ALS symptoms typically appear in one’s 40s and 50s, although juvenile cases in people younger than 25 do occur. In addition to the early onset, the young man in the case study was unusual in that his weakness and muscle atrophy affected mostly his right arm, shoulder and neck. However, he did have mild weakness elsewhere, convincing Rouleau that ALS was the correct diagnosis.

The team sequenced three likely candidate ALS genes: FUS, superoxide dismutase 1 (SOD1), and TAR DNA Binding Protein 43 (TDP-43). They discovered a single base pair deletion near the end of the FUS gene, causing a frameshift that swapped the final 32 correct codons for 33 new, incorrect ones. This genetic defect joins several others found previously in the carboxyl terminus of FUS, confirming the importance of that region for the protein’s function (see ARF related news story on Dormann et al., 2010).

The young man’s mother, the team found, also carried the FUS mutation, yet had no ALS symptoms at 47. Several mutations in FUS and other ALS genes have a wide range of onset (see table), causing disease in individuals as young as 13 or as old as 72 (see ARF related news story on Yan et al., 2010).

“The major message here is that there is a good deal of heterogeneity in all forms of ALS,” wrote Richard Bedlack of the Duke ALS Clinic in Durham, North Carolina, in an email to ARF (see full comment below). “This includes variability in penetrance, age and site of onset, co-morbidities (for example dementia) and progression rate.” Bedlack did not participate in the current case study.

ALS Genes Associated With Juvenile Onset

Gene	Age of Onset	Reference
ALSIN	3-20	Hadano et al., 2001; Yang et al., 2001
SETX	<25	Chen et al., 2004
UBQLN2	16-71	Deng et al., 2011
SIGMAR	1-2	Al-Saif et al., 2011
FUS	20, 65	DeJesus-Hernandez et al., 2010
	17-22	Bäumer et al., 2010
	13, 21	Huang et al., 2010
	13-72	Yan et al., 2010
	20	Belzil et al., 2011
	19	Belzil et al., 2012

The variable onset, Rouleau said, could be the result of genetic or environmental factors. In the man in Montréal, the researchers used further sequencing to eliminate one potential genetic risk factor, that is, lengthy repeats in the ataxin 2 gene (see ARF related news story on Elden et al., 2010). In his case, they suspect that environmental toxins could have hastened disease. The young man had worked on farms around organophosphate fertilizers linked to neurodegeneration, as well as around heavy metals during a job in toxic waste recycling and training as a welder.

“I think we now need to focus hard on this variability... Mother Nature is trying to tell us something here!” Bedlack wrote. “We could, for example, use new tools such as whole genome microarray to compare fast and slow progressors.”—Amber Dance.

Reference:
Belzil VV, Langlais JS, Daoud H, Dion PA, Brais B, Rouleau GA. Novel FUS deletion in a patient with juvenile amyotrophic lateral sclerosis. Arch Neurol. 2012 Jan 16. Abstract