Cone Dysfunction and Congenital Day Blindness in Awassi Sheep Is Caused by a Mutation in the CNGA3 Gene

Abstract

Purpose: : To study a flock of Improved Awassi sheep in which lambs with congenital impaired day vision were reported.

Methods: : Epidemiology of the flock was studied through breeding records. Six affected lambs and six age-matched normal controls were selected for detailed functional investigation. Behavioral changes were quantified by measuring maze passage time under scotopic and photopic conditions. Flash electroretinography (ERG) was used to investigate rod and cone function. Light microscopy and immunohistochemistry were used to study retinal structure. The coding regions of the CNGA3, CNGB3, and GNAT2 genes, known to cause achromatopsia in humans, were sequenced using RNA extracted from the retinas of 4 affected and 8 non-affected lambs; design of the PCR primers was based on the orthologous bovine genes.

Results: : Out of 7,099 lambs that were born in the flock between 11/2004-6/2008, 124 lambs (1.7%) were diagnosed with impaired vision. The genealogy of the affected lambs suggested an autosomal recessive mode of inheritance. Daytime maze passage time of affected animals was significantly longer than normal controls, but there were no differences between groups in scotopic maze passage time. Electroretinography showed normal rod function and diminished, but not abolished, cone function in affected animals. Histological and immunohistochemical evaluation of affected retinas revealed the physical presence of both red-green and blue cones in large numbers at least up to the age of 7 months, suggesting that the behavioral day blindness and reduced cone ERGs reflect cone dysfunction rather than severe cone photoreceptor loss. Affected lambs were found to be homozygous for a single nucleotide substitution in the cone photoreceptor cGMP-gated channel alpha-subunit (CNGA3) gene, changing amino acid R236 to a stop codon.

Conclusions: : Our results show that a mutation in the ovine CNGA3 gene causes impaired daytime vision and diminished cone function in sheep while cone photoreceptors are histologically present in large numbers at least until the age of 7 months. We propose that affected sheep can serve as a naturally-occurring large animal model for human achromatopsia, including the study of gene therapy for this disease.

Keywords: retinal degenerations: hereditary • genetics • photoreceptors: visual performance