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Acute Promyelocytic Leukemia - Diagnosis, Prognosis, and Role of Ancillary Testing

Posted on May 14 2014

Case Study

The characteristic chromosomal abnormality associated with APL is t(15;17) which is identified in ~90% of cases. This translocation results in the fusion of promyelocytic leukemia (PML) and the retinoic acid receptor alpha (RARA) protein. However, in approximately 10% of APL cases, t(15;17) is not detected due to cytogenetic failures, variant translocation, or more complex rearrangements (6). It is important to recognize these APL variants as they differ in prognosis and in their sensitivity to ATRA and to arsenic trioxide (see Table #1). For example, PLZF-RAR-alpha and STAT5b-RAR-alpha have been associated with retinoid resistance and a relatively poor prognosis.  In the setting of strong pathologic suspicion for APL and negative t(15;17) studies, molecular testing for variant translocations should be considered at diagnosis for proper therapy and for appropriate molecular testing in minimal residual disease.

History:

A 73 year old man presents with pancytopenia.

Flow cytometry was performed on the bone marrow aspirate.

Microscopic Findings:

The bone marrow aspirate showed involvement by acute promyelocytic leukemia with prototypic morphological features. Note the characteristic morphology with bilobed nuclei, cytoplasmic granularity, and Auer rods indicated by the arrow.

Special Studies:

Flow Cytometry

+/- CD33+ CD34- CD117+ HLA-DR- MPO+ (bright) CD11b- CD11c-

 CD117+/CD34

CD117+/CD34

 CD117+/HLA-DR-

CD117+/HLA-DR-

FISH Studies

POSITIVE PML/RARA FISH with a translocation between PML (15q24) and RARA (17q21.1) in 84% of cells.

Conventional Cytogenetics

46,XY,t(15;17)(q24;q21)[20]

Testing by PCR

 Not performed at diagnosis, see post therapy studies

Differential Diagnosis:

Acute promyelocytic leukemia (AML M3), acute myeloid leukemia with maturation, acute myeloid leukemia not otherwise specified.

Diagnosis:

Acute promyelocytic leukemia (AML M3)

Post Therapy Follow up Studies:

Post therapy studies showed a negative flow cytometry result with immunophenotypically unremarkable myeloblasts and immature myeloid cells (~2.0%); however, FISH studies showed a low-level fusion (~4%) and RT-PCR studies were positive for PML-RARA translocation. Subsequent post therapy PCR studies showed conversion to molecular negativity.

Educational Comments:

Diagnosis:

Optimal treatment of APL requires rapid initiation of treatment with all-trans retinoic acid (ATRA) and supportive care measures. Initial diagnosis is made with morphology and/or flow cytometry and is confirmed by molecular studies (FISH and/or PCR and cytogenetics). Flow cytometry findings can strongly support the diagnosis of APL. In one series, flow cytometry showed APL cases demonstrated a unique immunophenotypic profile (CD34- CD117+ HLA-DR-) with characteristic CD11b and CD11c negativity, and when encorporated into a flow cytometric diagnostic panel, these APL-associated antigens profiles resulted in a diagnostic specificity and positive predictive value of 98% (1). Significantly, the APL-specific immunophenotypic profile is independent of the underlying cytogenetic abnormalities, and is seen in cases with classic PML-RARA and alternative translocations (1). Thus, morphologic evaluation with flow cytometry is critical to the diagnosis of APL and provide sufficient diagnostic certainty to warrant ATRA therapy prior to molecular confirmation.

Prognosis:

Cases of APL are biologically heterogenous with multiple clinical and pathological risk factors associated with prognosis. In addition to ATRA, arsenic-based regimens have been developed, and a recent phase III study showed that ATRA plus arsenic trioxide is non-inferior, and may be superior to ATRA plus chemotherapy in low-intermediate risk APL patients (70 years of age who present with a low white count), (2). Given these treatment options, risk stratification is critically important in APL. Clinical and pathological risk factors associated with a relative poor prognosis are listed below:

  1. High WBC count at presentation, seen in 20-30% of patients
  2. Advanced age (>70YO)
  3. CD56 expression assessed by flow cytometry
  4. PML/RARA molecular positivity following consolidation therapy
  5. Detection of FLT3 mutations (best assessed at time of diagnosis)

