The <i>Atoh7</i> remote enhancer provides transcriptional robustness during retinal ganglion cell development.

Abstract

The retinal ganglion cell (RGC) competence factor ATOH7 is dynamically expressed during retinal histogenesis. <i>ATOH7</i> transcription is controlled by a promoter-adjacent primary enhancer and a remote shadow enhancer (SE). Deletion of the <i>ATOH7</i> human SE causes nonsyndromic congenital retinal nonattachment (NCRNA) disease, characterized by optic nerve aplasia and total blindness. We used genome editing to model NCRNA in mice. Deletion of the murine SE reduces <i>Atoh7</i> messenger RNA (mRNA) fivefold but does not recapitulate optic nerve loss; however, SE^del/knockout (KO) <i>trans</i> heterozygotes have thin optic nerves. By analyzing <i>Atoh7</i> mRNA and protein levels, RGC development and survival, and chromatin landscape effects, we show that the SE ensures robust <i>Atoh7</i> transcriptional output. Combining SE deletion and KO and wild-type alleles in a genotypic series, we determined the amount of <i>Atoh7</i> needed to produce a normal complement of adult RGCs, and the secondary consequences of graded reductions in <i>Atoh7</i> dosage. Together, these data reveal the workings of an evolutionary fail-safe, a duplicate enhancer mechanism that is hard-wired in the machinery of vertebrate retinal ganglion cell genesis.