Date of Award
Doctor of Philosophy (PhD)
Amyotrophic lateral sclerosis (ALS) is fatal neurodegenerative disease for which there is no cure. The only treatment available extends survival by only a matter of months. There are over 20 genes that are known to cause ALS. Over half of the ALS cases with a family history of disease (FALS) can be explained by mutations in known ALS genes with hexanucleotide repeat expansions in C9ORF72 accounting for 40% of families. However roughly 90% of cases have no family history of disease (sporadic ALS or SALS) and a much smaller proportion (10%) of these cases can be explained by mutations in known ALS genes. Understanding the genetic factors that cause ALS or influence its progression will help us understand the cellular pathways involved in disease and identify potential therapeutic targets.
We used a pooled-sample sequencing approach to identify mutations in 17 ALS genes in a cohort of FALS and SALS patients to investigate the contribution of these genes to SALS, including the role of rare variants and the effect of mutations in multiple ALS genes in an individual. We identified potentially pathogenic mutations in 64.3% of familial and 27.8% of sporadic subjects. 3.8% of subjects had mutations in more than one ALS gene and these individuals on average had onset 10 years earlier than those with mutations in only one ALS gene (p=0.0046). There were no individual rare variants that were significantly associated with sporadic ALS, but rare variants in SOD1 were cumulatively more common in SALS subjects.
In addition we investigated the genetic background and stability of C9ROF72 repeat expansions in ALS. The presence of a risk haplotype shared between all expansion-carriers led to the prevailing idea of a founder expansion event, however this shared haplotype also supports the hypothesis of a genetic background that is more prone to expansion. We identified a rare variant rs147599399 on this genetic background that is present in some expansion carriers and some non-expansion carriers, indicating that the expansion arose on at least two separate occasions. This raises the possibility that C9ORF72 repeat expansions in sporadic ALS could be the result of de novo expansions on the risk haplotype. Furthermore we showed that expansion carriers with rs147599399 minor allele had longer survival than expansion carriers without the SNP (p=0.00047), indicating that the genetic background surrounding the C9ORF72 influences the effects of the expansion.
We performed Southern blotting to explore the size and stability of C9ORF72 repeat expansions. There was a high degree of somatic instability and instability in transmissions between families. There was no difference between expansion sizes in symptomatic and asymptomatic expansion carriers in families an there was no correlation between expansion size in any patient tissues and any clinical characteristics. These results need to be confirmed in a larger sample cohort, but suggest that expansion size alone doesn’t determine pathogenicity of C9ORF72 repeat expansions.
Lastly we examined the candidate gene TREM2 as a risk factor for ALS. This gene is involved in regulation of microglial activity, which is a known component of ALS pathogenesis, and the rare variant p.R47H was recently associated with risk Alzheimer’s disease. We found that the same p.R47H variant was significantly associated with ALS in our cohort and that expression of TREM2 was increased in ALS patients and SOD1 mutant mice compared to controls. A variant in the related gene TREML4 was marginally associated with ALS, but the effect of this variant is unknown. Mutations in the TREM genes provide a genetic link between to the neuro-inflammatory component of ALS and suggest other genes involved in microglial activation are good candidates for novel variant identification.
Chair and Committee
Matthew B Harms
Timothy Miller, John Rice, Joseph Dougherty, Christina Gurnett
Cady, Janet Elizabeth, "Genetic Factors that Contribute to the Pathogenesis of Amyotrophic Lateral Sclerosis" (2015). Arts & Sciences Electronic Theses and Dissertations. 639.