MLL domain histone methyltransferase that catalyzes the methylation

MLL associated
translocations are found in 70% of infant leukaemia’s less than 2 years of age.
Normally, the MLL gene encodes for a SET domain histone methyltransferase that
catalyzes histone H3 lysine 4 methylation at particular loci. In mixed lineage leukemia,
the catalytic SET domain responsible for H3K4 methyltransferase activity is
lost and the remaining MLL protein is fused  to a variety of partners such as AF4, AF9,
AF10 and ENL by chromosomal translocations in a balanced manner which causes
the overexpression of leukemia promoting genes..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,histone methyltransferase, apoptosis, differentiation

I. Introduction

Mixed Lineage Leukemia pathology associated with
haemopoetic cells is under a hot bebate from the last two decades. MLL
associated translocations are found in 70% of infant leukaemia’s under the age
of 2 years with a very poor prognosis1. Mixed lineage
leukemia display co-expression of lymphoid as well as myeloid antigens hence
infants with MLL translocation show both myeloid and lymphoid blast cell
population2. Normally, the MLL
gene encodes for a SET domain histone methyltransferase that catalyzes the
methylation of lysine 4 of histone H3 (H3K4) at particular loci 3. In MLL, the catalytic
SET domain responsible for H3K4 methyltransferase activity is lost and the
remaining MLL protein is fused  to a
variety of partners such as AF4, AF9, AF10 and ENL by balanced chromosomal translocations
and rearrangements4. Amino terminal
portion of MLL protein is fused to 50 different binding partners5.The fusion
products retain the gene specific recognition domains even after translocation
and interact directly or indirectly with other histone methyltransferaes like
DOTIL6. DOT1L
interacts with six unique MLL fusion proteins created by chromosomal translocations
i.e. MLL-AF4, MLL-AF9, MLL-ENL, MLL-AF10, SET-NUP214, CALM-AF107.The fusion
products gain the ability to recruit Dot1L to the aberrant gene locations and
increase the expression of leukaemia promoting genes8.There is still
lack of good quality therapeutics for mixed lineage leukemia due to lack of
small molecule inhibitors that will directly target MLL9.The focus of
the review will be on the recent published work as well as therapeutic targets
from the last 10 years.

II.
PI3K as a therapeutic target of MLL

Dual
inhibition PI3K/mTOR shows anticancer activity in MLL rearranged leukaemias. In
vivo PI3K/mTOR inhibition reduced tumour progression and increased survival in
MLL-AF9 xenograft mouse model10.BEZ, rapamycin
and MK-2206 have shown good in vitro activity as well as have shown good
activity in mice tumour models by inhibiting PI3K, mTOR and AKT pathways10. 

III.
CDK4/CDK6 as a therapeutic target of MLL

In
MLL there is a cell differentiation block which can be broken by using small
molecules like CDK6 inhibitors11.CDK6 as a
therapeutic target for mixed lineage leukemia was identified by Plakle et al.,
201412. PD-0332991 is a dual
inhibitor of CDK4/CDK6 which is clinical trials for  treatment of breast cancers as well as
PD-0332991 have shown strong growth inhibition in MLL rearranged  leukemic cells 12. Current
treatment of MLL is chemotherapy and allogenic stem cell transplantation in
selected cases13.

IV.
Small molecule inhibitors of histone deacetylases as treatment of MLL

Inhibition
of histone deacetylases by HDAC inhibitors induce apoptosis in MLL rearranged
cell by autophagy inhibition14. Inhibition of
histone deacetylase by valproic acid induced cell cycle arrest (G1-phase) and
apoptosis in MLL-AF9 expressing cell lines15.

