Acute promyelocytic leukemia with PML-RARA t(15;17).
Acute promyelocytic leukemia (APML) is a subtype of acute myeloid leukemia (AML) in which abnormal promyelocytes predominate. APML accounts for approximately 5-8% of AML . It differs from other subtypes of AML in that patients are often younger, with a median age of 40 years old. Also, most patients present with a pancytopenia rather than an elevated white count, and white blood cell counts higher than 5000/ L are associated with a poor prognosis. Disseminated intravascular coagulation (DIC) frequently occurs and is considered to be a medical emergency .
More than 95% of APML patients have a t(15;17)(q21;q11),which is diagnosed by cytogenetics and fluorescence in-situ hybridization (FISH) or by RT-PCR . The use of cytogenetics alone is not advisable because it may not detect cryptic PML-RARA rearrangements; however, cytogenetics can be helpful by revealing additional chromosomal abnormalities. It is imperative that the diagnosis be made quickly . This makes RT-PCR for t(15;17) one of the only "stat" tests in molecular diagnostics. Patients who have t(15;17) are immediately started on all-trans retinoic acid (ATRA) therapy, as well as anthracycline-based chemotherapy .
The t(15;17) involves the retinoic acid receptor alpha (RARA) gene on chromosome 17 and the PML gene on chromosome 15. Two novel fusion genes are formed: a PML-RARA gene on the der(15) chromosome and a RARA-PML gene on the der(17). The PML-RARA fusion protein functions as an aberrant retinoid receptor with altered DNA binding and transcriptional regulatory properties. This fusion protein has leukemogenic potential . Breakpoints in RARA usually occur within intron 2 . Breakpoints in PML can occur in intron 3, exon 6 or intron 6, which produces short, variable, and long forms, respectively (see figure 3). There appear to be different clinical characteristics depending on the isoform. For instance, patients with the short form often have increased white blood cell counts and a microgranular form of APML . The variable form is the least common isoform, accounting for 5-6% in most series. Studies suggest that having the variable form indicates an adverse prognosis ; however, these studies have had small sample sizes due to the low frequency of APML patients with the variable form. Currently, the only prognostic indicators that are used clinically are white blood cell count and platelet count. A white blood cell count < 10,000/ L and platelet count > 40,000/ L are considered to be favorable prognostic factors.
Figure 3. This schematic representation of the PML gene shows how different splice sites result in different products (S= short, V= variable, L= long). The stippled boxes represent exons. Courtesy of Li et al. Blood, vol 90, No 1, 1997: pg 308.
In this case, RT-PCR analysis of the patient's peripheral blood for the PML-RARA t(15;17) translocation showed a breakpoint in exon 6 of the PML gene, which is consistent with the variable form. The translocation is also detected as a series of alternatively-spliced bands using primers from exon 3 of the PML gene and exon 3 of the RARA gene (intron 3 breakpoint analysis). Based on the presence of t(15;17) and the morphology of the cells seen in the patient's bone marrow, the patient was treated with ATRA and idarubicin. The patient is being followed for residual disease with molecular testing for the t(15;17).
Molecular testing is often used to assess a patient's remission status. The International Working Group defines the therapeutic endpoint of APML as a complete molecular remission when using RT-PCR with a sensitivity threshold of one cell in 103 or 104 . Not all patients, however, who remain PCR+ will suffer a relapse . Quantitative RT-PCR can also be employed to monitor patients following consolidation therapy to detect fluctuating levels of minimal residual disease so that proper treatment can be established .
Contributed by Deborah Marks, MD and Jeffrey Kant, MD, PhD