**4.1. Histone mutations**

Recent studies and advances in DIPG and biopsy specimens available at the time of the diagnosis have permitted researchers to identify the mutations encoding for histones H3.1 (*HIST1H3B* and *HIST1H3C*), H3.2 (*HIST2H3C*), and H3.3 (*H3F3A*)—proteins known for packaging DNA into chromatin. Histone mutations are found in nearly 80% of children with DIPG, and its high frequency strongly suggests its potential as a driver mutation [45,46]. Clear evidence also indicates that the molecular pathogenesis of DIPG is distinct from non-brain‐ stem HGGs [46].

The *K27M* (lysine replaced by methionine at amino acid 27) or *K27I* (lysine replaced by isoleucine at amino acid 27) mutations result from a gain of function and have the potential to lower overall amounts of wild-type H3 with trimethylated lysine 27 (*H3K27me3*). This results in a loss of methylation at this site. Also, sequestration of the polycomb repressive complex 2 (*PRC2*) further results in overall histone hypomethylation. Normally, the *PRC2* complex represses gene expression through histone methylation. In the absence of *PRC2* complex member *EZH2*, genes that should be silent by methylation are expressed and transcriptional‐ ly active, leading to the mechanism of *K27M*/*K27I* tumorigenicity [47].

Studies analyzing the differences between H3.3 and H3.1 subgroups are showing that they can have distinct cells of origin [48]. A distinct genomic expression pattern between these two subgroups, in addition to the higher frequency of H3.1 mutation in a younger age, could imply that H3.3 and H3.1 mutations target distinct progenitors. Another interesting finding is that *PDGFRA* amplification is seen mainly in combination with H3.3 mutation, while *ACVR1* is only seen mainly in combination with mutant H3.1 [18,48].

It is known that the type of histone H3 mutation can predict the prognosis and OS of DIPG patients in a more accurate way than clinical, histological, or radiological characteristics of the tumor [29]. The discovery of the histone mutations and its importance are an incentive to the reintroduction of biopsy at the time of diagnosis, permitting to identify the genomic land‐ scape of the patient and determination of a better treatment plan.