V.
Retinoic acid and Vitamin D as important drugs for MLL

MLL-AF9 expressing cell line MOLM-14 undergoes
differentiation when exposed to ATRA or 1, 25-dihydroxyvitamin D316. Retinoic acid in combination with 5-azacytidine
inhibit growth MLL positive leukemic cells17

VI.
Glycogen Synthase kinase 3 is an important target to control MLL

Glycogen
Synthase kinase3 supports MLL leukemia proliferation and maintainance.GSK3
inhibition induces G1 arrest and cell death in MLL transformed cells. GSK3-?
inhibition increased survival in mouse model of MLL associated leukaemic. Specific GSK-3 inhibitor
SB-415286 inhibits growth and induces apoptosis in leukemic cells18.

VII.Combination
of Sirt1 activators and DOT1L inhibitor for the treatment of mixed lineage
leukemia

Activation
of SIRT1 and inhibition of DOTIL can be an effective therapy for mixed lineage leukemia.SIRT1
mediates silencing of the MLL-AF9 leukemic program upon DOT1L inactivation by
H3K9 deacetylation 19.SIRT1 activator
SRT1720 in combination with DOTIL inhibitor augment apoptosis induction in
mixed lineage leukemia cells19.

VIII. ?-catenin as a therapeutic target of MLL

Leukemic stem
cells show more self renewal and drug résistance20. ?-catenin establishes the growth of MLL Leukemic
stem cells21. Reversal of LSC to PLSC significantly reduces the
growth of MLL-transformed cells by suppression of ?-catenin 22.

IX. TET1 is a direct target of MLL-fusion proteins and
is an important therapeutic target

TET1 is significantly up-regulated in MLL-rearranged
leukemia, leading to a global increase of 5-hydroxymethylcytosine level23.TET1 is a fusion partner of MLL. TET1 is
overexpressed in MLL rearranged leukemia and increases the expression of
leukemia promoting genes Hoxa9, Meis1 and Pbx324.TET1 overexpression increases proliferation
and inhibits apoptosis of MLL cells25.TET1 knockdown or therapeutic intervention
of TET  prevent MLL rearranged leukemia26.

X.
BET family members and MLL

Bromodomain and extra terminal protein (BET) family
of proteins (BRD2, BRD3, BRD4 and BRDT) recruit MLL fusion oncogene proteins to
aberrant gene locations and increase the expression of  leukemia promoting  genes BCL2, CDK6 and C-MYC27. So inhibition
of bromodomain proteins provide a new approach for the treatment of mixed
lineage leukemia.

XI.
DOTIL inhibitors for the treatment of MLL

Small molecule inhibition of DOTIL kill mixed
lineage leukemia cells by inhibiting H3K79 hypermethylation at the promoters of
leukemia promoting genes28.Small molecule inhibition
of DOT1L increases apoptosis in cells carrying MLL rearrangement as well as in
mouse model of MLL29. EPZ5676 and
EPZ004777 are the currently available DOT1L inhibitors which are in research
and development for the treatment of mixed lineage leukemia30.

XII. Lysine specific demethylase
inhibitors for the treatment of MLL

LSD1 is essential for proliferation of leukemic
stem cells containing MLL-Fusion oncogene LSD1 (Lysine specific demethylase1)
is highly up regulated in mixed lineage leukemia 31. LSD1 inhibitors
promote differentiation and apoptosis of MLL cells32.

 

XIII. Menin and MLL interaction
blockers

Borkin et al.
recently developed highly potent and orally bioactive small molecule inhibitors
that block the interaction between menin and MLL protein33. These compounds inhibit the growth of leukemia cells
in vitro as well as prolonged the survival MLL leukemic mice.Inhibiting
the Menin-MLL Interaction Causes Hox Gene downregulation and differentiation of
MLL-Rearranged Leukemia33. MI-463 and
MI-503 block the MLL binding site on menin, resulting in downregulation of Hox
genes and differentiation of leukemic blasts33.

XIV. Conclusion

 

Various
small molecule inhibitors are in research and development for the treatment of
mixed lineage leukemia. All Currently available treatments for mixed lineage leukemia
have low efficacy as well as high toxicity. So there is a need to develop new
drugs as well as to identify new therapeutic targets for mixed lineage leukemia