Identification of a new mutant allele, Grm6(nob7), for complete congenital stationary night blindness

Electroretinogram (ERG) studies identified a new mouse line with a normal a-wave but lacking the b-wave component. The ERG phenotype of this new allele,nob7, matched closely that of mouse mutants forGrm6,Lrit3,Trpm1, andNyx, which encode for proteins expressed in depolarizing bipolar cells (DBCs). To identify the underlying mutation, we first crossednob7mice withGrm6nob3mutants and measured the ERGs in offspring. All the offspring lacked the b-wave, indicating thatnob7is a new allele forGrm6:Grm6nob7. Sequence analyses ofGrm6nob7cDNAs identified a 28 base pair insertion between exons 8 and 9, which would result in a frameshift mutation in the open reading frame that encodes the metabotropic glutamate receptor 6 (Grm6). Sequencing both the cDNA and genomic DNA from exon 8 and intron 8, respectively, from theGrm6nob7mouse revealed a G to A transition at the last position in exon 8. This mutation disrupts splicing and the normal exon 8 is extended by 28 base pairs, because splicing occurs 28 base pairs downstream at a cryptic splice donor. Consistent with the impact of the resulting frameshift mutation, there is a loss of mGluR6 protein (encoded byGrm6) from the dendritic tips of DBCs in theGrm6nob7retina. These results indicate thatGrm6nob7is a new model of the complete form of congenital stationary night blindness, a human condition that has been linked to mutations ofGRM6.