Histone demethylases are a new class of emerging epigenetic modulators. The first described histone demethylase enzyme in this group is lysine-specific demethylase 1A. It removes methyl groups from histones H3K4me1/2 and H3K9me1/2. LSD1 is abnormally overexpressed in various types of solid tumors and acute leukemias, where it impedes differentiation and contributes to cancer cell proliferation, cell metastasis and invasiveness, and is associated with inferior prognosis. LSD1 inhibitors significantly attenuate tumor progression in vitro and in vivo in a range of solid tumors and acute myeloid leukemia.
Abbreviations: ESC: Embryonic Stem Cell; MAOs: Monoamine Oxidases; FAD: Flavin Adenine Dinucleotide; LSD1: Lysine-Specific Demethylase 1; TCP: Tranylcypromine; AML: Acute Myeloid Leukemia; JMJD: JUMONJI C Domain-Containing; DNMT1: DNA-Methyltransferase 1; STAT3: Signal Transducer and Activator of Transcription 3; E2F1: E2F Transcription factor 1; 2-OG: 2-Oxoglutarate; ATRA: All Trans Retinoic Acid.
Epigenetic events serve an important function in carcinogenesis. Post-translational Histone modifications are critical for control of transcription and chromatin architecture. Histone demethylases is a new class of emerging epigenetic modulators  that catalyze N-demethylation of histone lysine’s . Demethylation differs from histone acetylation in the fact that methylation occurs on both lysine and arginine (R), and in being linked to both transcriptional activation and repression . Demethylases are divided into two subgroups based on their catalytic mechanisms: the flavin adenine dinucleotide (FAD)-dependent LSD1 and LSD2 and JMJD family containing JmjC domain .
Lysine-specific demethylase 1A (LSD1), is known also as KDM1A, KIAA0601, BHC110, and AOF2) . It is the first described histone demethylase enzyme. LSD1 enzyme consisted of 3 major domains: an amine oxidase (AO)-like domain, which is FAD cofactor dependent, a SWIRM domain, which is unique to chromatin-associated proteins and a coiled-coil TOWER domain . It is located in the cell nucleus and is a key epigenetic regulator that acts as a histone methylation eraser. It demethylates di- and mono-methyl groups from the fourth and nine positions on histone 3 protein (H3K4me2/1 and H3K9me2/1), but not H3K4me3. This results in transcriptional repression or activation, respectively . LSD1 can also remove mono- and di-methylated lysine residues from few non-histone targets (such as DNMT1, p53, E2F1, HIF-1α and STAT3) which are associated with angiogenesis, cell cycle arrest, chromatin remolding and proliferation of cancer cells . In addition, LSD1 functions as a H3K9 demethylase when recruited by androgen or estrogen receptor . LSD2 is also important in epigenetic regulation. LSD2 is a histone H3K4me1/2 demethylase with different structural organization and functions relative to LSD1 .
The JMJD family has key roles in cell differentiation, proliferation, and stem cell self-renewal. They oxidatively remove the trimethyl group of histone lysine residues preferably in a Fe2+ and 2-oxoglutarate (2-OG) dependent manner .
LSD1 is highly expressed in embryonic stem cells (ESC) and is downregulated during differentiation . LSD1 shows differential expression in adult tissues . It regulates vital cellular processes, ranging from embryonic development to adult tissue homeostasis
. LSD1 maintains self-renewal potential of embryonic stem
cells and adult stem cells and regulates cellular differentiation of
stem cells in various tissues, including myogenic differentiation,
adipogenesis, hematopoiesis and epithelial differentiation .
In hematopoiesis, LSD1 negatively regulates HSCs self-renewal
and help terminal erythroid, granulocytic, and megakaryocytic
maturation . LSD1-defeciency causes severe pancytopenia and
impaired HSCs differentiation . LSD1 also plays a significant
role in development, thermogenesis, inflammation, neuronal and
cerebral physiology, and the maintenance of stemness in stem
In emergency hematopoiesis as in severe viral or bacterial
infections, the inflammatory cytokines TNFα and IL1β suppress
LSD1 activity and induce HSCs proliferation, differentiation, and
immune cells production . LSD1 participates in pathological
conditions including cancer and viral infection. LSD1 is involved
in various stages of cancer including development, progression,
metastasis, and recurrence after therapy . LSD1 is not a potent
oncogene . It promotes tumor-cell growth  and facilitates
cancer cells survival  by regulating gene expression in favor of
cancer cells adaptation to a pro-cancer microenvironment .
