A crystallin mutant cataract with mineral deposits

Connexin mutant mice develop cataracts containing calcium precipitates. To test whether pathologic mineralization is a general mechanism contributing to the disease, we characterized the lenses from a non-connexin mutant mouse cataract model. By co-segregation of the phenotype with a satellite marker and genomic sequencing, we identified the mutant as a 5-bp duplication in the γC-crystallin gene (Crygc). Homozygous mice developed severe cataracts early, and heterozygous animals developed small cataracts later in life. Immunoblotting studies showed that the mutant lenses contained decreased levels of crystallins and of connexin46 and connexin50, but increased levels of resident proteins of the nucleus, endoplasmic reticulum, and mitochondria. The reductions of fiber cell connexins were associated with a scarcity of immunoreactive gap junction punctae as detected by immunofluorescence, and significant reductions in gap junction-mediated coupling between fiber cells in Crygc lenses. Particles that stained with the calcium deposit dye, Alizarin red, were abundant in the insoluble fraction from homozygous lenses, but nearly absent in wild-type and heterozygous lens preparations. Whole-mount homozygous lenses also stained with Alizarin red in the cataract region. Mineralized material with a regional distribution similar to the cataract was detected in homozygous lenses (but not wild-type lenses) by micro-computed tomography. Attenuated total internal reflection micro-Fourier transform infrared spectroscopy identified the mineral as apatite. These results are consistent with previous findings that loss of gap junctional coupling between lens fiber cells leads to formation of calcium precipitates. They also support the hypothesis that pathologic mineralization contributes to formation of cataracts of different etiologies.