# Splicing variants in MYRF cause partial loss of function in the retinal pigment epithelium leading to nanophthalmos

## TL;DR

This study shows that specific MYRF gene variants cause small eye size without other symptoms, but complete loss of MYRF function leads to more severe disease.

## Contribution

Identifies hypomorphic MYRF alleles as the cause of isolated nanophthalmos and reveals a tissue-specific threshold effect.

## Key findings

- dG-MYRF reduces target gene expression in RPE cells without affecting cleavage or localization.
- MyrfhumdG/humdG mice are embryonic lethal, showing a milder effect than complete MYRF knockout.
- Splicing variants in MYRF create nonfunctional isoforms linked to isolated nanophthalmos.

## Abstract

Improper light focus on the retina, refractive error, is primarily caused by eye size differences and is the leading cause of vision loss worldwide. C-terminal variants in the Myelin regulatory factor (MYRF) gene, a retinal pigment epithelium–derived (RPE-derived) transcription factor, lead to isolated nanophthalmos characterized by a small, though structurally sound eye. However, other MYRF loss-of-function variants cause syndromic disease. To address this discrepancy, in vitro and animal studies were performed on a pathogenic C-terminal variant dG-MYRF (p.Gly1126fs30*, c.3376-1G>A). Human RPE cells or primary RPE transduced with dG-MYRF showed reduced target gene expression, with decreased steady-state levels of the C-terminal cleavage product, but normal cleavage and localization. A homozygous humanized MYRF C-terminal mouse model (MyrfhumdG/humdG) was embryonic lethal by E18.5, while WT (MyrfhumWT/humWT) mice were viable. Single-cell RNA-seq from E17.5 MyrfhumdG/humdG and KO RxCre;Myrffl/fl (E15.5 and P0) mice revealed shared differentially expressed genes, with decreased effect size in the MyrfhumdG/humdG eyes. These findings support dG-MYRF as a hypomorphic allele. Additionally, 2 MYRF splicing variants creating nonfunctional isoforms were found in families with isolated nanophthalmos. Overall, hypomorphic MYRF alleles underlie isolated nanophthalmos, supporting a tissue-specific threshold effect and highlighting unique roles for the MYRF C-terminus in the RPE.

Dominant variants in MYRF partially altering gene function or dosage may lead to ocular disease only while total loss-of-function may lead to syndromic disease.