In-depth proteomic analysis of the human cerumen-a potential novel diagnostically relevant biofluid

Human cerumen, also called earwax, is a substance secreted by various glands in the outer ear canal. Although the variation of texture and color during otorhinolaryngological diseases is a generally known phenomenon, cerumen as biofluid remains relatively unexplored. However, there is an emerging interest for protein biomarkers which are easily accessible and predictive for diagnostics and therapy outcome. Here we provide a thorough investigation of human cerumen applying two different prefractionation techniques: i) 1D-PAGE prefractionation with subsequent LC-MS/MS, and ii) online SCX-fractionation coupled to LC-MS/MS. Additionally, individual variation was addressed by shotgun LC-MS/MS of specimens from 5 subjects. In total, we identified 11,562 distinct peptides representing 2013 proteins in human cerumen. The in-depth characterization revealed a high complexity of cerumen comparable with other human biofluids such as urine, plasma, or saliva. A probiotic or antibiotic character of cerumen has previously been discussed. In this study we provide further evidence for the important role of cerumen as an antimicrobial barrier and in local immune response, e.g. by assessing high amounts of zinc-alpha-2-glycoprotein.

Cerumen analysis might have promising potential as diagnostic body fluid for biomarker characterization and disease specific objectives. Disease-associated or infection-specific changes may support diagnostics in otorhinolaryngology and may lead to a better understanding of human cerumen's function in immune response. An easy-to-handle and standardized sample collection and preparation of cerumen can further improve individualized medicine strategies.