Clinical and genetic investigation of a large Tunisian family with complete achromatopsia: identification of a new nonsense mutation in GNAT2 gene

Genetic variants of accessory proteins and G proteins in human genetic disease

We present a series of three articles on the genetics and pharmacogenetics of G protein- coupled receptors (GPCR). In the first article, we discuss genetic variants of the G protein subunits and accessory proteins that are associated with human phenotypes; in the second article, we build upon this to discuss "G protein-coupled receptor (GPCR) gene variants and human genetic disease" and in the third article, we survey "G protein-coupled receptor pharmacogenomics". In the present article, we review the processes of ligand binding, GPCR activation, inactivation, and receptor trafficking to the membrane in the context of human genetic disease resulting from pathogenic variants of accessory proteins and G proteins. Pathogenic variants of the genes encoding G protein α and β subunits are examined in diverse phenotypes. Variants in the genes encoding accessory proteins that modify or organize G protein coupling have been associated with disease; these include the contribution of variants of the regulator of G protein signaling (RGS) to hypertension; the role of variants of activator of G protein signaling type III in phenotypes such as hypoxia; the contribution of variation at the RGS10 gene to short stature and immunological compromise; and the involvement of variants of G protein-coupled receptor kinases (GRKs), such as GRK4, in hypertension. Variation in genes that encode proteins involved in GPCR signaling are outlined in the context of the changes in structure and function that may be associated with human phenotypes.

Clinical and Genetic Features of Korean Patients with Achromatopsia

This multicenter study aimed to characterize Korean patients with achromatopsia. The patients' genotypes and phenotypes were retrospectively evaluated. Twenty-one patients (with a mean age at the baseline of 10.9 years) were enrolled and followed up for a mean of 7.3 years. A targeted gene panel or exome sequencing was performed. The pathogenic variants of the four genes and their frequencies were identified. CNGA3 and PDE6C were equally the most prevalent genes: CNGA3 (N = 8, 38.1%), PDE6C (N = 8, 38.1%), CNGB3 (N = 3, 14.3%), and GNAT2 (N = 2, 9.5%). The degree of functional and structural defects varied among the patients. The patients' age exhibited no significant correlation with structural defects. During the follow-up, the visual acuity and retinal thickness did not change significantly. In CNGA3-achromatopsia patients, a proportion of patients with a normal foveal ellipsoid zone on the OCT was significantly higher than that of patients with other causative genes (62.5% vs. 16.7%; p = 0.023). In PDE6C-achromatopsia patients, the same proportion was significantly lower than that of patients with other causative genes (0% vs. 58.3%; p = 0.003). Korean patients with achromatopsia showed similar clinical features but a higher prevalence of PDE6C variants than those of other ethnic groups. The retinal phenotypes of the PDE6C variants were more likely to be worse than those of other genes.

Photoreceptor Structure in GNAT2-Associated Achromatopsia

Purpose

The purpose of this study was to report GNAT2-associated achromatopsia (GNAT2-ACHM) natural history, characterize photoreceptor mosaic, and determine a therapeutic window for potential intervention.

Methods

Patients with GNAT2-ACHM were recruited from a single tertiary referral eye center (Moorfields Eye Hospital, London, UK). We performed longitudinal clinical evaluation and ophthalmic examination, and multimodal retinal imaging, including adaptive optics scanning light ophthalmoscopy, quantitative analysis of the cone mosaic, and outer nuclear layer (ONL) thickness, including cone densities evaluation in selected regions of interest and comparison with reported healthy controls.

Results

All nine subjects (3 women) presented with nystagmus, decreased visual acuity (VA), light sensitivity, and highly variable color vision loss. One patient had normal color vision and better VA. Mean VA was 1.01 (±0.10) logarithms of the minimal angle of resolution (LogMAR) at baseline, and 1.04 (±0.10) LogMAR after a mean follow-up (range) of 7.6 years (1.7−12.8 years). Optical coherence tomography showed preservation of the foveal ellipsoid zone (EZ; n = 8; 88.9%), and EZ disruption (n = 1; 11.1%). Mean ONL thickness (range, ± SD) was 84.72 µm (28.57−113.33, ± 25.46 µm) and 86.47 µm (28.57−113.33, ± 24.65 µm) for right and left eyes, respectively. Mean cone densities (±SD) at 190 µm, 350 µm, and 500 µm from the foveal center, were 48.4 (±24.6), 37.8 (±14.7), and 30.7 (±9.9), ×10³ cones/mm², respectively. Mean cone densities were lower than these of unaffected individuals, but with an overlap.

