A QTL on distal Chromosome 3 that influences the severity of light-induced damage to mouse photoreceptors Michael Danciger, 1,2 Michael T. Matthes, 3 Douglas Yasamura, 3 Novrouz B. Akhmedov, 1 Tammy Rickabaugh, 1 Susan Gentleman, 4 T. Michael Redmond, 4 Matthew M. La Vail, 3 Debora B. Farber 1,5 1 Jules Stein Eye Institute, UCLA School of Medicine, Los Angeles, California 90095, USA 2 Department of Biology, Loyola Marymount University, Los Angeles, California 90045, USA 3 Beckman Vision Center, UCSF School of Medicine, San Francisco, California 94143, USA 4 Laboratory of Retinal Cell and Molecular Biology, National Eye Institute, National Institutes of Health, Bethesda, Maryland 20892, USA 5 Molecular Biology Institute, UCLA School of Medicine, Los Angeles, California 90095, USA Received: 14 December 1999 / Accepted: 18 February 2000 Abstract. C57BL/6J-c 2J (c2J) albino mice showed much less damage to their photoreceptors after exposure to prolonged light than BALB/c mice and seven other albino strains tested. There were no gender differences, and preliminary studies suggested that the c2J relative protective effect was a complex trait. A genome- wide scan using dinucleotide repeat markers was carried out for the analysis of 194 progeny of the backcross (c2J × BALB/c)F 1 × c2J and the thickness of the outer nuclear layer (ONL) of the retina was the quantitative trait reflecting retinal damage. Our results revealed a strong and highly significant quantitative trait locus (QTL) on mouse Chromosome (Chr) 3 that contributes almost 50% of the c2J protective effect, and three other very weak but significant QTLs on Chrs 9, 12, and 14. Interestingly, the Chrs 9 and 12 QTLs corresponded to relative susceptibility alleles in c2J (or relative protection alleles in BALB/c), the opposite of the relative protective effect of the QTLs on Chrs 3 and 14. We mapped the Rpe65 gene to the apex of the Chr 3 QTL (LOD score  19.3). Northern analysis showed no difference in retinal ex- pression of Rpe65 message between c2J and BALB/c mice. How- ever, sequencing of the Rpe65 message revealed a single base change in codon 450, predicting a methionine in c2J and a leucine in BALB/c. When the retinas of aging BALB/c and c2J mice reared in normal cyclic light were compared, the BALB/c retinas showed a small but significant loss of photoreceptor cells, while the c2J retinas did not. Finding light damage-modifying genes in the mouse may open avenues of study for understanding age-related macular degeneration and other retinal degenerations, since light exposures may contribute to the course of these diseases. Introduction Rod and cone photoreceptors, located at the most distal portion of the neural retina, lie within projections of the retinal pigment epi- thelium (RPE). These cells are packed so tightly together that their nuclei cannot lie abreast of one another. Instead, they intercalate such that the outer nuclear layer (ONL) is about 10 nuclei deep. When photoreceptors degenerate, their outer segments become dis- organized and the ONL thins as the nuclei disappear. This process occurs during the course of many inherited retinal degenerations as well as after toxic light exposures. The retinas of C57BL/6J-c 2J (c2J) mice, unlike eight other albino strains tested (including BALB/c), show very little photo- receptor damage after prolonged light exposure (La Vail et al. 1987a, 1987b). Light overexposure is known to influence the course of inherited retinal degenerations in mice and rats (Noell 1965; Sanyal and Hawkins 1986; Wang et al. 1997; La Vail et al. 1999; Organisciak et al. 1999) and has been suggested as a factor in human retinal degenerations (Cideciyan et al. 1998; Hecken- lively et al. 1991), including age-related macular degeneration (AMD) (Young 1988; Cruickshanks et al. 1993; Winkler et al. 1999). Consequently, we became interested in this c2J protective background characteristic. In a preliminary study (data not shown), we produced a backcross between c2J and BALB/c mice and ex- posed the progeny to constant light. We then scored the progeny phenotypically as homozygous c2J or heterozygous based on the appearance of their retinas, and conducted a genome-wide scan with markers comprising a “10-cM map.” We found no significant correlation with any marker, suggesting that this characteristic is governed by more than one gene. To determine which genes in- fluence the severity of light-induced damage, we set up a test cross with c2J and BALB/c mice and carried out a quantitative trait locus (QTL) study. Materials and methods Mice. BALB/cByJ and C57BL/6J-c 2J mice were purchased from The Jackson Laboratory, Bar Harbor, Me. Determination of ONL thickness. After exposure to constant light for 2 weeks at 115–130 ft-c (equivalent to a well-lit room), eyes were enucle- ated from anesthetized mice, fixed in a mixture of 2% formaldehyde and 2.5% glutaraldehyde in phosphate buffer, and cut along the vertical me- ridian through the optic nerve head. On a single 1-m section from each mouse, measurements of the thickness of the ONL were made; three mea- surements, spaced 50 m apart, were taken at nine 0.25-mm intervals both in the superior and inferior hemispheres starting from the optic nerve head (see Fig. 1) as described previously (La Vail et al. 1987a). The mean of 12 measurements from the posterior retina in the superior hemisphere (areas marked 2–5 in Fig. 1) were used to compare the mice. All test and control mice were 70–88 days old (fully mature) at the start of the constant light exposure. All procedures involving the mice adhered to the ARVO (As- sociation for Research in Vision and Ophthalmology) Resolution on the Use of Animals in Research and the guidelines of the UCSF Committee on Animal Research. Genotyping. Markers that were reported to be polymorphic between C57BL/6J and BALB/c mice were selected from the Whitehead database (Dietrich et al. 1992; 〈http://www-genome.wi.mit.edu/〉) and tested. A map of markers spanning the genome was set up according to the following criteria: one marker was within 10 cM of the most proximal, and one within 10 cM of the most distal marker on each chromosome, and all internal chromosome markers were no more than 20 cM from each other. Primer Correspondence to: M. Danciger; E-mail: danciger@ucla.edu Mammalian Genome 11, 422–427 (2000). © Springer-Verlag New York Inc. 2000 Incorporating Mouse Genome