AFG3L2 mutations cause autosomal dominant ataxia SCA28 and reveal an essential role for the mitochondrial m-AAA protease complex in the cerebellum

Objectives: Dominant spinocerebellar ataxias (SCA) are a heteroge- neous group of neurological disorders characterized by cerebellar dysfunction mostly due to Purkinje cell degeneration. We previously mapped SCA type 28 to chr. 18p11 in an Italian family. Here, we report the identification of the gene causing SCA28. Methods: Candidate genes within the SCA28 critical region were sequenced in a proband from the original family. Subsequent molecular screening was performed in a large sample of families (n = 129) and sporadic patients (n = 150) with spinocerebellar ataxia negative for mutations in known SCA genes. Functional studies were performed in a yeast cellular model. Expression studies were performed in human and rat central nervous system. Results:Wehave found thatAFG3L2 (ATPase family gene 3-like 2) is the disease-causing gene. Along with paraplegin, which causes recessive spastic paraparesis SPG7, AFG3L2 is a component of the mitochondrial m-AAA complex, an evolutionarily conserved protease involved in pro- tein quality control.We have identified 4 heterozygousAFG3L2missense mutations in the original kindred and in other 3/129 (*2%) unrelated SCA families. Interestingly, concurrent heterozygosity for a recessive paraplegin mutation negatively modulated the phenotype in one family. All theAFG3L2mutations are located in the FtsH-like protease domain at highly conserved amino acids. Modeling on FtsH structure indicates that theymay affect substrate interaction.AFG3L2 protein and transcriptwere found to be highly and selectively expressed in cerebellar Purkinje cells. Expression of normal and mutant AFG3L2 homocomplex in m-AAA- deficient yeast cells demonstrate that the mutations cause respiratory deficiency and proteolytic defect. Preliminary phenotypic analysis of the patients harbouring a mutation in the SCA28 gene showed a relatively uniform ADCA1-type clinical presentation characterized by slowly-pro- gressive cerebellar ataxia with cerebellar atrophy and a frequent occurrence of oculomotor dysfunction and pyramidal signs. Conclusion: This work identifies AFG3L2 as a novel cause of dominant neurodegenerative disease and indicates an essential role for the mitochondrial m-AAA protease complex in protecting the cerebellum against neurodegeneration. Italian Ministry of Health grant (ex art 56) to FT.