Inherited eye diseases are a group of disorders affecting various structures of the eye, such as the retina, optic nerve, cornea, and lens, and are mostly caused by genetic mutations. Genetic testing is essential for definitive diagnosis, identification of inheritance patterns, and providing information to patients and families, and ophthalmologists play a significant role.
Genetic testing is performed for the following purposes.
There are “clinical testing” and “research-based testing,” which differ in purpose, cost, and certainty of results.
The following principles serve as guidelines when performing genetic testing.
Test results are returned in the following three categories.
| Result | Meaning |
|---|---|
| Positive | Detects a mutation that causes the disease |
| Negative | No mutation detected in the target region |
| VUS (Variant of Uncertain Significance) | Association between variant and disease not established |
VUS requires interpretation and further evaluation by specialists and cannot be used as a diagnostic basis as is.
A VUS is a result where the relationship between the variant and the disease is currently unknown, and it cannot be determined as “positive” or “negative.” Interpretation by a specialized genetic counselor or ophthalmic genetic specialist is necessary, and reclassification may occur through database updates or additional family testing.
Commonly used terms in ophthalmic clinical practice are listed below.
An overview of the main genetic testing methods is provided below.
| Test method | Target | Features |
|---|---|---|
| Chromosome karyotyping | Chromosome number and structure | Detects large-scale chromosomal abnormalities |
| Chromosomal microarray | DNA copy number variation | High-resolution detection of choroidal neovascularization |
| FISH | Specific chromosomal sequences | Visualization with targeted probes |
| Sanger sequencing | Single gene | High accuracy; suitable for confirming a small number of variants |
| Gene panel | Multiple genes | Simultaneous evaluation of related genes |
| WES | Whole exome | Comprehensive analysis of undiagnosed cases |
| WGS | Whole genome | Most comprehensive test including introns and regulatory regions |
| PCR | Short DNA/RNA sequences | Amplification and confirmation of specific mutations |
For the diagnosis of hereditary eye diseases, it is recommended to proceed with testing stepwise in the following order. 1)
If a diagnosis is not obtained through the stepwise approach of panel → WES → WGS, complementary methods such as RNA-seq, long-read sequencing, and functional assays are considered. 1)
Regular reanalysis of exome and genome data can newly detect causative variants that were unidentified in the initial analysis, with reports of up to a 20% improvement in diagnostic yield. 1)
WES targets the protein-coding exome region (about 1-2% of the entire genome) and is relatively low cost. WGS analyzes the entire genome, including introns, promoters, and regulatory regions, so it can detect mutations in non-coding regions, but both cost and data volume are larger.
Generally, a stepwise approach of panel testing → WES → WGS is recommended. 1) First, a panel test evaluates genes related to the suspected disease group, and if a diagnosis is not reached, proceed to WES. If diagnosis remains difficult, WGS is considered.
The following databases are used to determine the clinical significance of variants. 1)
Approved
voretigene neparvovec (Luxturna): The first ophthalmic gene therapy drug approved by the FDA and EMA in 2017 for inherited retinal dystrophy (LCA2) due to RPE65 gene mutations. 1) It is administered via subretinal injection using an AAV vector.
Effect in pediatric cohort: In AAV gene therapy (Phase III) for choroideremia (CHM gene mutation), stabilization of visual acuity has been reported in a pediatric cohort. 1)
Research and Clinical Trial Stage
Stargardt disease (ABCA4 mutation): Treatment using dual AAV vectors and antisense oligonucleotides (ASOs) is ongoing in Phase I/II. 1)
Achromatopsia (CNGA3/CNGB3 mutation): Gene therapy using an AAV vector is being evaluated in Phase I/II. 1)
Usher syndrome type 1B (MYO7A mutation): Dual AAV vector therapy for large genes is ongoing in Phase I/II. 1)
X-linked retinoschisis (RS1 mutation): Treatment using AAV8 vector is being evaluated in Phase I/II. 1)
In ophthalmic gene therapy, the AAV (adeno-associated virus) vector is the most commonly used delivery system. 1) The eye is an immune-privileged site and allows minimally invasive local administration, making it a major target organ for gene therapy.
Voretigene neparvovec for LCA2 (Leber congenital amaurosis due to RPE65 mutation) received FDA and EMA approval in 2017 and is available for clinical use. 1) Gene therapies for other IRDs (Stargardt disease, achromatopsia, Usher syndrome, etc.) are currently in Phase I to III trials.
