Acute Promyelocytic Leukemia – Symptoms, Treatment

Acute Promyelocytic Leukemia is a distinguished subset of acute myeloid leukemia which is characterized by fusion gene transcript PML-RAR-alpha and high cure rates with treatment. This entity was first described in 1957 in patients with severe bleeding tendencies with fibrinolysis, rapid deterioration of the clinical condition, and the presence of promyelocytes in peripheral blood and bone marrow.[rx][rx][rx]

Acute promyelocytic leukemia (APL) has become a curable disease by all-trans retinoic acid (ATRA)-based induction therapy followed by two or three courses of consolidation chemotherapy. Currently, around 90% of newly diagnosed patients with APL achieve complete remission (CR) and over 70% of patients are curable. To further increase the CR and cure rates, detection and diagnosis of this disease at its early stage is very important, hopefully before the appearance of APL-associated coagulopathy. In induction therapy, concomitant chemotherapy is indispensable, except for patients with low initial leukocyte counts. Prophylactic use of intrathecal methotrexate and cytarabine should be done, particularly for patients with hyperleukocytosis. If patients relapse hematologically or even molecularly, arsenic trioxide will be the treatment of choice under careful electrocardiogram monitoring. Am80, liposomal ATRA, gemtuzumab ozogamicin or ATRA in combination with cytotoxic drugs may be used at this stage or later. Allogeneic SCT will be the treatment of choice after patients of age <50 years have elapsed, provided that they have HLA-identical family donors or DNA-identical unrelated donors.

Acute promyelocytic leukemia is classified into low-risk (white blood cell count (WBC) 10,000/microL or less and platelets 40,000/microL or more), intermediate (WBC 10,000/microL or less and platelets 40,000/microL or less), and high-risk (WBC more than 10,000/microL) to guide treatment.[rx][rx][rx][rx]

Advances in the molecular pathology of this leukemia have led to the introduction of arsenic trioxide and all-trans retinoic acid therapies, which have led to improved prognosis.

Pathophysiology

Morphology

Upon investigation of the bone marrow and peripheral blood of a patient with APL, atypical promyelocytes are seen. Promyelocytes are large (usually greater than 20 microns in diameter) myeloid precursors, often with a high nucleus to cytoplasmic ratio, fine chromatin, and prominent nucleoli. The distinguishing feature of the promyelocyte is the presence of many violet granules in the cytoplasm, which can obscure the nucleus [rx]. The promyelocytes in APL differ from normal promyelocytes in that they are larger in size and have altered shapes of their nuclei [rx]. The abnormal promyelocyte population in a typical APL sample accounts for at least 30% of observed myeloid cells in the marrow. Very few cells are seen that have progressed past the promyelocyte stage of differentiation [rx].

The two morphologic variants of the disease are the hyper granular and microgranular forms. The hyper granular form accounts for 75% of cases [rx]. On Wright’s-stained smears, the cytoplasm of promyelocytes contain tightly packed bright pink, red-blue, or dark purple granules. The microgranular form accounts for the other 25% of APL cases and is characterized by a bilobed nucleus and no granularity visible with light microscopy [rx]. Clinicians should be aware of the two variants to improve the accuracy of diagnosis and expedite prompt treatment intervention.

Immunohistochemistry

With rare exceptions, the APL phenotype is CD34-negative/partial or weak positive, HLA-DR–negative, CD13-positive, CD33-positive, CD11b-negative, CD15-weak or negative, CD117-weak/variable, and sometimes CD2-positive and CD56-positive [rx].

Cytogenetics

Acute promyelocytic leukemia is defined by a particular cytogenetic finding. A reciprocal translocation between the long arms of chromosomes 15 and 17 at q24.1;q21.2 is found, with the subsequent creation of a fusion gene, PML/RARA. This genetic abnormality results in the linkage of the RARA gene on chromosome 17 with the PML gene on chromosome 15 [rx]. The PML/RAR protein functions as an aberrant receptor that blocks retinoic acid-induced myeloid differentiation; hence, the accumulation of promyelocytes arrested in a certain stage of development predominates in affected patients’ tissue samples [rx]. To identify this cytogenetic finding, a variety of testing strategies are routinely employed, including karyotyping, fluorescence in situ hybridization (FISH), and reverse transcription-polymerase chain reaction (RT-PCR) testing.

