Ift172 conditional knockout mice model human IFT172-associated retinal degeneration

Abstract

Purpose : Intraflagellar transport (IFT) is thought to be critical for building and maintaining photoreceptor outer segments, which can be regarded as specialized sensory cilia. The transition from anterograde to retrograde IFT in photoreceptor outer segments is believed to involve the IFT172 protein, and mutations in the IFT172 gene are associated with phenotypes ranging from isolated retinal degeneration to severe syndromic ciliopathies. The goal of this study was to create a mammalian model of IFT172-associated retinal degeneration to investigate the ocular disease mechanism.

Methods : A rod photoreceptor specific Ift172 knockout mouse model was created by breeding mice carrying a floxed Ift172 allele (Ift172^tm1.1Rama designated Ift172^loxP) with transgenic mice expressing Cre recombinase under the control of the rhodopsin promoter (Tg(Rho-icre)1Ck; designated iCre⁺). Ocular coherence tomography (OCT) was used to image the retina and electroretinography (ERG) was used to assess retinal function in mice up to 6 months of age. The wild type Ift172 allele is designated Ift172^WT, and wild type mice that do not express Cre recombinase are designated iCre^- .

Results : OCT imaging of mice homozygous for the floxed Ift172^loxP allele that also express iCre (Ift172^loxP/loxP-iCre⁺) showed thinning of the outer nuclear layer (ONL) beginning at 1 month of age (36±6µm) compared to controls (52±1.5µm). By two months of age, the ONL of the Ift172^loxP/loxP-iCre mice had completely degenerated while the control ONL remained full thickness (p<0.01). ERGs generated by Ift172^loxP/loxP-iCre⁺ mice corresponded with OCT data, with diminished retinal function by 1 month of age, and complete absence of function by 3 months of age (p<0.01). No significant changes in OCT or ERG were observed in control mice up to 6 months of age(Ift172^loxP/WT-iCre⁺, Ift172^loxP/WT-iCre^-, Ift172^WT/WT-iCre^-, Ift172^WT/WT-iCre⁺, Ift172^loxP/loxP-iCre^- ). This result is consistent with the recessive inheritance pattern seen in humans.

Conclusions : Ift172^loxP/loxP-iCre⁺ mice accurately model the rapid retinal degeneration observed in patients with IFT172-associated retinal degeneration,. Ift172^loxP/loxP-iCre^- mice showed no significant ERG or OCT changes, indicating that the presence of two floxed alleles alone does not lead to degeneration. This model can be utilized to gain better understanding IFT172's role in the maintenance of rod photoreceptor outer segments.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.