Fucosidosis is an ultra-rare and fatal lysosomal storage disease caused by the impaired lysosomal degradation of fucosylated glycoconjugates due to a deficiency in the lysosomal tissue α-L-fucosidase (FUCA1). The accumulation of fucosylated metabolites within lysosomes leads to a range of severe, primarily neurological, symptoms, including cognitive impairment and progressive motor dysfunction. In this study, we explored a therapeutic approach using translational readthrough (TR) for patients with premature termination codons resulting from nonsense mutations in the FUCA1 gene. We ectopically expressed several clinically identified FUCA1 nonsense variants in a cell line with low endogenous FUCA1 expression. Treatment with the aminoglycoside G418 induced TR, leading to partial recovery of the full-length enzyme and FUCA1 activity. Moreover, combining aminoglycoside treatment with CC-885-induced degradation of the eukaryotic release factor subunit eRF3a further enhanced FUCA1 restoration in two variants (p.Q82X and p.W188X). This study lays the groundwork for individualized TR therapy for patients with fucosidosis with FUCA1 nonsense variants.

Fucosidosis is a rare, autosomal recessive lysosomal storage disorder caused by deficiency of an alpha-L-fucosidase due to pathogenic variants in the FUCA1 gene, leading to the accumulation of fucoglyco-conjugates in the lysosomes of the liver, brain, skin and other organs. Its main clinical features include progressive neurological deterioration, seizures, coarse facies, visceromegaly, angiokeratoma, growth retardation, recurrent sinopulmonary infections and dysostosis multiplex. Three patients with fucosidosis from two unrelated families with severe developmental delay, hearing loss, coarse facies but no hepatosplenomegaly and angiokeratoma are presented. A homozygous, novel nonsense mutation c.236G>A (p.Trp79*) in exon 1 of the FUCA1 gene was identified in one family, and a homozygous novel 64.5 kb deletion, including HMGCL (exons 1-6), FUCA1, and CNR2 (exon 2) genes in the other. Fucosidosis should be considered in patients with delayed motor and cognitive development followed by progressive neurological deterioration, even in the absence of common features such as organomegaly and angiokeratoma. The pathogenic variants identified in both families were novel and consistent with fucosidosis type 1. To our knowledge, this is the first reported case of fucosidosis accompanied by 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase deficiency resulting from a contiguous gene deletion involving the HMGCL gene at the 1p36.11 locus.

α-L-fucosidases (EC 3.2.1.51) are of particular interest due to their ability to cleave terminal α-L-fucose residues from glycoconjugates, a property associated with numerous biological and therapeutic effects. They have also been investigated for their potential use in glycan remodeling, disease biomarker analysis, and particularly as therapeutic agents in the context of fucosidosis, a rare lysosomal storage disorder, caused by a deficiency in α-L-fucosidase activity. However, limitations in enzyme availability, stability, and substrate specificity highlight the need for novel and more efficient enzyme sources. Bifidobacterium castoris (B. castor is) is a newly identified species first discovered in the beaver gut microbiota in 2019. Phylogenetic studies have revealed its advanced metabolic capacity, and genomic analyses have demonstrated its extensive carbohydrate metabolism potential. This research article focuses on the recombinant production and biochemical characterization of a novel α-L-fucosidase from B. castoris LMG (Laboratorium voor Microbiologie Gent) 30937, predicted to belong to glycoside hydrolase family 29 (GH29) according to Universal Protein Resource (UniProt) annotation. Under optimized reaction conditions the recombinant α-L-fucosidase exhibited a specific activity of 0.264 U/mg to pNP-Fuc (4-Nitrophenyl-α-L-fucopyranoside). The results indicate that the enzyme is active in the pH range of 3.0-8.0 and temperatures of 24-42 °C, but its optimum conditions are the slightly acidic pH of 5.5 and the elevated temperature of 42 °C. This profile suggests that the enzyme is adapted to acidic intestinal-like environments. This novel enzyme expands the GH29 α-L-fucosidase repertoire and offers a promising new candidate for future biotechnological applications.

