Inherited Retinal Dystrophies (IRD)

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Inherited retinal dystrophies (IRD) are a unique group of disorders associated with the retina of the eye that manifest in various phenotypes due to underlying genetic variations. IRDs are categorized based on the retinal dysfunction’s primary locality (photoreceptor cells or retinal pigment epithelium), pattern of inheritance (autosomal dominant or recessive, X-linked,), genetic determinant and progression rate. IRDs are a highly heterogeneous group with over 250 genes studied so far (RetNet; Due to the overlaying genetic make-up between these disorders, genes are associated to multiple phenotypes that are significantly variable. Thus, the complexity of IRD hinders definitive clinical diagnosis.1
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RHO encodes rhodopsin, a photoreceptor protein which is light sensitive allowing vision particularly in depleted light environments.6 In the case of arRP, more than 30 genes have been implicated with mutation in USH2A gene being predominantly pathogenic. USH2A encodes usherin, a crucial protein present in the basement membrane that provides structural support within light-sensitive tissue.7 For xlRP, most of the disease-causing genes and its functions are unknown with only two genes identified on the X-chromosome. Mutations in the genes, RPGR and RP2 genes, account for most cases of xlRP however their function is poorly understood. RPGR and RP2 are thought to play a role in production of cilia which is essential for visual perception.8 The genes associated with RP are crucial in development of photoreceptor cells therefore alterations in these genes result in dysfunctional photoreceptors leading to various characteristic RP phenotypic qualities.
The majority of IRD incidences are considered to be engendered by RP as the overall frequency occurs 1 in every 4000 individuals worldwide.5 Genes related to RP have been substantially documented and investigated in the Western population. Molecular analysis of RP have been previously studied in Spanish, Italian, Northern Irish and English families9–12. Haim13 and Amman14 estimate the epidemiology
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