Unconvential Myosin VI motor protein is required for iris development and outer retinal function

Abstract

 

Purpose

 

To determine the molecular basis and the pathologic consequences of a chemically induced mutation in a mouse model with electroretinographic defects, tvrm89.

 

Methods

 

Mice from a G3 N-ethyl-N-nitrosourea mutagenesis program were screened for abnormal behavior and sensory defects by retinal imaging, slit lamp biomicroscopy and electroretinography (ERG). The recessive tvrm89 mutation was chromosomally mapped by a genome-wide linkage analysis using simple sequence length polymorphic markers in an F2 intercross, followed by direct sequencing of candidate genes in the critical region. The tvrm89 ocular phenotype was characterized by spectral-domain optical coherence tomography (SD-OCT), scanning laser ophthalmoscopy (SLO), detailed ERG-based studies of the retina and retinal pigment epithelium (RPE), and histological analysis of these structures.

 

Results

 

The tvrm89 mutation was localized to chromosome 9. Because homozygous tvrm89 mutants exhibited a circling behavior, the Snell’s waltzer mouse gene, myosin6, became an obvious candidate as it was within the critical region. Sequencing identified a T-C point mutation at amino acid 480 in myosin6 that is predicted to cause a deleterious leucine to proline change. The mutation was shown to segregate with affected mice in both the B6-myo6^tvrm89 and F2 mapping colonies and is a conserved residue across species. Mice homozygous for the myosin6^tvrm89 mutation display iris malformations and attenuation of the ERG a- and b-wave as well as dc-ERG components by 4 weeks of age; these abnormalities do not become more severe at later ages.

 

Conclusions

 

The myosin6^{tvrm89/tvrm89} phenotype is very similar to that reported for Myo6^sv/sv mice, which are effective null Myo6 mutants. The observation that homozygous myosin6^tvrm89 mice display reduced ERG components suggests that myosin6 is necessary for outer retinal function.

 

Keywords: 510 electroretinography: non-clinical • 571 iris • 551 imaging/image analysis: non-clinical