Genetic Variation in Ataxia Gene ATXN7 Influences Cerebellar Grey Matter Volume in Healthy Adults

AAV-Mediated CAG-Targeting Selectively Reduces Polyglutamine-Expanded Protein and Attenuates Disease Phenotypes in a Spinocerebellar Ataxia Mouse Model

Polyglutamine (polyQ)-encoding CAG repeat expansions represent a common disease-causing mutation responsible for several dominant spinocerebellar ataxias (SCAs). PolyQ-expanded SCA proteins are toxic for cerebellar neurons, with Purkinje cells (PCs) being the most vulnerable. RNA interference (RNAi) reagents targeting transcripts with expanded CAG reduce the level of various mutant SCA proteins in an allele-selective manner in vitro and represent promising universal tools for treating multiple CAG/polyQ SCAs. However, it remains unclear whether the therapeutic targeting of CAG expansion can be achieved in vivo and if it can ameliorate cerebellar functions. Here, using a mouse model of SCA7 expressing a mutant Atxn7 allele with 140 CAGs, we examined the efficacy of short hairpin RNAs (shRNAs) targeting CAG repeats expressed from PHP.eB adeno-associated virus vectors (AAVs), which were introduced into the brain via intravascular injection. We demonstrated that shRNAs carrying various mismatches with the CAG target sequence reduced the level of polyQ-expanded ATXN7 in the cerebellum, albeit with varying degrees of allele selectivity and safety profile. An shRNA named A4 potently reduced the level of polyQ-expanded ATXN7, with no effect on normal ATXN7 levels and no adverse side effects. Furthermore, A4 shRNA treatment improved a range of motor and behavioral parameters 23 weeks after AAV injection and attenuated the disease burden of PCs by preventing the downregulation of several PC-type-specific genes. Our results show the feasibility of the selective targeting of CAG expansion in the cerebellum using a blood-brain barrier-permeable vector to attenuate the disease phenotype in an SCA mouse model. Our study represents a significant advancement in developing CAG-targeting strategies as a potential therapy for SCA7 and possibly other CAG/polyQ SCAs.

The association of RAB18 gene polymorphism (rs3765133) with cerebellar volume in healthy adults

Genetic factors are responsible for the development of the human brain. Certain genetic factors are known to increase the risk of common brain disorders and affect the brain structure. Therefore, even in healthy people, these factors have a role in the development of specific brain regions. Loss-of-function mutations in the RAB18 gene (RAB18) cause Warburg Micro syndrome, which is associated with reduced brain size and deformed brain structures. In this study, we hypothesized that the RAB18 variant might influence regional brain volumes in healthy people. The study participants comprised 246 normal volunteers between 21 and 59 years of age (mean age of 37.8 ± 12.0 years; 115 men, 131 women). Magnetic resonance imaging (MRI) and genotypes of RAB18 rs3765133 were examined for each participant. The differences in regional brain volumes between T homozygotes and A-allele carriers were tested using voxel-based morphometry. The results showed that RAB18 rs3765133 T homozygote group exhibited larger gray matter (GM) volume in the left middle temporal and inferior frontal gyrus of the cerebrum than the A-allele carriers. An opposite effect was observed in both the posterior lobes and right tonsil of the cerebellum, in which the GM volume of RAB18 rs3765133 T homozygotes was smaller than that of the A-allele carriers (all P FWE < 0.05). Our findings suggest that RAB18 rs3765133 polymorphism affects the deve-lopment of specific brain regions, particularly the cerebellum, in healthy people.