Achromatopsia: On the Doorstep of a Possible Therapy

Achromatopsia (ACHM, synonym: rod monochromatism) is a rare autosomal recessive inherited retinal disorder with an incidence of approximately 1 in 30,000 [1,2]. The disease presents at birth or early infancy with nystagmus and photophobia, and further ophthalmological examinations typically reveal reduced visual acuity and very poor or absent color vision. The clinical diagnosis of ACHM is based on symptoms supported by morphological and functional - especially electrophysiological - findings [1,2]. They arise from dysfunction of all three types of cones (S, M, and L), and are caused by mutations in the CNGA3, CNGB3, GNAT2, PDE6C, and PDE6H genes, all of which encode crucial components of the cone phototransduction cascade [1,2,3,4,5,6,7,8,9,10,11]. Most recently, mutations in the activating transcription factor 6 (ATF6), a key regulator of the unfolded protein response (UPR), have also been asscociated with ACHM [28]. Although ACHM is considered a functionally stationary disease affecting only the cone system, recent studies have described age-dependent progressive changes in retinal architecture, including photoreceptor cell loss as well as macular changes [12,13,14].

Currently, no specific therapy is available for ACHM. Dark or special filter glasses or red-tinted contact lenses can be helpful in reducing photophobia and potentially improving visual acuity. Further, low-vision aids, such as high-powered magnifying glasses, can be useful for patients [1,2]. Most recently, gene replacement therapy performed on animal models for three ACHM genes (CNGA3, CNGB3, and GNAT2) have shown promising results [15,16,17,18,19]. Accurate genetic and clinical diagnosis of patients will therefore enhance and enable therapeutic intervention in the near future [1,3,4,12,13].

This short review summarizes the genetic background, pathophysiology, clinical findings, diagnostics, and therapeutic perspectives in ACHM.

The human retina has approximately 6 million cone photoreceptors and 120 million rod photoreceptors. Cones are primarily responsible for central, fine-resolution, and color vision under photopic conditions (i.e. photopic vision). They are primarily concentrated in the central macula, comprising nearly 100% of the photoreceptors residing in the fovea, the region responsible for the highest resolution and maximal visual acuity. In contrast, rods are responsible for peripheral, low-light, and night vision (i.e. scotopic vision) and are primarily found in the peripheral retina and perimacular region.

Six genes have been implicated, accounting for ∼75-80% of cases of ACHM so far. CNGA3 and CNGB3 encode the channel-forming α- and the modulatory β-subunits of the cyclic nucleotide-gated (CNG) cation channel of the cone photoreceptor [5,6,20], GNAT2 encode the catalytic α-subunit of the G-protein transducin [11,21], and PDE6C and PDE6H encode the catalytic α- and the inhibitory γ-subunits, respectively, of the cone photoreceptor-specific phosphodiesterase [10,11,22]. These polypeptides are fundamental components of the cone phototransduction cascade (opsin-transducin-phosphodiesterase-CNG channel) in the outer segment of the cone photoreceptor.

CNGA3 and CNGB3 are the most important ACHM genes in patients of Northern European descent, as they account for 40-50 and 25% of cases, respectively [7,23]. Of all mutations, a 1-bp deletion, i.e. c.1148delC, in CNGB3 is by far the most frequent, accounting for ∼75% of all CNGB3-mutant alleles. It is most likely due to a founder effect [24]. Mutations in GNAT2 and PDE6C are rare (∼1-2%), and a single homozygous nonsense mutation is to date the only genetic defect observed in PDE6H [11]. Very recently, ATF6 was identified as a new ACHM gene, suggesting a crucial and unexpected role of ATF6A in human foveal development and cone function. This was surprising as ATF6 encodes the ubiquitously expressed ATF6, a key regulator of the UPR, and cellular endoplasmic reticulum (ER) homeostasis. This adds to the list of genes that, despite a ubiquitous expression, when mutated can result in an isolated retinal photoreceptor phenotype [28].

Most mutations in the ACHM genes result in complete function loss of the respective polypeptide. In few cases, a partial function loss has been reported, resulting in milder forms like incomplete ACHM, oligocone trichromacy, and cone dystrophy [25,26,27].

