A recent paper in Nucleic Acid Research (2018, 46, 11251) describes a collaborative effort between scientists at the Netherlands Cancer Institute, Leiden University Medical Center, University Medical Center Utrecht, University of California and UbiQ, with the aim to gain more insight into the functions of Dot1 and its crosstalk with histone H2B. In order to directly demonstrate the site of ubiquitination on H2B, a monoclonal antibody was raised against yeast K123 monoubiquitinated H2B by using a synthetic K123 monoubiquitinated yeast H2B(115-130) peptide as antigen.

Conserved from yeast to human, the histone methyltransferase Dot1 methylates K79 of histone H3 on the nucleosome core. This methylation affects gene expression and DNA damage response, and is enhanced by monoubiquitination of the C-terminus of histone H2B. Given the promising clinical development of Dot1 inhibitors (e.g. for the treatment of acute leukemia in children), it is important to elucidate the regulatory mechanisms of Dot1. Using genetic interaction maps of increased-dosage alleles of Dot1, a functional relationship was identified between increased Dot1 dosage and loss of the deubiquitinating module of the SAGA co-activator complex, a deubiquitinating enzyme of monoubiquitinated H2B and as such negative regulator of H3K79 methylation (Figure 1). Increased Dot1 dosage was found to promote monoubiquitination of H2B in a dose-dependent manner and this was exacerbated by the loss of SAGA-deubiquitinating activity. Importantly, the stimulatory effect on H2B ubiquitination was mediated by the N-terminus of Dot1, independent of its methyltransferase activity. Thus, Dot1 and monoubiquitinated H2B are subject to bi-directional crosstalk and Dot1 has chromatin regulatory functions that are independent of its methyltransferase activity.

Figure 1. Left: van Welsem et al. Nucleic Acid Research 2018. Right: Model for the mutual crosstalk between Dot1 and monoubiquitinated H2B.