Novel Variant of RARB Gene in Familial Isolated Ocular Coloboma: A Case Report

Introduction

Ocular coloboma is a congenital malformation characterized by areas of missing tissue in the eye due to incomplete closure of the embryonic optic fissure during the fifth week of gestation [1]. Approximately 3.7 to 8 per 10 000 live births are affected by ocular coloboma [2]. The defects can be unilateral or bilateral, and the closure defects can affect any ocular structure, including the iris, retina, lens, choroid, and optic nerve [1]. Bilateral cases make up approximately 60% of total coloboma cases [3].

Ocular coloboma has been causally linked to environmental and genetic influences. Environmental factors associated with coloboma development include vitamin A deficiency, thalidomide exposure, increased paternal age, and fetal alcohol syndrome. The underlying genetic causes of coloboma have been difficult to identify outside of syndromic colobomas, such as CHARGE syndrome, Treacher Collins syndrome, and Patau syndrome [2,4]. More than 70% of individuals with non-syndromic colobomas do not have identified underlying genetic defects [4].

Pulmonary hypoplasia/agenesis, diaphragmatic hernia/eventration, anophthalmia/microphthalmia, and cardiac defect (PDAC) syndrome is a rare condition that can present with syndromic coloboma and other ocular abnormalities [5]. Previous cases have reported causative variants within genes important for eye development in the retinoic acid (RA) signaling pathway, such as STRA6 and RARB [6]. Pathogenic variants in the RARB gene, which encodes RA receptor beta, have been established in PDAC syndrome but have rarely been reported in isolated coloboma [6,7]. The purpose of this study is to contribute to the limited reports of isolated colobomas associated with pathogenic variants in the RARB gene. In the present study, we report a maternally inherited variant in the RARB gene that may cause non-syndromic ocular coloboma.

Case Report

The proband was a 6-month-old male infant with bilateral ocular colobomas. He had right eye retinal coloboma and left-sided retinal, iris, and optic nerve coloboma (Figure 1) that was discovered shortly after birth. At 6 months, he was evaluated by the ocular genetics team and did not show signs of visual impairment on physical examination. Visual acuity was central, steady, and maintained in each eye and had an equal response to induced tropia testing. His eye alignment was orthophoric. The patient reached age-appropriate developmental milestones, had no behavioral abnormalities, and had no concerns from his pediatrician. The family received genetic counseling to discuss the option of genetic testing. Given the otherwise normal examination, the parents opted out of genetic testing at that time.

At his 12-month visit, the patient was found to have bilateral undescended testicles, which were surgically managed with bilateral orchiopexy. He also had ear tubes placed for recurrent ear infections. At 18 months of age, the patient returned to the clinic for genetic evaluation. Chromosomal microarray analysis and exome sequencing were completed, and the chromosomal microarray analysis detected a 36 kb deletion within the cytogenetic band 3p24.2 (NM_000965.4). It was reported as a copy number change of uncertain clinical significance on exon 2 of the RARB gene. Exome sequencing was completed, but the copy number detection was suboptimal and therefore did not detect the deletion.

Parental testing with next-generation sequencing confirmed the same small deletion containing exon 2 of the RARB gene (Figure 2) in the mother and that the variant was absent in the asymptomatic father. The mother was a 38-year-old woman with right eye iris and retinal coloboma resulting in profound vision impairment. Besides the ocular coloboma, the mother had no significant medical history. Neither the proband nor mother presented with additional features of PDAC syndrome. The phenotype genotype correlation suggested that the maternally inherited deletion in the RARB gene may have been associated with the patients’ colobomas. Evaluation for other midline defects using magnetic resonance imaging, as well as pituitary hormone testing, was discussed. Given there were no other health concerns presenting at the time in our patient, the family decided to hold off on additional imaging and laboratory testing. The patient was continuing to follow-up with ocular genetics and ophthalmology outpatient services for clinical changes.

Discussion

To date, there are limited reports of isolated colobomas associated with the RARB gene in humans. However, several RARB variants have been reported in association with PDAC syndrome [7–10]. These cases were affected by abnormalities beyond coloboma, such as sclerocornea, severe developmental delays, initial hypotonia, progressive motor impairment in early childhood, Chiari 1 malformation, cardiac defects, diaphragmatic hernia, severe feeding difficulties, intestinal malrotation, failure to thrive, recurrent apneas, and hypoglycemia. The RARB variants reported in PDAC were de novo, unlike our case of maternal inheritance. Since our patient had no additional PDAC features, we propose that the deletion within exon 2 of the RARB gene may be responsible for isolated ocular coloboma.

Previous zebrafish studies have indicated RA receptor signaling, specifically RARB, as crucial for optic fissure closure during eye development [11]. RARB knockout mice appear morphologically normal except for ocular defects, including cataracts, congenital retinal folds, reduced eye weight and size, and retrolenticular membrane with persistent hyaloid vasculature [12,13]. When RARB mutants are paired with RARA or RARG receptor gene mutants, mice exhibit systemic developmental organ defects and severe ocular defects [12]. These studies display the importance of heterodimerization between RARB and RA receptor signaling during embryonic ocular morphogenesis.

A separate case of maternally inherited coloboma identified an Arg144Gln missense mutation in the DNA-binding domain of RARB in exon 3 [6]. The 38-year-old son was described to have a right retina coloboma and left retinal coloboma, with nonspecific color vision deficits and mild phacodonesis. In addition, this patient had mild asymmetric sensorineural hearing loss of unclear etiology and severe gastroesophageal reflux resulting in Barrett’s esophagus. The mother was described to have bilateral uveal colobomas, mature cataract, sensory exotropia and microcornea in the left eye, with likely microphthalmia [6]. Given our patient’s younger age and milder symptoms, the findings of color vision loss, severe reflux, and sensorineural hearing loss in the 38-year-old patient may be of interest to monitor over time in our patient.

The use of chromosomal microarray analysis in our study identified a novel variant in the RARB gene that is likely pathogenic for isolated coloboma. Recognition of this variant is clinically important for predicting disease progression and prognosis. Limitations of this case include the lack of additional genetic and laboratory testing, given the family’s preference. The absence of functional validation and extended familial data restricts our ability to establish pathogenicity. However, these findings contribute to the limited knowledge of the genetic causes of non-syndromic coloboma.

Conclusions

In this case report, we described a mother and son with isolated ocular coloboma and identified a potentially disease-causing deletion within exon 2 of the RARB gene. These findings are important because, although variants in the RARB gene have been reported in the disease spectrum, they have rarely been implicated in non-syndromic coloboma inheritance. Based on the findings of our case, we hypothesize that RARB function, dependent on the variant type and location, can determine whether a system-restricted or extra-orbital phenotype will develop in humans. Due to the lack of functional genetic testing, our ability to establish pathogenicity of the variant was limited. Further exploratory research on the role of the RARB gene in eye development is needed to understand the pathogenesis of familial isolated coloboma.