Novel risk genes and their clinical impact in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset, fatal neurodegenerative disease mainly caused by the loss of upper and lower motor neurons, resulting in progressive muscle atrophy and paralysis. Although most ALS cases are sporadic (SALS), ~10% of them are familial (FALS), usually with an autosomal-dominant inheritance pattern. Although genetic studies have partially elucidated the genetic background of the disease, a large part of ALS heritability is still missing.

The aim of this thesis was the identification of genetic risk factors in ALS, and to understand their role in shaping the clinical phenotype. To achieve this goal, we initially screened a large cohort of FALS and SALS patients of Italian descent for mutations in the known ALS genes FUS, ATXN2 and PFN1. We established a mutational frequency for FUS of ~5% in familial cases, and described a novel phenotype associated with mutations within the nuclear localization signal of the gene. We confirmed the role of ATXN2 intermediate repeats in SALS susceptibility and refined the CAG-repeat range associated with the disease. We observed that PFN1 mutations are extremely rare in sporadic cases and probably do not play a role in FTD pathogenesis.

We then aimed to assess whether mutations in known ALS genes are associated to the appearance of extramotor phenotypes within motor neuron diseases, or contribute to the pathogenesis of neurodegenerative disorders other than ALS and FTD. Our results suggest that rare variants in the ANG gene, previously associated with ALS, also confer a susceptibility to idiopathic Parkinson’s disease (PD). We observed an increased presence of oligoclonal bands in the cerebrospinal fluid (CSF) of ALS patients carrying ANG and TARDBP mutations, suggesting a possible link with inflammatory diseases of the central nervous system. We also confirmed an association of c9orf72 (G4C2)n with ALS-plus syndromes, although we did not find evidence of a pathogenic role in other neurodegenerative disorders beyond TDP-43 protheinopathies.

Lastly, we aimed to identify novel causative genes in our ALS cohort, using different methodological approaches. We initially relied upon the candidate gene approach to screen a cohort of unrelated FALS index cases for mutations in TAF15 and EWS, selected because of their homology with FUS, and in the PON cluster, encoding for a family of proteins involved in detoxifying exogenous toxics. Although our results initially suggested an association of TAF15 and PON1-3 with ALS, subsequent evidence has casted significant doubt over these and other genes identified through the candidate gene approach.

Faced with these pitfalls, we were forced to adopt a different strategy for gene hunting in ALS, and resorted to an exome-wide, case-control, RVB analysis. The validity of such strategy is highlighted by the identification of the two novel ALS genes TUBA4A and NEK1.

Genetic studies in ALS have by far given the largest contribution to our understanding of the pathogenesis of the disease, and it is the hope that further advancements in the field will eventually lead to the identification of therapeutic targets toward finding a cure for ALS.