High huntingtin protein levels in saliva linked to worse disease
Noninvasive saliva tests could help predict Huntington's outcomes
Higher levels of healthy and mutant huntingtin protein in saliva, but not in blood, are significantly associated with worse motor symptoms of Huntington’s disease, a study showed.
According to the investigators, these findings support the use of noninvasive saliva tests to monitor Huntington’s progression and predict clinical outcomes — which, in turn, can be immediately applied to clinical research applications.
“Overall, our study presents a compelling argument for the inclusion and analysis of saliva [in clinical testing] to further the understanding, diagnosis and prognosis of [Huntington’s],” the researchers wrote.
The study, “Salivary Huntingtin protein is uniquely associated with clinical features of Huntington’s disease,” was published in the journal Scientific Reports.
Huntingtin protein levels in saliva tied to worse motor symptoms
Huntington’s is caused by an excess of repeats, called CAG triplets, within the HTT gene that provides instructions to produce the huntingtin (HTT) protein. Having 36 or more CAG triplets leads to the production of an abnormally long huntingtin protein that forms clumps and affects brain regions responsible for movement, cognitive function, and behavior.
In most cases, Huntington’s is diagnosed with the onset of motor symptoms, also called manifest disease. However, some people may experience subtle changes in cognitive, behavior, and fine motor skills seven to 10 years before clear motor symptoms emerge. This is a preclinical stage referred to prodromal or premanifest.
Researchers based in California previously reported that people with manifest Huntington’s had elevated levels of total HTT — both the normal and mutated forms (mHTT) of the protein — in their saliva relative to healthy people. As such, salivary HTT showed its potential as a non-invasive biomarker for disease progression.
Now, the same team of scientists has expanded their work to investigate total HTT and mHTT levels in saliva samples from Huntington’s patients and compare these levels with those found in the blood. The team also explored potential links between these markers and disease features and/or clinical measures.
People carrying a Huntington’s-causing HTT mutation were recruited from the University of California, San Diego’s Huntington’s Disease Society of America Center of Excellence.
Premanifest participants were those who had 38 or more CAG repeats but no signs of Huntington’s hallmark symptoms. Meanwhile, manifest patients were those showing signs of the disease, as confirmed by a clinician.
In total, blood and saliva samples were collected from 95 people: 19 with manifest Huntington’s, 34 with premanifest disease, and 42 unaffected individuals, who served as controls.
The results showed that higher salivary mHTT levels, but no other HTT measure, were significantly associated with older age, longer CAG repeats, and higher CAP score. The CAP score is a measure derived from a formula that accounts for CAG repeat number and a person’s age; it results in higher scores with older age and longer CAG repeats.
No significant association was observed between blood and salivary total HTT and mHTT levels among premanifest and manifest patients.
Also, total HTT levels in blood and saliva were not significantly different between the three groups.
However, significantly more salivary mHTT values were below the lower limit of quantification (LLoQ) in healthy controls, compared with premanifest and manifest Huntington’s patients. Still, there was no difference in the number of these values between the two groups of patients.
We show that [mutant huntingtin protein] is uniquely processed in saliva, compared to blood, and contend that further investigation into this biofluid will provide new information regarding the development and disease progression of Huntington’s disease.
Among premanifest patients, higher salivary total HTT levels were significantly associated with higher salivary mHTT levels. Higher levels of these proteins also were significantly linked to worse motor function, as assessed with validated measures, and more severe chorea, or involuntary muscle contractions.
In fully manifest patients, higher total HTT levels in saliva were significantly associated with worse motor function and chorea, while more salivary mHTT correlated with older age and higher CAG repeat number.
When researchers combined the two Huntington’s groups, higher salivary mHTT levels were significantly associated with higher total HTT levels, older age, longer CAG repeats, worse motor function, more severe chorea, and lower independence.
More mHTT levels in saliva also were linked to prognosis scores reflecting a faster progression to manifest disease.
All of these associations were maintained after the researchers adjusted for age, sex, and number of CAG repeats.
“Salivary [total HTT] levels may increase with motor symptoms … in [premanifest] individuals, and subsequently decrease with further motor deterioration, or the onset of rigidity, in manifest HD [Huntington’s disease] individuals,” the team wrote.
Lastly, preliminary tests suggest that HTT and mHTT are processed differently in blood and saliva — a finding that requires further study, the team noted.
“We show that mHTT is uniquely processed in saliva, compared to blood, and contend that further investigation into this biofluid will provide new information regarding the development and disease progression of Huntington’s disease,” the researchers concluded.
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