Mice with an induced mutation in collagen 8A2 develop larger eyes and are resistant to retinal ganglion cell damage in an experimental glaucoma model.
OBJECTIVE
To study susceptibility to glaucoma injury as it may be affected by mutations in ocular connective tissue components.


METHODS
Mice homozygous for an N-ethyl-N-nitrosourea induced G257D exchange (Gly to Asp) missense mutation (Aca23) in their collagen 8A2 gene were studied to measure intraocular pressure (IOP), axial length and width, number of retinal ganglion cells (RGC), and inflation responses. Three month old homozygous Aca23 mutant and wild type (WT) mice had 6 weeks exposure to elevated IOP induced by polystyrene microbead injection. Additional Aca23 and matched controls were studied at ages of 10 and 18 months.


RESULTS
Aca23 mice had no significant difference from WT in IOP level, and in both strains IOP rose with age. In multivariable models, axial length and width were significantly larger in Aca23 than WT, became larger with age, and were larger after exposure to glaucoma (n=227 mice). From inflation test data, the estimates of scleral stress resultants in Aca23 mice were similar to age-matched and younger WT C57BL/6 (B6) mice, while the strain estimates for Aca23 were significantly less than those for either WT group in the mid-sclera and in some of the more anterior scleral measures (p<0.001; n=29, 22, 20 eyes in Aca23, older WT, younger WT, respectively). With chronic IOP elevation, Aca23 eyes increased 9% in length and 7% in width, compared to untreated fellow eyes (p<0.05, <0.01). With similar elevated IOP exposure, WT eyes enlarged proportionately twice as much as Aca23, increasing in length by 18% and in nasal-temporal width by 13% (both p<0.001, Mann-Whitney test). In 4 month old control optic nerves, mean RGC axon number was not different in Aca23 and WT (46,905±7,592, 43,628±11,162, respectively; p=0.43, Mann-Whitney test, n=37 and 29). With chronic glaucoma, Aca23 mice had a mean axon loss of only 0.57±17%, while WT mice lost 21±31% (median loss: 1% versus 10%, n=37, 29, respectively; p=0.001; multivariable model adjusting for positive integral IOP exposure).


CONCLUSIONS
The Aca23 mutation in collagen 8α2 is the first gene defect found to alter susceptibility to experimental glaucoma, reducing RGC loss possibly due to differences in mechanical behavior of the sclera. Detailed study of the specific changes in scleral connective tissue composition and responses to chronic IOP elevation in this strain could produce new therapeutic targets for RGC neuroprotection.
Glaucoma Center of Excellence, Wilmer Ophthalmological Institute, Johns Hopkins University, Baltimore, MD, USA.
Matthew R
MR
Steinhart
Frances E
FE
Cone
Cathy
C
Nguyen
Thao D
TD
Nguyen
Mary E
ME
Pease
Oliver
O
Puk
Jochen
J
Graw
Ericka N
EN
Oglesby
Harry A
HA
Quigley
D000818:Animals; D058452:Axial Length, Eye / Q000187:drug effects; D001369:Axons / Q000187:drug effects / Q000473:pathology; D002452:Cell Count; D024102:Collagen Type VIII / Q000235:genetics*; D004195:Disease Models, Animal; D005038:Ethylnitrosourea; D005901:Glaucoma / Q000139:chemically induced / Q000235:genetics* / Q000473:pathology; D006720:Homozygote; D007429:Intraocular Pressure / Q000187:drug effects; D051379:Mice; D008810:Mice, Inbred C57BL; D008863:Microspheres; D020125:Mutation, Missense; D009798:Ocular Hypertension / Q000139:chemically induced / Q000235:genetics* / Q000473:pathology; D009900:Optic Nerve / Q000187:drug effects / Q000473:pathology; D009929:Organ Size; D011137:Polystyrenes; D020033:Protein Isoforms / Q000235:genetics; D012165:Retinal Ganglion Cells / Q000187:drug effects / Q000473:pathology*; D012590:Sclera / Q000187:drug effects / Q000473:pathology
C507463:Col8a2 protein, mouse; D024102:Collagen Type VIII; D011137:Polystyrenes; D020033:Protein Isoforms; D005038:Ethylnitrosourea