Nonsense Suppression Therapy: New Hypothesis for the Treatment of Inherited Bone Marrow Failure Syndromes

Somatic mosaicism in patients with Fanconi anaemia: Proposal of alternative tissue for inconclusive diagnoses

INTRODUCTION

Fanconi anaemia (FA) is a rare genetic disorder marked by progressive bone marrow failure, chromosomal fragility, and increased cancer susceptibility. Laboratory diagnosis includes chromosomal instability test and mutation investigation. A total of 15%-25% of all patients may have somatic mosaicism, characterized by two distinct haematopoietic cell populations, one resistant and one sensitive to agents that induce chromosomal breakage, which complicates the diagnosis by a high incidence of reverted cells leading to inconclusive or false-negative results. The study aimed to evaluate the use of bone marrow stromal mesenchymal cells (BM-MSCs) as an alternative, non-haematopoietic tissue for diagnosis.

METHODS

Bone marrow mesenchymal stromal cells from 12 patients with positive diepoxybutane (DEB) tests were cultivated and analysed by cytogenetics and mutation investigation.

RESULTS

The DEB test was performed at 0.1 and 0.01 μg/ml concentrations, with an index ranging from 0.24 to 1.00. At higher concentration, the metaphases number was lower, probably due to toxicity. Regarding the molecular investigation, all the mutations previously found in peripheral blood were identified on BM-MSC.

CONCLUSION

This study demonstrated the possibility of using BM-MSCs as an alternative tissue for cytogenetic and molecular investigation. Future tests using an intermediate DEB concentration may lead to an optimal protocol that could be non-toxic to cells but provides conclusive results.

Molecular Approaches Fighting Nonsense

Nonsense mutations are the result of single nucleotide substitutions in the DNA that change a sense codon (coding for an amino acid) to a nonsense or premature termination codon (PTC) within the coding region of the mRNA [...].

Nonsense suppression therapies in human genetic diseases

About 11% of all human disease-associated gene lesions are nonsense mutations, resulting in the introduction of an in-frame premature translation-termination codon (PTC) into the protein-coding gene sequence. When translated, PTC-containing mRNAs originate truncated and often dysfunctional proteins that might be non-functional or have gain-of-function or dominant-negative effects. Therapeutic strategies aimed at suppressing PTCs to restore deficient protein function-the so-called nonsense suppression (or PTC readthrough) therapies-have the potential to provide a therapeutic benefit for many patients and in a broad range of genetic disorders, including cancer. These therapeutic approaches comprise the use of translational readthrough-inducing compounds that make the translational machinery recode an in-frame PTC into a sense codon. However, most of the mRNAs carrying a PTC can be rapidly degraded by the surveillance mechanism of nonsense-mediated decay (NMD), thus decreasing the levels of PTC-containing mRNAs in the cell and their availability for PTC readthrough. Accordingly, the use of NMD inhibitors, or readthrough-compound potentiators, may enhance the efficiency of PTC suppression. Here, we review the mechanisms of PTC readthrough and their regulation, as well as the recent advances in the development of novel approaches for PTC suppression, and their role in personalized medicine.

Deciphering the molecular mechanism of stop codon readthrough

Recognition of the stop codon by the translation machinery is essential to terminating translation at the right position and to synthesizing a protein of the correct size. Under certain conditions, the stop codon can be recognized as a coding codon promoting translation, which then terminates at a later stop codon. This event, called stop codon readthrough, occurs either by error, due to a dedicated regulatory environment leading to generation of different protein isoforms, or through the action of a readthrough compound. This review focuses on the mechanisms of stop codon readthrough, the nucleotide and protein environments that facilitate or inhibit it, and the therapeutic interest of stop codon readthrough in the treatment of genetic diseases caused by nonsense mutations.

Targeting Nonsense: Optimization of 1,2,4-Oxadiazole TRIDs to Rescue CFTR Expression and Functionality in Cystic Fibrosis Cell Model Systems

Cystic fibrosis (CF) patients develop a severe form of the disease when the cystic fibrosis transmembrane conductance regulator (CFTR) gene is affected by nonsense mutations. Nonsense mutations are responsible for the presence of a premature termination codon (PTC) in the mRNA, creating a lack of functional protein. In this context, translational readthrough-inducing drugs (TRIDs) represent a promising approach to correct the basic defect caused by PTCs. By using computational optimization and biological screening, we identified three new small molecules showing high readthrough activity. The activity of these compounds has been verified by evaluating CFTR expression and functionality after treatment with the selected molecules in cells expressing nonsense–CFTR–mRNA. Additionally, the channel functionality was measured by the halide sensitive yellow fluorescent protein (YFP) quenching assay. All three of the new TRIDs displayed high readthrough activity and low toxicity and can be considered for further evaluation as a therapeutic approach toward the second major cause of CF.