this problem arises due to perturbations within neurons or supporting neuroglia. To address this question, we leveraged a rat model harboring the recessive TFG p.R106C mutation (mRATBN7.2, g.11:43897639C>T, c.316C>T), which recapitulates multiple phenotypes associated with HSP in humans, including progressive motor deficits, leg spasticity, and indications of an inflammatory response within the motor cortex. In particular, we took advantage of cell type-specific gene therapies to demonstrate that the reintroduction of wild-type TFG into synapsin 1-positive neurons provides robust protection against MND, whereas its expression in GFAP-positive glial cells provides no significant improvement in quantitative measures of gait, despite a dramatic reduction in the presence of reactive astrocytes throughout the brain. These data strongly suggest that therapeutic approaches targeting neurons should be pursued in cases of TFG-HSP, with our animal model offering a unique platform for preclinical assessment.

Multiple roles for TFG ring complexes in neuronal cargo trafficking

Pathological variants in Trk-fused gene (TFG) have been implicated in a variety of neurodegenerative conditions. In particular, mutations within its amino-terminal PB1 domain have been suggested to cause hereditary spastic paraplegia (HSP), resulting in progressive lower limb spasticity and weakness. The structural basis for this effect is unknown. Here, we combine X-ray crystallography and cryo-electron microscopy to determine a structural model of TFG, demonstrating the mechanism by which it forms octameric ring complexes. A network of electrostatic and hydrophobic interactions defines the interface between protomers. Moreover, we show that mutations identified previously in HSP patients disrupt this interface, destabilizing octamers, which ultimately leads to axonopathy. Surprisingly, the impacts of these variants are not equivalent in vivo, highlighting the existence of multiple, distinct mechanisms by which TFG mutations contribute to neurodegenerative disease.

Novel TFG mutation causes autosomal-dominant spastic paraplegia and defects in autophagy

BACKGROUND

Mutations in the tropomyosin receptor kinase fused (TFG) gene are associated with various neurological disorders, including autosomal recessive hereditary spastic paraplegia (HSP), autosomal dominant hereditary motor and sensory neuropathy with proximal dominant involvement (HMSN-P) and autosomal dominant type of Charcot-Marie-Tooth disease type 2.

METHODS

Whole genome sequencing and whole-exome sequencing were used, followed by Sanger sequencing for validation. Haplotype analysis was performed to confirm the inheritance mode of the novel TFG mutation in a large Chinese family with HSP. Additionally, another family diagnosed with HMSN-P and carrying the reported TFG mutation was studied. Clinical data and muscle pathology comparisons were drawn between patients with HSP and patients with HMSN-P. Furthermore, functional studies using skin fibroblasts derived from patients with HSP and patients with HMSN-P were conducted to investigate the pathomechanisms of TFG mutations.

RESULTS

A novel heterozygous TFG variant (NM_006070.6: c.125G>A (p.R42Q)) was identified and caused pure HSP. We further confirmed that the well-documented recessively inherited spastic paraplegia, caused by homozygous TFG mutations, exists in a dominantly inherited form. Although the clinical features and muscle pathology between patients with HSP and patients with HMSN-P were distinct, skin fibroblasts derived from both patient groups exhibited reduced levels of autophagy-related proteins and the presence of TFG-positive puncta.

CONCLUSIONS

Our findings suggest that autophagy impairment may serve as a common pathomechanism among different clinical phenotypes caused by TFG mutations. Consequently, targeting autophagy may facilitate the development of a uniform treatment for TFG-related neurological disorders.

Parkinson's Disease, Parkinsonisms, and Mitochondria: the Role of Nuclear and Mitochondrial DNA

PURPOSE OF REVIEW

Overwhelming evidence indicates that mitochondrial dysfunction is a central factor in Parkinson's disease (PD) pathophysiology. This paper aims to review the latest literature published, focusing on genetic defects and expression alterations affecting mitochondria-associated genes, in support of their key role in PD pathogenesis.

RECENT FINDINGS

Thanks to the use of new omics approaches, a growing number of studies are discovering alterations affecting genes with mitochondrial functions in patients with PD and parkinsonisms. These genetic alterations include pathogenic single-nucleotide variants, polymorphisms acting as risk factors, and transcriptome modifications, affecting both nuclear and mitochondrial genes. We will focus on alterations of mitochondria-associated genes described by studies conducted on patients or on animal/cellular models of PD or parkinsonisms. We will comment how these findings can be taken into consideration for improving the diagnostic procedures or for deepening our knowledge on the role of mitochondrial dysfunctions in PD.

A novel TFG variant of uncertain significance in amyotrophic lateral sclerosis: A case report and review of literature

Introduction

Amyotrophic lateral sclerosis is a neurodegenerative disease with wide variation of genetics associated with it. Among the different genes described, mutation in TFG is a rare finding in amyotrophic lateral sclerosis.

Case presentation

A 35 years old right-handed male presenting with ipsilateral weakness was diagnosed with amyotrophic lateral sclerosis. He was found to have missense variant of TFG with uncertain significance on exome sequencing.

