Case report: ocular manifestations of a gain-of-function mutation in CLCN6, a newly diagnosed disease

Background: In 2020, a new disease was reported by Polovitskaya et al., caused by a monoallelic, gain-of-function mutation in CLCN6, encoding the ClC-6 Cl−/H±exchanger. Methods: Here, we report the ophthalmic findings of one of the first three patients with this disease (the proband) and review the findings in the other two patients in the literature. Results: The CLCN6 gene is part of the voltage-dependent chloride channel protein family. It functions as either a chloride channel aiding in cell-volume regulation and acidification of intracellular organelles or as an antiporter, which are membrane proteins involved in the transport of molecules across a phospholipid membrane. This particular gene is found in late endosomes. Ion transport across endosome membranes is essential for endosomal function. The proband carried a de novo c.1658A>G (p.Tyr553Cys) mutation in CLCN6. The patient reported herein has a notable optic nerve appearance. The nerve initially appeared elevated. Over time, the optic nerve elevation appearance decreased, associated with progressive vision loss with a visual acuity of 20/470 at last follow-up. Conclusion: While Clcn6−/− mice have been found to have a mild neuronal lysosomal storage phenotype, the three reported children with a de novo c.1658A>G (p.Tyr553Cys) variant displayed significant developmental delay and neurodegeneration.