The document summarizes the pathogenesis and treatment of acute promyelocytic leukemia (APL). APL results from the t(15;17) translocation forming the PML-RARA fusion protein, which disrupts normal retinoic acid signaling and blocks differentiation of promyelocytes. Treatment uses all-trans retinoic acid (ATRA) to induce differentiation and arsenic trioxide to degrade the PML-RARA protein. While ATRA and arsenic trioxide can achieve high remission rates, resistance can develop, requiring alternative treatments or chemotherapy.

1. APL
Hamzeh Nusairat PharmD, JUST university

2. MOA : APL pathogenesis
 In normal cell:
 (RARα) and (RXR) forms a heterodimer.
 This heterodimer binds to regions of DNA called (RAREs)
 This binding mediate the transcription of hundreds of
genes (responsible for self-renewal and differentiation).

3.  In the absence of ligand (retinoic acid) the heterodimer recruits :
 1) corepressors such as nuclear receptor corepressor (NCoR)
 2) silencing mediator for retinoid and thyroid hormone receptor (SMRT)
 3)enzymes particularly histone deacetylases (HDAC)
 Upon RA binding to heterodimer  triggers a conformational change 
release corepressors  recruit coactivators
 due to t (15,17) 
 PML-RARA fusion protein disrupts this coactivator recruitment  NO gene
transcription  NO self-renewal and differentiation

4. PML-RARA tetramers
 1) Bind to a large number of target DNA sites may contribute to widespread
transcriptional deregulation
 2) Has a functional domains : (DNA binding domain, hormone binding domain, RXR-
binding domain , PML coiledcoil and the RING finger domains) contribute to :
 leukemogenesis
 sensitivity to retinoic acid and arsenic trioxide
 3) Disrupts the formation of PML nuclear bodies (implicated in the transformation of APL
cells)

5. PML nuclear bodies
 sphere-shaped domains that localize to the nuclear matrix
 Has many functions:
 epigenetic
 silencing, transcription, and replication
 modulate p53 signaling
 Senescence (cessation of cell division)  through their ability to control sumoylation
[Modifier proteins] and proteolysis

6. PML protein
 undergoes several posttranslational modifications, such as phosphorylation and
sumoylation
 important in the formation of nuclear bodies
 Recruit a number of partner proteins:
 DAXX: a repressor of transcription and modulator of apoptosis

7. RXRA
 co-localizes at PML-RARA bound DNA promoter regions  enhance the latter’s
DNA binding ability
 PML-RARA-RXR complex has been associated with APL pathogenesis
 Sumoylation of RXRA  transformation to APL cells

8. Treatment with RA
 1) induces blast differentiation
 2) degradation of the fusion protein PML-RARA
 3) Inhibition of RARα (involve a normal negative feedback loop of RA on its own
receptors)
 4) alleviate a repressive chromatin environment  allowing transcriptional
activators to bind to RARE (retinoic acid response elements)

9. Treatment with Arsenic trioxide
 Degradation of both the PML-RARA fusion protein
 Degradation the normal PML protein
 The primary mechanism : the targeting of the PML-RARA or normal PML
towards nuclear bodies
 enhances the reformation of nuclear bodies
 ATO bind to cysteine residues on the PML moiety of the fusion protein 
 this binding triggers or enhances the binding of ubiquitin-conjugating enzyme 9 (UBC9) to the
PML RING finger domain
 UBC9  then allows the PML-RARA moiety to undergo sumoylation
 UBC9 attachment recruits ring finger protein 4 (RNF4) onto PML nuclear bodies

10. Retinoic acid resistance
 1) the presence of the PML-RARA fusion protein is required to give sensitivity to RA
 Exposure to compounds such as vitamin D3 partially restores responsiveness to RA ‫مهمم‬
 2) increased degradation through proteasomal pathways of PML-RARA  leads to RA resistance.
 the presence of the PML-RARa protein role :
1) Its expression leads to the initiation of APL
2) while its presence is necessary to confer sensitivity to pharmacological doses of RA
 C-Raf, Vav-1 and Fgr also responsible for resistance
 If PML-RARa NOT present  NO translocation t (15;17) // high risk  chemo

