Novel mutation as a cause of anterior segment dysgenesis leading to blindness in progeny: the genetics decoded!

Abstract

Background

Anterior segment dysgenesis (ASD) disorders comprises of spectrum of developmental conditions affecting the structures of angle of anterior chamber including cornea, iris, and lens. These conditions are characterized by both autosomal dominant and recessive patterns of inheritance often with incomplete penetrance/variable expressivity. A significant overlap among phenotypes attributed to mutations in different ASD genes is well recognized.

Case presentation

We present a case involving a 29-year-old pregnant woman referred for genetic screening and counseling. She had a 7-year-old male child with congenital bilateral corneal opacity, and his elder sister also exhibited similar findings. Exome sequencing identified a novel variant in the CYP1B1 gene in a homozygous state, which was associated with anterior segment dysgenesis. Both parents were found to be carriers of the same variant, while the sister had the same variant in a homozygous state. Genotype–phenotype correlation was performed, and it was concluded that the novel variant could be responsible for the eye changes in both siblings. The parents sought prenatal diagnosis for the current pregnancy, which was deemed possible.

Conclusions

This case underscores the importance of genetic testing in such rare diseases, as it can assist in early diagnosis, management, and prognosis. It also aids clinicians and parents in making decisions regarding the continuation of the pregnancy at the appropriate time.

Background

Anterior segment dysgenesis (ASD) is the failure of normal development of tissues in the anterior segment of the eye, affecting anterior chamber structures and associated with an increased risk of glaucoma and corneal opacity [1]. ASD includes a spectrum of developmental conditions affecting the cornea, iris, and lens. The cause is multifactorial, with many genes involved in eye development [2].

ASD is a subject of paramount importance within the field of ophthalmology due to its potential to cause visual impairment and its association with an increased risk of ocular comorbidities, including glaucoma and corneal opacity. Understanding the genetic underpinnings and molecular mechanisms behind these disorders is crucial for accurate diagnosis, appropriate management, and the development of potential therapeutic interventions.

These anomalies may manifest in isolation or in various combinations, leading to distinct subtypes of ASD with diverse clinical characteristics. Additionally, ASD disorders can sometimes extend their impact beyond the eye, affecting other organ systems, further emphasizing the complexity of these conditions.

Case presentation

A 29-year-old woman, in her third pregnancy at 14 weeks of gestation, was referred to our center for genetic screening. She had a non-consanguineous marriage and had previously delivered two term pregnancies, with both children presenting congenital bilateral cataracts resulting in near-complete blindness. Both parents appeared phenotypically normal, and no other family members were affected. The first child was an 11-year-old girl, and the second child was a 7-year-old boy. Antenatal and natal histories were uneventful, with no history of infections during pregnancy. Both children were delivered normally at full term. Ocular examination of the affected male child (Fig. 1) revealed that he could perceive light and hand movements in the right eye, but the projection of rays was defective. He could count fingers up to 2 m with his left eye. The intraocular pressure (IOP) in the left eye was 22 mmHg. Slit lamp examination of the left eye showed central leucomatous corneal opacity, ectropion uveae, a deep anterior chamber, and buphthalmos (Fig. 2). In the right eye, there was total corneal opacity with a failed corneal graft, rendering it non-functional (Fig. 3). Ultrasound findings of the left eye revealed a few moderate-intensity dot-like echoes, while the right eye showed high-intensity dot-like echoes filling the entire vitreous cavity, retino-choroidal thickening, and the absence of the T sign (Figs. 4, 5). Fundus evaluation was inconclusive due to media opacity. The patient was healthy systemically, with normal stature and build, no facial dysmorphism, no hearing abnormalities, normal mental health, and no cardiac, neurological, skeletal, or genitourinary abnormalities. A diagnosis of unilateral Peters anomaly Type I was made, with trabeculodysgenesis in the fellow right eye.

Fig. 1

Affected male child with right eye phthisis bulbi and left eye congenital cataract

Fig. 2

Slit lamp view of Left eye showing Central leucomatous corneal opacity, ectropion uvae, deep Anterior chamber and buphthalmos

Fig. 3

Slit lamp view of Right eye showing total corneal opacity, failed corneal graft; Pthysical eye

Fig. 4

USG of left eye showing few dot like echoes of moderate intensities

Fig. 5

USG image of Right eye showing high intensity dot like echoes filling entire vitreous cavity, retini-choroidal thickening, No T sign

The clinical exome sequencing report of the elder sibling, using next-generation sequencing (NGS), revealed a homozygous variant c.92C > A (p.Ala31Asp) in exon 2 of the CYP1B1 gene, classified as a variant of uncertain significance (VUS). Sanger analysis confirmed the same variant in a homozygous state in the other affected male child (Fig. 6). Carrier screening showed that the parents both carried the same variant in a heterozygous state, indicating autosomal recessive inheritance. Genetic screening was sought to assess the risk to the current fetus. Amniocentesis of the pregnant woman, followed by Fluorescence in situ hybridization (FISH) and Sanger variant analysis, indicated that the current pregnancy was unaffected.

