Whole Exome Sequencing widens the genetic landscape of Hereditary Optic Neuropathies

Hereditary optic neuropathies (HON) are a genetic cause of visual impairment characterized by degeneration of retinal ganglion cells. The majority of HON are caused by pathogenic variants in mtDNA genes and in gene OPA1. However, several other genes can cause optic atrophy and can only be identified by high throughput genetic analysis. Whole Exome Sequencing (WES) is becoming the primary choice in rare disease molecular diagnosis, being both cost effective and informative. We performed WES on a cohort of 106 cases, of which 74 isolated ON patients (ON) and 32 syndromic ON patients (sON). The total diagnostic yield amounts to 27%, slightly higher for syndromic ON (31%) than for isolated ON (26%). The majority of genes found are related to mitochondrial function and already reported for harbouring HON pathogenic variants: ACO2, AFG3L2, C19orf12, DNAJC30, FDXR, MECR, MTFMT, NDUFAF2, NDUFB11, NDUFV2, OPA1, PDSS1, SDHA, SSBP1, and WFS1. Among these OPA1, ACO2, and WFS1 were confirmed as the most relevant genetic causes of ON. Moreover, several genes were identified, especially in sON patients, with direct impairment of non-mitochondrial molecular pathways: from autophagy and ubiquitin system (LYST, SNF8, WDR45, UCHL1), to neural cells development and function (KIF1A, GFAP, EPHB2, CACNA1A, CACNA1F), but also vitamin metabolism (SLC52A2, BTD), cilia structure (USH2A), and nuclear pore shuttling (NUTF2). Functional validation on yeast model was performed for pathogenic variants detected in MECR, MTFMT, SDHA, and UCHL1 genes. For SDHA and UCHL1 also muscle biopsy and fibroblast cell lines from patients were analysed, pointing to possible pathogenic mechanisms that will be investigated in further studies. In conclusion, WES proved to be an efficient tool when applied to our ON cohort, for both common disease-genes identification and novel genes discovery. It is therefore recommended to consider WES in ON molecular diagnostic pipeline, as for other rare genetic diseases.

Neuro-ophthalmological and electrophysiological characterization of mitochondrial diseases.

Aims and methods: 1) characterization of patients with Dominant Optic Atrophy (DOA) associated with mutations in AFG3L2 and ACO2 genes in comparison with classical OPA1-DOA; 2) characterization of patients with mtDNA mutations causing MELAS and MERRF syndromes and correlation with heteroplasmy; 3) longitudinal evaluation of subacute m.11778G>A/MTND4 Leber’s Hereditary Optic Neuropathy (LHON) patients co-treated with rAAV2/2-ND4 gene therapy and idebenone. We performed a comprehensive neuro-ophthalmological assessment coupled with electrophysiological examination. Results: 1) We described and compared 23 ACO2 and 13 AFG3L2 patients with 72 OPA1 patients. All patients presented temporally predominant optic atrophy, with ACO2 showing higher RNFL and GCL thicknesses at OCT, while AFG3L2 was virtually-indistinguishable from OPA1. 2) Retinopathy was the most common manifestation in 17/33 MELAS patients, conversely, optic atrophy was the most common finding in 7/8 MERRF patients. Correlation of heteroplasmy with neuro-ophthalmological parameters failed to disclose any significance in MELAS, while it negatively correlated with OCT parameters in MERRF. 3) We compared modifications in visual acuity, OCT and electrophysiological parameters at 3 timepoints in 9 LHON patients. We observed significant decrease of RNFL thickness and reduction of PhNR amplitude. Visual acuity improved of about -0.37 LogMAR, correlating significantly with time from onset and from injection, but not with idebenone therapy duration. Discussion: 1) ACO2 seems associated to better preservation of retinal ganglion cells, depending on a different pathogenic mechanism involving mtDNA maintenance, as opposed to AFG3L2 which is involved in OPA1 processing. 2) MELAS and MERRF patients presented with a clearly distinct ocular phenotype, possibly reflecting a selective susceptibility of different retinal cell types to global energy defect or oxidative stress. 3) Follow up of LHON patients treated with gene therapy confirmed the deterioration in OCT and electrophysiological parameters, while the amount of visual improvement was similar to the one observed in recent clinical trials.