In this issue: Dr. Bezzerri provides an update on a therapeutic approach targeting premature termination codon (PTC) mutations in SDS, or the c.183-184TA>CT mutation in the SBDS gene to be more specific.
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Progress on the development of novel therapies targeting the c.183-184TA>CT mutation in the SBDS gene.
Thank you Dr. Valentino Bezzerri for the detailed overview and summary, to put your latest publication into context.
What is the significance of the c.183-184TA>CT mutation in SBDS gene?
A large percentage of SDS patients carry the nonsense mutation c.183-184TA>CT in addition to the most common … “splice site” mutation. For instance, the c.183-184TA>CT mutation is present in more than half ( 56%) of SDS patients in the Italian SDS registry. This genetic variant leads to the generation of a premature termination codon (PTC) on the SBDS mRNA, which leads to a stop of protein translation at the 62nd amino acid position (K62X). mRNA that harbor such nonsense mutations are generally unstable and are rapidly degraded by a cellular mechanism known as nonsense mediated decay (NMD). If any mRNA could withstand the NMD, the resulting protein would be truncated and lose most (if not all) its function.
Is there any way to address this problem?
Stop codons can occasionally be overridden by near-cognate aminoacyl-tRNA, whose anticodon is complementary just for two of the three nucleotides. This process has been defined as translational read-through and may lead to abnormal termination of translation in 0.001-0.1% of total neo-synthesized proteins. Interestingly, this process is able to endogenously promote the read-through of PTC in 0.01-1% of cases. Even though this process is quite ineffective, it served to prove that mutations generating PTC may be corrected.
Could this mechanism be exploited pharmacologically with potential clinical benefit?
Aminoglycosides are a class of natural or semisynthetic antibiotics derived from actinomycetes. In eukaryotic cells, aminoglycosides may promote the binding of a near-cognate tRNA to a PTC, displacing eRF1, thus resulting in nonsense mutation suppression. The aminoglycoside geneticin (G418) was initially investigated in cystic fibrosis (CF) cell models harboring nonsense mutated CFTR gene. Further studies demonstrated the efficacy of aminoglycosides G418 and gentamicin in restoring a significant amount of functional CFTR and dystrophin proteins in CF and Duchenne muscular dystrophy (DMD), respectively. This reports therefore represented the proof of concept that the development of translational read-through inducing drugs (TRIDs) is feasible. Unfortunately, severe adverse effects caused by prolonged treatments with aminoglycosides, including auditory and vestibular toxicities have been reported, limiting the widespread clinical use of aminoglycosides for nonsense suppression therapy.
Are there any success stories for these types of drugs?
Ataluren (PTC124) was launched in 2007 by PTC Therapeutics (NJ, USA), as a potent TRID, without antibiotic properties. Compared with classical aminoglycosides, ataluren may promote a more selective readthrough of PTC, without affecting endogenous stop codons. Furthermore, ataluren has shown less toxicity and better safety than aminoglycosides. The use of ataluren as a potential therapeutic agent for genetic disorders has been early proposed for the treatment of DMD and CF. Most importantly, ataluren has been approved for the treatment of DMD in Europe. Data from clinical trials showed that chronic ataluren treatment is beneficial to DMD patients undergoing standard care, because it delays the progression of ambulation impairment and the worsening of pulmonary and cardiac functions. Interestingly, clinical studies revealed that the best results are observed in younger individuals, suggesting major benefits of early ataluren administration.
Why is ataluren not widely used?
Despite promising pre-clinical results, ataluren unfortunately failed in clinical studies of CF. The clinical development of ataluren for CF was therefore discontinued. These premises highlight that ataluren clinical benefit in people with CF may be highly variable. Consistent with this, also approximately 39% of patients with DMD did not exhibit protein resynthesis after treatment in a Phase 2a clinical trial.
The major pitfall of ataluren readthrough efficacy remains therefore its variable efficacy, which may depend on the sequence of the PTC (UAA<UAG<UGA), as well as the PTC flanking regions. Moreover, the amount of target nonsense mutated transcript may be affected by NMD.
Has anyone tried this strategy on SDS?
To address Ataluren’s variable efficacy, an Italian research team headed by Dr. Valentino Bezzerri and Dr. Marco Cipolli (Cystic Fibrosis Center, University Hospital of Verona, Italy) tested a panel of ataluren analogues with improved in vitro efficacy. The authors tested the efficacy of analogues NV848, NV914, NV930, NV2445, and 5i on the restoration of SBDS protein expression in SDS cell models, in order to improve the clinical performance of this class of drugs. Second-generation analogues NV848, NV914, and NV930 were optimized from the lead compound NV2445, which showed promising results in correcting nonsense mutated CFTR expression in vitro, whereas the 5i compound was directly derived from ataluren. Molecule NV848 can restore SBDS protein synthesis in vitro to the same extent as ataluren, improving in vitro myelopoiesis, promoting neutrophil maturation and reducing the expression of dysplastic markers in these cells.
In addition, NV848 may have some advantages over ataluren, since it is quite hydrophilic, thus facilitating the selection of administration routes, and did not show any appreciable toxicity up to 1mM concentration in zebrafish experiments. Most importantly, NV848 has shown to be the best candidate for further development in SDS therapy, due to superior absorption, distribution, metabolism, and excretion (ADME) properties, compared to ataluren and other analogues.
There are three scientific publications on PubMed on this topic. You can find the latest article by Dr. Bezzerri, below, published last month.
Bezzerri V, Lentini L, Api M, Busilacchi EM, Cavalieri V, Pomilio A, Diomede F, Pegoraro A, Cesaro S, Poloni A, Pace A, Trubiani O, Lippi G, Pibiri I, Cipolli M.Biomedicines.
2022 Apr 12;10(4):886.