Karyotyping and FISH are usually used at the time of diagnosis and throughout the treatment course, while RT-PCR testing becomes more relevant later in the treatment course to detect minimal residual disease. Bone marrow specimens are preferred for cytogenetic testing. First, traditional karyotyping is routinely done on diagnostic bone marrow specimens. In this methodology, technicians use programs to analyze a complete karyotype to identify the size, shape, and number of chromosomes. The downside of this method is that it can be time-consuming, taking days to complete. It is, however, highly specific. Fluorescence in situ hybridization should also be utilized in bone marrow specimens, and occasionally in peripheral blood samples when bone marrow sample acquisition is expected to be delayed. In FISH analysis, labeled DNA probes are hybridized to either metaphase chromosomes or interphase nuclei, and the hybridized probes are detected with fluorochromes. This technique is a rapid and sensitive means of detecting recurring numerical and structural abnormalities [rx] The result of the test can usually be available within 24 hours, which is an important advantage since early diagnosis of this disease subtype is critical.

Finally, RT-PCR testing is another methodology used to detect the PML/RARA gene sequences. The reverse transcription-polymerase chain reaction is a highly sensitive technique used for the detection and quantitation of messenger RNA, which consists of two parts: the synthesis of complementary DNA from RNA by reverse transcription and the amplification of a specific complementary DNA by the polymerase chain reaction. This methodology can also detect PML breakpoint location and can be utilized to monitor minimal residual disease (MRD) status. Currently, available PCR methods are able to detect approximately one leukemia cell diluted 10⁵ to 10⁶ times, or one blast or less per 100,000 nucleated cells [rx]. Monitoring of MRD status serially at regular intervals after initial induction therapy can guide treatment choices and identify those patients at higher risk of relapse.

Causes of Acute Promyelocytic Leukemia

The RAR-alpha (Retinoic acid-alpha) gene which encodes nuclear hormone receptor transcription factors is present on the long arm of chromosome 17 and is invariably involved in APL. It promotes the expression of various genes after binding to retinoic acid. In the majority (90% to 95%) of the cases, APL results from a t (15;17) (q22;q21) translocation resulting in the head to the tail fusion of promyelocytic leukemia (PML) gene to RAR-alpha to generate two fusion genes, PML–RARalpha and a reciprocal RAR-alpha-PML (80%) that encode a protein, which functions as an aberrant retinoid receptor. The other cytogenetic abnormalities associated with APL include t(5;17)(q35;q21), t(11;17)(q23;q21), t(11;17)(q13;q21), and t(17;17)(q11;q21) fuse RAR-alpha to the Nucleophosmin (NPM), Promyelocytic Leukemia Zinc Finger (PLZF), Nuclear Mitotic Apparatus (NuMA), and STAT5b genes, respectively, leading to expression of their fusion proteins. These translocations also have clinical significance due to their responsiveness (NPM/RAR-alpha, NuMA/RAR-alpha) or partial/complete refractoriness to retinoids (STAT5B/RAR-alpha, PLZF/RAR-alpha).[rx][rx]

The mechanisms of formation of the above chromosomal rearrangements and initiation of leukemia are unknown. Chemotherapy, ionizing radiation, industrial solvents, and other toxic agents are some of the known risk factors.

or

The mutation that causes acute promyelocytic leukemia involves two genes, the PML gene on chromosome 15 and the RARA gene on chromosome 17. A rearrangement of genetic material (translocation) between chromosomes 15 and 17, written as t(15;17), fuses part of the PML gene with part of the RARA gene. The protein produced from this fused gene is known as PML-RARα. This mutation is acquired during a person’s lifetime and is present only in certain cells. This type of genetic change, called a somatic mutation, is not inherited.

