Identification of CDHR1 variants that disrupt retinal structure and the potential for gene therapy
Background
Retinal degeneration, a leading cause of vision loss, significantly impacts patients' quality of life. Recent research has identified the CDHR1 gene as a major cause of autosomal recessive retinal degeneration, garnering considerable attention. Mutations in this gene manifest as three distinct phenotypes: macular dystrophy, cone-rod dystrophy, and retinitis pigmentosa. Macular dystrophy, in particular, closely resembles age-related macular degeneration (AMD) in terms of morphology, leading to frequent misdiagnoses in clinical settings. This is largely due to the presence of silent nucleotide substitutions, which are difficult to detect with conventional genetic testing. Consequently, patients have been denied appropriate treatment opportunities for an extended period.
Key Findings
CDHR1 is a non-classical cadherin protein expressed in photoreceptor cells, including cones and rods, which receive visual information. This protein plays a crucial role in maintaining the alignment of the outer segment, a critical structure for light detection. Researchers utilized a CDHR1 knockout mouse model (Cdhr1 Knockout Mouse) to evaluate the potential of adeno-associated virus (AAV) gene supplementation therapy. Mice treated with the therapeutic agent via subretinal injection exhibited long-term improvements in retinal structure and visual behavioral function. This was attributed to the restoration of the original length of the shortened and disorganized photoreceptor outer segments and increased cell survival in the disease model. The researchers demonstrated that patients with silent nucleotide substitutions and hypomorphic variants, which retain some gene function, may also benefit from the treatment. Furthermore, the study presents a clinically applicable diagnostic flowchart, which is noteworthy. The flowchart establishes criteria for accurately distinguishing CDHR1 variants from similar ABCA4, PRPH2, and GUCY2D gene mutations associated with macular dystrophy.
Significance and Prospects
This research is recognized for overcoming the limitations of gene therapy delivery and expanding the possibilities of personalized medicine. The CDHR1 gene's entire coding sequence is of a size that can be readily inserted into a standard AAV vector, making therapeutic development relatively straightforward. This genetic characteristic is likely to be a positive factor in accelerating the drug approval process in the future. The possibility that some patients previously diagnosed with dry AMD may actually have CDHR1 variants necessitates a comprehensive re-evaluation of existing genetic diagnostic systems. Adding silent nucleotide substitutions to the diagnostic gene panel is essential to ensure that patients receive appropriate treatment. However, there are challenges to overcome before the results obtained in animal models can be applied to humans. Subsequent research is needed to address concerns about ocular damage associated with subretinal injection and to demonstrate the long-term safety of the therapeutic gene.
CDHR1 is a recently identified cause of autosomal recessive retinal degeneration manifesting as three distinct clinical phenotypes: macular dystrophy, cone-rod dystrophy or retinitis pigmentosa. In this review, we summarise the discovery and characterisation of CDHR1, clinical phenotypes, natural history and therapeutic approaches including gene supplementation and CRISPR gene editing. CDHR1 is a non-classical cadherin that is highly expressed in cone and rod photoreceptors and is essential for the higher-order organisation of the functionally critical outer segments. Promising pre-clinical data show that AAV gene supplementation therapy delivered by subretinal injection can lead to long-term morphological, structural, functional and behavioural improvements in the Cdhr1 knockout mouse model. Notably, CDHR1 supplementation restored full-length photoreceptor outer segments that are usually shortened and disorganised in disease models and prolonged photoreceptor survival - key mechanisms for the functional and behavioural rescue effects that were observed. The disease is likely to be underdiagnosed because CDHR1-associated macular dystrophy - likely to be the most common disease phenotype - is most often caused by a 'silent' nucleotide substitution that has previously been overlooked by genetic testing. Since the macular dystrophy phenotype has phenotypic similarities to advanced dry age-related macular degeneration (AMD), misdiagnoses are common. CDHR1-associated macular dystrophy also shares phenotypic features with a variety of monogenic masquerades such as ABCA4, PRPH2 and GUCY2D-associated macular dystrophies; we present a flowchart to guide the clinical distinction of these disorders which is now critical for patients as their treatments diverge. AAV gene therapy may be beneficial across the CDHR1 disease spectrum - including those with hypomorphic variants associated with macular dystrophy or retinitis pigmentosa. The coding sequence fits into AAV and with the
In clinical practice, this research provides ophthalmologists with a clear diagnostic guideline to avoid misdiagnosis. In particular, the diagnostic flowchart for distinguishing CDHR1 variants from similar macular diseases is a key factor in improving the success rate of personalized treatment. From an industrial perspective, the gene's size, which is suitable for standard AAV vectors, is expected to accelerate the development of new drug pipelines. Furthermore, by identifying previously overlooked patients with silent mutations, the study facilitates the recruitment of patients for clinical trials. Accurate diagnostic techniques and the development of dedicated therapies work together to provide tangible benefits by preventing vision loss in patients.