The clinical diagnosis of ACHM is based on the medical history and psychophysical (visual acuity, visual field, and color vision), electrophysiological, and morphological examinations. Since these individuals see the world only with their rods, the clinical findings arise consequentially from missing cone and intact rod functions [1,2,25].

Congenital ACHM is usually recognized at birth or in the first months due to striking pendular nystagmus and an increased sensitivity to light (photophobia). On functional examinations, reduced visual acuity, a small central scotoma, eccentric fixation, and reduced or complete loss of color discrimination can be detected. Most individuals have complete ACHM with a total lack of cone function. Rarely, individuals have incomplete ACHM in which one or more cone types may be partially functioning [1,2,25]. The clinical findings in incomplete ACHM are similar to those in the complete form, but milder. Hyperopia is common, but not obligatory. The best visual acuity varies with the severity of the disease; it is 20/200 or less in complete ACHM and may be as high as 20/80 in incomplete cases. Visual acuity is usually stable over time; both nystagmus and photophobia may improve slightly. Electroretinographic examinations typically show absent or markedly reduced cone responses, while rod responses are reported to remain normal or nearly normal [1,2,25]. However, deficits in rod and rod-mediated function have also been demonstrated by Moskowitz et al. [26].

Although the fundus is usually normal, macular changes and vessel narrowing may be present. Several clinical studies in recent years have investigated outer retinal architecture and foveal morphology in detail using high-resolution optical coherence tomography (OCT) and adaptive optics [12,13,14,27,29,30,31,32,33,34]. The macular appearance in OCT can show either normal lamination or variable degrees of disruption of the photoreceptor layers and loss of retinal pigment epithelium. Sundaram et al. [12] categorized ACHM patients into the following five groups based on outer retinal OCT findings: (1) continuous inner segment ellipsoid (Ise), (2) Ise disruption, (3) absent Ise, (4) foveal hyporeflective zone, and (5) outer retinal atrophy.

Thiadens et al. [14] concluded that ACHM is not a stationary disease but is rather a disorder that shows a progressive loss of cone photoreceptors. Thomas et al. [13] also showed progressive longitudinal changes in retinal morphology in children and described an interesting OCT sign, i.e. a hyperreflective zone which appears to be the precursor of the hyporeflective zone seen in later ages. On the contrary, Genead et al. [29] provided evidence that cone loss is not age dependent. To draw valid conclusions about the continuous presence of cones in ACHM, further studies with a substantial number of patients from different age groups, but with a homogenetic background, are needed. Typical findings of retinal morphology are shown in figure 1.

The technology of adaptive optics enables direct visualization of rod and cone photoreceptors in the human retina in vivo. In recent years, several studies have investigated photoreceptor morphology in ACHM [29,30,31,32,33,34]. Carroll et al. [32] observed a severely disrupted cone mosaic in the fovea and parafoveal area of a single patient with mutations in CNGB3. Genead et al. [29] reported residual cone structures in all 9 patients carrying mutations in either CNGA3 or CNGB3, though the majority of cones had a reduced reflectance. While methods for quantifying cones are well established, further tools and studies including larger patient groups are still needed in order to assess residual photoreceptor integrity.

The newly introduced ATF6- related ACHM phenotype was found to show severe foveal hypoplasia with a poorly formed or absent foveal pit. This may be a hallmark of the ATF6- related disease which distinguishes it from other forms of ACHM, where the pathogenesis relates to abnormalities of the cone phototransduction pathway [28].

Although the retinal structure seems to show great variability and a slow progressive cone loss has been suggested [12,13,14,33,35,36,37,38,39], no clear association between retinal structure and function could be described [12]. Therefore, ACHM can be best described as a functionally stationary disorder with slow degeneration of cone photoreceptors.

The clinical diagnosis is easily determined based on characteristic features and typical functional and morphological findings; however, some retinopathies may be confused with ACHM. These differential diagnoses include blue-cone monochromatism, KCNV2 retinopathy, oligocone trichromacy, hereditary green-red color vision defects, and cone dystrophies [2,35,36].

In order to make an early and confident diagnosis, knowledge of detailed genotype-phenotype correlations is crucial. This information may become important when identifying suitable candidates and the optimal timing for therapy and when assessing the effect of intervention in the future.

