Measuring concentrations of mutant huntingtin (mHTT) protein and another protein, called neurofilament light protein (NfL), can provide early and specific indications of Huntington’s-related changes, according to a new study.
The findings suggest the potential of these two markers in both clinical practice and clinical research.
Huntington’s is caused by a mutation in the HTT gene, which leads to the production of mHTT. Given recent efforts to develop huntingtin-lowering therapies, having sensitive biomarkers of disease progression and classification of mutation carriers before symptom onset is essential.
In particular, although robust clinical, cognitive, and neuroimaging biomarkers of disease progression have been established, biochemical markers have been more challenging.
mHTT concentration in the cerebrospinal fluid (CSF) — liquid found in the brain and spinal cord — has been associated with Huntington’s clinical severity and has been assessed in a Phase 1/2 clinical trial (NCT02519036) to determine the levels of huntingtin protein. However, this approach’s sensitivity, specificity and variability within each individual has not been analyzed.
NfL protein is a component of the neuronal cytoskeleton — a key structure to maintain cells’ shape and organization — that is released into the CSF upon neuronal damage. Similar to mHTT, NfL has been linked to clinical severity in Huntington’s. Also, blood levels of NfL have been proposed as a prognostic biomarker for this disease, including rates of atrophy (shrinkage) in Huntington’s-associated brain areas and cognitive decline.
Despite this information, no study has determined mHTT and NfL levels in parallel in the CSF and blood of mutation carriers and controls, along with clinical and magnetic resonance imaging (MRI) data. Also lacking is research on the temporal sequence in which biochemical markers are altered during Huntington’s disease course.
To address these gaps, researchers conducted the HD-CSF study, in which scientists determined mHTT levels in CSF and NfL levels in CSF and plasma, comparing all three measurements head-to-head against clinical and MRI measures.
Of note, within-subject stability of mHTT and NfL were assessed in parallel in a subgroup of patients.
A total of 80 participants — 20 controls, 20 pre-Huntington’s (mutation carriers not manifesting symptoms) and 40 with early-to-moderate disease — were recruited at the University College London HD Multidisciplinary Clinic, in London, U.K.
Individuals with manifest Huntington’s had worse motor, cognitive and functional scores than those with pre-Huntington’s.
In both pre- and manifest Huntington’s groups, levels of mHTT in CSF and NfL in CSF and plasma positively correlated with age. Also, the higher the CSF mHTT concentration, the greater the number of CAG repeats — the typical HTT gene alterations in Huntington’s patients.
Matching previous data, the concentrations of CSF mHTT, CSF NfL, and plasma NfL were all significantly higher in Huntington’s mutation carriers compared to controls. Also, CSF NfL and plasma NfL were higher in manifest than in pre-Huntington’s individuals after accounting for age and number of CAG repeats.
However, only plasma NfL correlated with clinical measures – total functional capacity, total motor score, symbol digit modalities test (an assessment of cognitive function), color naming, word reading and verbal fluency – after taking age and CAG repeats into account.
Unlike CSF mHTT, NfL concentrations in the CSF and plasma were associated with brain volume on MRI. In particular, CSF levels correlated with all specific measures, including whole brain, gray matter (containing cell bodies and synapses), white matter (made of nerve fibers connecting grey matter areas) and caudate, a brain area typically reduced in Huntington’s patients.
“All correlations with clinical and imaging measures were stronger for NfL in CSF and plasma than for mHTT in CSF,” the team wrote. “This perhaps reflects that NfL, as a marker of [nerve fiber] damage, has a more direct relationship with the development of clinical manifestations and brain atrophy.”
Researchers found that mHTT in the CSF had almost perfect accuracy, while CSF and plasma NfL both displayed high accuracy to differentiate controls from mutation carriers. In turn, to distinguish pre- from manifest Huntington’s, mHTT displayed moderate accuracy, while NfL had high accuracy in both CSF and plasma.
The findings also revealed that CSF mHTT and both CSF and plasma NfL concentrations were highly stable in individuals for four to eight weeks. Then, a subsequent analysis suggested that fewer than 35 participants per group would be required to incorporate these measures into clinical trials.
Also, computer modeling indicated that mHTT in the CSF is the earliest detectable change in Huntington’s, followed by plasma and CSF NfL. Subsequent changes included total motor score and whole brain volume.
“These findings provide evidence that biofluid concentrations of mHTT and NfL have potential for early and sensitive detection of alterations in [Huntington’s] and could be integrated into both clinical trials and the clinic,” researchers concluded.