The pathological risk factors merit additional discussion. CD56-expression has been shown to be an independent adverse prognostic risk factor for relapse in APL patients treated with ATRA plus idarubicin-derived regimens (3). Molecular assessment of response can be clinically relevant when performed at the end of consolidation therapy. The reported incidence of molecular positivity following consolidation therapy is variable. Nevertheless, MRD molecular positive patients after consolidation have a poor prognosis and are likely progress to relapse if no therapeutic intervention is taken (4). Finally, FLT3 mutations (FLT3-D835 and FLT3-ITD) have been associated with various hematologic paramenters, including high white counts.   A recent large study showed higher relapse rates in FLT3-ITD+ patients while no impact was observed for FLT3-D835+ patients on univariate analysis. However, multivariate analysis did not confirm the independent prognostic value of FLT3-ITD in patients treated with ATRA and anthracycline-based regimens (5).

Alternate translocations - Therapeutic and molecular diagnostic implications.

The characteristic chromosomal abnormality associated with APL is t(15;17) which is identified in ~90% of cases. This translocation results in the fusion of promyelocytic leukemia (PML) and the retinoic acid receptor alpha (RARA) protein. However, in approximately 10% of APL cases, t(15;17) is not detected due to cytogenetic failures, variant translocation, or more complex rearrangements (6). It is important to recognize these APL variants as they differ in prognosis and in their sensitivity to ATRA and to arsenic trioxide (see Table #1). For example, PLZF-RAR-alpha and STAT5b-RAR-alpha have been associated with retinoid resistance and a relatively poor prognosis.  In the setting of strong pathologic suspicion for APL and negative t(15;17) studies, molecular testing for variant translocations should be considered at diagnosis for proper therapy and for appropriate molecular testing in minimal residual disease.

Table 1.

Translocation ATRA Sensitivity Arsenic Sensitivity Note
PML-RARA, t(15,17) Sensitive Sensitive Arsenic modulates effect through modulation on PML (7)
NPM-RARA, t(5;17) Sensitive * Most common variant translocation
PLZF-RARA, t(11;17) Resistant * Associated with a poor prognosis
NUMA-RARA, t(11;17) Sensitive *  
BCOR-RARA,  t(X;17) Sensitivity * Morphologic correlate-associated with round inclusions and rectangular cytoplasmic bodies
Stat5b-RARA t(3;17) Resistant * Associated with a poor prognosis
FIP1L1-RARA, t(4;17) Sensitive *  
PRKAR1A-RARA fusion see note see note Reported sensitivity to ATRA, idarubicin, and arsenic trioxide (8)

* Not demonstrated

References:

  1. Am J Clin Pathol. 2011 Jan;135(1):76-84. Flow Cytometry Rapidly Identifies All Acute Promyelocytic Leukemias With High Specificity Independent of Underlying Cytogenetic Abnormalities. Dong HY, Kung JX, Bhardwaj V, McGill J
  2. N Engl J Med. 2013 Jul 11;369(2):111-21. Retinoic acid and arsenic trioxide for acute promyelocytic leukemia. Lo-Coco et al.
  3. Blood 2011 Feb 10;117(6):1799-805. Clinical significance of CD56 expression in patients with acute promyelocytic leukemia treated with all-trans retinoic acid and anthracycline-based regimens. Montesinos P et al.
  4. J Clin Oncol. 2011 Feb 10;29(5):495-503. Modern approaches to treating acute promyelocytic leukemia. Sanz et al.
  5. Haematologica. 2011 Oct;96(10):1470-7. Prognostic value of FLT3 mutations in patients with acute promyelocytic leukemia treated with all-trans retinoic acid and anthracycline monochemotherapy. Barragán E , et al.
  6. Front Oncol. 2011 Oct 25;1:35. Development of real-time quantitative polymerase chain reaction assays to track treatment response in retinoid resistant acute promyelocytic leukemia. Jovanovic JV et al.
  7. Nature Cell Biology 10, 547 - 555 (2008) Arsenic degrades PML or PML–RARα through a SUMO-triggered RNF4/ubiquitin-mediated pathway. Lallemand-Breitenbach V et al.
  8. Blood. 2007 Dec 1;110(12):4073-6. The PRKAR1A gene is fused to RARA in a new variant acute promyelocytic leukemia. Catalano Aet al.

 

Case study by Mark Stonecypher, MD


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