Fucose-containing glycoproteins and glycolipids broadly occur in humans as well as in many other species and are essential for a wide range of physiological processes, such as cell adhesion, fertilization, and tumor development. In humans, the cellular degradation of various fucosylated glycoconjugates depends on the FUCA1-encoded lysosomal tissue α-L-fucosidase (FUCA1). The crucial role of FUCA1 is reflected by the severe lysosomal storage disease fucosidosis, which causes a massive accumulation of fucosylated glycans, glycolipids, and α(1,6)-fucosylated glycoasparagines. Therefore, it is reasonable to assume that FUCA1 is predominantly responsible for the degradation of fucosylated glycoconjugates, although a second, functionally uncharacterized α-L-fucosidase, the plasma α-L-fucosidase (FUCA2), is known. To investigate the impact of both fucosidases in more detail, we generated two different monoclonal antibodies as useful tools for the detection of human and murine FUCA1 and utilized a FUCA2-specific antibody to demonstrate that FUCA2 is a bona fide lysosomal protein that is sorted in a mannose 6-phosphate (M6P)-dependent manner. We then compared FUCA1 and FUCA2 upon ectopic expression and evaluated their enzyme activity profiles under various conditions. Untagged and differently tagged versions of FUCA1 exhibited α-L-fucosidase activity, while various FUCA2 derivatives, even after affinity purification, did not show any fucosidase activity against commonly used pseudo-substrates. Our findings suggest that FUCA1 and not FUCA2 is exclusively responsible for the lysosomal de-fucosylation of glycoconjugates.

Fucosidosis is a rare lysosomal storage disease caused by α-L-fucosidase deficiency following a mutation in the FUCA1 gene. This enzyme is responsible for breaking down fucose-containing glycoproteins, glycolipids, and oligosaccharides within the lysosome. Mutations in FUCA1 result in either reduced enzyme activity or complete loss of function, leading to the accumulation of fucose-rich substrates in lysosomes. Lysosomes become engorged with undigested substrates, which leads to secondary storage defects affecting other metabolic pathways. The central nervous system is particularly vulnerable, with lysosomal dysfunction causing microglial activation, inflammation, and neuronal loss, leading to the neurodegenerative symptoms of fucosidosis. Neuroinflammation contributes to secondary damage, including neuronal apoptosis, axonal degeneration, and synaptic dysfunction, exacerbating the disease process. Chronic neuroinflammation impairs synaptic plasticity and neuronal survival, leading to progressive intellectual disability, learning difficulties, and loss of previously acquired skills. Inflammatory cytokines and lysosomal burden in motor neurons and associated pathways contribute to ataxia, spasticity, and hypotonia, which are common motor symptoms in fucosidosis. Elevated neuroinflammatory markers can increase neuronal excitability, leading to the frequent occurrence of epilepsy in affected individuals. So, fucosidosis is characterized by rapid mental and motor loss, along with growth retardation, coarse facial features, hepatosplenomegaly, telangiectasis or angiokeratomas, epilepsy, inguinal hernia, and dysostosis multiplex. Patients usually die at an early age. Treatment of fucosidosis is a great challenge, and there is currently no definitive effective treatment. Hematopoietic cell transplantation studies are ongoing in the treatment of fucosidosis. However, early diagnosis of this disease and treatment can be effective. In addition, the body's immune system decreases due to chemotherapy applied after transplantation, leaving the body vulnerable to microbes and infections, and the risk of death is high with this treatment. In another treatment method, gene therapy, the use of retroviral vectors, is promising due to their easy integration, high cell efficiency, and safety. In another treatment approach, enzyme replacement therapy, preclinical studies are ongoing for fucosidosis, but the blood-brain barrier is a major obstacle in lysosomal storage diseases affecting the central nervous system. Early diagnosis is important in fucosidosis, a rare disease, due to the delay in the diagnosis of patients identified so far and the rapid progression of the disease. In addition, enzyme replacement therapy, which carries fewer risks, is promising.