We read with interest Uyar and colleagues' recent report on the association between
diabetes, nondiabetic elevated glycated hemoglobin levels (HbA1c), and neuroaxonal
damage in Parkinson's disease (PD) patients from the MARK‐PD study.
1
The authors confirmed previously established findings of an inverse association between
diabetes and cognitive and motor status. The authors also demonstrated higher serum
neurofilament light (NfL) levels (a marker of neuroaxonal damage)
2
in PD patients with prevalent type 2 diabetes and in PD patients with nondiabetic
elevated HbA1c levels. These associations persisted after adjustment for age, body
mass index (BMI), and vascular risk factors (prevalent arterial hypertension, hypercholesterolemia,
and history of stroke). We recently noted similar motor and cognitive associations
in PD patients with diabetes
3
in the Tracking Parkinson's study, although only a nonsignificant trend toward an
association in the overall PD cohort between NfL levels and more severe motor and
cognitive status at baseline,4
which may reflect the reduced disease duration in the Tracking Parkinson's cohort,
compared with the MARK‐PD cohort.
Considering the authors' novel findings of an association between diabetes and neuroaxonal
damage, we explored the relationship between serum NfL and diabetes in our previously
defined subgroup of the Tracking Parkinson's study.
4
The analysis was performed using Stata V.17.0 (Stata, RRID:SCR_012763), and differences
were compared using Kruskal–Wallis tests for continuous data and χ2 tests for categorical
data, whereas the association between NfL and diabetes was further explored using
univariate and multivariate (age, BMI, and vascular risk factors) linear regression
analysis.
Of the 280 patients studied, 29 suffered from prevalent type 2 diabetes. PD‐DM patients
were older (74.1 years ± SD 7.7 vs. 68.1 years ± 8.7, P < 0.001), with higher BMIs
(31.1 ± SD [standard deviation] 5.7 vs. 27.1 ± SD 4.4, P < 0.001), whereas a higher
proportion had coexistent vascular risk factors than PD patients without diabetes
(P = 0.032). Serum NfL levels were higher in PD‐DM patients (39.5 ± SD 18.9 vs. 29.6 ± SD
16.0, P < 0.001). Using regression analysis, NfL levels were significantly associated
with patients' diabetic status (coefficient: 0.82, 95% CI [confidence interval]: 0.45–1.19,
P < 0.0001), which persisted (coefficient: 0.52, 95% CI: 0.18–0.86, P = 0.003) after
adjustment for age, BMI, and vascular risk factors (history of angina, myocardial
infarction, stroke, hypertension, and hypercholesterolemia).
Our findings affirm Uyar et al's report of an association between PD‐DM and more severe
neuroaxonal damage. Furthermore, the data indicate that the more severe phenotype
in PD‐DM noted to date by several studies is likely to be mediated by additional factors
other than vascular risk factor burden that tends to coexist in these cases. T2DM
and PD share several pathological processes encompassing neuroinflammation, lysosomal
dysfunction, mitochondrial dysfunction, and the development of central insulin resistance
that leads to neurodegeneration.
5
This process is in part mediated by hyperglycemia as demonstrated by the MARK‐PD study
and its downstream impact on α‐synuclein aggregation.
6
It is also possible that some of the observed associations are explained by diabetic
neuropathy, as other peripheral neuropathies are known to increase blood NfL concentrations.
7
Disentangling the mechanistic factors that contribute to this more rapidly progressive
axonal damage is of critical importance in the development of disease‐modifying therapies
for PD.
Full financial disclosures for the previous 12 months
N.V. has received unconditional educational grants from Ipsen and Biogen; travel grants
from Ipsen, AbbVie, and the International Parkinson's Disease and Movement Disorders
Society; and speaker's honorarium from AbbVie and Stada and served on advisory boards
for AbbVie and Brittania outside of the submitted work.
M.L. has no competing interest.
R.R. has no competing interest.
A.J.H. has no competing interest.
T.G. has no competing interest.
D.A. has no competing interest.
C.G. has no competing interest.
Y.B.‐S. has no competing interest.
H.Z. has served at scientific advisory boards for AbbVie, Alector, Eisai, Denali,
Roche Diagnostics, Wave, Samumed, Siemens Healthineers, Pinteon Therapeutics, Nervgen,
AZTherapies, and CogRx; has given lectures in symposia sponsored by Cellectricon,
Fujirebio, Alzecure, and Biogen; and is a cofounder of Brain Biomarker Solutions in
Gothenburg AB (BBS), which is a part of the GU Ventures Incubator Program (outside
the submitted work).
D.G.G. has received honoraria from Bial Pharma, GE Healthcare, and Vectura plc and
consultancy fees from the Glasgow Memory Clinic.
H.R.M. is employed by UCL. In the past 24 months he reports paid consultancy from
Biogen, UCB, AbbVie, Denali, Biohaven, and Lundbeck; lecture fees/honoraria from Biogen,
UCB, C4X Discovery, GE‐Healthcare, Wellcome Trust, and Movement Disorders Society;
and research grants from ASAP, Parkinson's UK, Cure Parkinson's Trust, PSP Association,
CBD Solutions, Drake Foundation, and Medical Research Council. Dr. Morris is a co‐applicant
on a patent application related to C9ORF72—Method for diagnosing a neurodegenerative
disease (PCT/GB2012/052140).
T.F. has received grants from the National Institute of Health Research, The Michael
J. Fox Foundation, John Black Charitable Foundation, Cure Parkinson's Trust, Innovate
UK, Van Andel Research Institute, and Defeat MSA. He has served on advisory boards
for Voyager Therapeutics, Handl Therapeutics, Living Cell Technologies, Bial, and
Profile Pharma. He has received honoraria for talks sponsored by Bial, Profile Pharma,
and Boston Scientific.
Author Roles
Research project: A. Conception, B. Organization, C. Execution;
Manuscript preparation: A. Writing of the first draft, B. Review and critique.
N.V.: 1A, 1B, 1C, 2A, 2B
M.L.: 1C, 2B
R.R.: 1C, 2B
A.J.H.: 1C, 2B
T.G.: 1C, 2B
D.A.: 1C, 2B
C.G.: 1C, 2B
Y.B.‐S.: 1C, 2B
H.Z.: 1C, 2B
D.G.G.: 1A, 1B, 1C, 2B
H.R.M.: 1A, 1B, 1C, 2B
T.F.: 1A, 1B, 1C, 2B.