The PML-RARα protein functions differently than the protein products of the normal PML and RARA genes. The protein produced from the RARA gene, RARα, is involved in the regulation of gene transcription, which is the first step in protein production. Specifically, this protein helps control the transcription of certain genes important in the maturation (differentiation) of white blood cells beyond the promyelocyte stage. The protein produced from the PML gene acts as a tumor suppressor, which means it prevents cells from growing and dividing too rapidly or in an uncontrolled way. The PML-RARα protein interferes with the normal function of both the PML and the RARα proteins. As a result, blood cells are stuck at the promyelocyte stage, and they proliferate abnormally. Excess promyelocytes accumulate in the bone marrow and normal white blood cells cannot form, leading to acute promyelocytic leukemia.

The PML-RARA gene fusion accounts for up to 98 percent of cases of acute promyelocytic leukemia. Translocations involving the RARA gene and other genes have been identified in a few cases of acute promyelocytic leukemia.

Symptoms of Acute Promyelocytic Leukemia

If your child has APL, the following symptoms may be present:

• Anemia – breathlessness, fatigue
• Low white cells – frequent, persistent infections
• Low platelets – bruising and/or bleeding
• DIC – bruising/bleeding which may be very severe
• Bleeding that is hard to stop, even from a small cut
• Blood in the urine
• Heavy nosebleeds
• Bleeding gums and easy bruising
• Fever and infections
• Low red blood cell count
• Paleness
• Tiring easily
• Poor appetite
• Unexplained weight loss

Similar Symptoms

The symptoms tend to be similar to AML in general with the following being possible symptoms:^[rx]

Easy bleeding from low platelets may include

• Bruising (ecchymosis)
• Gingival bleeding
• Nose bleeds (epistaxis)
• Increased menstrual bleeding (menorrhagia)

Diagnosis of Acute Promyelocytic Leukemia

Histopathology

Acute promyelocytic leukemia is characterized by the presence of the large atypical promyelocytes and other myeloid precursors in various stages of development in the peripheral blood. The bone marrow is hypercellular, and APL promyelocytes account for about 30% of the myeloid cells in the classic variant. The typical acute promyelocytic leukemia promyelocyte has a creased, folded, bilobed, kidney-shaped, or dumb-bell shaped nuclei with a high nucleus-cytoplasmic ratio, fine chromatin, and prominent nucleoli, in addition to many violet granules (which coalesce to form Auer rods) in the cytoplasm and intense myeloperoxidase activity. In the microgranular variant (20% to 30%), pellets and Auer rods are less prominent, and the nucleus has a characteristic bilobed, folded appearance. Other less common variants, hyperbasophilic variant, M3r (associated with the PLZF) have also been described in the literature.

On immunophenotyping, the premature malignant promyelocytes express bright cytoplasmic myeloperoxidase, early myeloid markers, CD13 and CD33 but do not express HLA-DR, CD11b and are weakly positive/negative for CD15, CD117 (expressed in mature myelocytes) and CD34 (early myeloid progenitor cells). CD9 is expressed in acute promyelocytic leukemia but not in other AML subtypes. Co-expression of CD 2 is commonly seen in the hypogranular variant of acute promyelocytic leukemia.

History and Physical

Patients usually present with generalized weakness and fatigue, gingival bleeding, petechiae or ecchymoses, visual changes secondary to retinal hemorrhages, epistaxis, or menorrhagia or infections. Patients may also present with thrombotic complications such as deep venous thrombosis, pulmonary embolism, cerebrovascular accident, etc. Some patients with advanced disease may present with overt disseminated intravascular coagulation and frank bleeding.

Pancytopenia is common at presentation. The key difference between APL and AML is that many patients with the former ae at risk for disseminated intravascular coagulation and associated hyperfibrinolysis. The coagulopathy has to be managed as a medical emergency otherwise it often leads to CNS and pulmonary hemorrhage.

