Spectral-Domain Optical Coherence Tomography Analysis in Syndromic and Nonsyndromic Forms of Retinitis Pigmentosa due to USH2A Genetic Variants.

INTRODUCTION: This study aimed to analyze macular structure by using spectral-domain optical coherence tomography (SD-OCT) in a cohort of patients affected by autosomal recessive retinitis pigmentosa and Usher syndrome, due to genetic variants in USH2A gene, and to correlate optical coherence tomography (OCT) parameters with functional and genetic data. METHODS: The subjects of this study were 92 patients, 46 syndromic (Usher syndrome type IIa [Ush2]) and 46 nonsyndromic (autosomal recessive RP [arRP]), with clinical and genetic diagnosis of USH2A-related retinal dystrophy, who underwent a complete ophthalmic examination and spectral-domain OCT analysis. The study focused on evaluating the differences between the 2 groups in the following parameters: best-corrected visual acuity (BCVA), ellipsoid zone (EZ) width, presence of epiretinal membrane (ERM), and cystic macular lesions (CMLs). Variants in USH2A gene were divided into 3 categories, according to the expected impact (low/high) at protein level of the different variants on each allele. RESULTS: BCVA and EZ width were significantly lower in Ush2 than in arRP patients (p < 0.0001 and p = 0.001). ERM was detected in 34.8% (16/46) of arRP patients and in 65.2% (30/46) of Ush2 patients (p = 0.003). CML was detected in 17.4% (8/46) of arRP patients and 30.4% (14/46) of Ush2 patients (p = 0.14). The allelic distribution was statistically different (p = 0.0003) by dividing the 2 diseases: for Ush2 patients it was 45.7% (high/high), 39.1% (low/high) and 15.2% (low/low); for arRP patients it was 8.7% (high/high), 56.5% (low/high), and 34.8% (low/low). The severity class of the variants significantly affected visual acuity and EZ width parameters (p = 0.004 and p = 0.002, respectively). CONCLUSION: Retinal disease, as evaluated by means of SD-OCT, shows more advanced degeneration signs in the syndromic than the nonsyndromic form of retinal dystrophy related to USH2A gene. Variant types and allelic profiles are determining factors for the onset of syndromic features. However, since the 3 allelic profiles can be found in both Usher and RP patients, other factors must necessarily play a determining role.

A Novel GUCA1A Variant Associated with Cone Dystrophy Alters cGMP Signaling in Photoreceptors by Strongly Interacting with and Hyperactivating Retinal Guanylate Cyclase.

Guanylate cyclase-activating protein 1 (GCAP1), encoded by the GUCA1A gene, is a neuronal calcium sensor protein involved in shaping the photoresponse kinetics in cones and rods. GCAP1 accelerates or slows the cGMP synthesis operated by retinal guanylate cyclase (GC) based on the light-dependent levels of intracellular Ca2+, thereby ensuring a timely regulation of the phototransduction cascade. We found a novel variant of GUCA1A in a patient affected by autosomal dominant cone dystrophy (adCOD), leading to the Asn104His (N104H) amino acid substitution at the protein level. While biochemical analysis of the recombinant protein showed impaired Ca2+ sensitivity of the variant, structural properties investigated by circular dichroism and limited proteolysis excluded major structural rearrangements induced by the mutation. Analytical gel filtration profiles and dynamic light scattering were compatible with a dimeric protein both in the presence of Mg2+ alone and Mg2+ and Ca2+. Enzymatic assays showed that N104H-GCAP1 strongly interacts with the GC, with an affinity that doubles that of the WT. The doubled IC50 value of the novel variant (520 nM for N104H vs. 260 nM for the WT) is compatible with a constitutive activity of GC at physiological levels of Ca2+. The structural region at the interface with the GC may acquire enhanced flexibility under high Ca2+ conditions, as suggested by 2 µs molecular dynamics simulations. The altered interaction with GC would cause hyper-activity of the enzyme at both low and high Ca2+ levels, which would ultimately lead to toxic accumulation of cGMP and Ca2+ in the photoreceptor outer segment, thus triggering cell death.

Expanding the Clinical and Genetic Spectrum of RAB28-Related Cone-Rod Dystrophy: Pathogenicity of Novel Variants in Italian Families.

The small Ras-related GTPase Rab-28 is highly expressed in photoreceptor cells, where it possibly participates in membrane trafficking. To date, six alterations in the RAB28 gene have been associated with autosomal recessive cone-rod dystrophies. Confirmed variants include splicing variants, missense and nonsense mutations. Here, we present a thorough phenotypical and genotypical characterization of five individuals belonging to four Italian families, constituting the largest cohort of RAB28 patients reported in literature to date. All probands displayed similar clinical phenotype consisting of photophobia, decreased visual acuity, central outer retinal thinning, and impaired color vision. By sequencing the four probands, we identified: a novel homozygous splicing variant; two novel nonsense variants in homozygosis; a novel missense variant in compound heterozygous state with a previously reported nonsense variant. Exhaustive molecular dynamics simulations of the missense variant p.(Thr26Asn) in both its active and inactive states revealed an allosteric structural mechanism that impairs the binding of Mg2+, thus decreasing the affinity for GTP. The impaired GTP-GDP exchange ultimately locks Rab-28 in a GDP-bound inactive state. The loss-of-function mutation p.(Thr26Asn) was present in a compound heterozygosis with the nonsense variant p.(Arg137*), which does not cause mRNA-mediated decay, but is rather likely degraded due to its incomplete folding. The frameshift p.(Thr26Valfs4*) and nonsense p.(Leu13*) and p.(Trp107*) variants, if translated, would lack several key structural components necessary for the correct functioning of the encoded protein.