Emerging Treatments for Achromatopsia

A rare inherited cause of vision loss with clinical evaluation of gene therapies under way

ANDRÁS M. KOMÁROMY, DrMedVet, PhD

Cone photoreceptor dystrophies are a group of rare eye disorders that cause visual impairments from early in life. Achromatopsia is an inherited cone dystrophy with visual symptoms that usually are present from birth. These symptoms include markedly reduced visual acuity, extreme light sensitivity, and the absence of color discrimination.¹

Achromatopsia is caused by mutations in any of six genes that have been identified to date,^2-7 with an estimated 75% of cases caused by mutations in two of these genes. Achromatopsia is estimated to occur in approximately 27,000 people in the United States and Europe combined.¹ There are currently no approved treatments for patients with achromatopsia, although deep red tinted spectacles or contact lenses can reduce symptoms of light sensitivity.

CLINICAL FEATURES OF ACHROMATOPSIA

Achromatopsia is characterized by an absence of cone photoreceptor function, which manifests as decreased cone photoreceptor responses on photopic electroretinogram (ERG) (Figure 1). The only functioning photoreceptors are rod photoreceptors, although some patients also have decreased rod photoreceptor response on ERG.^8,9

Figure 1. Achromatopsia is characterized by decreased cone photoreceptor responses on electroretinograms (ERG). The ERG recordings on the left are from a person without achromatopsia. Those on the right are from a person with achromatopsia. Each of the four rows shows an ERG recording that was obtained in response to unique conditions of adaptation and flash stimulus (from the International Society for Clinical Electrophysiology of Vision standard). The light-adapted single-flash and flicker ERGs show decreased cone photoreceptor responses in the retina with achromatopsia. ERG recordings provided courtesy of David Birch, MD, Retina Foundation of the Southwest.

Patients with achromatopsia have an absence of color discrimination and are photophobic, or extremely sensitive to light. In general, best-corrected visual acuity is approximately 20/200 in dimly lit conditions and worse in brighter situations, such as outside during daylight (Figure 2).¹

Figure 2. The effect of achromatopsia on vision. Images are representations of vision from an individual with unaffected sight (left) or from an individual with complete achromatopsia (right). In a person with unaffected sight, cone photoreceptors mediate high-acuity color vision in brightly lit conditions (top left image). Rod photoreceptors mediate low-acuity grayscale vision in dimly lit conditions (bottom left image). Individuals with complete achromatopsia have a total absence of cone photoreceptor function, causing achromatic vision that is limited to black, white and shades of gray. The only functioning photoreceptors are rod photoreceptors, which have a high sensitivity to light. Individuals with complete achromatopsia are extremely sensitive to light, and rod photoreceptors mediate highly saturated low-acuity grayscale vision in brightly lit conditions (top right image). Vision in dimly lit conditions is thought to be similar to that of persons with unaffected sight (bottom right image), with best-corrected visual acuity approximately 20/200. Original image from Thawat Tanhai © 123RF.com.

Cases of achromatopsia are classified as complete or incomplete, depending on whether the patient has a total or partial loss of color discrimination. The variability across this spectrum is believed to be due, in part, to the degree of residual cone photoreceptor function.

Patients with complete achromatopsia have a total absence of color discrimination, and they are only able to perceive black, white, and shades of gray. Complete achromatopsia is caused by the total loss of function of all three types of cone photoreceptors (long-wavelength, medium-wavelength, and short-wavelength).¹⁰

Patients with incomplete achromatopsia have better visual acuity and color discrimination, compared to patients with complete achromatopsia. Some patients with incomplete achromatopsia have specific types of color vision impairment, such as a loss of photoreceptor function in medium-wavelength cones but residual function in long-wavelength and short-wavelength cones.^1,10 This example of incomplete achromatopsia is associated with deuteranopia-like green color blindness, but it is distinct from true X-linked deuteranopias, which are caused by mutations in the gene that encodes medium-wavelength-sensitive opsin.

