ACRT Journal

Introduction

Achromatopsia refers to reduced visual acuity, photophobia, hyperopia, nystagmus, and impaired color vision resulting from dysfunction of cone photoreceptors inherited in an autosomal manner [1]. Typically, all three cone types are involved (complete type), but rarely, dysfunction is restricted to one or more types (incomplete type). Light-adapted (LA) full-field electroretinograms (ffERGs) that isolate cone-mediated function are non-recordable, while dark-adapted (DA) rod responses are normal or slightly abnormal [1,2]. Anatomically, retinal appearance is mostly normal, although mild foveal variants, absent foveal reflex, or a normal macula on fundoscopy have been reported [1,2]. Complete achromatopsia has an estimated prevalence of about 1 in 30,0000 people [3]. Variants in ATF6, CNGA3, CNGB3, GNAT2, PDE6C, or PDE6H have been associated with diagnosis [1,4]. Current treatment strategies include dark or tinted lenses to alleviate photo-aversion [1,5]. Establishing variant-specific strategies to identify retinal dystrophies through medical history, physical examination, laboratory and genetic testing, and family history can aid in prognosis, genetic counseling, and disease management.

Variants in the CNGA3 gene have exclusively been associated with achromatopsia [1,4], with only one clear report of Leber congenital amaurosis (LCA) in literature [6]. LCA is a severe retinal dystrophy diagnosed at birth or within the first year of life [7,8]. Clinical features of LCA include severe vision impairment or blindness, nystagmus, poor pupillary light responses, roving eye movements, oculodigital sign (eye poking, rubbing, or applying pressure), and an extinguished or irregular ffERG under both DA and LA conditions [7,8]. Fundoscopy may reveal pigmentary retinopathy, pallor of the optic disc, and vascular attenuation [7,8]. Disease prevalence in the literature is between 1 in 30,000 to 1 in 81,000 people [7,9]. LCA is predominantly autosomal recessive and genetically heterogeneous, and variants in over twenty genes are known to be associated with disease [8], though up to 30% of cases will go undiagnosed molecularly [10].

Herein, we describe the medical history, clinical manifestations, and clinical course of a 6-month-old male patient with a homozygous pathogenic variant in the CNGA3 gene with an initial ffERG presentation indistinguishable from LCA. Subsequent improvement in the rod-mediated DA ffERG led to the patient’s diagnosis of achromatopsia. We present this case to document early developmental changes that can present as LCA in young patients with achromatopsia.

Case Presentation

A male infant of Mennonite background was initially evaluated at age 6 months after his parents noted nystagmus, lack of eye contact, and his eyes intermittently not tracking together. His parents also had concerns about his distance vision. His evaluation was completed at a genetic eye disease clinic in Lafarge, Wisconsin, by an ophthalmologist specializing in inherited retinal degeneration (MAS). Visual acuity was central, unsteady, and maintained bilaterally. Cycloplegic refraction yielded acuity of +5.00D OD and +4.50D OS. The patient also shook his head with a fixed target, especially with a near target. On exam, pigmentary changes in the macula and a blunted foveal reflex were seen bilaterally. The ISCEV standard DA ffERGs were nondetectable using the RetEval™ ERG system and small-size sensor strips skin electrodes (LKC Technologies, Inc, Gaithersburg, MD). The rod ERG was recorded following a 20-minute dark adaptation to capture responses from each eye. Scotopic ERG showed a nondetectable response for both eyes at low flash intensity (DA 0.05). At higher flash intensities, severely reduced ERG responses were observed for both eyes (Figure 1. Upper panel) compared to normal scotopic and photopic ERG waveforms obtained for a 6-month-old healthy patient in our recording conditions (Figure 1. Lower panel). The patient was prescribed glasses to correct refractive errors at the end of the initial evaluation.

The patient’s family history was significant for high myopia in his mother and hearing loss at age 25 in his father, who worked in construction. The patient also has a maternal first cousin, once removed, with achromatopsia and a paternal great-uncle and greatgrandmother with possible retinitis pigmentosa. Follow-up genetic testing revealed a homozygous variant of CNGA3 c.1126G>A, p.(Glu376Lys). At the time, there is one individual heterozygous for this variant in the Genome Aggregation Database (gnomAD, n>120,000 exomes and >15,000 genomes) [11]. No homozygotes were observed in the dataset. The affected amino acid is well conserved in mammals, which suggests that this position may not tolerate variation. In silico modeling predicted toleration of the amino acid change.

Koeppen et al. reported a patient with achromatopsia homozygous for the same p.(Glu376Lys) variant [12]. Cell culture studies showed macroscopic currents for cyclic nucleotide-gated channels carrying variant p.(Glu376Lys) were absent when the variant was expressed alone and significantly reduced when the variant was co-expressed with the wild-type, similar to other tested missense alterations, suggesting that this variant affects the channel function. We realized this is a known variant within the Mennonite community associated with achromatopsia with a mean allele frequency in that population of 2.84% (Dr. Erik Puffenberger, personal correspondence). A total of 266 nuclear genes were evaluated, and additional findings included heterozygous variants of uncertain significance in autosomal recessive genes, CNGB3 c.1513A>G, p.(Thr505Ala) and IFT172 c.3674G>A, p.(Arg1225Gln) and deemed unlikely to be relevant to the case.

