Neurofilament light chain (NfL), a biomarker previously measured in blood or cerebrospinal fluid and used to indicate neurodegeneration, is detectable in the vitreous humor of the eye, 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, and higher levels of NfL were associated with higher levels of other biomarkers known to be associated with Alzheimer’s disease (AD), including amyloid-β and tau proteins.
“The study had three primary findings,” lead author Manju L. Subramanian, MD, associate professor of ophthalmology at Boston University School of Medicine, Massachusetts, told Medscape Medical News.
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, 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 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 September 17 in Alzheimer’s Research & Therapy.
Verge of Clinical Applicability?
Early diagnosis of neurodegenerative diseases remains a challenge, the investigators note. As such, there is a palpable need for reliable biomarkers that can help with early diagnosis, prognostic assessment, and measurable response to treatment for AD 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 AD. In previous studies, for example, elevated levels of NfL in CSF and blood have been shown to reliably distinguish between patients with AD and healthy volunteers.
Because certain eye diseases have been associated with AD in epidemiological studies, they may share common risk factors and pathological mechanisms at the molecular level, the researchers note. In an earlier study, the current investigators found that cognitive function among patients with eye disease was significantly associated with amyloid-β 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,” 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-β, 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.
Significant Correlations
Results showed that 55 of the 77 participants (71.4%) had at least 20 pg/ml of NfL protein present in the vitreous humor. The median level was 68.65 pg/ml.
Statistically significant associations were found between NfL levels in the vitreous humor and Aβ40, Aβ42, and total tau; higher NfL levels were associated with higher levels of all three biomarkers. On the other hand, NfL levels were not positively associated with increased vitreous levels of p-tau181.
Vitreous NfL concentration was significantly associated with inflammatory cytokines, including interleukin-15, interleukin-16, and monocyte chemoattractant protein-1, as well as vascular proteins such as vascular endothelial growth factor receptor-1, VEGF-C, vascular cell adhesion molecule-1, Tie-2, and intracellular adhesion molecular-1.
Despite these findings, NfL in the vitreous humor was not associated with patients’ clinical ophthalmic conditions or systemic diseases such as hypertension, diabetes, and hyperlipidemia. Similarly, NfL was not significantly associated with APOE genotype E2 and E4, the alleles most commonly associated with AD.
Finally, no statistically significant associations were found between NfL and Mini-Mental State Examination (MMSE) scores.
A “First Step”
Most research currently examining the role of the eye in neurodegenerative disease is focused on retinal biomarkers imaged by optical coherence tomography, the investigators note. Although promising, data obtained this way have yielded conflicting results.
Similarly, while the diagnostic potential of the core CSF biomarkers for AD (Aβ40, Aβ42, p-tau, and total tau) is well established, the practical utility of testing CSF for neurodegenerative diseases is limited, write the researchers.
As such, an additional biomarker source such as NfL — which is quantifiable and protein-based within eye fluid — has the potential to play an important role in predicting neurodegenerative disease in the clinical setting, they add.
“The holy grail of neurodegenerative-disease diagnosis is early diagnosis. Because if you can implement treatment early, you can slow down and potentially halt the progression of these diseases,” Subramanian said.
“This study is the first step toward determining if the eye could play a potential role in early diagnosis of conditions such as Alzheimer’s disease,” she added.
That said, Subramanian was quick to recognize the findings’ preliminary nature and that they do not offer reliable evidence that vitreous NfL levels definitively represent neurodegeneration. As such, the investigators called for more research to validate the association between this type of biomarker with other established biomarkers of neurodegeneration, such as those found in CSF fluid or on MRI and PET scans.
“At this point, we can’t look at eye fluid and say that people have neurodegenerative diseases,” she noted. “The other thing to consider is that vitreous humor is at the back of the eye, so it’s actually a fairly invasive procedure.
“I think the next step is to look at other types of eye fluids such as the aqueous fluid in the front of the eye, or even tear secretions, potentially,” Subramanian said.
Other study limitations include the lack of an association between NfL levels and MMSE scores and that none of the study participants were actually diagnosed with AD. Validation studies are needed to compare vitreous levels of NfL in patients with mild cognitive impairment/AD to normal controls, the investigators note.
Fascinating but Impractical?
Commenting on the findings for Medscape Medical News, Sharon Fekrat, MD, professor of ophthalmology, Duke University Medical Center, Durham, North Carolina, agreed that there’s potential importance of the eye in diagnosing neurodegeneration. However, she suggested that vitreous humor may not be the most expedient medium to use.
“I commend the authors for this fascinating work. But practically speaking, if we ultimately want to use intraocular fluid to diagnose Alzheimer’s and perhaps other neurodegeneration, I think aqueous humor might be more practical than the vitreous humor,” said Fekrat, who was not involved with the research.
“What might be even better is to have a device that can be held against the eyeball that measures the levels of various substances inside the eyeball without having to enter the eye,” added Justin Ma, a Duke University medical student working under Fekrat’s guidance.
“It could be similar technology to what’s currently used to measure blood glucose levels,” Ma added.
The study was supported in part by the National Institute of Aging. Subramanian, Fekrat, and Ma have disclosed no relevant financial relationships. Disclosures for other study authors are listed in the original article.
Alzheimers Res Ther. Published online September 17, 2020. Full article
For more Medscape Psychiatry news, join us on Facebook and Twitter.