Subterranean Fishes of the World

3078

2015

AIM 2015

http://www.mexicancaves.org/other/otherpubs.html

Genetic analysis indicates that multiple genes and quantitative trait loci (QTL) control eye degeneration in Astyanax cavefish. We are studying the eye QTL on chromosome 21 (C21 QTL), which contains 24 protein-coding genes located in scaffold KB871589.1 of the Pachón cavefish genome sequence. Semi-quantitative RT-PCR of 21 C21 QTL genes during the period of cavefish eye degeneration showed 15 genes with no changes in expression, 3 upregulated genes, and 3 downregulated genes in cavefish relative to surface fish embryos. The three-downregulated genes, cystathionine ß- synthase a (cbsa), alpha A-crystallin (cryaa), and heat shock transcription factor 2 binding protein are contiguous in the center of the C21 QTL. In situ hybridization showed that only the cbsa and cryaa genes have eye specific expression in surface fish embryos. We previously reported that CRYAA serves as a survival factor in Astyanax eye development, but it is not detectably mutated in cavefish and probably under trans-acting regulation (Ma et al., 2014, EvoDevo 5:28). The cbsa gene encodes an enzyme catalyzing the first step of the transsulfuration pathway, which is responsible for producing cysteine and the cysteinecontaining tripeptide glutathione, a critical antioxidant in the eye and other tissues. In situ hybridization showed that cbsa expression is downregulated in the cavefish lens and retina, cbsa knockdown in surface fish embryos affects retina development, and the F1 hybrid test indicated that cbsa downregulation is likely caused by a cis-acting mutation. A nonsynonomous amino acid substitution (threonine > isoleucine) was also detected in the catalytic domain of cavefish CBSA, suggesting that mutations in both non-coding and coding regions may affect cbsa function. Similar coding region mutations in human CBS prevent proper enzyme folding and produce homocystinuria, a disease characterized by defective sulfur amino acid metabolism. One of the effects of this disease is on the eye. These results suggest that cavefish may suffer from a breakdown in the defense system protecting the eye against chemical and oxidative stress due to mutations in the cbsa gene and that this deficiency may be one of the causes of eye degeneration.

1