Cancers of the blood, or leukaemias, that involve mutations in a gene called Mixed Lineage Leukaemia (MLL) have a very poor prognosis and are particularly prevalent in young children. Due to the aggressive nature of this type of cancer, there is an acute need for the development of more effective therapies to help treat the children who suffer from this devastating condition. Professor Thomas Milne’s group in the MRC Molecular Haematology Unit recently published an article in Cell Reports detailing the mechanisms by which MLL translocations may drive malignancy and exploring the potential use of small molecule inhibitors to mediate this. In this blog post, two of the authors on the paper (Jon Kerry and Laura Godfrey) describe their findings.
The main cause of MLL leukaemias are translocations, where one chromosome breaks and joins to another, causing the aberrant fusion of two genes. Translocations of the MLL gene result in MLL fusion proteins (MLL-FPs).
MLL-FPs cause leukaemia by binding to and increasing the level of activity of many important genes essential for normal cell function, which eventually results in the cell developing into a cancerous cell. The mechanism by which MLL-FPs are recruited to genes is currently unknown, but by understanding this mechanism in detail we can potentially develop new therapeutic strategies to block the recruitment of MLL-FPs to their target genes, and in turn stop them transforming normal cells into cancer cells.
Our study initially investigated two proteins, Menin and PAF1, that interact with MLL-FPs and are both thought to be essential to recruit MLL-FPs to their gene targets, therefore contributing to the development of leukaemia. By scanning the entire genome of MLL-FP leukaemia cells, our data supports the idea that Menin is pivotal for MLL-FP recruitment, but we found that the role of PAF1 is significantly less than previously thought and may only have a role at a small subset of genes, if at all. Menin inhibitors have been recently shown to be effective against MLL-leukaemias and this is supported by our findings.
Importantly, we identified unusually large binding domains of MLL-FPs. Typically, MLL-FPs are found bound to a small defined region at the beginning of the gene, known as the promoter. However, we identified a unique binding behaviour of MLL-FPs where, as well as binding at gene promoters, the MLL-FPs fusion protein also bound at large areas in the middle of the gene, creating a large ‘spread’ of MLL-FP binding.
We showed that spreading of MLL-FPs correlated with increased gene expression and occurred at a number of genes which have previously been shown to be overexpressed in MLL-FP patients as well as being associated with a poor prognosis for patients diagnosed with the condition. Together, this indicates that MLL-FP spreading could act as a central pathogenic mechanism in the development of MLL leukaemias.
The spreading MLL-FP binding patterns were reminiscent of a broad genomic feature termed super enhancers (regulatory regions with unusually strong enrichment for the binding of transcriptional activators). However, we showed that the regions of the genome where spreading of MLL-FPs was observed are clearly distinct from super enhancers.
Recent advances in the treatment of MLL leukaemias have seen the introduction of small molecule inhibitors that target DOT1L, a protein that is important for gene expression of MLL-FP gene targets and interacts with the MLL-FP complex. We showed that spreading MLL-FP gene targets were more sensitive to DOT1L inhibition, showing the strongest decrease in gene expression and therefore were the most dependent upon DOT1L activity. This implicates spreading MLL-FP gene targets as an Achilles heel in the maintenance of MLL leukaemias. Small molecule inhibitors specifically targeting MLL-FP regulatory proteins such as Menin and DOT1L could be a potential therapeutic strategy.
In conclusion, we have identified a set of gene targets which display a previously uncharacterised MLL-FP spreading pattern that is coupled with Menin and DOT1L activity. These genes were the most responsive to DOTL1 inhibition and seem to be critical with regard to disease progression and maintenance.. Ultimately, this in-depth analysis provides a potential mechanistic explanation for the specificity of DOT1L and Menin inhibitors currently being tested in the treatment of MLL leukaemias and may provide a platform for the development of even more efficient and combinatorial therapies.
New treatments for MLL leukaemias are desperately needed by the children who have been diagnosed with the condition, and studies like this lay the fundamental groundwork for the development of such therapeutic strategies.
Post edited by Emma Mee-Hayes.