Melanin and Neuromelanin Fluorescence Studies Focusing on Parkinson's Disease and Its Inherent Risk for Melanoma

Abstract

Parkinson's disease is associated with an increased risk of melanoma (and vice versa). Several hypotheses underline this link, such as pathways affecting both melanin and neuromelanin. For the first time, the fluorescence of melanin and neuromelanin is selectively accessible using a new method of nonlinear spectroscopy, based on a stepwise two-photon excitation. Cutaneous pigmentation and postmortem neuromelanin of Parkinson patients were characterized by fluorescence spectra and compared with controls. Spectral differences could not be documented, implying that there is neither a Parkinson fingerprint in cutaneous melanin spectra nor a melanin-associated fingerprint indicating an increased melanoma risk. Our measurements suggest that Parkinson's disease occurs without a configuration change of neuromelanin. However, Parkinson patients displayed the same dermatofluorescence spectroscopic fingerprint of a local malignant transformation as controls. This is the first comparative retrospective fluorescence analysis of cutaneous melanin and postmortem neuromelanin based on nonlinear spectroscopy in patients with Parkinson's disease and controls, and this method is a very suitable diagnostic tool for melanoma screening and early detection in Parkinson patients. Our results suggest a non-pigmentary pathway as the main link between Parkinson's disease and melanoma, and they do not rule out the melanocortin-1-receptor gene as an additional bridge between both diseases.

Keywords: Parkinson’s disease; dermatofluoroscopy; melanin; neuromelanin.

Figure 1

The principle of dermatofluoroscopy. Different modes of excitation of fluorescence in the visible spectral region: (a) conventional fluorescence excitation by one photon (e.g., 400 nm); (b) excitation by simultaneous absorption of two photons via an only virtual energy level (e.g., 800 nm, preferably from a femtosecond laser); (c) excitation by stepwise absorption of two photons (e.g., 800 nm, preferably from a nanosecond laser) via a real intermediate energy level.

Figure 2

Simplified energy level scheme of melanin in melanosomes of nevi and of melanoma illustrates the process responsible for fluorescence spectra in the excited state: radiationless relaxation in nevi k (N) and in melanoma k (M). The relaxation includes ultrafast Franck–Condon relaxation (tuning of the core configuration to the changed excited-state electron distribution) and nonradiative vibrational relaxation.

Figure 3

Dermatofluoroscopy of skin from Caucasian patients (Fitzpatrick type 2, 3) in vivo. The four representative classes of melanin-dominated spectra: (a) normal pigmented skin (class 4), (b) benign nevus (class 3), (c) dysplastic nevus (class 2), and (d) melanoma (class 1).

Figure 4

Dermatofluoroscopy of a nevus from a Parkinson patient: (a) location of the measuring grid on the nevus, the grid pitch is 200 µm; (b) representative melanin-dominated spectra of this nevus. Green line: fluorescence from nevomelanocytes of a benign nevus area. Yellow line: fluorescence corresponding to that from nevomelanocytes of a dysplastic nevus area. Red line: fluorescence corresponding to that from melanoma cells. The wavelength (horizontal line) is given in nanometers (nm), counts of fluorescence intensity on the vertical line.

Figure 4

Dermatofluoroscopy of a nevus from a Parkinson patient: (a) location of the measuring grid on the nevus, the grid pitch is 200 µm; (b) representative melanin-dominated spectra of this nevus. Green line: fluorescence from nevomelanocytes of a benign nevus area. Yellow line: fluorescence corresponding to that from nevomelanocytes of a dysplastic nevus area. Red line: fluorescence corresponding to that from melanoma cells. The wavelength (horizontal line) is given in nanometers (nm), counts of fluorescence intensity on the vertical line.

Figure 5

Dermatofluoroscopy of the postmortem substantia nigra pars compacta (SNpc) of a Parkinson patient. The grid above the specimen shows the individual measuring areas; the colored dots indicate the local neuromelanin concentration resulting from the measurements, here for the sake of clarity only as a rough differentiation (blue, no neuromelanin; yellow-green, neuromelanin). Scale 1:5.

Figure 6

Examples of fluorescence spectra of postmortem SNpc of control case. The blue line represents a spectrum from a largely neuromelanin (NM)-free region; this fluorescence is the tissue autofluorescence from NAD(P)H. The other two spectra represent—from light to dark green—increasing contribution of NM fluorescence to the autofluorescence; the dark-green lined spectrum results from regions with maximum NM content.

Figure 7

Fluorescence spectrum of NM in the postmortem SNpc of controls (blue line) and in a Parkinson patient (green line).