Abstract
Purpose :
Photoreceptor cell death in both patients and mouse models of inherited retinal degeneration (IRD) diseases often occurs over an extended period of time (several decades in case of patients). For example, in Spata7 mutant mice, an LCA3 model, although a large proportion (60%) of photoreceptors (PRs) die in the first 3 months, some PRs can survive up to 1.5 years of age. The goal of the project is to investigate this mechanism of prolonged survival in a subset of PR cells, which can form the basis for developing a new therapeutic approach to delay or prevent PR degeneration.
Methods :
To address the heterogeneity of PR survival in the Spata7 mutant retina, single-cell RNA-Seq (scRNA-Seq) was performed at four time points. PRs were identified based on gene expression profile of individual cells and associated markers. Genes/pathways that are differentially expressed between mutant and wild-type PRs were identified and analyzed. Immunofluorescence based chromatin assessment was done at P180 to profile the state of surviving rods. Histone modification is modulated by injecting HDACi at onset of degeneration (P14, N=5) in the Spata7 mutant mice.
Results :
Single cell transcriptome analysis revealed that mature Spata7 mutant rod cell clusters exhibit distinct transcriptome profile compared to age-matched WT rods. These distinct Spata7 mutant rods display a significant increase in the expression of non-lineage specific genes and reduction in the expression of photoreceptor lineage-specific genes. These transcriptomic changes stem from altered chromatin profiles of the surviving PRs including upregulation of repressive histone marks in the PR euchromatin. Furthermore, we observed that experimental manipulation of the chromatin state by injecting HDACi accelerates PR degeneration in the Spata7 mutant eye compared to contralateral control, suggesting an epigenetic basis for the surviving rod cells in the Spata7 mutant retina to adapt to the stress.
Conclusions :
Single cell transcriptome profiling allows us for the first time to probe the heterogeneity in degenerating photoreceptor cells. Our data suggest an adaptation model where surviving PRs prolong cell survival primarily by down-regulating PR lineage-specific gene expression. Downregulation of the PR-specific genes is mediated by modulating chromatin state, an intrinsic mechanism that is induced by stress.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.