Using exome sequencing, which is functionally the most relevant part of the genome, scientists identified an amino acid change in the valosin-containing protein (VCP) gene in an Italian family published in the December 9, 2010, edition of Neuron Report. In this study of this family, SOD1, TDP-43, and FUS mutations were previously excluded in an attempt to identify the underlying genetic abnormality responsible for the disease.
“It is very exciting to see that this new sequencing technology has resulted in a significant finding for ALS,” said ALS Association Chief Scientist Lucie Bruijn, Ph.D. “Discovery of VCP provides another piece of the puzzle in understanding the disease mechanism.”
The identification of genes underlying rare familial forms of ALS has had significant impact on the understanding of the molecular mechanisms underlying typical ALS. Much of the ongoing molecular biology work in the ALS field is based on the discovery of mutations in genes encoding SOD1, TDP-43, and FUS. Each new gene implicated in the etiology of ALS provides fundamental insights into the pathogenesis of motor neuron degeneration and facilitates disease modeling and the design and testing of targeted therapeutics; hence, there is much interest in the identification of novel genetic mutations.
“Mutations in VCP have previously been identified in families with Inclusion Body Myopathy, Paget disease, and Frontotemporal Dementia,” said Bryan Traynor, M.D., of Laboratory of Neurogenetics, National Institute on Aging, a part of the National Institutes of Health and of the Department of Neurology at Johns Hopkins and one of the lead contributors to this report. “Screening of VCP in a cohort of 210 familial ALS cases and 78 autopsy-proven ALS cases identified four additional mutations including a p.R155H mutation in a pathologically proven case of ALS.”
Other key contributors include: Janel O. Johnson and Yevgeniya Abramzon of the Neuromuscular Diseases Research Group, Jessica Mandrioli of University of Modena, Italy, Michael Benatar of Emory University, Atlanta Georgia, Gabriella Restagno, Molecular Genetics Unit, A.S.O. O.I.R.M.-S. Anna, Turin, Italy and Adriano Chiò of University of Turin, Italy.
VCP protein is essential for maturation of ubiquitin-containing autophagosomes, and mutant VCP toxicity is partially mediated through its effect on TDP-43 protein, a major constituent of ubiquitin inclusions that neuropathologically characterize ALS. The study data broaden the phenotype of IBMPFD to include motor neuron degeneration, suggest that VCP mutations may account for approximately 1%–2% of familial ALS, and provide evidence directly implicating defects in the ubiquitination/protein degradation pathway in motor neuron degeneration.
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease clinically characterized by upper and lower motor neuron dysfunction resulting in rapidly progressive paralysis and death from respiratory failure. The pathological hallmarks of the disease include paleness of the corticospinal tract due to loss of motor neurons, the presence of ubiquitin-positive inclusions (accumulation of protein waste products) within surviving motor neurons, and the deposition of pathological TDP-43 aggregates.
"Our study potentially widens the clinical spectrum associated with ALS and provides new insight into this fatal disease," added Dr Traynor.
Population-based epidemiological studies estimate that approximately 5% of ALS cases are hereditary. Of these, approximately 15% are caused by mutations in the SOD1 gene, and a further 3%–4% of cases are due to pathogenic variants in either the TDP-43 or FUS gene. Linkage and positional cloning studies aimed at finding additional familial ALS genes have been complicated by a lack of samples from large, multigenerational families, mainly due to the dramatically shortened lifespan associated with the diagnosis.
Whole-exome sequencing is a new technique that exploits the massively parallel sequencing capabilities of next-generation platforms to rapidly identify rare variants in the approximately 1% of the genome that codes for proteins. The power of exome sequencing stems from the fact that the majority of monogenic diseases arise from mutations within this protein-coding portion of the genome, and the ability of this technology to find new causative genes has already been demonstrated. Furthermore, whole-exome sequencing is now a realistic strategy for detecting pathogenic variants in small families where linkage analysis would not be possible due to a shortage of DNA samples from affected individuals.
These new findings will be presented at the 21st International Symposium on ALS/MND in Orlando, Florida, December 11-13, which is hosted by The ALS Association.
This research was also funded by National Institute of Neurological Disorders and Stroke, the Robert Packard Center for ALS Research at Johns Hopkins and the Muscular Dystrophy Association.