Clinical discussion

The genetics involved in amyotrophic lateral sclerosis is ever-evolving. The identification of new TFG variant in this disease adds another evidence to the role of TFG in neurodegenerative disease.

Conclusions

The finding of TFG variant of uncertain significance is a rare finding in amyotrophic lateral sclerosis. And with the identification of new TFG variant, it leads to further understanding of spectrum of TFG and its pathophysiology in amyotrophic lateral sclerosis.

TFG regulates secretory and endosomal sorting pathways in neurons to promote their activity and maintenance

Molecular pathways that intrinsically regulate neuronal maintenance are poorly understood, but rare pathogenic mutations that underlie neurodegenerative disease can offer important insights into the mechanisms that facilitate lifelong neuronal function. Here, we leverage a rat model to demonstrate directly that the TFG p.R106C variant implicated previously in complicated forms of hereditary spastic paraplegia (HSP) underlies progressive spastic paraparesis with accompanying ventriculomegaly and thinning of the corpus callosum, consistent with disease phenotypes identified in adolescent patients. Analyses of primary cortical neurons obtained from CRISPR-Cas9-edited animals reveal a kinetic delay in biosynthetic secretory protein transport from the endoplasmic reticulum (ER), in agreement with prior induced pluripotent stem cell-based studies. Moreover, we identify an unexpected role for TFG in the trafficking of Rab4A-positive recycling endosomes specifically within axons and dendrites. Impaired TFG function compromises the transport of at least a subset of endosomal cargoes, which we show results in down-regulated inhibitory receptor signaling that may contribute to excitation-inhibition imbalances. In contrast, the morphology and trafficking of other organelles, including mitochondria and lysosomes, are unaffected by the TFG p.R106C mutation. Our findings demonstrate a multifaceted role for TFG in secretory and endosomal protein sorting that is unique to cells of the central nervous system and highlight the importance of these pathways to maintenance of corticospinal tract motor neurons.

Parkinsonian Syndromes in Motor Neuron Disease: A Clinical Study

Background

Parkinsonian syndromes may rarely occur in motor neuron disease (MND). However, previous studies are heterogeneous and mostly case reports or small case series. Therefore, we aimed to identify and characterize patients with concurrent parkinsonian syndromes extracted from a cohort of 1,042 consecutive cases diagnosed with MND at a tertiary Italian Center.

Methods

Diagnosis of Parkinson's disease (PD), progressive supranuclear palsy (PSP) and corticobasal syndrome (CBS) was made according to current criteria. Clinical characterization included: upper and lower motor neuron disease features, typical and atypical parkinsonian features, oculomotor disorders, cognitive testing, MRI features, and, when available molecular neuroimaging. Genetic testing was carried out for major MND and PD-associated genes.

Results

Parkinsonian syndromes were diagnosed in 18/1042 (1.7%) of MND patients (7 PD, 6 PSP, 3 CBS, 2 other parkinsonisms). Based on phenotype, patients could be categorized into amyotrophic lateral sclerosis (ALS)-parkinsonism and primary lateral sclerosis (PLS)-parkinsonism clusters. Across the whole database, parkinsonism was significantly more common in PLS than in other MND phenotypes (12.1 vs. 1.1%, p = 5.0 × 10⁻¹⁰). MND patients with parkinsonian features had older age of onset, higher frequency of oculomotor disorders, cognitive impairment, and family history of parkinsonism or dementia. Two patients showed pathogenic mutations in TARDBP and C9orf72 genes.

Conclusion

Specific patterns in MND-parkinsonism were observed, with PLS patients often showing atypical parkinsonian syndromes and ALS patients more frequently showing typical PD. Systematic clinical, genetic, and neuropathologic characterization may provide a better understanding of these phenotypes.

Mutation Screening of TFG in α-Synucleinopathy and Amyotrophic Lateral Sclerosis

BACKGROUND

Recently, p.R383H in TFG was identified as the disease cause in a family with α-synucleinopathy and amyotrophic lateral sclerosis (ALS). However, no further replication has been conducted in larger cohorts.

OBJECTIVE

The aim was to explore the genetic role of TFG in α-synucleinopathy and ALS.

METHODS

We analyzed the rare protein-coding variants in patients with Parkinson's disease (PD), ALS, multiple system atrophy (MSA), spastic paraplegia (N = 2709), and 7536 controls with whole-exome sequencing.

RESULTS

Nine rare variants were identified in PD and two in MSA. One PD patient carried the same variant p.R383H. Similarly, this patient developed early-onset PD with bradykinesia and rigidity on the left side as the initial symptoms. However, at the gene level, rare variants of TFG were not enriched in patients.

CONCLUSIONS

Rare variants of TFG were not enriched in α-synucleinopathy and ALS. However, we could not deny the potential pathogenicity of specific variants such as p.R383H. Further exploration is still necessary. © 2022 International Parkinson and Movement Disorder Society.