MLL associated translocations are found in 70% of infant leukaemia’s less than 2 years of age1.Variousstudies have that In mixed lineage leukemia balanced chromosome translocations occurs lead to expression ofleukemia promoting genes2.Various cellular proteins like PI3K, GSK3?, mTOR, cyclin dependent kinases,histone deacetylases and histone methyltransferases are targeted for the treatment of mixed lineage leukemia.Keywords-Mixed lineage leukemia, translocation, PI3K, mTOR, histone methyltransferase.I.
INTRODUCTIONMixed Lineage Leukemia pathology associated with haemopoetic cells is under a hot bebate from the last twodecades. MLL associated translocations are found in 70% of infant leukaemia’s under the age of 2 years with avery poor prognosis1. Mixed lineage leukemia display co-expression of lymphoid as well as myeloid antigenshence infants with MLL translocation show both myeloid and lymphoid blast cell population3, 4. Normally,the MLL gene encodes for a SET domain histone methyltransferase that catalyzes the methylation of lysine 4 ofhistone H3 (H3K4) at particular regions 5.
In MLL, the active SET Histone 3 lysine 4 methyltransferaseactivity domain is lost and truncated MLL protein is fused to number of other protein partners such as AF4,AF9, AF10 and ENL by balanced chromosomal translocations and rearrangements2, 6. Amino terminalportion of MLL protein is fused to fifty distinct binding protein partners 7.The fusion products retain theabilty to locate gene specific recognition regions even after translocation and interact directly or indirectly withother histone methyltransferaes like DOTIL8.
DOT1L interacts with six unique MLL fusion proteins createdby chromosomal translocations i.e. MLL-AF4, MLL-AF9, MLL-ENL, MLL-AF10, SET-NUP214, CALMAF109,10.
The fusion products gain the ability to recruit Dot1L to the aberrant gene regions and increase theexpression of genes responsible for promotion of leukaemia 11.There is still lack of good quality therapeuticsfor mixed lineage leukemia due to lack of small molecule inhibitors that will directly target MLL12.The focusof the review will be on the recent published work as well as therapeutic targets from the last 2 decades.
II. PI3K AS A THERAPEUTIC TARGET OF MLLRecent reports have shown that Simaltaneous inhibition of PI3K/mTOR has shows anticancer activity in MLLrearranged leukaemias13. In vivo PI3K/mTOR inhibition has shown to reduce tumour progression and alsoshown to increase survival in MLL-AF9 xenograft mouse model13.BEZ, rapamycin and MK-2206 haveshown good in vitro activity as well as have shown good activity in mice tumour models by inhibiting PI3K,mTOR and AKT pathways13.III. CDK4/CDK6 AS A THERAPEUTIC TARGET OF MLLIn MLL there is a cell differentiation block which can be broken by using small molecules like CDK6inhibitors14.CDK6 as a therapeutic target for mixed lineage leukemia was identified by Plakle et al., 201415.
PD-0332991 is a dual inhibitor of CDK4/CDK6 which is clinical trials for treatment of breast cancers as well as 714 | P a g ePD-0332991 have shown strong growth inhibition in MLL rearranged leukemic cells 15. Current treatment ofMLL is chemotherapy and stem cell theray16.IV. SMALL MOLECULE INHIBITORS OF HISTONE DEACETYLASES AS TREATMENTOF MLLIt has been shown recently that HDAC inhibitors induce apoptosis in MLL rearranged cell by autophagy 17.Inhibition of histone deacetylase by VPA (valproic acid) in cells harbouring MLL induced cell cycle arrest (G1-phase) and apoptotic cell death in MLL-AF9 expressing cell lines18.
V. RETINOIC ACID AND VITAMIN D AS IMPORTANT DRUGS FOR MLLMLL-AF9 expressing leukemic cell line MOLM-14 undergoes differentiation when exposed to ATRA or 1, 25-dihydroxyvitamin D319. Simultaneous treatment of MLL cells with Retinoic acid and epidrug 5-azacytidinehas shown to inhibit growth of MLL positive leukemic cells20VI. GLYCOGEN SYNTHASE KINASE 3 IS AN IMPORTANT TARGET TO CONTROLMLLGlycogen Synthase kinase3 has shown to support MLL leukemia proliferation21.GSK3 inhibition has shownto induce G1 growth arrest and cell death in MLL transformed cells21.
