Effects of UV-B radiation on a hereditary suture cataract in mice

UVR-B induced cataract development in C57 mice

The evolution of the morphological appearance and intensity of light scattering in C57 mice lenses after exposure to ultraviolet radiation type B (UVR-B) was investigated. A total of 80, 6-week-old female C57BL/6 mice were divided into four groups (n=20). One eye in each animal was exposed in vivo to UVR-B in the 300 nm wavelength region (UVR-B-300 nm) to a dose of 5 kJm(-2) for 15 min. The radiation output had lambda(max) at 302 nm with 5 nm [FWHM]. The animals were consecutively sacrificed at 1, 2, 4 and 8 days after the exposure. Macroscopic lens changes were documented using grid- and dark field illumination photography. Light scattering in the exposed and contralateral not exposed lens was measured quantitatively. Morphological lens changes were documented using grid- and dark field illumination photography. In vivo exposure to UVR-B-300 nm induced subcapsular cataract in all exposed lenses and occasionally cortical and nuclear cataract at all investigated time points. Exposed lenses scattered light significantly higher on all investigated days compared to contralateral non-exposed lenses. A transient increase of light scattering peaking at day 2 in exposed as well as in contralateral not exposed lenses was identified. Light scattering of the lenses varies with latency time after exposure. A dose of 5 kJm(-2) UVR-B-300 nm induces light scattering in C57 mice lenses. The increase has a transient peak at 2 days after exposure. The variation of light scattering among days 1, 2, 4, and 8 indicates a dynamic change of scattering characteristics in the mouse lens following unilateral in vivo exposure to 5 kJm(-2) UVR-B-300 nm.

Mouse models of congenital cataract

Mouse mutants affecting lens development are excellent models for corresponding human disorders. The mutant aphakia has been characterised by bilaterally aphakic eyes (Varnum and Stevens, J Hered 1968;59:147-50); the corresponding gene was mapped to chromosome 19 (Varnum and Stevens, Mouse News Lett 1975;53:35). Recent investigations in our laboratory refined the linkage of 0.6 cM proximal to the marker D19Mit10. Several candidate genes have been excluded (Chuk1, Fgf8, Lbp1, Npm3, Pax2, Pitx3). The Cat3 mutations are characterised by vacuolated lenses caused by alterations in the initial secondary lens fibre cell differentiation. Secondary malformations develop at the cornea and iris, but the retina remains unaffected. The mutation has been mapped to chromosome 10 close to the markers D10Mit41 and D10Mit95. Several candidate genes have been excluded (Dcn, Elk3, Ldc, Mell8, Tr2-11). The series of Cat2 mutations have been mapped close to the gamma-crystallin genes (Cryg; Löster et al., Genomics 1994;23:240-2). The Cat2nop mutation is characterised by a mutation in the third exon of Crygb leading to a truncated gamma B-crystallin and the termination of lens fibre cell differentiation. The Cat2 mutants are interesting models for human cataracts caused by mutations in the human CRYG genes at chromosome 2q32-35.

Cataract mutations and lens development

The lens plays an essential role for proper eye development. Mouse mutants affecting lens development are excellent models for corresponding human disorders. Moreover, using mutations in particular genes the process of eye and lens development can be dissected into distinct steps. Therefore, three mouse mutants will be described in detail and discussed affecting three essential stages: formation of the lens vesicle, initiation of secondary lens fiber cell formation, and terminal differentiation of the secondary fiber cells. The mutant aphakia (ak) has been characterized by bilaterally apakic eyes [Varnum and Stevens (1968) J. Hered. 59, 147-150], and the corresponding gene was mapped to chromosome 19 [Varnum and Stevens (1975) Mouse News Letters 53, 35]. Recent investigations in our laboratory refined the linkage 0.6 +/- 0.3 N cm proximal to the microsatellite marker D19Mit10. The linked gene Pax2, responsible for proper development of the posterior part of the eye and the optic nerve, was excluded as candidate gene by sequence analysis. Histological analysis of the homozygous ak mutants revealed a persisting lens stalk and subsequently the formation of lens rudiments. The lens defects led to irregular iris development and retinal folding. Congenital aphakia is known as a rare human anomaly. Besides a corneal dystrophy (CDTB), no corresponding disease is localized at the homologous region of human chromosome 10q23. The Cat3 mutations are characterized by vacuolated lenses caused by alterations in the beginning of secondary lens fiber cell differentiation at embryonic day 12.5. Secondary malformations develop at the cornea and the iris, but the retina remains unaffected. Two mutant alleles of the Cat3 locus have been mapped to mouse chromosome 10 very close to the microsatellite markers D10Mit41 and D10Mit95 (less than 0.3 cM). Since Cat3 is mapped to a position, which is homologous to human chromosome 12q21-24, the disorder cornea plana congenita can be considered as a candidate disease. The series of Cat2 mutations have been mapped close to the locus encoding the gamma-crystallin gene cluster Cryg [Löster et al. (1994) Genomics 23, 240-242]. The Cat2nop mutation is characterized by a deletion of 11 bp and an insertion of 4 bp in the 3rd exon of Crygh leading to a truncated gamma B-crystallin. The defect in the Crygh gene is causative for the stop of lens fiber cell differentiation from embryonic day 15.5 onward. Besides the lens, no further ocular tissue is affected. The Cat2 mouse mutants are interesting models for human cataracts caused by mutations in the gamma-crystallin genes at human chromosome 2q32-35. The ak, Cat3 and Cat2 mutants are discussed in the context of other mutants affecting early eye and lens development. Additionally, human congenital cataracts are discussed, which have been characterized similar to the mouse models. The overview of the three types of mutants demonstrates that genes, which affect the early eye development, e.g. at the lens vesicle stage, have consequences for the development of the whole eye. In contrast, if the mutation influences later steps of lens differentiation, the consequences are restricted to the lens only. These data indicate a decreasing effect of the lens for the regulation of eye development during embryogenesis.