Photoreceptor degeneration, azoospermia, leukoencephalopathy, and abnormal RPE cell function in mice expressing an early stop mutation in CLCN2.
OBJECTIVE
To determine the molecular basis and the pathologic consequences of a chemically induced mutation in a mouse model of photoreceptor degeneration, nmf240.


METHODS
Mice from a G3 N-ethyl-N-nitrosourea mutagenesis program were screened by indirect ophthalmoscopy for abnormal fundi. A chromosomal position for the recessive nmf240 mutation was determined by a genome-wide linkage analysis by use of simple sequence length polymorphic markers in an F2 intercross. The critical region was refined, and candidate genes were screened by direct sequencing. The nmf240 phenotype was characterized by histologic analysis of the retina, brain, and male reproductive organs and by electroretinogram (ERG)-based studies of the retina and retinal pigment epithelium (RPE).


RESULTS
Clinically, homozygous nmf240 mutants exhibit a grainy retina that progresses to panretinal patches of depigmentation. The mutation was localized to a region on chromosome 16 containing Clcn2, a gene associated with retinal degeneration. Sequencing identified a missense C-T mutation at nucleotide 1063 in Clcn2 that converts a glutamine to a stop codon. Mice homozygous for the Clcn2(nmf240) mutation experience a severe loss of photoreceptor cells at 14 days of age that is preceded by an elongation of RPE apical microvilli. Homozygous mutants also experience leukoencephalopathy in multiple brain areas and male sterility. Despite a normal retinal histology in nmf240 heterozygotes, the ERG light peak, generated by the RPE, is reduced.


CONCLUSIONS
The nmf240 phenotype closely resembles that reported for Clcn2 knockout mice. The observation that heterozygous nmf240 mice present with a reduced ERG light peak component suggests that CLCN2 is necessary for the generation of this response component.
Edwards
Malia M
MM
Jackson Laboratory, Bar Harbor, Maine, USA. medwar28@jhmi.edu
Marín de Evsikova
Caralina
C
Collin
Gayle B
GB
Gifford
Elaine
E
Wu
Jiang
J
Hicks
Wanda L
WL
Whiting
Carrie
C
Varvel
Nicholas H
NH
Maphis
Nicole
N
Lamb
Bruce T
BT
Naggert
Jürgen K
JK
Nishina
Patsy M
PM
Peachey
Neal S
NS
D000818:Animals; D053713:Azoospermia / Q000235:genetics* / Q000473:pathology; D015153:Blotting, Western; D001921:Brain / Q000473:pathology; D000090007:CLC-2 Chloride Channels; D018118:Chloride Channels / Q000235:genetics*; D018389:Codon, Nonsense; D004596:Electroretinography; D005038:Ethylnitrosourea / Q000633:toxicity; D005260:Female; D055106:Genome-Wide Association Study; D056784:Leukoencephalopathies / Q000235:genetics* / Q000473:pathology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D016296:Mutagenesis / Q000187:drug effects; D020419:Photoreceptor Cells, Vertebrate / Q000473:pathology*; D012162:Retinal Degeneration / Q000235:genetics* / Q000473:pathology; D055213:Retinal Pigment Epithelium / Q000473:pathology*
D000090007:CLC-2 Chloride Channels; D018118:Chloride Channels; C000715428:Clcn2 protein, mouse; D018389:Codon, Nonsense; D005038:Ethylnitrosourea