RESUMO
Spinocerebellar ataxia type 8 (SCA8), a dominantly inherited neurodegenerative disorder caused by a CTGâ¢CAG expansion, is unusual because most individuals that carry the mutation do not develop ataxia. To understand the variable penetrance of SCA8, we studied the molecular differences between highly penetrant families and more common sporadic cases (82%) using a large cohort of SCA8 families (n = 77). We show that repeat expansion mutations from individuals with multiple affected family members have CCGâ¢CGG interruptions at a higher frequency than sporadic SCA8 cases and that the number of CCGâ¢CGG interruptions correlates with age at onset. At the molecular level, CCGâ¢CGG interruptions increase RNA hairpin stability, and in cell culture experiments, increase p-eIF2α and polyAla and polySer RAN protein levels. Additionally, CCGâ¢CGG interruptions, which encode arginine interruptions in the polyGln frame, increase toxicity of the resulting proteins. In summary, SCA8 CCGâ¢CGG interruptions increase polyAla and polySer RAN protein levels, polyGln protein toxicity, and disease penetrance and provide novel insight into the molecular differences between SCA8 families with high vs. low disease penetrance.
Assuntos
Degenerações Espinocerebelares , Expansão das Repetições de Trinucleotídeos , Ataxia , Humanos , Proteínas do Tecido Nervoso/genética , Penetrância , Proteínas , RNA Longo não Codificante/genética , Degenerações Espinocerebelares/genéticaRESUMO
We have discovered that beta-III spectrin (SPTBN2) mutations cause spinocerebellar ataxia type 5 (SCA5) in an 11-generation American kindred descended from President Lincoln's grandparents and two additional families. Two families have separate in-frame deletions of 39 and 15 bp, and a third family has a mutation in the actin/ARP1 binding region. Beta-III spectrin is highly expressed in Purkinje cells and has been shown to stabilize the glutamate transporter EAAT4 at the surface of the plasma membrane. We found marked differences in EAAT4 and GluRdelta2 by protein blot and cell fractionation in SCA5 autopsy tissue. Cell culture studies demonstrate that wild-type but not mutant beta-III spectrin stabilizes EAAT4 at the plasma membrane. Spectrin mutations are a previously unknown cause of ataxia and neurodegenerative disease that affect membrane proteins involved in glutamate signaling.