The lens opacity (Lop) mutation on mouse chromosome 10 causes mistargeting of Major Intrinsic Protein (MIP) in lens fiber cells

Purpose. To define the mutation in the MIP gene underlying cataract development in the Lop mouse and to determine the fate of the mutant protein. Methods. MIP cDNA was amplified from Lop and wild-type lens RNA using the reverse transcriptase polymerase chain reaction, then TA-subcloned and sequenced by the dideoxy-termination method. The distribution of MIP in embryonic Lop and wild-type lenses was determined by confocal immunofluorescence microscopy. Results. The Lop MIP cDNA sequence was identical to that of wild-type, except for a G to C nucleotide change that resulted in the non-conservative substitution of proline for alanine at position 51 (A51P). In MIP from other species, alanine is the consensus residue at this position, whereas proline is never encoded. Immunofluorescence revealed that wild-type MIP was restricted to the fiber cell plasma membranes, whereas Lop MIP had an intracellular distribution. Double immunolabeling studies revealed that Lop MIP co-localized with protein disulfide isomerase, an abundant constituent of the endoplasmic reticulum (ER). Conclusions. Sequence data support the hypothesis that a point (missense) mutation in the coding region of the MIP gene underlies the Lop phenotype. Mutant MIP is translated in Lop lenses but fails to reach the plasma membrane, instead accumulating in the fiber cell ER. These data indicate that MIP is essential for lens transparency and identify this gene as a promising candidate for congenital cataract in humans.