Although the refractive properties of
Epha2 −/− lenses were disturbed, the tissue remained transparent. This finding conflicts with that of an earlier report, in which disruption of the
Epha2 locus in mice using a secretory gene-trapping strategy resulted in progressive cortical cataract formation.
15 In those animals, microscopic cellular disruptions were visible at 2 weeks. The incidence of cataract increased from 26% at 5 months of age to 83% by 14 months. Mature cataracts often featured frank rupture of the posterior capsule and extrusion of lens material. In the present study, we examined >100 lenses from
Epha2 −/− animals ranging from P2 to 1 year of age and did not observe cataracts. Occasionally, small flecklike opacities were present in the lens nucleus, but the incidence of such opacities was not greater in
Epha2 −/− lenses than in age-matched wild-type controls. The present data suggest that
Epha2 is not required for lens transparency, at least not on the mixed genetic background used here. The discrepancy between our findings and those in the previous study may reflect differences in genetic background. Several inbred strains of mice (e.g., 129) harbor mutations in the
Bfsp2 gene, which encodes CP49, an intermediate filament protein with important structural roles in the lens.
27 Lenses lacking CP49 are more fragile and prone to damage than wild-type lenses.
28 We excluded the
Bfsp2 mutation from our mice, but it is not clear from the methodological description of mice used in the earlier studies whether the
Bfsp2 mutation was present. The cataract-prone strain used previously (Epha2
Gt(KST085)Byg) was generated with the secretory gene trap approach. In that strategy, a fusion protein comprising a portion of the Epha2 ectodomain (encoded by exons 1–5) fused to a neomycin/β-galactosidase reporter cassette was trapped within the secretory pathway.
29 It is possible that the presence of the fusion protein could have contributed to the severity of the lens phenotype. In the present study,
Epha2 was disrupted by vector insertion into exon 5, introducing an in-frame translational stop codon within the vector sequence. It is unlikely that such a severely attenuated sequence would be translated, and neither of the antibodies used in the study detected Epha2 protein expression in the knockout mice. If a truncated mutant protein were to be translated, it is predicted to lack the transmembrane domain, the kinase domain, and the SAM (sterile alpha motif) domain of wild-type Epha2.