Four months after the initial evaluation, his parents reported his vision had improved when wearing lenses and noted less nystagmus. On exam, horizontal nystagmus alternated with lateral head bobbing, and there was a new rotary component of the nystagmus. Ten months after our initial evaluation, his parents reported increased light sensitivity with improved vision on cloudy days. On exam, horizontal nystagmus was noted without previously noted rotary component or head bobbing. A blunted foveal reflex with increased macular pigmentation was noted bilaterally on the exam. Two years after the presentation, the pen light exam noted no head bobbing or nystagmus. The blunted foveal reflex present bilaterally was unchanged. The patient consistently wore sunglasses outside three years after the presentation, and his vision appeared stable, per his mother. Fundus examination revealed unchanged blunted foveal reflexes bilaterally (Figure 2). No nystagmus or head bobbing was noted. Follow-up ffERG performed at three years of age showed significant changes from previous tests. The previously undetectable DA ERG improved and were outside the expected 2.5th and 97.5th percentile limits for the b-wave in DA 0.01, and both a- and b-wave in DA 3.0 and DA 10.0 flash strengths established for the RETeval device with no response to LA 3.0 and LA 28 Hz conditions (Figure 1, 2nd row compared to control shown in Figure 1, lower panel). Management of the patient’s retinal dystrophy consisted of a corrective lens and avoidance of sunlight.

Four years after the initial presentation, his mother reported no further changes in vision and that he consistently wore his glasses. Unsteady fixation and blunted foveal reflex remained unchanged bilaterally. The rod-mediated DA ffERG showed improvement compared with the ffERG at three years of age, with scotopic response outside the normal adult limits. Still, photopic ERGs remained undetectable (Figure 1, 3rd row compared to control shown in Figure 1, lower panel).

Discussion

Mutations in genes associated with retinal dystrophies can have different and overlapping phenotypes [13]. Therefore, it is important to understand the structural and functional variations associated with specific mutations. An infant with nystagmus, reduced vision, and a flat ERG is typically diagnosed with LCA [14,15], although there may be considerable overlap with congenital stationary night blindness (CSNB) and achromatopsia. With the advent of gene and other therapies for these disorders, it will be critical to understand variations in the developmental history of these disorders in very young patients, who may be the best candidates for intervention. Recently, a Phase I retinal gene therapy trial for CNGA3-mutated achromatopsia showed functional improvement and no adverse effects three years after participants received subretinal AAV8. CNGA3 gene therapy [16].

Achromatopsia is an autosomal recessive retinal dystrophy, and variants of it have been found in six genes [1,4]. CNGA3 is a gene associated almost exclusively with achromatopsia [1,4]. Solaki et al. compiled an overview of the CNGA3 variant spectrum [17]. They gathered data from 889 independent families with achromatopsia. In this, they commented on the clinical and diagnostic aspects of achromatopsia. Usually, the diagnosis is given in the first months or years of age via confirmation with molecular genetic testing, as children may have poor cooperation on clinical exams. The ffERG is a tool for diagnosing scotopic recordings within normal limits, while photopic responses are unmeasurable. Imaging of the retina usually shows a normal fundus. Differential diagnoses can include early onset cone dystrophy, blue cone monochromacy, bradyopsia, or other diseases that may present with nystagmus or photophobia. The course of achromatopsia has historically been considered stationary, but retinal imaging has revealed progressive changes in cone cells [17].

A previous report found the CNGA3 variant in a 10-month-old with congenital nystagmus, sluggish pupils, and a flat scotopic and photopic ERG consistent with LCA6. No serial ffERGs were reported, but there was no visual response at 2 years of age. The variant reported in that case was a homozygous unique missense variant in a consanguineous family. ACMG variant classification guidelines suggest a likely pathogenic classification for the reported variant due to other pathogenic variants affecting the same codon, the rarity of the variant in the general population, and agreement across in silico modeling tools that this variation would lead to disrupted protein function. This case of a 6-monthold male from a Mennonite background with a CNGA3 variant is consistent with the existing literature on a relationship between this CNGA3 variant and achromatopsia [1,4]. Additionally, approximately six Mennonite individuals have been documented to have achromatopsia due to the homozygosity of this variant (Dr. Erik Puffenberger, personal correspondence). Although reduced in amplitude relative to adults, rod-mediated ERGs are usually evident in the first weeks of life [18]. The marked developmental delay in the appearance of rod-mediated ERGs in this cone dystrophy may be a feature of this CNGA3 variant and some CACNA1F variants of CSNB [15]. Thus far, the patient has been managed with corrective lenses and sunlight avoidance. Four years postdiagnosis, ffERG revealed flat photopic and scotopic responses within normal limits. This late improvement in DA ERG response has not been previously reported. The fact this report is based on a single subject over a relatively short period of observation represents significant limitations to the case study. 

Figure 1: ERG waveforms were recorded under scotopic and photopic conditions. Six ISCEV standard recording traces during three developmental time points (6, 36, and 48 months) using a skin electrode for the patient (upper three rows) compared to the control (lower row). At six months, developmental time point recordings from both left and right eyes are measured for scotopic conditions only. For the normal subject, ERG traces at 5 months and 24 months are presented to indicate the normal range and typical waveform in our recording protocol.

Figure 2: (A) Fundus photograph at 3 years old. (B) Autofluorescence photograph at 3 years old.

Conclusion

In summary, we describe an autosomal recessive CNGA3 variant in a male Mennonite child initially presenting at 6 months of age with nystagmus, poor visual responses, and non-recordable LA and DA ERGs, typically associated with LCA. Photopic ERGs remained nonrecordable, but scotopic ERG response markedly improved across a four-year follow-up period. At three years of age, the ERG pattern was typical achromatopsia. This report highlights the importance of serial ffERG testing in very young infants to understand the natural history of rod and cone functional development in inherited retinal disorders.

Acknowledgments: Erik Puffenberger, PhD, for his insights on the variant in the Mennonite community.