Disease Progression in Myeloproliferative Neoplasms
Disease Progression in Myeloproliferative Neoplasms
CML is an MPN originating in a hematopoietic stem cell carrying the BCR-ABL1 fusion gene. BCR-ABL1 is a result of the t(9;22)(q34;q11.2) in which ABL1 and BCR genes on chromosomes 9 and 22, respectively, are split and form a fusion gene residing on the minute derivative chromosome 22, also known as the Philadelphia chromosome (Ph). The BCR-ABL1 fusion leads to a constitutively activated tyrosine kinase and cytokine-independent myeloid proliferation. The annual incidence of CML is one to two cases per 100,000 persons per year. The median age at diagnosis is 50 to 60 years; however, pediatric and adolescent patients have been reported. The natural history of the disease prior to the availability of tyrosine kinase inhibitors (TKIs) was bi- or triphasic with a chronic phase (CP) followed by an accelerated phase (AP) or blast phase (BP). Survival rates and prevention of progression to the advanced phase have dramatically improved in the era of TKIs. This therapy is not curative and requires long-term maintenance to suppress the Ph-positive clone since a CML stem cell persists despite targeted treatment. With the advent of TKIs, additional mechanisms of disease progression have emerged. These mechanisms include tyrosine kinase domain (TKD) mutations and the emergence of BCR-ABL1–negative clones. The discussion of the former is beyond the scope of this report. The latter and other typical and unusual presentations of CML progression have been illustrated by the workshop cases and are discussed in the following paragraphs.
Approximately 20% to 50% of patients with CML are asymptomatic at the time of initial diagnosis, which is often made on routine physical examination with a CBC demonstrating an elevated WBC count. Approximately 90% of patients present in CP and, when symptomatic, show fatigue, malaise, weight loss, night sweats, and abdominal pain related to splenomegaly. Less common findings include bleeding or thrombosis related to abnormal platelet counts and/or function, gouty arthritis due to elevated levels of uric acid, gastric ulcers due to increased histamine levels, and leukostasis due to high WBC, leading to dyspnea, confusion, and priapism. In most untreated patients with CML, the CP is typically prolonged, is slowly progressive, and gradually evolves into the AP. Approximately 20% of patients in CP will progress directly into BP without a transition through AP. Both AP and BP are characterized by more prominent symptoms, and the latter may include extramedullary blast proliferation (myeloid sarcoma) most commonly in lymph nodes, skin, and soft tissues.
Patients with CP-CML typically have leukocytosis (12–500 × 10/L; median, 100 × 10/L) with neutrophilia and a left shift with overrepresentation of myelocytes ("myelocyte bulge") and segmented neutrophils. Rare blasts can be seen. Dysgranulopoiesis is absent. Absolute basophilia is a constant feature. Absolute eosinophilia and monocytosis also may be present. Monocytes typically constitute less than 3% of the peripheral blood (PB) differential count, and p190 fusion protein (minor breakpoint region of BCR gene, exons 1–2) is associated with significant monocytosis resembling that seen in chronic myelomonocytic leukemia. Mild to moderate normocytic normochromic anemia is typically present, and poikilocytosis, including teardrop forms and/or nucleated RBCs, is associated with bone marrow (BM) fibrosis. The platelet count can be normal or elevated. Thrombocytosis can be observed in patients with CML associated with p230 fusion protein, when the breakpoint occurs in the u-BCR region.
BM is hypercellular due to markedly increased granulopoiesis with a prominent left shift. The left shift is easily appreciated in the BM biopsy samples due to increased immature precursors along bony trabeculae. Eosinophilia may be present. Blasts constitute less than 5% of a BM differential count in CP; however, the exact cutoff signifying progression to AP varies in different classification schemes. Erythropoiesis is decreased. The characteristic "dwarf" megakaryocytes are small with hypolobated nuclei. The number of megakaryocytes ranges from decreased to markedly increased. In a proportion of patients, aggregates and sheets of megakaryocytes are seen that are often accompanied by reticulin fibrosis and may herald disease progression. Pseudo-Gaucher cells and sea-blue histiocytes may be present.
Immunophenotypic abnormalities reported in CP-CML include decreased expression of CD16 and CD32 on mature neutrophils and aberrant expression of CD56 by the myeloid series. Myeloid blasts can show decreased expression of CD62L and aberrant expression of both CD56 and CD7. The major diagnostic utility of flow cytometric or immunohistochemistry-based immunophenotyping is limited to AP or BP, when the determination of lineage of the blast population is necessary. In a limited number of cases with prominent reticulin fibrosis and hemodilute aspirate smears, a CD34 immunohistochemical stain can assist in quantifying blasts in sections of BM biopsy and aspirate clot specimens.
The defining t(9;22)(q34;q11.2) can be seen in BM and PB cultures of more than 90% of patients with CP-CML. A small subset of patients shows variant translocations involving additional chromosome(s), such as 1p36.1, 3p21, 11q13, 12p13, and 17q25. Finally, in rare cases, patients with CML have cryptic abnormalities that cannot be appreciated by conventional cytogenetics. In these cases, BCR-ABL1 genes can only be identified by fluorescence in situ hybridization (FISH) or polymerase chain reaction (PCR). The most common additional chromosomal abnormalities observed at the time of diagnosis of CP-CML are loss of chromosome Y, gain of chromosome 8, a second Ph chromosome, and isochromosome 17q. Genomic microarray studies have provided additional insights into the spectrum of abnormalities in CP-CML. Single-nucleotide polymorphism array analysis showed clonal alterations in approximately 20% of patients. Most genomic losses are observed at or around the BCR and ABL1 genes. Interestingly, select gene polymorphisms, such as the T allele of BIM exon 5, were associated with delayed major molecular response to imatinib therapy, potentially leading to more frequent TKD mutations and TKI resistance.
