The TARDBP gene is closely associated with pathogenesis of amyotrophic lateral sclerosis (ALS), but is it an angel or a demon? [Ten Deadly Sins of Rare Diseases Ⅲ]

Step into the 'Ten Deadly Sins of Rare Diseases' column, where we unravel the mechanisms underlying the occurrence and development of rare diseases, delve into gene therapy advancements, and explore innovative preclinical model development and drug screening  strategies to drive effective translational research outcomes. Let's take a look at the previous reviews:

1. Exploring RHO-Related Pathogenic Mechanisms and RHO Gene Therapy Research Progress [Ten Deadly Sins of Rare DiseasesⅠ]

2. Why are humanized mice more suitable for hemophilia research, which has a higher prevalence in men? [Ten Deadly Sins of Rare Diseases Ⅱ]

In this issue, we focus on a rare progressive neurodegenerative disease called Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig's disease or motor neuron disease. Early symptoms of ALS typically include weakness in the limbs and muscle spasms; eventually it may progress to include neuropathic pain or cognitive/behavioral impairments in late stages. Following disease progression, muscle strength diminishes as ALS even affects control of muscles needed to speak, eat and breathe, which leads to respiratory insufficiency, severe weight loss, and eventually reaching the late stage of the disease and necessitating full-time care for the patient.

Nine years ago, the ALS Ice Bucket Challenge sparked a wave of social media responses that turned the world's attention towards ALS. Today, research on ALS continues to make relentless progress. As one of the important pathogenic genes associated with ALS, TARDBP (TDP-43) plays both positive and negative roles under different physiological and pathological conditions, reminiscent of a "fallen angel."

Pathogenic Mechanisms of TARDBP

TARDBP, also known as TAR DNA-binding protein 43 (TDP-43), is a protein encoded by the TARDBP gene. Approximately 95% of ALS cases and about 50% of FTD (frontotemporal dementia) cases exhibit cytoplasmic inclusions containing TDP-43 in neurons. Pathological hallmarks of ALS include abnormal accumulation of TDP-43 protein and mis-localization in the cytoplasm.

TDP-43 is primarily expressed in the cell nucleus and acts as a DNA and RNA binding protein. Under normal physiological conditions, it plays multiple important roles in overall cell function, including  transcription, translation, mRNA transport, mRNA stability, microRNA (miRNA) and long non-coding RNA (lncRNA) processing.

Why refer to TARDBP as a "fallen angel"? Under normal physiological conditions, TDP-43 has the ability to shuttle between nucleus and cytoplasm. However, under pathological conditions, TDP-43 aggregates in the cytoplasm, exhibiting high levels of phosphorylation or ubiquitination, which leads to a significant reduction in solubility^[1]. The "angel" from "heaven" (the nucleus) turns into a "demon" when TDP-43 becomes mis-localized in the cytoplasm, resulting in cellular toxicity. Additionally, within the cytoplasm, TDP-43 continues to exert its RNA-binding function, leading to abnormal functionality which contributes to the neurodegenerative changes in neurons.

This metaphor of the “fallen angel” highlights the transformation of TDP-43 from its normal role in the nucleus to its pathological behavior in the cytoplasm, where it generates cellular toxicity and contributes to neurodegeneration.

Mutations in the TARDBP Gene

Most cases of ALS are sporadic ALS (sALS), which means they occur without a family history and accounts for approximately 90% of cases. A smaller percentage of cases (~10%) are familial ALS (fALS), which means at least one individual within a family is an ALS patient. There are about 50 genes that have been reported to be associated with ALS pathogenesis. Among them, the following genes have been extensively studied for their mutations: superoxide dismutase 1 (SOD1) gene, chromosome 9 open reading frame 72 (C9orf72) gene, fused in sarcoma (FUS) gene, and TARDBP gene.

Most ALS-related mutations occur in exon 6 of the TARDBP gene. The most common missense mutations are A382T and M337V. Mutations in the C-terminal region of TDP-43 enhance its intrinsic tendency to aggregate. The following diagram (Figure 1) illustrates the main distribution of mutations in the TARDBP gene. Pathogenic mutations have also been identified in intronic and UTR regions of TARDBP^[2].

Figure 1. The major distribution of mutations in the TARDBP gene^[2].

Targeting TARDBP and other Genes for ALS Therapy

Regarding the drug pipelines for ALS treatment, the SOD1 gene is predominantly targeted, followed by TARDBP. However, there is one antisense oligonucleotide (ASO) therapy targeting FUS mRNA in Phase 3 Clinical trials as of April 2023, as shown in the table below.^[3] Currently, the majority of TARDBP-targeted drugs are at the preclinical stage, and related research is experiencing a continuous rise in interest and momentum — a very promising combination of trends for the potential treatment of ALS.

Therapies in Clinical Trials (As of April 2023)

* Also being tested for people with sporadic ALS
** Also being tested for people with frontotemporal dementia (FTD) ^[3]

To accelerate ALS drug development, Cyagen has launched the Next-Generation Humanized Mouse Model Construction Program - HUGO-GT™ (Humanized Genomic Ortholog for Gene Therapy) Program.  Currently, the most commonly-used humanized models only achieve partial insertion of the human gene into the mouse genome, which is . Our gene humanization approach involves the in situ replacement of mouse genes by whole human genomic DNA: as done with our full-length genomic DNA humanized TARDBP (hTARDBP) and hFUS mice. This approach allows for the construction of preclinical research models for ALS that closely mimic the biological mechanisms in the real world and encompass a broader range of intervention targets. To this end, we have introduced hot-spot mutations (MT) based on the wild-type (WT) humanized models for Fus and Tardbp, as well as developed transgenic (TG) disease models that provide more ideal humanized mouse models for studying genetic diseases and developing effec gene therapy drugs.

HUGO-GT™ mouse models encompass a broader range of intervention targets and provide more ideal models for genetic disease studies and gene therapy drug development for diseases including ALS.

Contact us to request a quote for our hTARDBP, hFUS and Sod1 models of ALS or a free consultation for your custom model.

References:

[1]Ederle H , Dormann D .TDP‐43 and FUS en route from the nucleus to the cytoplasm[J].FEBS Letters, 2017, 591(11).DOI:10.1002/1873-3468.12646.

[2]Prasad A , Bharathi V , Sivalingam V ,et al.Molecular Mechanisms of TDP-43 Misfolding and Pathology in Amyotrophic Lateral Sclerosis[J].Frontiers in Molecular Neuroscience, 2019, 12:25-.DOI:10.3389/fnmol.2019.00025.

[3] https://www.als.org/research/als-research-topics/genetics/therapies-targeting-als-linked-genetic