GSK3-? inhibition has shown toincrease survival in mouse model of MLL associated leukaemic 21. Specific GSK-3 inhibitor SB-415286 hasbeen repored to inhibit growth by induction of apoptosis in leukemic cells22.VII.COMBINATION OF SIRT1 ACTIVATORS AND DOT1L INHIBITOR FOR THE TREATMENTOF MIXED LINEAGE LEUKEMIAActivation of SIRT1 and at same time inhibition of DOTIL has been shown be an effective therapy for mixedlineage leukemia23.SIRT1 activation mediated silencing of the MLL-AF9 leukemia has been shown to beenhanced by simultaneous DOT1L inhibition23.
SIRT1 activation by SIRT1 activator SRT1720 incombination with DOTIL inhibitor has been reported to augment apoptosis induction in mixed lineage leukemiacells23.VIII. ?-CATENIN AS A THERAPEUTIC TARGET OF MLLIt has been reported that Leukemic stem cells have a more self renewal and drug résistance property 24.
?-catenin establishes the growth of mixed lineage leukemia Leukemic stem cells25. Reversal of LSC to PLSChas shown to significantly reduces the growth of mixed lineage leukemia cells by ?-catenin downregulation orsuppression 26.IX. TET1 IS A DIRECT TARGET OF MLL-FUSION PROTEINS AND IS AN IMPORTANTTHERAPEUTIC TARGETTET1 has shown to be highly expressed in MLL-rearranged leukemia cells with leads to drastic increase of5-hydroxymethylcytosine levels27.TET1 has shown to be an associated parter of MLL which is leads toincreased growth 27. Overexpressed of TET1 in MLL rearranged leukemia has shown to be responsible foroverexpression of leukemia promoting genes Hoxa9 Pbx3 and Meis1 28.TET1 overexpression has shown 715 | P a g eincreases proliferation and inhibit cell death of MLL cells29.Recent report suggested that TET1 knockdown ortherapeutic intervention of TET subside MLL rearranged leukemia 30.
X. BET FAMILY MEMBERS AND MLLIt has been shown that BET family members i.eBromodomainT,Bromodomain2,Bromodomain 3 andBromodomain 4 recruit MLL fusion oncogene proteins to diverged genic regions and increase the expression ofleukemia inducing genes BCL2, CDK6 and C-MYC31. It has been shown that inhibition of bromodomainproteins could provide a new novel approach for the treatment of mixed lineage leukemia31 .XI. DOTIL INHIBITORS FOR THE TREATMENT OF MLLIt has been shown that inhibition of DOTIL by small molecules kill mixed lineage leukemia cells by inhibitingH3K79 hypermethylation at the promoters of leukemia promoting genes32.
Inhibition of DOT1L has shown toincrease apoptosis in cells carrying MLL rearrangement cells as well as in mouse model of MLL33. EPZ5676and EPZ004777 are the currently available DOT1L inhibitors which are in research and development for thetreatment of mixed lineage leukemia34.XII. LYSINE SPECIFIC DEMETHYLASE INHIBITORS FOR THE TREATMENT OF MLLLSD1 is shown to be essential for proliferation and growth of leukemic stem cells containing MLL-Fusedoncogenes35 LSD1 (Lysine specific demethylase1) is shown to be highly up regulated in mixed lineageleukemia 35. It has been shown that Lysine specific demethylase inhibitors promote differentiation andapoptotic cell death of MLL cells36.XIII.
MENIN AND MLL INTERACTION BLOCKERSBorkin et al. recently developed potent inhibitors blocking interaction of leukemia associated protein MLLand menin 37. These compounds showed to inhibit the growth of leukemia cells in vitro as well as prolongedthe survival of MLL leukemic mice37. Inhibiting the interaction between Menin and MLL has shown tocause downregulation of Hox A genes and differentiation of MLL-Rearranged Leukemic cells37. Borkin et al.
Showed that MI-463 and MI-503 blocked the MLL Menin interaction, resulted in increased cell death anddifferentiation 37.