15 Targeting an in-frame nonsense variant in <i>ABHD12</i> causing PHARC syndrome with translational read-through-inducing drugs (TRIDs)

Background Premature termination codons (PTC) due to nonsense variants in ABHD12 represent almost one third of patients in PHARC syndrome, characterised by retinitis pigmentosa, early onset cataracts, hearing loss and ataxia.Lipid dysregulation has been highlighted as the central pathophysiology mechanism in PHARC.Translational readthrough-inducing drugs (TRIDs) can enhance the read-through of PTC leading to expression of full-length ABHD12 protein.We created two cell models, PHARC fibroblasts and iPSC-RPE, to assess the efficacy of two TRIDs (amlexanox and ataluren/PTC124).Methods Fibroblasts from one PHARC patient harbouring a homozygous nonsense variant in ABHD12 c.193C>T, p. (Arg65*) were dosed with TRIDs.ABHD12 expression was assessed by qPCR and by immunoblotting.Lipidomic profile was analysed by LC-MS.Induced pluripotent stem cells derived from PHARC patient and WT individual were reprogramed from fibroblasts and differentiated into RPE.Results PHARC fibroblasts only express the alternative splicing ABHD12-b isoform, compared to WT which express both ABHD12-a and ABHD12-b.At protein level, amlexanox treatment led to an increase in the ABHD12-b isoform production in PHARC fibroblasts.PHARC patient fibroblasts exhibited alterations in glycerophospholipids and sphingolipids that were restored by amlexanox treatment.PHARC and WT iPSC-RPE exhibited key characteristics of native RPE.Conclusions A proportion of ABHD12 transcripts are escaping the nonsense mediated decay in PHARC fibroblasts, a favourable prognostic indicator of readthrough potential.The restoring effect on lipid dysregulation in PHARC fibroblasts by amlexanox, suggests TRIDs could be a potential therapeutic option for individuals carrying in-frame nonsense variants in ABHD12.