OPN1MW2 — Opsin 1, Medium Wave Sensitive 2

The OPN1MW2 gene provides instructions for making a protein called green cone photopigment, which is found in the retina of the eye. This protein is essential for normal color vision, specifically for detecting green light. It works by absorbing light and sending signals to the brain to help you see colors accurately. When there are mutations or changes in the OPN1MW2 gene, it can lead to problems with color vision. These changes can cause conditions ranging from mild color blindness, where it is difficult to distinguish between red and green, to more severe vision problems like Bornholm eye disease or blue cone monochromacy. In severe cases, individuals may experience poor visual acuity, sensitivity to light, and involuntary eye movements.

Gene description: OPN1MW2 encodes for a light-absorbing visual pigment of the opsin gene family, specifically the green cone photopigment or medium-wavelength sensitive opsin. It is part of a tandem array of opsin genes on the X chromosome, and defects in this gene cluster are associated with various color vision deficiencies.

Patient and family guide: The OPN1MW2 gene provides instructions for making a protein called green cone photopigment, which is found in the retina of the eye. This protein is essential for normal color vision, specifically for detecting green light. It works by absorbing light and sending signals to the brain to help you see colors accurately. When there are mutations or changes in the OPN1MW2 gene, it can lead to problems with color vision. These changes can cause conditions ranging from mild color blindness, where it is difficult to distinguish between red and green, to more severe vision problems like Bornholm eye disease or blue cone monochromacy. In severe cases, individuals may experience poor visual acuity, sensitivity to light, and involuntary eye movements.

Gene function: The encoded protein enables G protein-coupled photoreceptor activity and is involved in the G protein-coupled receptor signaling pathway. It mediates visual perception of green light in cone photoreceptor cells by absorbing visible light and triggering a phototransduction cascade.

Protein structure: Opsins are G-protein coupled receptors characterized by seven transmembrane domains. They contain an N-terminal extracellular domain and a C-terminal cytoplasmic domain, and bind to a cis-retinal chromophore.

Molecular function: The protein functions as a G protein-coupled photoreceptor that selectively activates G(i) proteins in response to medium-wavelength (green) light. This activation decreases intracellular cAMP levels. The process begins when the opsin-bound cis-retinal chromophore absorbs a photon and isomerizes to all-trans-retinal, inducing a conformational change that triggers the phototransduction cascade.

Mutation spectrum: The mutation spectrum includes structural variants such as deletions of the locus control region (LCR) and nonhomologous recombination leading to hybrid opsin genes. Pathogenic single nucleotide variants (e.g., missense mutations like p.Cys203Arg) and specific combinations of variants in exon 3 that induce incorrect splicing are also found.

Clinical significance: Mutations in the OPN1MW2 gene and its cluster cause a range of X-linked color vision deficiencies. These include deutanopic colorblindness (green color deficiency), Bornholm eye disease (BED), and blue cone monochromacy (BCM), which can present with reduced visual acuity, high myopia, and photophobia.

Inheritance: X-Linked

Chromosomal location: Xq28

Research and therapeutic approaches: While specific gene therapies for OPN1MW2 are still largely experimental, gene therapy is a promising approach for inherited retinal diseases. Current strategies involve delivering functional copies of opsin genes or using CRISPR-Cas9-based gene editing to correct specific mutations in the retina.