11. MUTATIONAL LANDSCAPE
 The Fms like tyrosine kinase 3 (FMLT3) gene most commonly mutated
 NOTE: The genetic alterations that occur in APL differ at diagnosis and at
relapse.
 Other : Recurrent mutations in FLT3, WT1, NRAS, KRAS, ARID and PML genes
 In relapse: ARID1B and RUNX1

12. Pin 1 ( Peptidyl-prolyl cis/trans isomerase
(PPIase))
 overexpressed in many human cancers  ability to activate oncogenes and inhibit tumor
supressors
 a direct target of RA (RA cuz destruction to pin 1)
 RA has carboxyl moiety that mimics the pSer/Thr-Pro (pin 1 substrate binding site)  inactivate
 (pSer/Thr-Pro) = Proline-directed phosphorylation  play a role in many cellular signaling pathways,
some of which are involved in oncogenesis

13. pathogenesis : Autophagy
 highly conserved degradative process
 regulated by genes (ATG’s)
 Decreased expression of ATG’s has been noted in primary AML blasts
 RA  increase the expression of ATG’s as well as restore the process of
autophagy
 Arsenic trioxide ATO  upregulate ATG’s via the mTOR pathway
 Upregulation of autophagy  allows mature granulocytes to complete
differentiation while limiting non-essential aggregate proteins

14. Autophagy: sequestosome 1
 The p62 or sequestosome 1  adapter protein plays a crucial role in directing
ubiquitinated proteins towards autophagosomal vesicles
 RA  increases its level  increases autophagy

15. Autophagy : Beclin-1
 part of the PI3K complex that is involved in the initiation and maturation of
autophagosomes
 ATRA and ATO  upregulate Beclin-1  increases autophagy

16. differentiation syndrome
 50% of patients undergoing treatment with RA
 due to :
 1) cytokine secretion.
 2) up-regulation of certain adhesion molecules on the surface of APL cells.
 expression of high-affinity β2 integrins (LFA-1) (ICAM-2)
 3) surface molecule CD13 on APL

17. Tamibarotene
 A new synthetic retinoid Tamibarotene (Am80) with higher binding affinity to
PML-RARA
 potent inducer of differentiation
 Its efficacy seem to vanish at 5 years of treatment

18.  APL patients (~95%) have PML-RARA translocation
 5% have a non-characteristic translocation:
 the PLZF/ RARA translocation  less responsive to ATRA and have a poorer
 STAT5b-RARA and ZBTB16-RARA  most common
 NuMaRARa, NPM-RARa

19. FLT3-ITD in APL
 associated poorer outcomes in primary APL
 Contribute to the development of RA resistance
 Disruption of P53 signaling
 combination treatment with ATRA/ ATO  for RA resistance

20. SYNERGISTIC EFFECTS OF ATRA & ATO
 ATRA and ATO leads to a sustained expression of target genes (TGM2 &
RARβ) leading to terminal differentiation
 cells express differentiation markers (CD11b) well after treatment termination
(ATRA & ATO)
 induce to demethylation in CpG islands of TGM2 and RARβ promoter regions

21. References
 1. Jimenez JJ, Chale RS, Abad AC, Schally AV. Acute promyelocytic leukemia
(APL): a review of the literature. Oncotarget. 2020 Mar 17;11(11):992-1003. doi:
10.18632/oncotarget.27513. PMID: 32215187; PMCID: PMC7082115.
 2. DeRemer D, & Higgins T (2020). Acute leukemias. DiPiro J.T., & Yee G.C., &
Posey L, & Haines S.T., & Nolin T.D., & Ellingrod V(Eds.), Pharmacotherapy: A
Pathophysiologic Approach, 11e. McGraw-Hill.
https://accesspharmacy.mhmedical.com/content.aspx?bookid=2577&sectionid=23
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