Neurodegenerative diseases represent a major threat to human health. Their prevalence increased in the last years due to the rise of the elderly population and, despite our understanding of mechanisms leading to neurodegeneration has increased in the past 25 years, there are no available treatments that significantly modify the course of these diseases. Therefore, there is an urgent need to identify more effective therapies.
One of the main problems is that most genes identified as altered in the expression are not readily targeted with current small-molecule therapeutics because the exact spatial conformation of these targets is unknown. In this sense, targeting the RNA (ribonucleotide nucleic acid) represents a unique strategy because it allows the modulation of the expression of a protein without the need of knowing its tridimensional structure.
Antisense oligonucleotides (ASOs) are synthetic oligonucleotides that are designed to bind to RNA by Watson-Crick base pairing. After binding to the targeted RNA, ASOs can modulate the function of the targeted RNA by two different mechanisms. The first one is the degradation of the pre-mRNA in the nucleus or mature RNA in the cytoplasm by the RNase H1 enzyme or the RISC complex (Ago2). This strategy is useful to decrease the expression of a pathologically overexpressed protein. Differently, ASOs can be used to modulate RNA function by regulating the splicing or polyadenylation of the pre-messenger RNA (pre-mRNA). In the last few years, a series of clinical trials highlighted the impact of ASOs in the treatment of different neurological disorders.
The first FDA-approved ASO is Nusinersen and was used to treat spinal muscular atrophy (SMA). SMA is an inherited disease characterized by muscle atrophy and weakness. Symptoms manifest in children at 6 months of age or younger and life expectancy is less than 2 years. SMA is caused by mutations in the survival motor neuron 1 (SMN1) gene that eventually lead to motor neurons degeneration.
SMN1 gene has a paralogous gene, called SMN2 that also encodes for the SMN protein. However, 90 to 95% of this protein is truncated and non-functional because of aberrant splicing. It has been shown that children with SMA that have a higher copy number of SMN2 generally have also a milder phenotype. Therefore, correction of SMN2 pre-mRNA splicing to promote increased production of SMN protein was investigated as a putative new therapeutic strategy and the new drug Nusinersen came out. Nusinersen is an antisense oligonucleotide drug that binds to SMN2 pre-mRNA and modifies its splicing to promote increased production of full-length SMN protein. Two phase 3 clinical studies were initiated (Finkel et al. 2017, Mercuri et al. 2018) that demonstrated the efficacy of Nusinersen in the treatment of SMA. Patients treated with the drug had improved motor-milestone response and likelihood of survival without overall side effects. Based on the striking results from the phase 3 studies the US Food and Drug Administration (FDA) approved Nusinersen for the treatment of SMA.
The Nusinersen case demonstrated that ASOs can be safely used in humans to reduce symptoms in patients with severe neurodegenerative diseases. Since then, several ASOs have been developed for the treatment of other neurodegenerative diseases and are now in clinical trials. Some examples are amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD), and Alzheimer’s disease (AD).
Another interesting peculiarity of ASOs is the possibility to be customized in a sequence-specific fashion and their relative simplicity of manufacturing if compared to small molecule-based drugs.
These characteristics make ASOs perfect for the rapid delivery of individualized treatments in critical patients. An astonishing example is the development of a customized ASO for a child with a unique mutation causing a rare and fatal neurodegenerative disease within 1 year from the first contact with the patient. The child showed a unique mutation in the ceroid lipofuscinosis 7 (CLN7) gene that led to a form of Batten’s disease, a rare severe disorder. The patient had blindness, ataxia, seizures, and developmental regression and after one year she was hospitalized after the rapid development of these symptoms. The authors identified a unique insertion of an SVA (SINE–VNTR–Alu) retrotransposon modulating the splicing of the gene that resulted in premature translational termination. A 22-nucleotide antisense oligonucleotide that masked the SVA site and restored normal splicing of the pre-mRNA was developed and called Milasen. Treatment of the child with Milasen has been accompanied by objective reductions in the frequency and duration of seizures. Moreover, the drug appears to have had an acceptable side-effect profile, with no safety concerns.
This study illustrates the ability to deploy a novel therapeutic agent for a patient with a rare disease based on the understanding of her specific pathogenic mutation in a limited time. It is an example of individualized genomic medicine and highlights, together with previous examples in this article, the broad potential of antisense oligonucleotide therapy for the treatment of neurological disorders.