To date, no established therapy is available; therefore, social and professional consequences are essential tasks to deal with. The current standard of care consists of managing symptoms by limiting retinal light exposure with tinted contact lenses and/or very darkly tinted sunglasses. Tinted contact lenses and cutoff filters typically transmit light at wavelengths between 400 and 480 nm, thereby reducing photophobia. Furthermore, magnifying aids and contact lenses with magnification are recommended to improve visual acuity. However, even with the best external aid techniques in place, daily tasks such as driving and going to school present significant obstacles [1,2].

Enormous efforts have been made in research in recent years to develop a possible cure for ACHM, and the new gene replacement therapeutic approaches are promising. ACHM has long been considered an ideal target for gene therapy based on the observation that - in contrast to other progressive retinal dystrophies - functioning photoreceptors are expected to be largely preserved [27,28,29,30,31,32,33,34]. A further advantage is that small- (mouse) and large- (dog and sheep) animal models exist for all three major genetic forms (CNGA3, CNGB3, and GNAT2) of the disease in which this hypothesis could be tested [15,16,17,18,19]. In recent years, multiple successful applications of gene replacement therapy using adeno-associated viral vectors in these animal models have been published [15,16,17,18,19]. These preclinical studies showed that such a therapy could restore cone-specific visual processing in the central nervous system even if cone photoreceptors had been nonfunctional from birth. Treated Cnga3^-/- mice were able to generate cone photoreceptor responses and transfer these signals to bipolar cells [16]. Furthermore, ganglion cells from treated, but not from untreated, Cnga3^-/- mice displayed cone-driven, light-evoked, spiking activity, indicating that signals generated in the outer retina are transmitted to the brain. This newly acquired sensory information was translated into cone-mediated, vision-guided behavior. The therapeutic effect was stable for at least 8 months after the treatment [16]. Another study in a naturally occurring mouse model of ACHM with a CNGA3 mutation, i.e. the cpfl5 mouse, confirmed these results, showing restored cone-mediated function and arrested cone degeneration; the observed therapeutic effect lasted for at least 5 months after the injection [19].

AAV-mediated gene replacement has also been employed in animal models of CNGB3- related ACHM. Carvalho et al. [18] successfully tested the efficacy of gene therapy in Cngb3^-/- mice using an AAV8 vector containing a human cone arrestin promoter driving the expression of human CNGB3. The therapy resulted in long-term improvement of retinal function in Cngb3^-/- mice, which was shown by restoration of cone electroretinographic amplitudes of up to 90% of the wild-type level and a significant improvement in visual acuity. Although successful restoration of cone function was also observed when the treatment was initiated at 6 months of age, restoration of normal visual acuity was only possible in 2- to 4-week-old animals [18].

Komaromy et al. [15] showed restoration of cone function and day vision in two canine models of CNGB3- related ACHM. However, an age-dependent reduction in the cone therapy success rate was also observed. In a subsequent study, combination of the gene therapeutic approach with administration of ciliary neurotrophic factor (CNTF) could solve this problem and led to a successful gene therapy in all dogs aged 14-42 months [39]. This success initiated a human study including five CNGB3 achromats, where intraocular microcapsules releasing CNTF were implanted into 1 eye in each of the participants. After 1 year of follow-up, CNTF did not measurably enhance cone function, which reveals a species difference between human and canine CNGB3- ACHM-mutant cones in response to CNTF [40].

In conclusion, the positive effects observed in preclinical studies on animal models of ACHM hold promise for future therapeutic approaches in human patients. Gene replacement trials for both CNGA3 and CNGB3 are in preparation. The observations of the human phenotype showing only a very slow progression in cone degeneration and no specific correlation between functional and morphological findings suggest a wider therapeutic window than presumed earlier. However, there is still a need to elucidate the relationship between cone structure and function, and to study the ability of the visual system to respond to the novel input in order to establish the most appropriate protocols to identify suitable patients and follow the therapeutic effect.

This work was supported by the Tistou and Charlotte Kerstan Foundation (RD-CURE Project).

The authors have no commercial interests related to the subject of this paper or the entities discussed in this paper.