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Xiaohua Gong, Jing Zeng, Catherine Cheng, Chun-hong Xia; The Role of Genetic Modifier in Cataract Formation Caused by Gja3 Connexin Knockout. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1727.
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Previous studies have shown that cataract formation is associated with calcium-dependent protein degradation in Gja3 (α3 connexin or Cx46) knockout mice and the cataract severity is dramatically different between C57/B6J (B6) and 129SvJae (129) mouse strain genetic backgrounds. This project aims to investigate new regulatory mechanism of lens calcium homeostasis and to identify an unknown genetic modifier that controls the severity of cataract formation.
Genome-wide linkage analysis, chromosome substitution by homologous counterparts from 129 and B6 inbred strains, and identification of SNPs of candidate genes were used to map and to identify candidate genes for the genetic modifier. Cataract formation was evaluated by slit-lamp examination in vivo, quantitative measurement of lens scattered light in vitro and confocal imaging of living lenses. Candidate genes and calcium channels were analyzed by RT-PCR, Western blotting and immunohistology.
A genetic modifier has been mapped into a 2.1 million base-pair interval on mouse Chr 7, and several candidate genes with missense mutations have been identified. This modifier in the B6 strain background acts as a semi-dominant suppressor for cataracts in Gja3 knockout mice. Cataract severity is correlated with the amount of calcium-dependent cleaved crystallin proteins. The expression of alpha and beta subunits of calcium channels is detected in the lens. More importantly, membrane translocation of calcium channel beta subunits is correlated with lens fiber cell maturation in wild-type and Gja3 knockout lenses in the B6 strain background, but not in the 129 strain background. Confocal images of Gja3 knockout lenses confirm that differentiated fiber cells undergo maturation before the cataract formation in mature fiber cells.
Calcium channels are utilized in lens fiber cell maturation regardless of the presence or absence of gap junction channels. Calcium homeostasis seems to be independently regulated by gap junction communication and calcium channels in the lens. The genetic modifier on mouse chromosome 7 influences cataract severity probably by regulating the membrane translocation of calcium channel beta subunits during lens fiber cell maturation, which controls intracellular calcium level to affect crystallin protein degradation in lens mature fibers. Thus, we will characterize candidate genes that are important for the function of lens calcium channels.
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