To determine the functional relevance of NMP-mediated structural rescue of H-R172W
+/−/
rds +/− mice, ERG studies were undertaken.
Figure 7Ashows representative scotopic and photopic ERG wave forms from single- and double-transgenic mice taken at 1 month of age. As reported previously, the introduction of the H-R172W transgene onto an
rds +/− background (H-R172W
+/−/
rds +/−) alleviates rod functional defects associated with the haploinsufficiency phenotype as early as 1 month of age,
25 whereas expression of the R172W mutant protein causes a significant (
P < 0.05) reduction in cone function
(Fig. 7B) . This early-onset phenotype makes H-R172W a perfect model for our rescue studies. In H-R172W mice, the enhancement in rod function persisted over time, whereas cone function continued to decline in an age-dependent manner. As shown in
Figure 7B , the addition of one NMP allele significantly improved rod and cone ERG amplitudes; at 1 month of age, NMP/H-R172W double-transgenic mice had scotopic and photopic amplitudes (90%–91% of WT), significantly higher than H-R172W single-transgenic mice (71% and 50% of WT, respectively). Rescue continued in the short term; at 3 months of age, H-R172W animals carrying an NMP allele had scotopic and photopic ERG amplitudes 82% and 73% of WT levels, respectively, whereas H-R172W animals without NMP continued to degenerate (56% vs. 34% of WT responses, respectively). At later time points (11 months), we found double-transgenic (H-R172W
+/−/NMP
+/−/
rds +/−) animals continued to have significantly (
P < 0.05) better rod function than single-transgenic (R172W
+/−/
rds +/−) mice (
Fig. 7C , left panel). Although Rds supplementation showed a statistically significant (
P < 0.001) amelioration of the cone defect in H-R172W
+/−/
rds +/− mice at 1 and 3 months of age
(Fig. 7B) , this rescue did not persist over the long-term, and cone ERG amplitudes in the H-R172W
+/−/NMP
+/−/
rds +/− were not different from H-R172W
+/−/
rds +/− by the age of 11 months (
Fig. 7C , right panel).