Evaluation

When acute promyelocytic leukemia is suspected, evaluation of peripheral blood smear and FISH for the fusion of PML/RARA should be expedited for rapid diagnosis of this time-sensitive disease. A prompt coagulopathy workup including a platelet count, prothrombin time (PT), activated partial thromboplastin time (PTT), d-dimer or fibrin split products, and fibrinogen should also be performed. Bone marrow biopsy and immunophenotyping should also be performed. Conventional karyotyping should also be performed as a part of the initial workup as it detects rare molecular subtypes of acute promyelocytic leukemia and other additional coexistent cytogenetic abnormalities- t(15:17). Reverse transcriptase-polymerase chain reaction (RT-PCR) for PML-RARA RNA is also used for confirming the diagnosis of acute promyelocytic leukemia and can also be used can for monitoring minimal residual disease.

Definitive diagnosis requires testing for the PML/RARA fusion gene. This may be done by polymerase chain reaction (PCR), fluorescent in situ hybridization (FISH), or conventional cytogenetics of peripheral blood or bone marrow. This mutation involves translocation of the long arm of chromosomes 15 and 17. On rare occasions, a cryptic translocation may occur which cannot be detected by cytogenetic testing; on these occasions, PCR testing is essential to confirm the diagnosis.^[rx]

CBC tests – A complete blood count (CBC) shows how many of each type of blood cell you have. A peripheral blood smear checks for blast cells. Specific types include tests for

• RBC – the numbers, size, and types of RBC in the blood
• WBC – the numbers and types of WBC in the blood
• Platelets – the numbers and size of the platelets
• Hemoglobin – an iron-rich protein in red blood cells that carries oxygen
• Hematocrit – how much space red blood cells take up in your blood
• Reticulocyte count – how many young red blood cells are in your blood
• Mean corpuscular volume (MCV) – the average size of your red blood cells

The complete blood count (CBC) includes most or all of these. CBC is one of the most common blood tests.

• Blood tests – Most people with acute myelogenous leukemia have too many white blood cells, not enough red blood cells, and not enough platelets. The presence of blast cells — immature cells normally found in bone marrow but not circulating in the blood — is another indicator of acute myelogenous leukemia.
• Imaging tests – X-rays, CT scans, MRIs, and ultrasounds give a clearer picture of what’s going on inside you. They can help find infections or show when cancer has spread to other parts of your body.
• Bone marrow tests – Your doctor uses a needle to take a sample of marrow, blood, and bone from your hip or breastbone. A specialist looks at it under a microscope for signs of leukemia.
• Spinal tap – This is also called a lumbar puncture. Your doctor uses a needle to take some cerebrospinal fluid from around your spinal cord. A specialist checks it for leukemia cells.
• Genetic tests – A laboratory can look at your leukemia cells for gene or chromosome changes. The results will tell your doctor more about your AML so they can help you decide on the best treatment.
• Lumbar puncture – is done in high-risk patients with elevated WBC count if intrathecal therapy is contemplated. Further, a cardiac evaluation is necessary before administering anthracyclines.
• The Genetic Testing Registry (GTR) – provides information about the genetic tests for this condition. The intended audience for the GTR is health care providers and researchers. Patients and consumers with specific questions about a genetic test should contact a health care provider or a genetics professional.

Treatment of Acute Promyelocytic Leukemia

APL is unique among leukemias due to its sensitivity to all-trans retinoic acid (ATRA; tretinoin), the acid form of vitamin A.^[rx] Treatment with ATRA dissociates the NCOR-HDACL complex from RAR and allows DNA transcription and differentiation of the immature leukemic promyelocytes into mature granulocytes by targeting the oncogenic transcription factor and its aberrant action.^[rx] Unlike other chemotherapies, ATRA does not directly kill the malignant cells.^[rx]

