Widespread errors in 'proofreading' cause inherited blindness

Widespread errors in 'proofreading' cause inherited blindness

Mistakes in "proofreading" the genetic code of retinal cells is the cause of a form of inherited blindness, retinitis pigmentosa (RP) caused by mutations in splicing factors. This new understanding of the disease process, published in Nature Communications, is leading to the development of a gene therapy for RP caused by splicing factor defects.

Splicing factors are important protein components of the gene proofreading or "splicing" mechanism that is found in all cells. Some sections of our DNA, known as introns, are removed or spliced out by the cell during protein construction, so that only the final intelligible genetic code remains. This is because the introns do not actually provide any meaningful instructions for making proteins. Variations in splicing can cause very different consequences on the formation or function of cells, including retinal cells.

The scientists were able to create a "retina in a dish" using stem cells derived from the skin samples donated by retinitis pigmentosa patients.This cell model enabled the team to compare retinal cells to others in the body. These cells are normally very hard to obtain as they would previously have had to be donated from the retina, usually after death.

Using this model, the researchers have shown that defects in splicing factors cause defective proofreading of components of the editing machinery itself. This counter-intuitive effect results in a "vicious cycle" of disruptive misinterpretation of the genetic code. The formation and functions of a special type of retinal cells, retinal pigment epithelial (RPE) cells, are the most severely affected. These cells are essential for supporting and nourishing photoreceptors (rod and cone cells), so when they go wrong the light-processing function of the retina breaks down, resulting in sight loss.

The study shows, for the first time, how genetic defects in splicing factors cause variations in the proofreading of retinal genes, leading to defects in retinal cell function and their eventual degeneration in retinitis pigmentosa. The team went on to show that CRISPR-Cas9 gene editing could be used to correct the genetic defects in a particular splicing factor. This also corrected the function of the RPE and rod and cone cells in their laboratory model, indicating a potential pathway to future treatments.