Bridging the Gap of ALS Missing Heritability: Are Neurogenes Being Overlooked?

Background: To date, over 40 genes have been associated with Amyotrophic Lateral Sclerosis (ALS), with C9orf72, SOD1, TARDBP, and FUS being the most frequently involved. ALS genetics experts collaborate through consortia to refine the list of ALS-related genes used by specialized centers for genetic testing. However, despite the best efforts, 30% of familial and 85% of sporadic ALS patients do not receive genetic confirmation. Methods: 805 Italian patients diagnosed with ALS based on the revised El Escorial criteria (79 familial, 726 sporadic), who tested negative for C9orf72 repeat expansion (RE) and for whom extensive clinical data were available, were enrolled at IRCCS Istituto Auxologico Italiano. Short-read whole-genome sequencing data were generated within the ALS Compute Project. We analysed 452 genes linked to late-onset neurological disorders, according to the Online Mendelian Inheritance in Man (OMIM) catalog and gene panels of Genomics England PanelApp, to identify pathogenic (P) or likely pathogenic (LP) single nucleotide variants (SNVs), based on the American College of Medical Genetics and Genomics, and pathogenic REs. Results: We identified 115 patients (14.3%) with P/LP mutations in neurogenes: 84 with P/LP SNVs and 37 with REs (1 ATXN10, 4 ATXN8OS, 27 ATXN2 intermediate RE, 3 ATXN2 full RE, 1 FMR1 premutation, and 1 HTT pathological RE with reduced penetrance). Six patients (0.7%) carried dual pathogenic mutations (DNAJC7/ATXN2, ATXN2/FMR1, POLG2/SOD1, MYH7/ATXN2, TBK1/DES, SOD1/ATXN8OS). Beyond the 79 patients (65.0%) with mutations in ALS/FTD-related genes, the highest proportion of cases had mutations in genes associated with neuropathies (n=12; 9.9% - SORD, GDAP1, MFN2, DHTKD1, ATL3, FBLN5, LITAF, TTR), myopathies (n=11; 9.1% - BAG3, MYH7, SYNE2, DES, TPM2, COL6A2), and ataxias (n=9; 7.4% - STUB1, TGM6, ATXN8OS, ATXN10). Less frequently, mutations were found in genes linked to movement disorders (n=6; 5.0% - LRRK2, HTT, GCH1, ATP1A3, SPR), mitochondrial diseases (n=3; 2.5% - TWNK, POLG2) and hereditary spastic paraplegia (n=1; 0.8% - ALDH18A1). Conclusion: Our findings highlight the extensive genetic and phenotypic overlap between ALS and other neurological disorders. Restricting genetic testing to ALS/FTD genes misses up to 35% of patients with mutations in other neurological diseases‘ genes, some of which are targets for emerging therapies. Expanding genetic screening could improve diagnosis and therapeutic opportunities.