New insights into the mechanisms of retinal degeneration due to Phosphodiesterase 6 (PDE6) deficiency

Abstract

Purpose : Loss-of-function mutations in the Pde6 gene lead clinically in most cases to retinitis pigmentosa (RP). The onset of the disease in rod photoreceptors has been attributed to either a massive increase in cyclic guanosine monophosphate (cGMP) or a presumed deviation of the Ca²⁺ metabolism. As the underlying pathophysiology has not yet been satisfactorily resolved, the aim of this work was to gain insight in the disease mechanisms by discerning between (a) the direct impact of the intracellular rise in cGMP levels and (b) presumed Ca²⁺-related effects mediated by rod cyclic nucleotide gated (CNG) channels in the outer segment membrane. For this purpose, Pde6a mutants were crossed into a CNGB1-deficient background (Cngb1^-/-) to protect them from any deleterious Ca²⁺-related effects, and these doubly mutant mice were compared to the single-mutant PDE6A-deficient lines.

Methods : Pde6a mutant models of cGMP-related RP were cross-bred with B6.129SvJ-Cngb1^tm mice lacking rod CNG channels. The two lines in this work include: (1) A.B6-Tyr⁺/J-Pde6a^{nmf282/nmf282}-Cngb1^tm and (2) A.B6/J-Pde6a^R562W/R562W-Cngb1^tm. Lines (1) and (2) were evaluated in vivo using non-invasive diagnostic techniques and in vitro via a histological work-up. The results were compared to those in unprotected Pde6a single-mutants and wild-type mice.

Results : The newly generated mouse lines show that a removal of CNG channels from rods by cross-breeding with Cngb1^-/- does substantially protect PDE6A-deficent lines from developing cGMP-related RP. While the outer retina was entirely lost within ca. 15-30 days after birth in unprotected lines, at least one layer of photoreceptor rows remained until 3-6 months in protected lines, an age where the CNGB1 deficiency alone significantly contributes to degeneration. Importantly, cone function in both lines remained detectable until the end of the current study at the age of 6 months.

Conclusions : In both Pde6a mutants, the results clearly indicate that the dominant deleterious effect requires functional rod CNG channels. Further, these findings suggest that the impaired Ca²⁺ influx mediated by CNG channels is causative for disease development. Therefore, a specific modulation of rod CNG channels, but not cone channels, is a most promising symptomatic approach to treat otherwise incurable forms of cGMP-related RP, and may also widen the therapeutic window for gene therapeutic approaches.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.