Partial rescue of the ocular retardation phenotype by genetic modifiers

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Abstract
The orJ allele of the murine ocular retardation mutation is caused by a premature stop codon in the homeodomain of the Chx10 gene. When expressed on an inbred 129/Sv strain, the orJ phenotype is characterized by microphthalmia and a thin, poorly differentiated retina in which the peripheral portion is affected to a greater extent than the central portion. Such mutant retinae lack differentiated bipolar cells and the optic nerve typically fails to form, leading to blindness. Here, we show that progeny from an outcrossed backcross between 129/Sv-orJ /orJ and Mus musculus castaneus produce animals that are homozygous for the orJ mutation and exhibit a much ameliorated eye phenotype. Although not of normal size, such modified orJ eyes are significantly larger than those in 129/Sv-orJ /orJ mice, and contain a better organized retina which includes bipolar cells. Furthermore, optic nerves are frequently present, and the eyes show a degree of function as reflected by electroretinogram and pupillary response. As in 129/Sv-orJ /orJ mice, however, modified orJ eyes show incomplete growth and a lack of cell differentiation in the periphery of the retina. The selective, and apparently nonmodifiable, effect of the ocular retardation phenotype on the periphery of the retina indicates that Chx10 plays an important role in the central-to-peripheral gradient of retinal development. These findings demonstrate that the ocular retardation phenotype can be greatly modified by the genetic background, and help to define a role for Chx10 in ocular development. © 2000 John Wiley & Sons, Inc. J Neurobiol 42: 232–247, 2000