Some studies have suggested that achromatopsia is a progressive disease in a subset of patients.^11,12 While achromatopsia is generally characterized by a lack of cone photoreceptor function without necessarily having a loss of cone photoreceptor cells, some cases have foveal atrophy that worsens with age.^12,13

Average retinal thickness is decreased among patients with achromatopsia, and the difference has been attributed to thinning of the outer retinal layers.^12,14

The retina usually appears normal by fundus examination, but there may sometimes be signs of foveal atrophy, macular granularity, or vascular attenuation.¹³ Optical coherence tomography imaging studies have reported that abnormalities in the outer fovea are present in more than three-quarters of patients with achromatopsia. These abnormalities include disruption or absence of the ellipsoid zone and/or retinal pigment epithelium, and in some cases, a complete absence of cells at the base of the fovea leaves an optically empty cavity, called a hyporeflective zone (Figure 3).^8,15

Figure 3. Optical coherence tomography analysis shows structural abnormalities in the outer fovea of some patients with achromatopsia. The left scan is through the fovea from a person with unaffected vision. The right scan is through the fovea from a person with achromatopsia. The white arrow marks a hyporeflective zone at the base of the fovea with a complete absence of cells. Approximately three quarters of individuals with achromatopsia have structural abnormalities in the outer fovea (ellipsoid zone and/or retinal pigment epithelium) when analyzed by OCT. Left image provided courtesy of David Birch, MD, Retina Foundation of the Southwest. Right image provided courtesy of Mark Pennesi, MD, Casey Eye Institute, Oregon Health and Science University.

MOLECULAR BASIS OF ACHROMATOPSIA

Achromatopsia is an autosomal recessive disease that is caused by mutations in any of six associated genes that have been identified to date: CNGA3, CNGB3, GNAT2, PDE6C, PDE6H, and ATF6.^2-7 Approximately half of achromatopsia cases are caused by mutations in the CNGB3 gene,¹⁶ and about a quarter are caused by mutations in the CNGA3 gene.¹⁷

The distribution of disease-causing mutations, however, varies between populations. For example, in certain subpopulations in the Middle East and China, the predominant causes of achromatopsia are mutations of the CNGA3 gene.^18,19

Five of the genes that have been linked to achromatopsia encode proteins that impact phototransduction in cone photoreceptors. The CNGA3 and CNGB3 genes encode the α and β subunits of the cyclic nucleotide-gated (CNG) ion channel, respectively.^3,20 CNG channels in the cone photoreceptor are composed of three α subunits and one β subunit, and are located in the plasma membrane of the outer segment.^21,22 When open, CNG channels allow the nonselective flow of cations across the photoreceptor membrane.

The CNG channel is gated by the small molecule cyclic guanosine monophosphate (cGMP).²³ When cGMP is bound to the intracellular domain of CNG, the channel is open to free cation flow. When cGMP is sparse within the cell and there is no longer enough to bind CNG, the channel is closed.

Under dark conditions, cGMP levels build up and CNG channels are open. The free flow of cations depolarizes the cone photoreceptor membrane, leading to the release of the excitatory neurotransmitter glutamate from the synaptic terminal of the photoreceptor. Downstream bipolar cells receive the glutamate, then interpret and transmit the message that the cone photoreceptor is not detecting a photon.

When a photoreceptor is exposed to light, the chemical transformation of rhodopsin in rod photoreceptors, or an analogous opsin in cone photoreceptors, triggers a cascade of events that ultimately decreases the intracellular levels of cGMP and closes the CNG channels. This creates a barrier to free cation flow and the photoreceptor membrane becomes hyperpolarized due to the constant activity of sodium-potassium pumps.²⁴ Hyperpolarization of the membrane inhibits glutamate release from the synaptic terminal, signaling to downstream bipolar cells that the cone photoreceptor is detecting light.

POTENTIAL NEW OPTIONS FOR TREATMENT

There are currently no approved treatment options for patients with achromatopsia. Tools are available to help patients manage the symptoms of visual impairment, such as deep red tinted spectacles or contact lenses to reduce symptoms of light sensitivity, and magnifiers to deal with poor visual acuity.