• Chemotherapy – Acute promyelocytic leukemia is a medical emergency with a very high pre-treatment mortality. All-Trans Retinoic Acid (ATRA) is the mainstay in the treatment of acute promyelocytic leukemia and used in all modern regimens. ATRA should be initiated without any delay even before cytogenetic confirmation is obtained. Before the introduction of ATRA in the 1980s, the prognosis of this disease was poor with chemotherapy alone. ATRA was then used in combination with anthracycline-based regimens with increased survival and cure rate. ATO (arsenic trioxide) also induces differentiation of the malignant myeloid clone by dissociating the PML/RAR-alpha-RXR complex from the target genes and found to have a synergistic action with ATRA. ATRA-ATO was also shown to have comparatively lesser toxicities than ATRA-chemo. Hence, ATRA-ATO for induction and consolidation has emerged as the new standard of care for patients with low-(to-intermediate) risk acute promyelocytic leukemia. ATRA- Idarubicin or ATRA – ATO plus gemtuzumab ozogamicin (antibody-drug conjugate) are preferred in patients with high risk without cardiac dysfunction. ATRA-ATO therapy with or without gemtuzumab ozogamicin is also a reasonable choice for patients with severe comorbidities, older adults, patients with cardiac dysfunction who cannot tolerate anthracycline-based regimens or overall poor functional status. Maintenance therapy after the initial consolidation is widely debated. Maintenance may not be necessary for patients receiving intensive induction/consolidation including ATO. Treatment and post-treatment monitoring up to 2 years with PCR are recommended. Treatment of relapsed APL is beyond the scope of this article.
• Supportive Therapy  – Supportive therapy plays a very important role in the survival of patients with acute leukemia. Bleeding diathesis is a known complication, especially in patients receiving treatment and platelets, should be maintained above 30 to 50 × 10/l and fibrinogen above 100 mg/dl to 150 mg/dl, with aggressive blood product support. High suspicion should be maintained for systemic infections as the patients are routinely immunosuppressed. In granulocytopenia patients with fever, an empiric antibiotic regimen to treat gram-negative bacteria should be instituted. Vancomycin should be started, if there is suspicion of catheter-related infection or based on blood cultures or if there is a suspicion of severe unknown infection. Antifungal should be considered if fever persists 5 days after the initiation of empiric antibiotics with appropriate testing. Because cure rates for APL are high, bone marrow transplantation is not the first option. It is only offered to patients who relapse or are resistant to therapy.
• Remission induction therapy – The purpose of the first phase of treatment is to kill the leukemia cells in your blood and bone marrow. However, remission induction usually doesn’t wipe out all of the leukemia cells, so you need further treatment to prevent the disease from returning.
• Consolidation therapy – Also called post-remission therapy, maintenance therapy, or intensification, this phase of treatment is aimed at destroying the remaining leukemia cells. It’s considered crucial to decreasing the risk of relapse.
• Radiation – High-energy X-rays can also stop cancer cells. Your doctor might use a large machine to send radiation toward cancer. Or they may insert a radioactive needle, seed, or wire into your body, on or near cancer.
• Stem cell transplant – Because AML treatment can also kill healthy cells, you might get stem cells that can grow into blood cells. They might come from you or from another person.
• Targeted therapy – This uses drugs to attack specific genes and proteins involved with the growth and spread of cancer cells.
• Bone marrow transplant – A bone marrow transplant, also called a stem cell transplant, may be used for consolidation therapy. A bone marrow transplant helps re-establish healthy stem cells by replacing unhealthy bone marrow with leukemia-free stem cells that will regenerate healthy bone marrow. Prior to a bone marrow transplant, you receive very high doses of chemotherapy or radiation therapy to destroy your leukemia-producing bone marrow. Then you receive infusions of stem cells from a compatible donor (allogeneic transplant).

FDA-Approved Treatments

The medication(s) listed below have been approved by the Food and Drug Administration (FDA) as orphan products for the treatment of this condition.

• Arsenic trioxide (Brand name: Trisenox) – Manufactured by Cephalon
  FDA-approved indication: In combination with tretinoin for treatment of adults with newly-diagnosed low-risk acute promyelocytic leukemia (APL) whose APL is characterized by the presence of the t(15;17). Also approved for induction of remission and consolidation in patients with acute promyelocytic leukemia (APL) who are refractory to, or have relapsed from, retinoid and anthracycline chemotherapy, and whose APL is characterized by the presence of the t(15;17) translocation.
  National Library of Medicine Drug Information Portal
  Medline Plus Health Information
• Tretinoin (Brand name: Vesanoid) – Manufactured by Roche Pharmaceuticals
  FDA-approved indication: Induction of remission in patients with acute promyelocytic leukemia who are refractory to or unable to tolerate anthracycline-based cytotoxic chemotherapeutic regimens.

References

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