Breaking Boundaries in Histone Modification MS-Based Detection: A Tailored Search Strategy for Unrestricted Identification of Novel Epigenetic Marks

Histone post-translational modifications (PTMs) play a crucial role in regulating gene expression and maintaining DNA integrity, and their aberrations are linked to various diseases, including cancer. While lysine acetylation and methylation have been extensively studied, recent research has uncovered additional PTMs that signifi- cantly contribute to chromatin structure and function. Mass spectrometry is the most effective analytical method for studying histone PTMs; however, computa- tional limitations often restrict the analysis to common modifications. Unrestrictive search strategies have the potential to enable a more comprehensive characteriza- tion of the histone modification landscape. In this work, we systematically assess the application of unrestrictive search approaches to histone data. After evaluating the limitations of these methods, we develop a novel bioin- formatics workflow, named HiP-Frag (histone PTM anal- ysis with FragPipe), which enables the identification of 96 sites decorated with uncommon PTMs on core histones- 60 of which were previously unreported-as well as 55 histone marks on linker histones, including 13 novel ones, purified from human cell lines and primary samples. The expanded histone PTM analysis enabled by this strategy is among the first to extract previously unexplored epigenetic information from mass spectrometry raw data. This approach paves the way for a facilitated and more streamlined identification of uncommon and yet unan- notated histone modifications, supporting a deeper dissection of the histone code and the understanding of the potential biological role of the novel epigenetic marks.