Genetic Modifiers of Age-Related Photoreceptor Degeneration/Maintenance

Abstract

Purpose: : Previously, we identified several QTL that significantly modify age-related photoreceptor (PR) loss in 268 F2 progeny of an F1 intercross between B6(Cg)-Tyr^c-2J/J (B6a) and BALB/cByJ (BALB) albino mice at 8 months. The purpose was to refine the loci of two of the strongest QTL (Chrs 6 and 10) and to evaluate candidate genes.

Methods: : Utilizing genomic DNAs from the original 268 F2 mice, the Center for Inherited Disease Research genotyped a number of SNPs in each QTL in a follow-up study. The data was analyzed with the Map Manager QTX program. Recombinant progeny testing was performed on BALB mice with introgressed B6a regions of the Chr 6 QTL. Candidate genes were sequenced by standard methods. Microarray studies were carried out between B6a and BALB eyecup RNAs at 4 months of age and at 8 months of age. QTL from this study were compared to QTL identified in an F1 intercross between B6 and A/J mice. N7, C57BL/6 (B6) Tnfaip3 +/- mice were kindly provided by Dr. Averil Ma (UCSF) and were bred to N8 to N10 on B6.

Results: : The B6a-dominant Chr 6 QTL was reduced from ~40 Mb in the original 15-cM map to a 1-LOD critical area of 3.6 Mb in the follow-up study utilizing 10 informative SNPs; the additive Chr 10 QTL from ~27 to 5.4 Mb with 22 SNPs. Recombinant progeny testing verified the proximal limit of the Chr 6 QTL. Eefsec and Antxr1 were selected for study from the Chr 6 QTL because of haplotype analysis for both, and because Antxr1 showed a significant difference in expression between the two strains in the microarray study. Ahi1 and Tnfaip3 (A20) from the Chr 10 QTL were selected because they have been implicated in retinal degeneration and/or showed expression differences. Sequencing revealed no differences in the coding regions of Eefsec, Antxr1 or Ahi1 between B6a and BALB so far. There were two amino acid differences in the A20 gene. However, A20 +/- and +/+ littermates showed no difference in PR loss at 8 months.

Conclusions: : No significant differences in coding sequence between B6a and BALB have been found in Eefsec, Antxr1 or Ahi1 so far. Although AA changes between the two strains were identified for A20, the lack of phenotype in the A20 +/- retina suggests that this gene is not the QTG for the Chr 10 QTL. Other genes will be selected for future study based on function and implication in PR degeneration, but the critical areas of the QTL must be refined further by recombinant progeny testing to reduce the number of candidate genes.

Keywords: degenerations/dystrophies • gene modifiers • retina: distal (photoreceptors, horizontal cells, bipolar cells)