Deficiency of Intraflagellar Transport Protein 139 in Rod Photoreceptors Causes Early-Onset Rod-Cone Degeneration in Mice

Abstract

Purpose: The specific role of retrograde intraflagellar transport protein 139 (IFT139), encoded by the TTC21B gene, in the development and viability of ciliated cells has yet to be defined. However, mutations in TTC21B have been linked to cilopathies with retinal phenotypes such as nephronophthisis and Jeune asphyxiating thoracic dystrophy. To investigate how this protein influences ciliated cell survival, including that of photoreceptors, a rod-specific conditional knockout mouse line was generated. Changes in retinal morphology and function were assessed in these mice, in vivo.

Methods: Mice engineered to have a targeted Ttc21b allele with loxP sites flanking exon 4 were crossed with rhodopsin-iCre transgenetic mice to knock out IFT139 in rods. All experiments performed on the Ttc21b^flox/flox/Rho-iCre⁺ conditional knockout mice (cKO) were compared to litter-mate Ttc21b^flox/flox control mice. Retinal function was measured by full-field electroretinography (ERG) using a broadband 4ms ON-OFF stimulus in dark-adapted and light-adapted conditions. Retinal morphology was assessed using fundus photography and optical coherence tomography (OCT).

Results: The cKO mice exhibited an early-onset retinal degeneration that resulted in a dramatic loss of photoreceptors by two months of age. At one month, the fundi of the cKO mice appeared normal, although OCT revealed that retinas were 22% (p=7x10^-13) thinner than those of control mice due to deterioration of the photoreceptor layer. At this age, ERG demonstrated that rod function was 55% (p<0.001) lower in cKO mice, whereas cone function had just started to decrease (-8%, p=0.37). At two months, fundus imaging in the cKO mice showed attenuated vasculature, abnormal optic disc pallor, and RPE that was visible through the neural retina; the cKO retinas were 43% (p=2x10^-12) thinner than those of control mice. Neither rod nor cone activating stimuli generated a detectable ERG in the two month old cKO mouse.

Conclusions: Rod photoreceptors with a deficiency of IFT139 developed and were still moderately functional in one month old mice. However, by two months, rod degeneration was widespread and, in this environment, the cones also failed. Future experiments will investigate, at the mechanism level, how IFT139 contributes to ciliated cell survival.