Unexpectedly mild phenotype in an ataxic family with a two-base deletion in the APTX gene

Phenotypic and molecular diversities of spinocerebellar ataxia type 2 in Japan

BACKGROUND

We intended to clarify the phenotypic and molecular diversities of spinocerebellar ataxia type 2 (SCA2) in Japan.

METHODS

DNA was extracted from the peripheral blood of 436 patients, including 126 patients with chronic neuropathy, 108 with amyotrophic lateral sclerosis, and 202 with cerebellar ataxia. We then PCR-amplified and sequenced the ATXN2 gene. The biopsied sural nerves of mutation-positive patients were subjected to light-microscopic and electron-microscopic analyses. Transfection analyses were performed using a Schwann cell line, IMS32.

RESULTS

We found PCR-amplified products potentially corresponding to expanded CAG repeats in four patients. Two patients in the chronic neuropathy group had a full repeat expansion or an intermediate expansion (39 or 32 repeats), without limb ataxia. The sural nerve biopsy findings of the two patients included axonal neuropathy and mixed neuropathy (axonal changes with demyelination). Schwann cells harbored either cytoplasmic or nuclear inclusions on electron microscopic examination. Both patients recently exhibited pyramidal signs. In the third patient in the cerebellar ataxia group, we identified a novel 21-base duplication mutation near 22 CAG repeats (c.432_452dup). The transfection study revealed that the 21-base-duplication mutant Ataxin-2 proteins aggregated in IMS32 and rendered cells susceptible to oxidative stress, similar to a CAG-expanded mutant. The fourth patient, with 41 repeats, had ataxia and spasticity. The two patients with cerebellar ataxia also had peripheral neuropathy.

CONCLUSIONS

Patients with expanded CAG repeats can exhibit a neuropathy-dominant phenotype not described previously. The novel 21-base-duplication mutant seems to share the aggregation properties of polyglutamine-expanded mutants.

PSP-Phenotype in SCA8: Case Report and Systemic Review

Spinocerebellar ataxia type 8 (SCA8) is a rare autosomal dominant neurodegenerative disease caused by expanded CTA/CTG repeats in the ATXN8OS gene. Many patients had pure cerebellar ataxia, while some had parkinsonism, both without causal explanation. We analyzed the ATXN8OS gene in 150 Japanese patients with ataxia and 76 patients with Parkinson's disease or related disorders. We systematically reassessed 123 patients with SCA8, both our patients and those reported in other studies. Two patients with progressive supranuclear palsy (PSP) had mutations in the ATXN8OS gene. Systematic analyses revealed that patients with parkinsonism had significantly shorter CTA/CTG repeat expansions and older age at onset than those with predominant ataxia. We show the imaging results of patients with and without parkinsonism. We also found a significant inverse relationship between repeat sizes and age at onset in all patients, which has not been detected previously. Our results may be useful to genetic counseling, improve understanding of the pathomechanism, and extend the clinical phenotype of SCA8.

Diminished OPA1 expression and impaired mitochondrial morphology and homeostasis in Aprataxin-deficient cells

Ataxia with oculomotor apraxia type 1 (AOA1) is an early onset progressive spinocerebellar ataxia caused by mutation in aprataxin (APTX). APTX removes 5'-AMP groups from DNA, a product of abortive ligation during DNA repair and replication. APTX deficiency has been suggested to compromise mitochondrial function; however, a detailed characterization of mitochondrial homeostasis in APTX-deficient cells is not available. Here, we show that cells lacking APTX undergo mitochondrial stress and display significant changes in the expression of the mitochondrial inner membrane fusion protein optic atrophy type 1, and components of the oxidative phosphorylation complexes. At the cellular level, APTX deficiency impairs mitochondrial morphology and network formation, and autophagic removal of damaged mitochondria by mitophagy. Thus, our results show that aberrant mitochondrial function is a key component of AOA1 pathology. This work corroborates the emerging evidence that impaired mitochondrial function is a characteristic of an increasing number of genetically diverse neurodegenerative disorders.

Noncoding repeat expansions for ALS in Japan are associated with the ATXN8OS gene

Objective

To assess the contribution of noncoding repeat expansions in Japanese patients with amyotrophic lateral sclerosis (ALS).

Methods

Sporadic ALS in Western countries is frequently associated with noncoding repeat expansions in the C9ORF72 gene. Spinocerebellar ataxia type 8 (SCA8) is another noncoding repeat disease caused by expanded CTA/CTG repeats in the ATXN8OS gene. Although the involvement of upper and lower motor neurons in SCA8 has been reported, a positive association between SCA8 and ALS remains unestablished. Spinocerebellar ataxia type 36 is a recently identified disease caused by noncoding repeat expansions in the NOP56 gene and is characterized by motor neuron involvement. We collected blood samples from 102 Japanese patients with sporadic ALS and analyzed the ATXN8OS gene by the PCR–Sanger sequencing method and the C9ORF72 and NOP56 genes by repeat-primed PCR assay.

Results

Three patients with ALS (3%) had mutations in the ATXN8OS gene, whereas no patient had a mutation in the C9ORF72 or NOP56 gene. The mutation-positive patients were clinically characterized by neck weakness or bulbar-predominant symptoms. None of our patients had apparent cerebellar atrophy on MRI, but 2 had nonsymptomatic abnormalities in the white matter or putamen.

Conclusions

Our finding reveals the importance of noncoding repeat expansions in Japanese patients with ALS and extends the clinical phenotype of SCA8. Three percent seems small but is still relatively large for Japan, considering that the most commonly mutated genes, including the SOD1 and SQSTM1 genes, only account for 2%–3% of sporadic patients each.