This report describes night blindness, congenital stationary, 1B, autosomal recessive, a subtype congenital stationary night blindness. The human gene implicated is GRM6, which encodes glutamate metabotropic receptor 6. There are two moderate-scoring fly orthologs, Dmel\mtt and Dmel\mGluR, for which loss of function allele, RNAi-targeting constructs, and alleles caused by insertional mutagenesis have been generated.
Human GRM6 has not been introduced into flies.
Retina-specific knockdown of Dmel\mtt, but not Dmel\mGluR, affects the morphology of the developing eye disc and of the axonal processes from the larval retina to the optic lobes, and results in dramatic disruption of the structure of the adult eye. Affected flies show a lower level of activity response to light stimulation compared to control flies.
[updated Feb. 2024 by FlyBase; FBrf0222196]
Congenital stationary night blindness (CSNB) is a clinically and genetically heterogeneous group of nonprogressive retinal disorders that can be characterized by impaired night vision, decreased visual acuity, nystagmus, myopia, and strabismus. CSNB can be classified into 2 groups based on electroretinography (ERG) findings: the Schubert-Bornschein type is characterized by an ERG in which the b-wave is smaller than the a-wave, whereas the Riggs type is defined by proportionally reduced a- and b-waves. In addition, Schubert-Bornschein CSNB is associated with decreased visual acuity, myopia, and nystagmus, whereas in Riggs CSNB patients have visual acuity within the normal range and no symptoms of myopia and/or nystagmus (summary by Riazuddin et al., 2010, pubmed:20850105). Additionally, Schubert-Bornschein CSNB can be subdivided into 'complete' and 'incomplete' forms (summary by Riazuddin et al., 2010), pubmed:20850105) [from MIM:310500; 2024.02.21]
[NIGHT BLINDNESS, CONGENITAL STATIONARY, TYPE 1B; CSNB1B](https://omim.org/entry/257270)
[GLUTAMATE RECEPTOR, METABOTROPIC, 6; GRM6](https://omim.org/entry/604096)
CSNB1B is a complete, or type 1, congenital stationary night blindness. Complete congenital stationary night blindness is characterized by the complete absence of rod pathway function, whereas the incomplete form is due to impaired rod and cone pathway function. Complete CSNB results from postsynaptic defects in depolarizing or ON bipolar cell signaling, whereas the hyperpolarizing or OFF bipolar cell pathway is intact (Van Genderen et al., 2009, pmid:20850105). [from MIM:310500 and MIM:257270; 2024.02.21]
Congenital stationary night blindness type 1B is caused by mutation in the GRM6 gene on chromosome 5q35. [from MIM:257270, 2024.02.21]
L-glutamate is the major excitatory neurotransmitter in the central nervous system and activates both ionotropic and metabotropic glutamate receptors. Glutamatergic neurotransmission is involved in most aspects of normal brain function and can be perturbed in many neuropathologic conditions. The metabotropic glutamate receptors are a family of G protein-coupled receptors, that have been divided into 3 groups on the basis of sequence homology, putative signal transduction mechanisms, and pharmacologic properties. Group I includes GRM1 and GRM5 and these receptors have been shown to activate phospholipase C. Group II includes GRM2 and GRM3 while Group III includes GRM4, GRM6, GRM7 and GRM8. Group II and III receptors are linked to the inhibition of the cyclic AMP cascade but differ in their agonist selectivities. Mutations in this gene result in congenital stationary night blindness type 1B. [provided by RefSeq, May 2018]
Like other metabotropic glutamate receptors, GRM6 is a G protein-coupled receptor that contains a signal peptide, a large extracellular domain, and 7 transmembrane segments. [from MIM:604096; 2024.02.21]
Moderate-scoring ortholog of human GRM2, GRM3, GRM4, GRM6, GRM7, GRM8 (2 Drosophila to many human).
High-scoring ortholog of human GRM3, moderate-scoring ortholog of GRM1, GRM2, GRM4, GRM5, GRM6, GRM7, GRM8 (2 Drosophila to many human).
