Smoking is a cause of amyotrophic lateral sclerosis. High low-density lipoprotein cholesterol levels? Unsure

Unraveling the complex interplay between genes, environment, and climate in ALS

Various genetic and environmental risk factors have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). Despite this, the cause of most ALS cases remains obscure. In this review, we describe the current evidence implicating genetic and environmental factors in motor neuron degeneration. While the risk exerted by many environmental factors may appear small, their effect could be magnified by the presence of a genetic predisposition. We postulate that gene-environment interactions account for at least a portion of the unknown etiology in ALS. Climate underlies multiple environmental factors, some of which have been implied in ALS etiology, and the impact of global temperature increase on the gene-environment interactions should be carefully monitored. We describe the main concepts underlying such interactions. Although a lack of large cohorts with detailed genetic and environmental information hampers the search for gene-environment interactions, newer algorithms and machine learning approaches offer an opportunity to break this stalemate. Understanding how genetic and environmental factors interact to cause ALS may ultimately pave the way towards precision medicine becoming an integral part of ALS care.

Common and rare variant association analyses in amyotrophic lateral sclerosis identify 15 risk loci with distinct genetic architectures and neuron-specific biology

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with a lifetime risk of one in 350 people and an unmet need for disease-modifying therapies. We conducted a cross-ancestry genome-wide association study (GWAS) including 29,612 patients with ALS and 122,656 controls, which identified 15 risk loci. When combined with 8,953 individuals with whole-genome sequencing (6,538 patients, 2,415 controls) and a large cortex-derived expression quantitative trait locus (eQTL) dataset (MetaBrain), analyses revealed locus-specific genetic architectures in which we prioritized genes either through rare variants, short tandem repeats or regulatory effects. ALS-associated risk loci were shared with multiple traits within the neurodegenerative spectrum but with distinct enrichment patterns across brain regions and cell types. Of the environmental and lifestyle risk factors obtained from the literature, Mendelian randomization analyses indicated a causal role for high cholesterol levels. The combination of all ALS-associated signals reveals a role for perturbations in vesicle-mediated transport and autophagy and provides evidence for cell-autonomous disease initiation in glutamatergic neurons.

Polygenic associations and causal inferences between serum immunoglobulins and amyotrophic lateral sclerosis

BACKGROUND

Due to the limitations like reverse causation and residual confounding commonly seen in the observational studies, the relationship between serum immunoglobulins and amyotrophic lateral sclerosis (ALS) remains unclear.

METHODS

Summary statistics from large-scale genome-wide association studies (GWAS) among European ancestry populations (~15,000 individuals for serum immunoglobulins, and more than 36,000 individuals for ALS) were accessed and used in the discovery and replication phase, respectively. Polygenic risk score analysis was performed to test the polygenic association, and Mendelian randomization analysis was used to infer the causality.

RESULTS

An inverse polygenic association was discovered between IgA and ALS, as well as between IgM and ALS. Such associations were however not replicated using a larger GWAS of ALS, and no causal association was observed for either IgA-ALS or IgM-ALS. For IgG and ALS, a positive polygenic association was both discovered [odds ratio (OR) = 1.18, 95% confidence interval (CI): 1.12-1.25, P = 5.9x10^-7] and replicated (OR = 1.13, 95% CI: 1.06-1.20, P = 0.001). A causal association between IgG and ALS was suggested in the discovery analysis (OR = 1.06, 95 %CI: 1.02-1.10, P = 0.009), but it was not statistically significant in the replication analysis (OR = 1.07, 95 %CI: 0.90-1.24, P = 0.420).

CONCLUSION

This study suggests a positive polygenic association between serum IgG and ALS.

Mendelian randomization implies no direct causal association between leukocyte telomere length and amyotrophic lateral sclerosis

We employed Mendelian randomization (MR) to evaluate the causal relationship between leukocyte telomere length (LTL) and amyotrophic lateral sclerosis (ALS) with summary statistics from genome-wide association studies (n = ~ 38,000 for LTL and ~ 81,000 for ALS in the European population; n = ~ 23,000 for LTL and ~ 4,100 for ALS in the Asian population). We further evaluated mediation roles of lipids in the pathway from LTL to ALS. The odds ratio per standard deviation decrease of LTL on ALS was 1.10 (95% CI 0.93–1.31, p = 0.274) in the European population and 0.75 (95% CI 0.53–1.07, p = 0.116) in the Asian population. This null association was also detected between LTL and frontotemporal dementia in the European population. However, we found that an indirect effect of LTL on ALS might be mediated by low density lipoprotein (LDL) or total cholesterol (TC) in the European population. These results were robust against extensive sensitivity analyses. Overall, our MR study did not support the direct causal association between LTL and the ALS risk in neither population, but provided suggestive evidence for the mediation role of LDL or TC on the influence of LTL and ALS in the European population.

Theme 1 Epidemiology and informatics

Background: Identifying mechanisms of neurodegenerative disease causation has for long seemed to be beyond the pale of traditional epidemiological tools. Elucidating a plausible mechanism for initiation of amyotrophic lateral sclerosis (ALS) has appeared particularly elusive (1). The impression, that environmental risk factors for ALS were not providing consistent direction, meant there was no sturdy epidemiologically-based "handle" to grasp when trying to envisage a biological mechanism for triggering sporadic ALS (2). There have been challenges with interpreting the data. At times, generic concerns over potential limitations of traditional epidemiological studies have appeared to overshadow the findings in circumstances where these limitations had been overcome largely. At other times, studies with different degrees of methodological limitations have been lumped together, thereby obscuring the results of the studies with less limitations. On occasion, methodological limitations have been downplayed or ignored entirely.Emergence of Mendelian Randomization (MR) methods has offered the promise of overcoming some of the potential limitations of epidemiological studies that used traditional methods. MR methods apply concepts developed in the field of economics to infer causality in the presence of unmeasured confounding (3). The principal idea is: 1) a genetic pattern is identified that predicts a suspected risk factor - a laboratory value in patients' blood, or a particular behavior; 2) that pattern is sought in patients and controls; 3) excess presence of the pattern in patients suggests that the risk factor plays a causal role in producing the disease.However, application of MR methods requires that several underlying assumptions, specific to these methods, have been satisfied (3). Moreover, epidemiological analyses using MR methods need to adhere to core epidemiological and statistical principles. Finally, findings from MR studies need to be interpreted critically, with close attention to the context from which they arise, and with utilization of internal and external comparators (4,5).This presentation will discuss the assumptions that need to be met to apply MR methods in general and how they relate to studies in patients with ALS, drawing on recently published reports.