Spatiotemporal control of genome engineering in cone photoreceptors.

BACKGROUND: Cones are essential for color recognition, high resolution, and central vision; therefore cone death causes blindness. Understanding the pathophysiology of each cell type in the retina is key to developing therapies for retinal diseases. However, studying the biology of cone cells in the rod-dominant mammalian retina is particularly challenging. In this study, we used a bacterial artificial chromosome (BAC) recombineering method to knock in the "CreERT2" sequence into the Gnat2 and Arr3 genes, respectively and generated three novel inducible CreERT2 mice with different cone cell specificities. RESULTS: These models (Gnat2CreERT2, Arr3T2ACreERT2, and Arr3P2ACreERT2) express temporally controllable Cre recombinase that achieves conditional alleles in cone photoreceptors. Cre-LoxP recombination can be induced as early as postnatal day (PD) two upon tamoxifen injection at varying efficiencies, ranging from 10 to 15% in Gnat2CreERT2, 40% in Arr3T2ACreERT2, and 100% in Arr3P2ACreERT2. Notably, knocking in the P2A-CreERT2 cassette does not affect cone cell morphology and functionality. Most cone-phototransduction enzymes, including Opsins, CNGA3, etc. are not altered except for a reduction in the Arr3 transcript. CONCLUSIONS: The Arr3P2ACreERT2 mouse, an inducible cone-specific Cre driver, is a valuable line in studying cone cell biology, function, as well as its relationship with rod and other retinal cells. Moreover, the Cre activity can be induced by delivering tamoxifen intragastrically as early as PD2, which will be useful for studying retinal development or in rapid degenerative mouse models.

Introduction and Discovery of Retinitis Pigmentosa.

Retinitis pigmentosa (RP) is the most common inherited retinal dystrophy. There are three main characteristics of RP: night blindness, retinal pigmentation, and visual field constriction. Among these three features, night blindness was the first to be discovered, which could be dated back to the ancient Egyptians at around 1500 BC. However, the night blindness described at that time was most likely associated with vitamin A deficiency rather than RP. Retinitis pigmentosa was first described in cadaver anatomic dissection before the invention of the ophthalmoscope. However, it was not linked to RP or night blindness. It was not until the invention of the ophthalmoscope that ophthalmologists could truly look into the eye and correlate the retinal pigmentation with clinical symptoms, such as night blindness and visual field constriction. In 1983, at a RP workshop that gathered together many experts, a consensus was reached regarding the terminology and guidelines for the diagnosis of RP. In this chapter, we will introduce the history and discovery of RP along with its characteristics.

A homozygous in-frame duplication within the LRRCT consensus sequence of CFAP410 causes cone-rod dystrophy, macular staphyloma and short stature.

Ciliopathies are a group of genetic dystrophies causing syndromic and non-syndromic retinal degeneration. We identified CFAP410 as the causative gene in a patient with childhood-onset retinal dystrophy without other systemic symptoms at the age of 20. This 20-year-old man presented with cone-rod dystrophy and CFAP410 homozygous in-frame duplication variants (c.340_351dup). His clinical features included early subnormal vision, posterior pole staphyloma, and short stature. Unlike the previously reported features of retinal ciliopathy, our patient showed no obvious retinal pigmentation and only a slight hyper-autofluorescent parafoveal ring at the 16-year follow up. This case report aims to characterize the clinical features in a patient with novel, homozygous and likely pathogenic in-frame duplication variants in the CFAP410 gene. Ultimately, this report will help contribute to the understanding of CFAP410-associated ciliopathies.

Mouse Models of Achromatopsia in Addressing Temporal "Point of No Return" in Gene-Therapy.

Achromatopsia is characterized by amblyopia, photophobia, nystagmus, and color blindness. Previous animal models of achromatopsia have shown promising results using gene augmentation to restore cone function. However, the optimal therapeutic window to elicit recovery remains unknown. Here, we attempted two rounds of gene augmentation to generate recoverable mouse models of achromatopsia including a Cnga3 model with a knock-in stop cassette in intron 5 using Easi-CRISPR (Efficient additions with ssDNA inserts-CRISPR) and targeted embryonic stem (ES) cells. This model demonstrated that only 20% of CNGA3 levels in homozygotes derived from target ES cells remained, as compared to normal CNGA3 levels. Despite the low percentage of remaining protein, the knock-in mouse model continued to generate normal cone phototransduction. Our results showed that a small amount of normal CNGA3 protein is sufficient to form "functional" CNG channels and achieve physiological demand for proper cone phototransduction. Thus, it can be concluded that mutating the Cnga3 locus to disrupt the functional tetrameric CNG channels may ultimately require more potent STOP cassettes to generate a reversible achromatopsia mouse model. Our data also possess implications for future CNGA3-associated achromatopsia clinical trials, whereby restoration of only 20% functional CNGA3 protein may be sufficient to form functional CNG channels and thus rescue cone response.