Fig. 6

Sanger sequencing chromatogram of younger male sibling showing homozygous status for c.92C > A (p.Ala31Asp) in exon 2 of the CYP1B1 gene

Discussion

Anterior segment dysgenesis (ASD) disorders encompass a spectrum of developmental conditions affecting the cornea, iris, and lens, which can have autosomal dominant or recessive inheritance patterns with incomplete penetration or variable expressivity [3]. The ASDs are complex and affect multiple structures which makes it difficult to classify. These can occur alone or in combination, with specific combinations identified such as Axenfeld–Rieger anomaly and Peters anomaly. It is easier to characterize the features of the malformation affecting each anterior segment structure as this avoids the use of different clinical diagnoses to describe the same condition [4]. Several clinical features are found in more than a single condition. Identification of the genes causing ASD has gradually revealed that these conditions are part of a disease spectrum [5].

In the early weeks of human development, morphogenetic movements result into bi-layered embryonic optic cup formation from the forebrain neuroectoderm, and the lens vesicle gets invaginated and separated from the overlying surface ectoderm [6]. By processes of tissue morphogenesis and differentiation, four types of embryonic tissue, the neuroectoderm, the surface ectoderm, the neural crest cells, and mesoderm-derived cells form the mature anterior segment structures giving rise to the cornea, iris, and drainage structures of the iridocorneal angle.

Currently, the identified phenotypic spectrum of ASD includes aniridia, iridogoniodysgenesis, Axenfeld–Rieger anomaly, Peters anomaly, and primary congenital glaucoma [7]. Peters anomaly is characterized by corneal opacity, defects in corneal epithelium and adhesions between iris and cornea or lens and cornea [8, 9]. Ocular anomalies may be associated with systemic symptoms involving single or multiple organs [10, 11]. Ocular defects can lead to increased intraocular pressure (IOP), resulting in glaucoma in around fifty percent of cases [2, 10, 11].

Peters anomaly can be classified into two types [9, 12,13,14,15,16,17,18]: Type I is unilateral, with iridocorneal adhesions, corneal opacity, and fewer vitreo-retinal and systemic abnormalities. Type II is usually bilateral, with the lens directly adherent to posterior corneal opacity and more commonly associated with systemic features like craniofacial defects, short stature, etc., also referred to as Peters plus syndrome [18].

Numerous genetic causes for ASD have been identified, with many yet to be accounted for. The most common inheritance pattern is autosomal dominant. Most characterized ASD genes encode transcription factors, with several representing extracellular matrix-related proteins. These genes are believed to act together to specify a population of mesenchymal progenitor cells, mainly of neural crest origin, as they migrate anteriorly around the embryonic optic cup. These genes are responsible for mesenchymal cell differentiation which eventually give rise to distinct anterior segment tissues. Development appears critically sensitive to gene dosage. Commonly identified genes associated with ASD and autosomal dominant inheritance include PAX6, JAG1, PITX2, PITX3, FOXC1, FOXE3, BMP4, and COL4A1. Those associated with autosomal recessive inheritance include CYP1B1, LAMB2, and B3GALTL [2, 19]. There is evidence that even more genes are involved. The range of ASD phenotypes is diverse, and mutations in the same gene may result in different phenotypes, with many novel mutations yet to be discovered.

PAX6 and FOXC1 are the most common gene mutations associated with Peters anomaly, while mutations in the CYP1B1 gene have also been reported [20, 21]. However, the missense c.92C > A (p.Ala31Asp) variant in the CYP1B1 gene has not been reported previously as a pathogenic variant or benign variant, to our knowledge. The p.Ala31Asp variant is novel in gnomAD Exomes and 1000 Genomes. The amino acid alanine at position 31 is changed to aspartate, altering the protein sequence and potentially its composition and physiochemical properties. Segregation analysis on the Sanger platform in both parents revealed the same mutation in a heterozygous state, and both affected children showed the same mutation in a homozygous state, confirming that this variant is pathogenic and responsible for the clinical manifestations in both children.

Primary congenital glaucoma has been found to be associated with homozygous or compound heterozygous mutations in CYP1B1 [22]. Few studies have identified mutations in individuals with isolated Peters or Axenfeld–Rieger anomaly, typically in association with glaucoma [23,24,25,26]. In a study, eight cases were identified with homozygous/compound heterozygous mutations having mild ectropion uveae, partial aniridia, and congenital glaucoma, further expanding the ocular phenotype associated with CYP1B1 mutations [27].

The use of novel technologies, such as whole-genome sequencing and exome sequencing is likely to expand the spectrum of mutations in known genes and assist in the identification of novel causative genes as well as modifiers that explain the phenotypic variability of ASD conditions [4, 19, 20]. These technologies are widely used in the diagnosis and deeper understanding of the genetics of anterior segment disorders [28]. The widespread availability and use of these genetic technologies also help clinicians and parents make informed decisions regarding the continuation of pregnancies, prognostication, and enrollment in targeted gene therapy trials.

Conclusions

Anterior segment dysgenesis disorders have heterogeneous presentations with incomplete penetration or variable expressivity. Clinical classification of such complex disorder with overlapping clinical features becomes extremely difficult. However, identification of the genes responsible for ASD has gradually led to the recognition that these conditions are part of a disease spectrum. Novel technologies such as whole-genome sequencing, exome sequencing, and comparative genomic hybridization are potentially effective tools for expanding our understanding of anterior segment dysgenesis disorders. Prenatal screening using these methods can assist clinicians and parents in making informed decisions regarding pregnancy.