Dhanvi Prajapati

Histone-modifying enzymes and histone post-translational modifications (PTMs) play key roles in the organization (euchromatin versus heterochromatin) and function (active versus silenced genes) of chromatin. The abundance and activity of these enzymes, along with their associated histone PTMs, are often altered in cancer cells, leading to deregulated gene expression. The expression of the KMT2A-MLLT3 protein, resulting from a chromosomal translocation in mixed-lineage leukemia (MLL), a subtype of acute myeloid leukemia, augments transcription elongation, promoting the expression of HOXA9 and MEIS1, genes that play critical roles in MLL development. Bortezomib, a proteasome inhibitor, has been effective at treating various cancers. In this study, we compared the impact of bortezomib on histone PTMs in the MLL cell line MOLM-13 and the chronic myeloid leukemic (CML) cell line K562. We report that MOLM-13 had a greater level of histone H2B monoubiquitinated at lysine 120 (H2BK120ub) and histone H3 dimethylated at lysine 79 (H3K79me2) (modifications involved in elongation) and similar levels of histone H2A monoubiquitinated at lysine 119 (H2AK119ub). Bortezomib treatment resulted in significant reductions in H2BK120ub and H2AK119ub levels, as well as in transcript levels of genes involved in MLL development.

Histone-modifying enzymes and histone post-translational modifications (PTMs) play key roles in the organization (euchromatin versus heterochromatin) and function (active versus silenced genes) of chromatin. The abundance and activity of these enzymes, along with their associated histone PTMs, are often altered in cancer cells, leading to deregulated gene expression. The expression of the KMT2A-MLLT3 protein, resulting from a chromosomal translocation in mixed-lineage leukemia (MLL), a subtype of acute myeloid leukemia, augments transcription elongation, promoting the expression of HOXA9 and MEIS1, genes that play critical roles in MLL development. Bortezomib, a proteasome inhibitor, has been effective in treating various cancers. In this study, we compared the impact of bortezomib on histone PTMs in the MLL cell line MOLM-13 and the chronic myeloid leukemic (CML) cell line K562. We report that MOLM-13 had a greater level of histone H2B monoubiquitinated at lysine 120 (H2BK120ub) and histone H3 dimethylated at lysine 79 (H3K79me2) (modifications involved in elongation) and similar levels of histone H2A monoubiquitinated at lysine 119 (H2AK119ub). Bortezomib treatment resulted in significant reductions in H2BK120ub and H2AK119ub levels, the loss of FAcilitates Chromatin Transcription (FACT) chromatin association in both cell lines, and the reduced transcript levels of genes involved in the development of MLL.

Acid extraction has been and remains one of the most widely used methods for preparing histones. Over the years, dilute hydrochloric or sulphuric acid solutions have been used alternately for histone extraction, followed by acetone or trichloroacetic acid precipitation, respectively. Although a sulphuric acid method has become commonplace in recent years, the protocol is quite involved and can pose problems when trying to solubilize the histones obtained this way. In this paper, we describe an alternative approach based on an early protocol using hydrochloric acid. The new method is technically straightforward and produces histones that are readily amenable to downstream uses, including sodium dodecyl sulfate, acetic acid–urea–Triton X-100, two-dimensional polyacrylamide gel electrophoresis, and mass spectrometry.

Doxorubicin is a chemotherapeutic drug for cancer that intercalates into nucleosome-free regions at promoters. Doxorubicin was reported to result in loss of histone H3 trimethylated lysine 4 (H3K4me3). To further explore doxorubicin’s mechanism of action, we determined the genomic location of the binding sites of doxorubicin in leukemic cells. The effect of doxorubicin intercalation into the chromatin of leukemic cells on histone modifications was also determined. We show that doxorubicin binding sites were present in the nucleosome-free regions associated with regulatory regions (promoters, enhancers, and super-enhancers) and in the gene body (introns). Doxorubicin treatment did not alter the levels of H3K4me3 and many other histone modifications but significantly lowered H2B ubiquitinated at lysine 120 (H2BK120ub), an elongation-dependent modification. Lastly, we demonstrate that doxorubicin results in the degradation of the largest subunit (RPB1) of RNA polymerase II.