The retinal pigmentation pathway in human albinism: Not so black and white.

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

Abstracts from the 2022 European Association for Vision and Eye Research Festival, 13-15 October 2022, Valencia.

PURPOSE: Albinism refers to a group of genetic disorders typically characterized by a loss/reduction of melanin in the hair, skin and eyes of affected patients. Apart from pigment changes, all albinism patients present with foveal hypoplasia and optic nerve misrouting, and have blurred vision. The molecular mechanisms that link this lack of pigment with neural retinal development are poorly understood, with foveal and optic tract development being difficult to model. To advance our knowledge, we developed a novel retinal organoid model of albinism, and characterized the development and outgrowth of retinal ganglion cells affected during albinism as a model for future studies. METHODS: Human oculocutaneous albinism 1 (OCA1) patient-derived stem cells were differentiated alongside controls into retinal organoids, as published previously1,2 . Early retinal ganglion cells develop in the first 4 weeks of differentiation, at which point whole organoids could be plated to allow for optic nerve-like outgrowth. Whole organoids were also fixed and analysed with immunohistochemistry (IHC) to visualize contralateral and ipsilateral ganglion cells present in the organoid. RESULTS: IHC analysis showed differences in the number of ipsilateral and contralateral retinal ganglion cells between the healthy control and albinism organoids, in line with in vivo observations. Further, optic nerve-like outgrowth could be achieved with both models, allowing for future research into optic nerve misrouting in albinism. CONCLUSIONS: We generated a novel retinal organoid model of oculocutaneous albinism, and characterized the retinal ganglion cell development and outgrowth. This will allow us in the future to study a different and sometimes overlooked aspect of albinism; optic nerve misrouting. References 1. Wagstaff, P. E., Ten Asbroek, A., Ten Brink, J. B., Jansonius, N. M. & Bergen, A. A. B. An alternative approach to produce versatile retinal organoids with accelerated ganglion cell development. Sci Rep 11, 1101, doi:10.1038/s41598-020-79651-x (2021). 2. Ohlemacher, S. K. et al. Stepwise Differentiation of Retinal Ganglion Cells from Human Pluripotent Stem Cells Enables Analysis of Glaucomatous Neurodegeneration. Stem Cells 34, 1553-1562, doi:10.1002/stem.2356 (2016).

Tcf4 is required for correct brain development during embryogenesis.

Tcf4 has been linked to autism, schizophrenia, and Pitt-Hopkins Syndrome (PTHS) in humans, suggesting a role for Tcf4 in brain development and importantly cortical development. However, the mechanisms behind its role in disease and brain development are still elusive. We provide evidence that Tcf4 has a critical function in the differentiation of cortical regions, corpus callosum and anterior commissure formation, and development of the hippocampus during murine embryonic development. In the present study, we show that Tcf4 is expressed throughout the developing brain at the peak of neurogenesis. Deletion of Tcf4 results in mis-specification of the cortical neurons, malformation of the corpus callosum and anterior commissure, and hypoplasia of the hippocampus. Furthermore, the Tcf4 mutant shows an absence of midline remodeling, underlined by the loss of GFAP-expressing midline glia in the indusium griseum and callosal wedge and midline zipper glia in the telencephalic midline. RNA-sequencing on E14.5 cortex material shows that Tcf4 functions as a transcriptional activator and loss of Tcf4 results in downregulation of genes linked to neurogenesis and neuronal maturation. Furthermore, many genes that are differentially expressed after Tcf4 ablation are linked to other neurodevelopmental disorders. Taken together, we show that correct brain development and neuronal differentiation are severely affected in Tcf4 mutants, phenocopying morphological brain defects detected in PTHS patients. The presented data identifies new leads to understand the mechanisms behind brain and specifically cortical development and can provide novel insights in developmental mechanisms underlying human brain defects.