Post-translational modifications (PTMs) affecting proteins during or after their synthesis play a crucial role in their localization and function. The modification of these PTMs under pathophysiological conditions, i.e., their appearance, disappearance, or variation in quantity caused by a pathological environment or a mutation, corresponds to post-translational variants (PTVs). These PTVs can be directly or indirectly involved in the pathophysiology of diseases. Here, we present the PTMs and PTVs of four major amyotrophic lateral sclerosis (ALS) proteins, SOD1, TDP-43, FUS, and TBK1. These modifications involve acetylation, phosphorylation, methylation, ubiquitination, SUMOylation, and enzymatic cleavage. We list the PTM positions known to be mutated in ALS patients and discuss the roles of PTVs in the pathophysiological processes of ALS. In-depth knowledge of the PTMs and PTVs of ALS proteins is needed to better understand their role in the disease. We believe it is also crucial for developing new therapies that may be more effective in ALS.

Abnormal cytoplasmic aggregates containing the TDP-43 protein and its fragments are present in the central nervous system of the majority of patients with amyotrophic lateral sclerosis (ALS) and in patients with frontotemporal lobar degeneration (FTLD). Many studies have focused on the C-terminal cleavage products of TDP-43 (CTFs), but few have focused on the N-terminal products (NTFs), yet several works and their protein domain composition support the involvement of NTFs in pathophysiology. In the present study, we expressed six NTFs of TDP-43, normally generated in vivo by proteases or following the presence of pathogenic genetic truncating variants, in HEK-293T cells. The N-terminal domain (NTD) alone was not sufficient to produce aggregates. Fragments containing the NTD and all or part of the RRM1 domain produced nuclear aggregates without affecting cell viability. Only large fragments also containing the RRM2 domain, with or without the glycine-rich domain, produced cytoplasmic aggregates. Of these, only NTFs containing even a very short portion of the glycine-rich domain caused a reduction in cell viability. Our results provide insights into the involvement of different TDP-43 domains in the formation of nuclear or cytoplasmic aggregates and support the idea that work on the development of therapeutic molecules targeting TDP-43 must also take into account NTFs and, in particular, those containing even a small part of the glycine-rich domain.

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterized by progressive loss of motor neurons in the brain and spinal cord. While most cases are sporadic, around 10% are familial. Recent genetic studies show that many apparently isolated cases carry pathogenic mutations, highlighting the importance of penetrance, the probability that a causal mutation manifests clinically. This review focuses on mutation penetrance in ALS (C9orf72, SOD1, TARDBP, FUS genes), its variability across genes, age, and environmental or genetic modifiers, and its implications for genetic counseling. Identification of pathogenic mutations informs the monitoring of relatives and, in some cases, gives access to targeted therapies or clinical trials. Counseling of asymptomatic relatives must consider incomplete penetrance, which can lead to delayed or absent disease manifestation. ALS exists on a clinical and genetic continuum including related disorders, such as frontotemporal dementia, further influencing risk interpretation. Advances in panel, whole-exome and whole-genome sequencing refine our understanding of penetrance and enable precise diagnostics, and potential tailored therapies. Understanding penetrance is therefore essential to translate mutation discovery into informed clinical decisions and genetic counseling in ALS.