Disease mechanisms of X- linked cone dystrophy caused by missense mutations in the red and green cone opsins

Cones are a type of photoreceptor cell that are responsible for visual acuity and colored vision. Cones are concentrated in the center of our retina in an area called the macula and help us see fine details. The retina has approximately 6 million cones. A significant proportion of these cells (90%) are the L-and M- cones and they are concentrated in the central macula. Their genes, L and M opsin genes (OPN1LW and OPN1MW respectively) are located on the X chromosome (Xq28). Therefore, retinal diseases resulting from mutations in these genes are considered X-linked and they are associated with various forms of visual defects which include red-green color vision deficiency, X-linked cone dystrophy, and blue cone monochromacy. Most of these inherited red-green color vision defects are associated with L and M opsin gene combinations. However, the remaining few are caused by missense mutations and intermixing of the L- and M- gene sequence within exon 3.

It’s been shown that congenital blue cone monochromacy is caused by cone opsin missense mutations C203R, W177R, and P307L. These mutations are manifested as disruption of retinal lamination and cone mosaic topography. C203R and W177R mutant genes cause misfolded opsonin which accumulates in the cone cells’ endoplasmic reticulum. This in turn has been hypothesized to cause toxicity of cone cells photoreceptors, leading to apoptosis. P307L mutant on the other hand impairs light absorption and cone phototransduction. Other mutants have also been identified which include N94K, R330Q, and G338E.

Previous studies have been carried out exclusively in-vitro on the mechanisms of the diseases caused by these gene mutations. However, the full in vivo effects of these mutants on the cone cells structures and physiological consequences have not been understood and how relevant the in vitro studies to the in vivo has not been confirmed. In the light of this, University of Florida researchers: Dr. Ping Zhu, Dr. Frank Dyka, Dr. Xiaojie Ma, Dr. Ling Yin, Ms. Heather Yu, Professor William Hauswirth, and Professor Wen-Tao Deng, together with Professor Wolfgang Baehr at University of Utah used AAV vector technology to express cone opsin mutants (N94K, W177R, P307L, R330Q, and G338E) specifically in M-Opsin knockout (Opn1mw^-/-)  mice with dysfunctional M-cones. They investigated in detail the subcellular localization of these mutants and their pathogenic effects on cone structure, function and viability. The original research article is now published in the FASEB Journal. Professor Wen-Tao Deng, the corresponding author is now affiliated with West Virginia University.

The opsin mutants were injected subretinally into one eye of 1 month old Opn1mw^-/-mice while the contralateral eyes were left intact and used as controls. They found out that the glycosylation of these mutants was not affected. Regarding their responses to light, uninjected Opn1mw^-/-mice had normal S-cone mediated light response but no M-cone electroretinograpy (ERG). Out of the 5 mutants, N94K, W177R and P307L showed no middle or long wavelength- mediated light response. The remaining 2 (R330Q and G338E) on the other hand showed ERG responses at both wavelengths. With respect to localization, N94K and W177R were detected in the inner segment and endoplasmic reticulum of cone cells while the other 3 were detected in the cone outer segment. Cone cell viability was also found to be reduced by all the mutant genes in Opn1mw^-/-mice. The corresponding rhodopsin mutations of N94K and W177R both cause autosomal dominant retinitis pigmentosa while that of P307L, R330Q,and G338E have not been identified.

The FASEB study provided an important understanding of the molecular mechanisms of the opsin mutant genes which pave the way to develop future advanced therapies for cone cell dystrophy.