LSD1 inhibition is a potential anti-cancer therapeutic strategy
 due to its high enzymatic activity and level of expression in
various types of tumors  as breast cancer, prostate cancer,
neuroblastoma as well as in hematological malignancies . LSD1
is considered a homology protein of MAO-A/B. Six MAO inhibitors
were tested for their ability to inhibit LSD1 activity; three were nonselective
(a) tranylcypromine/trans-2- phenyl cyclopropylamine
(TCP/2-PCPA), (b) phenelzine, and (c) nialamide and three were
selective for MAOA or MAOB (a) clorgiline, (b) deprenyl, and (c)
Several LSD1 inhibitors have been explored in the treatment of
lung cancer and other solid tumors in clinical trials or even clinical
use . LSD1 inhibition in cancer cells enhances immunogenicity
of tumor and secretion of chemokines that attract T cells, and
reverse resistance to PD-(L)1 blockade. LSD1 inhibitors upregulate
proinflammatory cytokines in Treg cells. It also promotes
macrophages polarization to anti-tumor M1-like macrophages
and enhances the infiltration of these macrophages and CD8 + T
cells into the tumor. Furthermore, LSD1 inhibition activates innate
immune cells, including macrophages and natural killer cells .
The insufficiency of LSD1 catalytic inhibition in some cancers may
be explained by LSD1 demethylase-independent activity during
LSD1 inhibitors are a promising epigenetic approach to
treat AML. They alter stem cell programs, inhibit proliferation,
and restore myeloid differentiation in AML cells. They have
increased efficacy in leukemia subtypes carrying AML1 and
MLL rearrangements, NPM1 mutations, and erythroid and
megakaryoblastic differentiation block. Their effects are enhanced
markedly when combined with histone deacetylase inhibitors
(HDACs) or ATRA with little toxicity. Clinical trials are currently
aim at treatment of refractory AML by blocking LSD1 activity .
ORY-1001 induces cell differentiation followed by reduction
of cell growth and clonogenicity particularly in M4 and M5
FAB subtypes and AML cells carrying MLL translocations .
INCB059872 (Incyte) is a potent selective LDS1 inhibitor that
delays cellular proliferation and induces human AML cells
differentiation . T3775440 is a potent selective LSD1 inhibitor
against erythroid and megakaryocytic leukemic cell lines
. SP2509 (Salarius Pharmaceuticals) is lethal against AML
expressing NMP1 mutation and MLL-rearrangements .
• The need to use more than single screening method due
to the high rate of false-positive or -negative results .
• The difficulty to develop selective LSD1 inhibitors due to
the high similarity of LSD1 with LSD2 and MAOs .
• Low anti-cancer potency of many LSD1 inhibitors, as
many tumor suppressor genes or oncogenes are often regulated
by multiple enzymes . Combining LSD1 inhibitors with other
drugs such as HDACs, or EZH2 (enhancer of zeste homolog 2)
inhibitors may be a solution. For example, combining SP2509 with
Panobinostat (HDAC inhibitor) shows promising results compared
to either agent alone in AML clinical trials. Combined inhibitors of
LSD1 and EZH2 act synergistically by inhibiting different histone
methylation-modulating proteins with apparently opposite
enzyme activities .
• Some LSD1 inhibitors are associated with acute anemia
and decreased platelets’ numbers .
• The different role played by LSD1 in different cancers
LSD1 acts as a key epigenetic regulator of gene expression
that controls cellular homeostasis. It can be a therapeutic target
in certain diseases. Gaining an accurate understanding of the
mechanism of action of LSD1 is essential for overcoming several