Early-Onset, Slow Progression of Cone Photoreceptor Dysfunction and Degeneration in CNG Channel Subunit CNGB3 Deficiency

Photoreceptor cyclic nucleotide-gated (CNG) channels are localized to the plasma membrane of photoreceptor outer segments and play a pivotal role in phototransduction.1,2 In the dark, cGMP binding activates rod CNG channels, allowing a steady cation current to flow into the outer segments. Light triggers a sequence of enzymatic reactions that leads to hydrolysis of cGMP, causing the channel to close. On channel closure, the inward current ceases and the cell hyperpolarizes, inhibiting synaptic transmission to second-order neurons. An analogous phototransduction scheme exists in cones, which are responsible for color vision and vision in bright illumination when rods are saturated. However, light sensitivity, cGMP sensitivity, Ca2+ permeation, and functional modulation differ profoundly between rod and cone CNG channels.3–5 CNG channels are composed of two structurally related subunit types, the A and B subunits. The rod channel consists of CNGA1 and CNGB1 subunits, whereas the cone channel is composed of CNGA3 and CNGB3 subunits. Heterologous expression studies have shown that the A subunits are responsible for the ion-conducting activity of the channel, whereas the B subunits function as modulators.2,6 Mutations in the rod CNG channel were identified in patients with retinitis pigmentosa,7 whereas mutations in the cone CNG channel have been associated with a variety of cone diseases, including achromatopsia, progressive cone dystrophy, and early-onset macular degeneration.8–11 To date more than 70 mutations have been identified in genes encoding the human CNGA3 and CNGB3 subunits, with mutations in CNGB3 alone found in more than 50% of patients with achromatopsia,9 a retinal disorder that affects approximately 1 in every 33,000 Americans. The condition is associated with color blindness, visual acuity loss, extreme light sensitivity, and nystagmus.8,11,12 The most frequently occurring mutation in CNGB3 is the Thr383fsx mutation, which accounts for more than 80% of all CNGB3 mutant alleles.9,13,14 This frame-shift mutation results in truncation of the pore-forming loop and the C-terminal cytoplasmic domain; as a result, no intact CNGB3 is formed. Thus, a loss-of-function defect is the primary mechanism of action for CNGB3 mutation in humans. Although CNGB3 shares a common topology with CNGA3 and has a pore-forming region, this subunit, when expressed alone, does not form a functional channel, in contrast to CNGA3, which forms a functional homomeric channel.6,15 Heteromeric CNGA3/CNGB3 channels display properties typical of native CNG channels,6,16 suggesting that CNGB3 modulates the channel's physiological properties in vivo. Our previous experimental work showing the interaction between CNGA3 and CNGB3 in the mouse retina17 and cone defects in CNGB3−/− mice18 strongly supports this view. Here we investigate the progression of cone defects caused by CNGB3 deficiency. CNGB3−/− mice displayed an early-onset, slow progression of cone dysfunction and degeneration, similar to the human achromatopsia found in patients with mutations in CNGB3. Furthermore, photoreceptor terminals appeared normal in the CNGB3−/− retina, suggesting that deficits in cone signaling result from disrupted phototransduction and cone loss rather than from synaptic defects. Thus, the CNGB3−/− mouse line will be a valuable model with which to study the functional and structural role of CNGB3 in cones and in retinal pathogenesis associated with achromatopsia.