Initial findings in a dog model of CNGB3 achromatopsia supported the therapeutic potential of ciliary neurotrophic factor (CNTF) when delivered as an intravitreal injection.²⁵

Despite these promising preclinical results, cone photoreceptor function was not enhanced in a small phase 1/2 clinical trial of five patients with achromatopsia who were treated with CNTF encapsulated cell technology implants (Neurotech, Inc.; Cumberland, RI).²⁶

Gene Therapies

Gene therapy is an emerging treatment option for patients with achromatopsia. Gene therapy often uses engineered viruses, or viral vectors, to deliver a functional copy of a gene into a patient’s cells.

Adeno-associated virus, or AAV, is well suited for use as a viral vector in a number of specific cell types, including retinal cells. AAV is a small simple virus that elicits only a weak immune response and has never been linked to disease in humans. AAV vectors have no viral genes remaining, which eliminates the possibility that any viral genes will cause an adverse event.²⁷ The functional gene delivered by an AAV vector is thought to remain stable for many years.²⁸ By addressing the underlying cause of an inherited disease, gene therapy has the potential for long-lasting therapeutic benefit following a one-time administration of the vector.

AAV GENE THERAPY FOR CNGB3 ACHROMATOPSIA

Animal Models and Preclinical Support

Animal models of achromatopsia have been extremely useful in the preclinical evaluation of potential gene therapy treatment options. Some of the models are naturally occurring genetic variants while others have been created through genetic engineering (Figure 4).

Figure 4. Animal models of achromatopsia caused by mutations in the CNGB3 or CNGA3 genes.

Researchers generated a mouse model of CNGB3 achromatopsia by introducing a null mutation in the CNGB3 gene. The phenotype of CNGB3 homozygous null mice is similar to the clinical profile of human patients with achromatopsia, including decreased visual acuity and decreased cone photoreceptor response on photopic ERG.^29,30

In these mice, a single subretinal injection of an AAV vector expressing the human CNGB3 gene led to increased cone responses on ERG.³¹

The same mouse model was used to evaluate the safety and therapeutic potential of a single subretinal injection of an AAV vector expressing an engineered version of the CNGB3 gene containing: (1) a CNGB3 gene sequence that was codon-optimized to maximize production of the normal CNGB3 protein; and (2) a robust “pan-cone” promoter that had been previously demonstrated to drive robust gene expression in all three types of cone photoreceptor.³²

A single injection of this AAV vector in the null CNGB3 mouse model was well tolerated, led to an increase in cone photoreceptor function, and at the end of the three month study was not associated with pathological changes in the retina after treatment.³³

There is an inherited form of achromatopsia that naturally occurs among multiple dog breeds, including the Alaskan malamute, German shorthaired pointer and miniature Australian shepherd. Affected dogs have features similar to patients with achromatopsia, such as day blindness and absence of cone photoreceptor function on ERG. In each of these dog breeds, the disease has been linked to mutations in the CNGB3 gene.^34,35

In two of these breeds with the canine form of achromatopsia caused by CNGB3 mutations, a single subretinal injection of an AAV vector expressing the human CNGB3 gene led to long-term increased cone responses on ERG and increased day vision as assessed by the ability to navigate an obstacle maze under bright light conditions (see Video).³⁶

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In addition to studies in mouse and dog models of achromatopsia, safety studies in normal nonhuman primates have also been important in supporting the development of gene therapy product candidates for patients with achromatopsia

These studies have shown that a subretinal injection of an AAV vector expressing the human CNGB3 gene was safe and well tolerated. Injection of the vector was associated with dose-dependent ocular inflammation that was self-limiting and diminished with time.³⁷

Clinical Trials

The cumulative findings of preclinical studies have supported the advancement of an AAV-based CNGB3 gene therapy product candidate into clinical development. Applied Genetic Technologies Corporation (AGTC; Gainesville, FL) is conducting a multicenter, nonrandomized, open-label, dose-escalation trial (NCT02599922) at five study sites in the United States.³⁸ One eye of each participant will receive a single subretinal injection of an AAV vector expressing a codon-optimized version of the human CNGB3 gene. The AGTC phase 1/2 trial of gene therapy for CNGB3 achromatopsia is divided into two stages, which will together enroll up to 24 participants.

