[The genetics of spinocerebellar ataxias]

The DNA repair enzyme, aprataxin, plays a role in innate immune signaling

Ataxia with oculomotor apraxia type 1 (AOA1) is a progressive neurodegenerative disorder characterized by a gradual loss of coordination of hand movements, speech, and eye movements. AOA1 is caused by an inactivation mutation in the APTX gene. APTX resolves abortive DNA ligation intermediates. APTX deficiency may lead to the accumulation of 5'-AMP termini, especially in the mitochondrial genome. The consequences of APTX deficiency includes impaired mitochondrial function, increased DNA single-strand breaks, elevated reactive oxygen species production, and altered mitochondrial morphology. All of these processes can cause misplacement of nuclear and mitochondrial DNA, which can activate innate immune sensors to elicit an inflammatory response. This study explores the impact of APTX knockout in microglial cells, the immune cells of the brain. RNA-seq analysis revealed significant differences in the transcriptomes of wild-type and APTX knockout cells, especially in response to viral infections and innate immune pathways. Specifically, genes and proteins involved in the cGAS-STING and RIG-I/MAVS pathways were downregulated in APTX knockout cells, which suggests an impaired immune response to cytosolic DNA and RNA. The clinical relevance of these findings was supported by analyzing publicly available RNA-seq data from AOA1 patient cell lines. Comparisons between APTX-deficient patient cells and healthy control cells also revealed altered immune responses and dysregulated DNA- and RNA-sensing pathways in the patient cells. Overall, this study highlights the critical role of APTX in regulating innate immunity, particularly in DNA- and RNA-sensing pathways. Our findings contribute to a better understanding of the underlying molecular mechanisms of AOA1 pathology and highlights potential therapeutic targets for this disease.

Role of dynamic and mitochondrial mutations in neurodegenerative diseases with ataxia: lower repeats and LNAs at multiple loci as alternative pathogenesis

Spinocerebellar ataxia is a growing group of hereditary neurodegenerative diseases for which ≥30 different genetic loci have been identified. In this study, we assessed the repeats at eight spinocerebellar ataxia (SCA) loci in 188 clinical SCA patients and 100 individuals without any neurological signs. Results from the present study were able to identify 16/188 (8.5%) clinical ataxia patients with repeat expansions in the pathological range of SCA genes, with the majority having expansion at the SCA1, 2, and 3 loci. The present study further evaluated two mitochondrial mutations associated with ataxia, i.e., T8993G and A8344G. Six patients were identified with A8344G mutation and none had the mutation in ATPase 6 gene; however, G8994A variation was found in three cases. Overall, three cases had triplet repeat expansions as well as mitochondrial (mt) mutations, which indicates potential association of triplet repeat expansions and mitochondrial mutations. Both the molecular analysis of several SCA loci and two relevant mt mutations indicated that the majority of ataxia cases were still undiagnosed; hence, the following hypotheses were proposed and tested based on available data: (i) lower repeats than normal range and (ii) large normal alleles (LNAs) at multiple loci may be an alternative basis for disease pathogenesis.