An assay designed to measure normal and abnormal forms of the huntingtin protein - the mutated form of which causes Huntington's disease (HD) - was successful in detecting levels of the mutant protein in a large multicenter study of individuals at risk for the devastating neurological disorder. The report from a team of Massachusetts General Hospital (MGH) investigators - which will appear in the Sept. 24 issue of Neurology and has been released online - also found changes in levels of the mutated protein that might predict when symptoms will appear.
"Our validation of this biomarker for measuring the huntingtin protein in the blood of individuals with HD supports its usefulness in monitoring treatments designed to affect the mutant protein," says Steven Hersch, MD, PhD, of the MassGeneral Institute for Neurodegenerative Disease (MGH-MIND), senior author of the report. "Our results also suggest that this assay might sense biological stresses in the body that occur as the disease becomes symptomatic in individuals who have inherited the HD mutation."
Inheriting a single mutated copy of the huntingtin gene causes deposition of the abnormal form of the protein within the brain. The toxic effects of the mutant protein eventually produce symptoms such as uncontrolled movements, erratic emotions and dementia. Symptoms of HD usually first appear in the middle years and worsen over the 10- to 30-year course of the disorder, leading to death from a variety of complications. A genetic test for the HD mutation has been available since the gene was discovered at MGH more that 20 years ago, but since there currently is no way to delay or slow the progression of symptoms, many individuals at risk for the disorder choose not to learn their gene status. Several potential therapies in development are designed to reduce levels of mutant huntingtin, but studies are currently hampered by difficulty in assessing the drugs' effectiveness, especially in early-phase trials.
Several years ago a collaboration led by Anne Young, MD, PhD, former chief of Neurology at MGH and a co-author of the Neurology paper, developed the assay used in the current study. The assay uses what is called HTRF technology and a set of three antibodies against the huntingtin protein - including one specific for the mutation site - to measure the relative levels of the mutant and total huntingtin proteins. Previous studies have confirmed the ability of HTRF assay to measure normal and mutant huntingtin in animal models of the disease and in blood cells and brain tissue samples from HD patients.
The current study was designed to validate those results in a large multicenter study of presymptomatic individuals known to be at risk for the disease as well as in patients with early symptoms. The researchers analyzed white blood cells collected from participants in PHAROS (Prospective Huntington At-Risk Observational Study). This National Institutes of Health investigation - led by Young and Ira Shoulson, MD, of Georgetown University, also a coauthor of the current study - was conducted from 1999 to 2009 and enrolled 1,000 individuals who had a parent or sibling with the disorder but had not learned their gene status. Blood samples from 342 participants, collected at 35 centers around the U.S., were suitable for analysis with the HTRF assay.