Further evidence that D90A-SOD1 mutation is recessively inherited in ALS patients in Italy

Advantages of routine next-generation sequencing over standard genetic testing in the amyotrophic lateral sclerosis clinic

BACKGROUND

Next-generation sequencing has enhanced our understanding of amyotrophic lateral sclerosis (ALS) and its genetic epidemiology. Outside the research setting, testing is often restricted to those who report a family history. The aim of this study was to explore the added benefit of offering routine genetic testing to all patients in a regional ALS centre.

METHODS

C9ORF72 expansion testing and exome sequencing was offered to consecutive patients (150 with ALS and 12 with primary lateral sclerosis [PLS]) attending the Oxford Motor Neuron Disease Clinic within a defined time period.

RESULTS

A total of 17 (11.3%) highly penetrant pathogenic variants in C9ORF72, SOD1, TARDBP, FUS and TBK1 were detected, of which 10 were also found through standard clinical genetic testing pathways. The systematic approach resulted in five additional diagnoses of a C9ORF72 expansion (number needed to test [NNT] = 28), and two further missense variants in TARDBP and SOD1 (NNT = 69). Additionally, 3 patients were found to carry pathogenic risk variants in NEK1, and 13 patients harboured common missense variants in CFAP410 and KIF5A, also associated with an increased risk of ALS. We report two novel non-coding loss-of-function splice variants in TBK1 and OPTN. No relevant variants were found in the PLS patients. Patients were offered double-blinded participation, but >80% requested disclosure of the results.

CONCLUSIONS

This study provides evidence that expanding genetic testing to all patients with a clinical diagnosis of ALS enhances the potential for recruitment to clinical trials, but will have direct resource implications for genetic counselling.

Cellular analysis of SOD1 protein-aggregation propensity and toxicity: a case of ALS with slow progression harboring homozygous SOD1-D92G mutation

Mutations within Superoxide dismutase 1 (SOD1) cause amyotrophic lateral sclerosis (ALS), accounting for approximately 20% of familial cases. The pathological feature is a loss of motor neurons with enhanced formation of intracellular misfolded SOD1. Homozygous SOD1-D90A in familial ALS has been reported to show slow disease progression. Here, we reported a rare case of a slowly progressive ALS patient harboring a novel SOD1 homozygous mutation D92G (homD92G). The neuronal cell line overexpressing SOD1-D92G showed a lower ratio of the insoluble/soluble fraction of SOD1 with fine aggregates of the misfolded SOD1 and lower cellular toxicity than those overexpressing SOD1-G93A, a mutation that generally causes rapid disease progression. Next, we analyzed spinal motor neurons derived from induced pluripotent stem cells (iPSC) of a healthy control subject and ALS patients carrying SOD1-homD92G or heterozygous SOD1-L144FVX mutation. Lower levels of misfolded SOD1 and cell loss were observed in the motor neurons differentiated from patient-derived iPSCs carrying SOD1-homD92G than in those carrying SOD1-L144FVX. Taken together, SOD1-homD92G has a lower propensity to aggregate and induce cellular toxicity than SOD1-G93A or SOD1-L144FVX, and these cellular phenotypes could be associated with the clinical course of slowly progressive ALS.

Individual Oligogenic Background in p.D91A-SOD1 Amyotrophic Lateral Sclerosis Patients

The p.D91A is one of the most common ALS-causing SOD1 mutations and is known to be either recessive or dominant. The homozygous phenotype is characterized by prolonged survival and slow progression of disease, whereas the affected heterozygous phenotypes can vary. To date, no genetic protective factors located close to SOD1 have been associated with the mild progressive homozygous phenotype. Using Next Generation Sequencing (NGS), we characterized a small cohort of sporadic and familial p.D91A-SOD1 heterozygous (n = 2) or homozygous (n = 5) ALS patients, to reveal any additional contributing variant in 39 ALS-related genes. We detected unique sets of non-synonymous variants, four of which were of uncertain significance and several in untranslated regions of ALS-related genes. Our results supported an individual oligogenic background underlying both sporadic and familial p.D91A cases irrespective of their p.D91A mutant alleles. We suggest that a comprehensive genomic view of p.D91A-SOD1 ALS patients may be useful in identifying emerging variants and improving disease diagnosis as well as guiding precision medicine.

A Systematic Review of Genotype-Phenotype Correlation across Cohorts Having Causal Mutations of Different Genes in ALS

Amyotrophic lateral sclerosis is a rare and fatal neurodegenerative disease characterised by progressive deterioration of upper and lower motor neurons that eventually culminates in severe muscle atrophy, respiratory failure and death. There is a concerning lack of understanding regarding the mechanisms that lead to the onset of ALS and as a result there are no reliable biomarkers that aid in the early detection of the disease nor is there an effective treatment. This review first considers the clinical phenotypes associated with ALS, and discusses the broad categorisation of ALS and ALS-mimic diseases into upper and lower motor neuron diseases, before focusing on the genetic aetiology of ALS and considering the potential relationship of mutations of different genes to variations in phenotype. For this purpose, a systematic review is conducted collating data from 107 original published clinical studies on monogenic forms of the disease, surveying the age and site of onset, disease duration and motor neuron involvement. The collected data highlight the complexity of the disease's genotype-phenotype relationship, and thus the need for a nuanced approach to the development of clinical assays and therapeutics.

Common mutations of interest in the diagnosis of amyotrophic lateral sclerosis: how common are common mutations in ALS genes?

INTRODUCTION

Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disease predominantly affecting upper and lower motor neurons. Diagnosis of this devastating pathology is very difficult because the high degree of clinical heterogeneity with which it occurs and until now, no truly effective treatment exists.

AREAS COVERED

Molecular diagnosis may be a valuable tool for dissecting out ALS complex heterogeneity and for identifying new molecular mechanisms underlying the characteristic selective degeneration and death of motor neurons. To date, pathogenic variants in ALS genes are known to be present in up to 70% of familial and 10% of apparently sporadic ALS cases and can be associated with risks for ALS only or risks for other neurodegenerative diseases. This paper shows the procedure currently used in diagnostic laboratories to investigate most frequent mutations in ALS and evaluating the utility of involved molecular techniques as potential tools to discriminate 'common mutations' in ALS patients.

EXPERT OPINION

Genetic testing may allow for establishing an accurate pathological diagnosis and a more precise stratification of patient groups in future drug trials.

Genetic analysis and SOD1 mutation screening in Iranian amyotrophic lateral sclerosis patients

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease, and the most common in European populations. Results of genetic analysis and mutation screening of SOD1 in a cohort of 60 Iranian ALS patients are here reported. Initially, linkage analysis in 4 families identified a disease-linked locus that included the known ALS gene, SOD1. Screening of SOD1 identified homozygous p.Asp90Ala causing mutations in all the linked families. Haplotype analysis suggests that the p.Asp90Ala alleles in the Iranian patients might share a common founder with the renowned Scandinavian recessive p.Asp90Ala allele. Subsequent screening in all the patients resulted in identification of 3 other mutations in SOD1, including p.Leu84Phe in the homozygous state. Phenotypic features of the mutation-bearing patients are presented. SOD1 mutations were found in 11.7% of the cohort, 38.5% of the familial ALS probands, and 4.25% of the sporadic ALS cases. SOD1 mutations contribute significantly to ALS among Iranians.