In the first stage, three dose levels are being evaluated, beginning with the lowest dose level and proceeding to higher dose levels after review of safety data by a Data and Safety Monitoring Committee. Participants in this initial dose-escalation stage are 18 years of age or older.

Additional participants will be enrolled during a second stage to evaluate the maximum tolerated dose that was determined during the first stage. Participants in the second stage will be six years of age or older.

The primary study endpoint is safety at one year, assessed by evaluation of ocular and non-ocular adverse events, and hematology and clinical chemistry parameters. The secondary endpoint is efficacy at one year, assessed by evaluation of visual acuity, light discomfort, color vision, static visual field, ERG, adaptive optics retinal imaging, and OCT.³⁸

Given that this is a rare disease, AGTC is also conducting an observational study (NCT01846052) funded in part by the National Eye Institute, to better characterize natural history features of achromatopsia over time in patients with mutations in the CNGB3 gene.³⁹

AAV GENE THERAPY FOR CNGA3 ACHROMATOPSIA

Animal Models and Preclinical Support

There are two mouse models of CNGA3 achromatopsia (Figure 4). One of the models was generated by introducing a null mutation into the CNGA3 gene.⁴⁰ The other model, referred to as cpfl5 (cone photoreceptor function loss type 5), contains a naturally occurring CNGA3 mutation.⁴¹ Both mouse models have phenotypic features that reflect the clinical profile of human patients with achromatopsia, including decreased cone photoreceptor response on ERG.^40,41

In both the null and naturally occurring mouse models of CNGA3 achromatopsia, a single subretinal injection of an AAV vector expressing the mouse CNGA3 gene led to increased cone responses on ERG.^42,43

There is also an inherited form of congenital day blindness that was found in a breed of sheep in Israel (Figure 4). The day blindness is associated with decreased cone photoreceptor function on ERG and has been linked to a mutation in the CNGA3 gene.^44,45 In these sheep, a single subretinal injection of an AAV vector expressing either a mouse or human CNGA3 gene led to increased cone responses on ERG and increased day vision as measured by the ability to navigate a barrier maze.⁴⁶

Clinical Trials

Positive findings from preclinical studies support the safety and potential therapeutic benefit of a gene therapy-based approach to the treatment of patients with CNGA3 achromatopsia.

AGTC is planning a phase 1/2 clinical trial of an AAV vector expressing the human CNGA3 gene in patients with CNGA3 achromatopsia. The trial will begin upon submission and approval of an IND application to the FDA. The study design is expected to follow a similar format to that used in the AGTC phase 1/2 trial of gene therapy for CNGB3 achromatopsia.³⁸

University Hospital Tubingen and Ludwig-Maximilian University of Munich (both in Germany) are also sponsoring a phase 1/2 single-center, single-arm, open-label, dose-escalation trial of a different AAV vector.⁴⁷ One eye of each participant is receiving a single subretinal injection of an AAV vector expressing the human CNGA3 gene. The primary endpoint is safety at one year. The secondary endpoint is efficacy at one year, assessed with an evaluation of visual function, patient-reported outcomes and retinal imaging.⁴⁷

FUTURE DIRECTIONS

Achromatopsia is an inherited retinal disease for which there is currently no approved treatment. AAV-based gene therapy product candidates are in clinical development for patients with achromatopsia caused by mutations in the CNGB3 or CNGA3 genes, which together account for approximately 75% of all cases. Achromatopsia is a disease that can slowly worsen with age in many patients.

With the knowledge and advancements gained from both observational studies and clinical trials of gene therapies for achromatopsia, we are on the brink of having the tools necessary to bring sight back to patients with this rare but severe cone dystrophy. RP

REFERENCES