Mutations in the long-wavelength (L) and middle-wavelength (M) cone opsin genes (designated OPN1LW
, respectively) have been associated with a wide range of visual defects including red-green color vision deficiency, blue cone monochromacy (BCM), X-linked cone dystrophy, X-linked cone dysfunction, and high myopia with abnormal cone function.1–16
While characterization of visual function in these individuals is relatively straightforward, less is known about how the presence of OPN1LW
mutations affects retinal structure. Such information will be of paramount importance for advancing efforts to restore cone function in individuals with OPN1LW
Recent studies have shown that OPN1LW
mutations resulting in congenital red-green color vision defects are associated with a variable retinal phenotype, with some individuals showing disrupted cone structure and/or thinning of the outer nuclear layer (ONL).8,14,17,18
It is difficult to draw definite conclusions about the pathogenicity of a specific mutant from comparisons of these individuals, as there may be other factors influencing the retinal phenotype. For example, during development, there is competition between the first two genes in the X-chromosome opsin array in the nascent L/M cones that ends with only one of the two genes being expressed in each cell.19
It has been shown that the relative proportion of cones expressing each of the two genes in the L/M array varies widely (over 40-fold).20,21
Thus previously observed differences in retinal phenotype may be confounded by differences in the relative expression of the mutant opsin with respect to the normal opsin. As the degree of retained cone photoreceptor structure is related to the therapeutic potential of a given retina,22
elucidation of genotype-specific retinal phenotypes is essential.
In one of the more serious vision disorders associated with OPN1LW
mutations, a single type of mutant opsin is expressed in all the cones that would have been L or M in a normal eye. In these subjects, rods and short-wavelength (S) cones are the only photoreceptors expressing normal photopigments. These individuals offer the opportunity to directly evaluate the effect of different OPN1LW
mutations. These mutations can be placed into one of three categories: (1) mutations that produced random nonhomologous missense substitutions at single amino acid positions1,3,12,16
; (2) partial or complete deletion of an exon15,23
; and (3) a recently identified category involving intermixing of ancestral OPN1LW
genes to produce “L/M interchange” mutations with deleterious combinations of nucleotides at normal polymorphic positions.7,8,10,13
While at least one L/M interchange mutation has been shown to directly cause cone malfunction (Greenwald SH, et al. IOVS
2012;53:ARVO E-Abstract 4643), it was recently shown that in addition to any functional changes in the photopigment caused by the mutations, many of the L/M interchange mutations also interfere with recognition of exon 3 by the splicing mechanism.24
Some of the variants incompletely interfere with splicing, so full-length mRNA is produced as well as the inappropriately spliced transcript. Whether there are structural differences between the mutation categories, or for different mutations within a category, has been unknown.
Here we used adaptive optics scanning laser ophthalmoscopy (AOSLO) and spectral-domain optical coherence tomography (SD-OCT) to examine 11 subjects for whom all cones except the S cones express one of six mutant opsins. There were differences in the anatomy and in the course and severity of vision loss across mutation categories. The subjects with L/M interchange mutations reported a later-onset progressive loss of visual function, while those with the C203R mutation showed a typical congenital BCM phenotype. We observed significant disruption of retinal lamination and of cone mosaic topography in all subjects, though the degree of disruption was generally greater for subjects with L/M interchange mutations than for those with random mutations. These differences provide insight into the underlying mechanisms responsible for loss of structure and function in these subjects. Furthermore, while the cone loss observed may limit success of any efforts to restore L/M cone function using gene therapy in any of these subjects, it may be possible to develop strategies to slow or halt the degenerative changes in people harboring L/M interchange mutations.