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

• Main Authors: Giancarlo Iarossi, Valerio Marino, Paolo Enrico Maltese, Leonardo Colombo, Fabiana D’Esposito, Elena Manara, Kristjana Dhuli, Antonio Mattia Modarelli, Gilda Cennamo, Adriano Magli, Daniele Dell’Orco, Matteo Bertelli
• Format: Article
• Language: English
• Published: MDPI AG 2021-12-01
• Series: International Journal of Molecular Sciences
• Subjects: autosomal recessive cone-rod dystrophy; compound heterozygosis; GTPase; molecular dynamics
• Online Access: https://www.mdpi.com/1422-0067/22/1/381
• ISSN: 1661-6596; 1422-0067

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 <i>RAB28</i> 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 <i>RAB28 </i>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.