Genetic retinal diseases, such as retinitis pigmentosa, affect approximately 5.5 million people worldwide. A study published in the journal Cell on February 26th reveals that this eye disease, which has long been believed to be purely genetic, may be partially caused by bacteria that "escape" from the intestine to the retina.

According to Martin Kriegel, a researcher at the University of Münster in Germany who was not involved in the study, it was surprising how this result contradicted the common belief that the eye is protected by a layer of tissue that bacteria cannot penetrate. He called it a "paradigm shift."

Previous research has shown that bacteria are not as rare in the eye as ophthalmologists once thought. This led Richard Lee, an ophthalmologist at University College London and one of the co-corresponding authors of the paper, to investigate whether bacteria could be implicated in retinal diseases.

The main cause of retinal diseases is mutations in the CRB1 gene, which can even lead to blindness. Lee and his colleagues discovered that besides weakening the protective barrier around the eyes, CRB1 gene mutations also weakened the connections between cells in the colonic lining.

The study indicates that the CRB1 gene is expressed in both the retinal pigment epithelium and the intestines, but mutations disrupt the cell connections between retinal pigment epithelial cells and colonic epithelial cells. Defects in the eye and colonic barrier functions result in the translocation of intestinal bacteria and retinal inflammation.

In this study, Wei Lai, a professor at the Second Affiliated Hospital of Guangzhou Medical University in China and one of the co-corresponding authors, bred mice with CRB1 mutations that had reduced levels of bacteria. Unlike mice with typical gut microbiota, these mice showed no evidence of deformation in the retinal cell layer.

In addition, treating the mutated mice with antibiotics reduced eye damage, suggesting that individuals carrying CRB1 gene mutations could benefit from antibiotics or anti-inflammatory drugs to mitigate the impact of bacteria. "If this is a treatable mechanism, it will change the lives of many families."
Jeremy Kay, a neurobiologist at Duke University, commented that although this paper proposed a "cool idea," individuals with CRB1 gene mutations should temper their excitement. "I am very concerned that patients, after reading this article, may think there is a simple answer." In reality, the situation remains complex.
Kay is uncertain whether the results from the mouse model can be applied to humans. Retinal diseases associated with the CRB1 gene typically take several years to fully develop, far exceeding the timeframe of this study. If intestinal bacteria can cause eye infection, it should also occur in other places, but this has not yet been observed in individuals with CRB1 gene mutations.
"Bacterial translocation is a big issue for humans," says Lee. Bacteria that migrate from the intestine may preferentially infect certain locations, although the reasons for this are still unclear. However, because bacteria are rare in the eye, even a small amount can have a significant impact.
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