The effect of PTC124 on choroideremia fibroblasts and iPSC-derived RPE raises considerations for therapy

Abstract

Inherited retinal dystrophies (IRDs) are caused by mutations in over 200 genes, resulting in a range of therapeutic options. Translational read-through inducing drugs (TRIDs) offer the possibility of treating multiple IRDs regardless of the causative gene. TRIDs promote ribosomal misreading of premature stop codons, which results in the incorporation of a near-cognate amino acid to produce a full-length protein. The IRD choroideremia (CHM) is a pertinent candidate for TRID therapy, as nonsense variants cause 30% of cases. Recently, treatment of the UAA nonsense-carrying CHM zebrafish model with the TRID PTC124 corrected the underlying biochemical defect and improved retinal phenotype. To be clinically relevant, we studied PTC124 efficiency in UAA nonsense-carrying human fibroblasts and induced pluripotent stem cell-derived retinal pigment epithelium, as well as in a UAA-mutated CHM overexpression system. We showed that PTC124 treatment induces a non-significant trend for functional rescue, which could not be improved by nonsense-mediated decay inhibition. Furthermore, it does not produce a detectable CHM-encoded protein even when coupled with a proteasome inhibitor. We suggest that drug efficiency may depend upon on the target amino acid and its evolutionary conservation, and argue that patient cells should be screened in vitro prior to inclusion in a clinical trial.

Novel CHM mutations in Polish patients with choroideremia – an orphan disease with close perspective of treatment

“…Therapy with translational read-through-inducing drugs (TRIDs) is an example of mutation-dependent therapy tested in cases with nonsense mutations. TRIDs promote ribosomal misreading of premature stop codons, which results in the incorporation of a near-cognate amino acid to produce a full-length protein [28]. Ataluren (PTC124) is one of the read-through promoting drugs.…”

“…PTC124 was also tested on human fibroblasts from patients with choroideremia and on RPE cells derived from patient induced pluripotent stem cells (iPSC). In human cells treated with ataluren, the recovery of prenylation activity was observed, although an increase in REP1 protein was not detected [28, 29]. Torriano et al suggest that the PTC124 efficiency may depend on the conservation and type of target amino acid and its localization, therefore a personalized approach is needed and in vitro screening of the patients’ cells should be considered before including a patient in a clinical trial [28].…”

In vitro metabolism, reaction phenotyping, enzyme kinetics, CYP inhibition and induction potential of ataluren

“…Ataluren has shown to produce full length, functional dystrophin ( 2 , 4 , 13 ) in the nonsense mutation mdx mouse and sapje zebrafish models of DMD. Ataluren activity has also been demonstrated in multiple cell‐based and animal disease models of other nonsense mutation genetic disorders, corroborating its ability to promote readthrough of premature stop codons and its potential for treating genetic disease caused by nonsense mutations 14–19 . Comprehensive nonclinical studies have been conducted in safety pharmacology and secondary pharmacodynamics, pharmacokinetics and metabolism, and toxicology programs.…”

Design of efficacious somatic cell genome editing strategies for recessive and polygenic diseases

“…Moreover, it is currently infeasible to generate unique transgenic animal models for every patient's specific mutation, and it is difficult, if not impossible, to recapitulate the human genetic background in animal models. In contrast, human iPSCs provide an important biological substrate to evaluate genome editing approaches, as data from iPSC studies have been used to inform human clinical trials for gene therapies 84,85 . Using allele-specific genome editors, we demonstrate biallelic gene correction of two distinct mutations in a single iPSC.…”