Congenital stationary night blindness in mice - a tale of two Cacna1f mutants.
BACKGROUND
Mutations in CACNA1F, which encodes the Ca(v)1.4 subunit of a voltage-gated L-type calcium channel, cause X-linked incomplete congenital stationary night blindness (CSNB2), a condition of defective retinal neurotransmission which results in night blindness, reduced visual acuity, and diminished ERG b-wave. We have characterized two putative murine CSNB2 models: an engineered null-mutant, with a stop codon (G305X); and a spontaneous mutant with an ETn insertion in intron 2 of Cacna1f (nob2).


METHODS
Cacna1f ( G305X ): Adults were characterized by visual function (photopic optokinetic response, OKR); gene expression (microarray) and by cell death (TUNEL) and synaptic development (TEM). Cacna1f ( nob2 ): Adults were characterized by properties of Cacna1f mRNA (cloning and sequencing) and expressed protein (immunoblotting, electrophysiology, filamin [cytoskeletal protein] binding), and OKR.


RESULTS
The null mutation in Cacna1f ( G305X ) mice caused loss of cone cell ribbons, failure of OPL synaptogenesis, ERG b-wave and absence of OKR. In Cacna1f ( nob2 ) mice alternative ETn splicing produced ~90% Cacna1f mRNA having a stop codon, but ~10% mRNA encoding a complete polypeptide. Cacna1f ( nob2 ) mice had normal OKR, and alternatively-spliced complete protein had WT channel properties, but alternative ETn splicing abolished N-terminal protein binding to filamin.


CONCLUSIONS
Ca(v)1.4 plays a key role in photoreceptor synaptogenesis and synaptic function in mouse retina. Cacna1f ( G305X ) is a true knockout model for human CSNB2, with prominent defects in cone and rod function. Cacna1f ( nob2 ) is an incomplete knockout model for CSNB2, because alternative splicing in an ETn element leads to some full-length Ca(v)1.4 protein, and some cones surviving to drive photopic visual responses.
Department Medical Genetics, University of Calgary, Calgary, Alberta, Canada. nlodha@ucalgary.ca
N
N
Lodha
S
S
Bonfield
N C
NC
Orton
C J
CJ
Doering
J E
JE
McRory
S C
SC
Mema
R
R
Rehak
Y
Y
Sauvé
R
R
Tobias
W K
WK
Stell
N T
NT
Bech-Hansen
D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D015220:Calcium Channels / Q000235:genetics*; D020746:Calcium Channels, L-Type; D055253:Color Vision / Q000502:physiology; D015350:Contrast Sensitivity / Q000502:physiology; D004195:Disease Models, Animal; D004596:Electroretinography; D015785:Eye Diseases, Hereditary; D040181:Genetic Diseases, X-Linked; D006801:Humans; D051379:Mice; D009154:Mutation / Q000235:genetics; D009216:Myopia / Q000235:genetics* / Q000503:physiopathology; D009755:Night Blindness / Q000235:genetics* / Q000503:physiopathology
C480684:Cacna1f protein, mouse; D015220:Calcium Channels; D020746:Calcium Channels, L-Type