, opening the door to a potential new method of predicting neurodegenerative disease, new research suggests.
In a study of 77 patients undergoing eye surgery for various conditions, more than 70% had more than 20 pg/mL of NfL in their vitreous humor. Higher levels of NfL were associated with higher levels of other biomarkers known to be associated with Alzheimer’s disease, including amyloid-beta and tau proteins.
“The study had three primary findings,” said lead author Manju L. Subramanian, MD, associate professor of ophthalmology at Boston University.
First, the investigators were able to detect levels of NfL in eye fluid; and second, those levels were not in any way correlated to the patient’s clinical eye condition, Dr. Subramanian said. “The third finding was that we were able to correlate those neurofilament light levels with other markers that have been known to be associated with conditions such as Alzheimer’s disease,” she noted.
For Dr. Subramanian, these findings add to the hypothesis that the eye is an extension of the brain. “This is further evidence that the eye might potentially be a proxy for neurodegenerative diseases,” she said. “So finding neurofilament light chain in the eye demonstrates that the eye is not an isolated organ, and things that happen in the body can affect the eye and vice versa.”
The findings were published online Sept. 17 in Alzheimer’s Research & Therapy.
Verge of clinical applicability?
Early diagnosis of neurodegenerative diseases remains a challenge, the investigators noted. As such, there is a palpable need for reliable biomarkers that can help with early diagnosis, prognostic assessment, and measurable response to treatment for Alzheimer’s disease and other neurologic disorders
Recent research has identified NfL as a potential screening tool and some researchers believe it to be on the verge of clinical applicability. In addition, increased levels of the biomarker have been observed in both the cerebrospinal fluid (CSF) and blood of individuals with neurodegeneration and neurological diseases, including Alzheimer’s disease. In previous studies, for example, elevated levels of NfL in CSF and blood have been shown to reliably distinguish between patients with Alzheimer’s disease and healthy volunteers.
Because certain eye diseases have been associated with Alzheimer’s disease in epidemiological studies, they may share common risk factors and pathological mechanisms at the molecular level, the researchers noted. In an earlier study, the current investigators found that cognitive function among patients with eye disease was significantly associated with amyloid-beta and total tau protein levels in the vitreous humor.
Given these connections, the researchers hypothesized that NfL could be identified in the vitreous humor and may be associated with other relevant biomarkers of neuronal origin. “Neurofilament light chain is detectable in the cerebrospinal fluid, but it’s never been tested for detection in the eye,” Dr. Subramanian noted.
In total, vitreous humor samples were collected from 77 unique participants (mean age, 56.2 years; 63% men) as part of the single-center, prospective, cross-sectional cohort study. The researchers aspirated 0.5 to 1.0 ml of undiluted vitreous fluid during vitrectomy, while whole blood was drawn for APOE genotyping.
Immunoassay was used to quantitatively measure for NfL, amyloid-beta, total tau, phosphorylated tau 181 (p-tau181), inflammatory cytokines, chemokines, and vascular proteins in the vitreous humor. The trial’s primary outcome measures were the detection of NfL levels in the vitreous humor, as well as its associations with other proteins.