Conclusions

The cone mosaic in GNAT2-ACHM is relatively well preserved, potentially allowing for a wide therapeutic window for cone-directed interventions.

Mutation spectrum and clinical investigation of achromatopsia patients with mutations in the GNAT2 gene

Achromatopsia (ACHM) is a hereditary cone photoreceptor disorder characterized by the inability to discriminate colors, nystagmus, photophobia, and low-visual acuity. Six genes have been associated with this rare autosomal recessively inherited disease, including the GNAT2 gene encoding the catalytic α-subunit of the G-protein transducin which is expressed in the cone photoreceptor outer segment. Out of a cohort of 1,116 independent families diagnosed with a primary clinical diagnosis of ACHM, we identified 23 patients with ACHM from 19 independent families with likely causative mutations in GNAT2, representing 1.7% of our large ACHM cohort. In total 22 different potentially disease-causing variants, of which 12 are novel, were identified. The mutation spectrum also includes a novel copy number variation, a heterozygous duplication of exon 4, of which the breakpoint matches exactly that of the previously reported exon 4 deletion. Two patients carry just a single heterozygous variant. In addition to our previous study on GNAT2-ACHM, we also present detailed clinical data of these patients.

Long-term retinal cone rescue using a capsid mutant AAV8 vector in a mouse model of CNGA3-achromatopsia

Adeno-associated virus (AAV) vectors are important gene delivery tools for the treatment of many recessively inherited retinal diseases. For example, a wild-type (WT) AAV5 vector can deliver a full-length Cnga3 (cyclic nucleotide-gated channel alpha-3) cDNA to target cells of the cone photoreceptor function loss 5 (cpfl5) mouse, a spontaneous animal model of achromatopsia with a Cnga3 mutation. Gene therapy restores cone-mediated function and blocks cone degeneration in the mice. However, since transgene expression delivered by an AAV vector shows relatively short-term effectiveness, this cannot be regarded as a very successful therapy. AAV2 and AAV8 vectors with capsid mutations have significantly enhanced transduction efficiency in retinas compared to WT AAV controls. In this study, AAV8 (Y447, 733F+T494V)-treated cpfl5 retinas showed greater preservation of short-term cone electroretinogram (ERG) responses than AAV8 (Y447, 733F)- or AAV2 (Y272, 444, 500, 730F+T491V)-mediated treatments. To explore the long-term rescue effect, AAV8 (Y447, 733F+T494V)-treated cpfl5 retinas were evaluated at 9 months following postnatal day 14 (P14) treatment. Rescued ERG responses in the cones of treated cpfl5 eyes decreased with increasing age, but still maintained more than 60% of the WT mouse responses at the oldest time point examined. Expression of CNGA3 and M/S-opsins was maintained in cone outer segments of the treated cpfl5 eyes and was equal to expression in age-matched WT retinas. Near-normal cone-mediated water maze behavior was observed in the treated cpfl5 mice. As these are the longest follow-up data reported thus far, AAV8 with capsid Y-F and T-V mutations may be one of the most effective AAV vectors for long-term treatment in a naturally occurring mouse model of CNGA3 achromatopsia.

Genetic testing for achromatopsia

We studied the scientific literature and disease guidelines in order to summarize the clinical utility of genetic testing for achromatopsia. The disease has autosomal recessive inheritance, a prevalence of 1/30000-1/50000, and is caused by mutations in the CNGB3, CNGA3, GNAT2, PDE6C, ATF6 and PDE6H genes. Clinical diagnosis is by ophthalmological examination, color vision testing and electrophysiological testing. Genetic testing is useful for confirming diagnosis and for differential diagnosis, couple risk assessment and access to clinical trials.

CNGB3 mutations cause severe rod dysfunction

PURPOSE

Congenital achromatopsia or rod monochromatism is a rare autosomal recessive condition defined by a severe loss of cone photoreceptor function in which rods purportedly retain normal or near-to-normal function. This report describes the results of electroretinography in two siblings with CNGB3-associated achromatopsia.

METHODS

Full field light- and dark-adapted electroretinograms (ERGs) were recorded using standard protocols detailed by the International Society for Clinical Electrophysiology of Vision (ISCEV). We also examined rod-mediated ERGs using series of stimuli that varied over a 6 log unit range of retinal illuminances (-1.9-3.5 log scotopic trolands).

