Residual Neuropathy Target Esterase activity predicts retinopathy among PNPLA6 disorders

Abstract

Purpose : Pathogenic variants in the PNPLA6 gene cause a broad spectrum of neurological disorders such as spastic paraplegia type 39 (SPG39), Gordon-Holmes syndrome (GHS), Boucher-Neuhauser syndrome (BNHS), Laurence-Moon syndrome (LMS), and Oliver-McFarlane syndrome (OMS). PNPLA6 encodes Neuropathy Target Esterase (NTE), yet it remains unclear why NTE enzymatic dysfunction causes retinopathy in a subset of patients. Here, we investigate the relationship between PNPLA6 enzymatic activity and retinal degeneration in human and mouse.

Methods : Clinical meta-analysis was performed on previously reported (PubMed, 2008-2022) and newly recruited patients. DNA constructs encoding full length NTE underwent site directed mutagenesis to produce the patient specific proteins. NTE activity was determined as previously published (PMID:12791540). Mouse visual function and structure assays were performed on Espion E2 system, Optodrum, and Spectralis. Statistical analysis was performed in Prism.

Results : Biallelic pathogenic PNPLA6 variants were detected in 20 patients with clinical diagnosis of SPG39, BNHS or OMS, including 17 novel variants. Clinical meta-analysis of 115 individuals indicated significant differences in ophthalmic and endocrinologic symptom onset between clinical diagnosis categories. Intriguingly, missense variants located within the enzymatic domain associated more frequently with more severe forms of the disease (BNHS,OMS). To examine effects of missense alleles on disease onset, we measured esterase activity in 74 variants observed across PNPLA6-associated clinical diagnoses. Residual esterase activity was significantly lower in patients with retinopathy and endocrinopathy. A mouse allelic series demonstrated that residual activity correlated with both viability and retinopathy. Mice with residual activity similar to OMS patients exhibit reduced retinal function, visual acuity, and retinal thickness compared to littermate controls.

Conclusions : The severity of PNPLA6 disorders and presence of retinal disease is driven by residual NTE activity. Enzyme activity of disease-associated missense variants indicates a relationship between enzymatic activity and likelihood of retinopathy and endocrinopathy, implicating missense variants as drivers of tissue-specific disease onset. This supports a novel genotype:activity:phenotype relationship among PNPLA6-opathies that may be valuable for both diagnosis and prognosis.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.