B6.Cg-Mfrp^rd6/J

Stock number: 003684
Research areas: Metabolism Research, Research Tools, Sensorineural Research

Description

Male and female mice homozygous for rd6develop a slowly progressive retinal degeneration affecting both rod and cone cells beginning at 3-4 weeks of age, soon after the retina develops. This mutant has been suggested as a model for the human flecked retinal disorder retinitis punctata albescens (RPA), which causes progressive visual loss similar to that characteristic of retinitis pigmentosa.

Detailed description

Retinal degeneration 6 (rd6) is a spontaneous 4 base pair deletion in the splice donor sequence of intron 4 of membrane frizzled-related protein (Mfrp) that does not cause a frameshift but rather a 58 amino acid deletion from the extracellular domain proximal to the transmembrane domain, a region believed to be glycosylated. MFRP is a single-pass transmembrane protein strongly expressed in the retinal pigment epithelium (RPE), where it localizes predominantly to the apical membrane, and in the ciliary epithelium. In addition to binding and localizing potassium channel KCNJ13 and the ceramidase ADIPOR1 to the RPE apical membrane, MFRP also binds the dicistronically expressed C1QTNF5 and lipids including phophatidylserine and phosphatidylinositol-4-phosphate. Mfrp^rd6 homozygotes have decreased ADIPOR1 protein and ADIPOR1 is absent from RPE apical membrane whereas KCNJ13 expression is normal but diffusely distributed rather than anchored to the apical membrane. Transcriptomics of RPE from Mfrp^rd6 homozygotes found upregulation of the complement and coagulation cascade genes and downregulation of genes involved in visual-chromophore regeneration and biosynthesis of fatty acids, including decreased expression of MFSD2A, ACSL6, ELOVL2, ELOVL4 and FADS2 which are involved in docosahexaenoic acid (DHA) processing. Consistent with this, the retinal lipidome was found to be abnormal, and increased DHA and osbond acid, and decreased linoleic acid were found in the RPE of Mfrp^rd6 homozygotes.

Mfrp^rd6 homozygotes develop a slowly progressive retinal degeneration affecting cone cells more than rod cells beginning at 3-4 weeks of age. Tworak et al found slow kinetics of visual chromophore regeneration at all times assessed, from one to six months of age. Ophthalmoscopic examination reveals small, evenly distributed white spots, from subretinal macrophage infiltration, throughout the retina from the time the mice are 8 weeks old. The spots remain clearly visible until about 7 months, when clinical signs of retinal degeneration become evident, and become less easily distinguishable as degeneration progresses. Retinal blood vessels appear pale and attenuated by 7 months and are undetectable by 15 months, when the fundus appears mottled, lightly pigmented and granular. Aberrations of the photoreceptor layer are histologically evident at 3-4 weeks; by one year, only one to three cell layers (of the normal 10-12) remain. Optical coherence tomography by Gogna et al (2025) found significantly shorter axial and posterior lengths with longer anterior chamber depth proving Mfrp^rd6 homozygotes have posterior microphthalmia-nanophthalmia. This strain provides a model for retinitis pigmentosa, fundus albipunctatus, isolated microphthalmia 5, microphthalmia-nanophthalmia and has been suggested as a model for the human flecked retinal disorder retinitis punctata albescens (RPA), which causes progressive visual loss similar to that characteristic of retinitis pigmentosa. (Hawes et al. 2000; Kameya et al 2002; Nawajes et al 2006; Won et al 2006; Sluch et al 2018; Kautzmann et al 2020; Tworak et al 2025; Gogna et al 2025)

Development

The Cat^b (catalase 1 null) mutation was identified in a screen for low catalase activity by Feinstein et al. (1964, 1966) at Argonne National Laboratory of offpring of (C3H x 101) male mice irradiated at Oak Ridge National Laboratory (600 R in fractionated exposures), then crossed to females of the Oak Ridge "multiple recessive testing stock" derived from a cross of NB mice (aabbc^ch p/c^ch p d se/d se) and mice of a non-inbred stock homozygous for three of the same recessive mutations, as well as for s, from which mice homozygous for all seven mutations were bred and maintained afterward by random breeding (Russell 1951). Putative heterozygotes for catalase deficiency were crossed to normal mice (not clearly identified, but presumably unirradiated progeny of (101 x C3H)F1 X ORNL testing stock), and offspring exhibiting low catalase activity were backcrossed to the heterozygous parent (Feinstein et al. 1966).Cat^b was subsequently transferred by backcrossing for eight generations onto C3H/HeAnl, a line carrying mouse mammary tumor virus (MMTV) and another line (C3Hf) free of the virus (Feinstein et al. 1978). The Jackson Laboratory received C3HfB/Ga-Cat^b, now C3Ga.Cg-Cat^b/J (stock #001906), from Allen H. Gates, then at the University of Rochester, in 1982.

Although all C3H strains carry the rd1 (retinal degeneration 1) mutation at Pde6b, the gene encoding the beta subunit of phosphodiesterase 1, retinas of these mice exhibited a peculiar, granular appearance distinct from the usual Pde6b^rd1 phenotype. F2 progeny of a cross of C3HfB/Ga-Cat^b and C57BL/6J mice exhibited three retinal phenotypes: 1) normal; 2) patches of pigment deposition and large, diffuse pigmented regions typical Pde6b^rd1 homozygotes; and 3) small, discrete, light-colored dots throughout the fundus. Mice exhibiting the last phenotype were intercrossed among themselves to produce a stock homozygous for Mfrp^rd6 and for the wild-type allele at Pde6b. During this process, a separate mutation with a distinct map position was identified and characterized as Crb1^rd8. STOCK Crb1^rd8 Mfrp^rd6/J (stock #004299) is homozygous for both Mfrp^rd6 and Crb1^rd8 on this mixed genetic background that derived in part from C57BL/6J, C3H, 101, and a linkage testing stock derived from non-inbred stocks. In order to isolate Mfrp^rd6 and Crb1^rd8 this strain was backcrossed to C57BL/6J, and selection for either Mfrp^rd6 or Crb1^rd8 yielded B6.C3Ga-Mfrp^rd6/J (stock #003684) and STOCK Crb1^rd8/J (stock #003392) respectively.

Allele

Symbol: Mfrp^rd6
Name: retinal degeneration 6
MGI accession ID: MGI:1888705
Generation method: Spontaneous
Marker symbol: Mfrp
Marker name: membrane frizzled-related protein
Marker synonyms: CTRP5; MCOP5; NNO2; RD6
Chromosome: 9
Strain of origin: C3fBAnl.Cg-Cat^b/AnlJ
Molecular note: The rd6 mutation was identified as a 4 bp deletion in the splice donor sequence of intron 4 in the Mfrp gene. The mutation results in the skipping of exon 4. While the mutation does not cause a frameshift, 58 amino acids are deleted from the protein and no protein was detected by western blot or immunohistochemistry of the eye. C1QTNF5, encoded dicistronically, was reported to have normal expression, localization, and dimer formation in the eye.