RESULTS

Dark-adapted ERGs in achromatopsia patients exhibited severely reduced b-wave amplitudes with abnormal b:a ratios (1.3 and 0.6). In comparison, the reduction in a-wave amplitude was less marked. The rod-mediated ERG took on an electronegative appearance at high-stimulus illuminances.

CONCLUSION

Although the defect that causes achromatopsia is primarily in the cone photoreceptors, our results reveal an accompanying disruption of rod function that is more severe than has previously been reported. The differential effects on the b-wave relative to the a-wave points to an inner-retinal locus for the disruption of rod function in these patients.

In vivo imaging of a cone mosaic in a patient with achromatopsia associated with a GNAT2 variant

PURPOSE

The 2 most common causative genes for achromatopsia (ACHM) are CNGA3 and CNGB3; other genes including GNAT2 account for only a small portion of ACHM cases. The cone mosaics in eyes with CNGA3 and CNGB3 variants are severely disrupted; the cone mosaics in patients with GNAT2-associated ACHM; however, have been reported to show a contiguous pattern in adaptive optics (AO) retinal images. The purpose of this study was to analyze the cone mosaic of another case of GNAT2-associated ACHM.

PATIENT AND METHODS

The patient was a 17-year-old Japanese boy. Comprehensive ocular examinations including fundus photography, electroretinography (ERGs), optical coherence tomography (OCT), and whole-exome analysis were performed. The cone mosaic was recorded with a flood-illuminated AO fundus camera, and the cone density was compared with those of 10 normal control eyes.

RESULTS

The patient had the typical phenotype of ACHM, and a novel homozygous variant, c.730_743del, in GNAT2 was identified. The fundus did not show any specific abnormalities, and the OCT images showed the presence of the ellipsoid zone. The AO fundus image showed a clearly defined cone mosaic around the fovea. The cone density at 500 μm from the fovea was reduced by 15-30 % as compared with those of the normal eyes.

CONCLUSIONS

This is the first description of a Japanese patient with ACHM with a novel GNAT2 variant. The eyes of this patient had a preserved cone structure with loss of function.

Achromatopsia caused by novel missense mutations in the CNGA3 gene

AIM

To identify the genetic defects in a Chinese family with achromatopsia.

METHODS

A 2.5-year-old boy, who displayed nystagmus, photophobia, and hyperopia since early infancy, was clinically evaluated. To further confirm and localize the causative mutations in this family, targeted region capture and next-generation sequencing of candidate genes, such as CNGA3, CNGB3, GNAT2, PDE6C, and PDE6H were performed using a custom-made capture array.

RESULTS

Slit-lamp examination showed no specific findings in the anterior segments. The optic discs and maculae were normal on fundoscopy. The unaffected family members reported no ocular complaints. Clinical signs and symptoms were consistent with a clinical impression of autosomal recessive achromatopsia. The results of sequence analysis revealed two novel missense mutations in CNGA3, c.633T>A (p.D211E) and c.1006G>T (p.V336F), with an autosomal recessive mode of inheritance.

CONCLUSION

Genetic analysis of a Chinese family confirmed the clinical diagnosis of achromatopsia. Two novel mutations were identified in CNGA3, which extended the mutation spectrum of this disorder.

G protein-coupled receptor accessory proteins and signaling: pharmacogenomic insights

The identification and characterization of the genes encoding G protein-coupled receptors (GPCRs) and the proteins necessary for the processes of ligand binding, GPCR activation, inactivation, and receptor trafficking to the membrane are discussed in the context of human genetic disease. In addition to functional GPCR variants, the identification of genetic disruptions affecting proteins necessary to GPCR functions have provided insights into the function of these pathways. Gsα and Gβ subunit polymorphisms have been found to result in complex phenotypes. Disruptions in accessory proteins that normally modify or organize heterotrimeric G-protein coupling may also result in disease states. These include the contribution of variants of the regulator of G protein signaling (RGS) protein to hypertension; the role variants of the activator of G protein signaling (AGS) proteins to phenotypes (such as the type III AGS8 variant to hypoxia); the contribution of G protein-coupled receptor kinase (GRK) proteins, such as GRK4, in disorders such as hypertension. The role of accessory proteins in GPCR structure and function is discussed in the context of genetic disorders associated with disruption of the genes that encode them. An understanding of the pharmacogenomics of GPCR and accessory protein signaling provides the basis for examining both GPCR pharmacogenetics and the genetics of monogenic disorders that result from disruption of given receptor systems.