Pathogenic NR2F1 variants cause a developmental ocular phenotype recapitulated in a mutant mouse model

Abstract

Pathogenic NR2F1 variants cause a rare autosomal dominant neurodevelopmental disorder referred to as the Bosch-Boonstra-Schaaf Optic Atrophy Syndrome. Although visual loss is a prominent feature seen in affected individuals, the molecular and cellular mechanisms contributing to visual impairment are still poorly characterized. We conducted a deep phenotyping study on a cohort of 22 individuals carrying pathogenic NR2F1 variants to document the neurodevelopmental and ophthalmological manifestations, in particular the structural and functional changes within the retina and the optic nerve, which have not been detailed previously. The visual impairment became apparent in early childhood with small and/or tilted hypoplastic optic nerves observed in 10 cases. High-resolution optical coherence tomography imaging confirmed significant loss of retinal ganglion cells with thinning of the ganglion cell layer, consistent with electrophysiological evidence of retinal ganglion cells dysfunction. Interestingly, for those individuals with available longitudinal ophthalmological data, there was no significant deterioration in visual function during the period of follow-up. Diffusion tensor imaging tractography studies showed defective connections and disorganisation of the extracortical visual pathways. To further investigate how pathogenic NR2F1 variants impact on retinal and optic nerve development, we took advantage of an Nr2f1 mutant mouse disease model. Abnormal retinogenesis in early stages of development was observed in Nr2f1 mutant mice with decreased retinal ganglion cell density and disruption of retinal ganglion cell axonal guidance from the neural retina into the optic stalk, accounting for the development of optic nerve hypoplasia. The mutant mice showed significantly reduced visual acuity based on electrophysiological parameters with marked conduction delay and decreased amplitude of the recordings in the superficial layers of the visual cortex. The clinical observations in our study cohort, supported by the mouse data, suggest an early neurodevelopmental origin for the retinal and optic nerve head defects caused by NR2F1 pathogenic variants, resulting in congenital vision loss that seems to be non-progressive. We propose NR2F1 as a major gene that orchestrates early retinal and optic nerve head development, playing a key role in the maturation of the visual system.

Structural and Functional Aspects of the Neurodevelopmental Gene NR2F1: From Animal Models to Human Pathology

“…Several clinical reports describe that a major clinical feature of BBSOAS patients is the profound impairment of visual performances (Bosch et al, 2014 ; Chen et al, 2016 ; Kaiwar et al, 2017 ; Rech et al, 2020 ; Starosta et al, 2020 ; Zou et al, 2020 ), due to optic nerve atrophy and/or optic nerve hypoplasia, decreased visual acuity and cerebral visual impairment, defined as impaired analysis and interpretation of visual stimuli (Bosch et al, 2016 ). While recent reports are detailing the ophthalmological features of visually impaired BBSOAS children (Zou et al, 2020 ; Jurkute et al, 2021 ), mouse models offer again the unique opportunity to further understand the role of Nr2f1 in vision, from the developmental and the functional points of view. Previous Nr2f1 conditional mouse models failed in reproducing key patient eye defects, such as malformed optic discs and optic atrophy (Tang et al, 2010 ), but constitutive models, better mimicking patients’ haploinsufficiency, efficiently recapitulate a plethora of BBSOAS-like symptoms (Bertacchi et al, 2019a ).…”

“…Optic nerve atrophy (ONA) and/or optic nerve hypoplasia (ONH), often reported in BBSOAS patients (Rech et al, 2020 ), are characterized by a reduced optic nerve (ON) exiting the retina, together with thinning of retinal layers, retinal ganglion cell (RGC) and retinal nerve fiber layers (RNFL), where RGCs and their axons reside, respectively (Jurkute et al, 2021 ). NR2F1 is dynamically expressed in different retinal cell types of the human eye, including RGCs (Bertacchi et al, 2019a ), which form the ON by elongating their axons from the retina to the brain.…”

“…The presence of Nr2f D-V gradients also suggests that these genes might be involved in the appropriate formation of the retinotopic projection map of RGC axons, the only long-range projections from the retina to the brain. Even if further experiments will be necessary to tackle this question, recent data about axonal misrouting in Nr2f1 -deficient mice point in this direction (Jurkute et al, 2021 ). Besides axonal guidance, the number of RGCs in the retina was also affected in mutants, since a stable decrease in RGC density was reported in adult HET mice (Jurkute et al, 2021 ).…”

“…Even if further experiments will be necessary to tackle this question, recent data about axonal misrouting in Nr2f1 -deficient mice point in this direction (Jurkute et al, 2021 ). Besides axonal guidance, the number of RGCs in the retina was also affected in mutants, since a stable decrease in RGC density was reported in adult HET mice (Jurkute et al, 2021 ). This defect originates early in development and possibly involves RGC apoptosis around birth (Bertacchi et al, 2019a ).…”

“…BBSOAS is caused by NR2F1 haploinsufficiency, implying that all patients so far identified carry a non-functional NR2F1 allele, either due to deletion or missense point mutations that compromise its expression levels and/or its molecular activity. Although reported mainly as de novo mutations, a few BBSOAS inherited variants have been recently described (Rech et al, 2020 ; Jurkute et al, 2021 ). The BBSOAS causative gene - NR2F1 - and its homolog NR2F2 code for transcriptional regulators belonging to the superfamily of steroid/thyroid hormone receptors.…”