In summary, diagnosis of CP-CML is typically straightforward, with most patients showing characteristic morphologic and laboratory features. The potential caveats include rare atypical presentations resembling other myeloproliferative or myelodysplastic/myeloproliferative neoplasms such as (1) CNL, when leukocytosis includes predominantly segmented neutrophils with no or minimal left shift; (2) ET, in cases with marked thrombocytosis and minimal leukocytosis; and (3) chronic myelomonocytic leukemia, in CML cases with significant monocytosis. Less often, a diagnosis of CML is made when a patient initially presents in the AP or BP.
Staging of CML at the time of initial diagnosis and classification of disease progression have been topics of ongoing discussions. Most published classification schemes rely on a combination of pathologic, laboratory, and clinical parameters. However, the cutoff criteria for advanced stage disease vary, and clinical validation of various staging systems is complex due to the ongoing changes in therapeutic regimens that have become available in the past 15 years. The 2008 World Health Organization (WHO) classification, most commonly used by pathologists, adopted staging criteria based on a literature review and the consensus of the Clinical Advisory Committee Table 1. Other staging systems have been proposed and clinically validated and continue to be used in TKI clinical trials. The most common are the MD Anderson and European LeukemiaNet staging systems.
Disease progression in CML can also be defined by cytogenetic or molecular events such as additional cytogenetic abnormalities in a BCR-ABL1–positive clone, TKD mutations, BCR-ABL1 amplification, and emergence of BCR-ABL1–negative clones. Many of these parameters are not currently included in a formal staging system. In the following paragraphs, we discuss manifestations of CML progression using WHO recommended criteria for CML staging. The CML cases submitted to the workshop are summarized in Table 2.
According to the WHO classification, the objective morphologic criteria of AP include the presence of 10% to 19% blasts in PB or BM and basophilia of more than 20% (Table 1). However, there is a concern that these parameters also may indicate more progressive disease. Other features such as megakaryocytic proliferation and fibrosis are more subjective and may require comparison with an original diagnostic BM specimen since approximately 30% of patients with CML have marked reticulin fibrosis at the time of original diagnosis. The morphologic features of AP frequently coincide with other staging criteria, such as progressive therapy-resistant leukocytosis, cytopenias, and/or splenomegaly. Case 140 illustrated the common coexistence of multiple features of AP and an interesting evolution of disease on imatinib therapy. This 82-year-old man originally presented with AP-CML with a WBC count of 200 × 10/L, 11% blasts in BM, and significant BM fibrosis. After 5 months of low-dose imatinib, the patient developed a prominent BM and PB basophilia of more than 20% Image 1. The BM blast count at that time decreased to 7%, which is below the threshold of 10% defining AP-CML. Flow cytometry immunophenotyping confirmed the presence of CD34- and/or CD117-positive precursors and a substantial population of atypical basophils positive for CD117; low-density CD45 and low side scatter; decreased-density CD123, CD13, and CD33; and negative for HLA-DR (Image 1). Of note, normal basophils are positive for dim CD45, CD123, CD9, CD22, bright CD38, dim CD33, dim CD25, and CD36 and negative for HLA-DR, CD34, CD117, CD64, and CD19. Abnormal basophils frequently show decreased-density CD38 and remain positive for CD34 or CD117, as seen in this case. The recognition of basophils is also important due to an overlap with a blast gate and a potential to overestimate blast percentage. In summary, this case illustrates that the features of AP-CML may change over time and that several characteristics of disease progression may be present simultaneously in a single case.
(Enlarge Image)
Image 1.
Accelerated phase chronic myelogenous leukemia defined by more than 10% blasts in bone marrow (BM) develops marked basophilia during therapy with imatinib. A, Initial BM showed 11% blasts and 4.5% basophils. B, Flow cytometry immunophenotyping demonstrated a prominent population of blasts (in red) and immunophenotypically aberrant basophils (in green). C, Subsequent BM showed mild decrease in the number of blasts and a significant increase in basophils (30%) and eosinophils (21%). D, Similar features were seen by flow cytometry. (Case 140, courtesy of B. Sander, MD, PhD.)
Similarly, case 201 showed several features of AP-CML in BM and PB. The patient had progressive fatigue, weight loss, low back pain, and "saddle" anesthesia. The CBC showed leukocytosis with 16% blasts and 25% basophils, anemia, and thrombocytopenia. The computed tomography scan showed a prominent soft tissue mass invading the right acetabulum and iliac muscle, diffuse "osseus metastases" with an epidural mass, and splenomegaly. BM was inaspirable. The BM biopsy specimen showed hypocellular BM with osteomyelosclerosis and collagen deposition (MF-3) Image 2. No increase in blasts was seen on CD34 immunostain. The karyotype and FISH confirmed the presence of t(9;22)(q34;q11.2)/BCR-ABL1. Based on BM and PB parameters alone, this case would be classified as AP-CML. However, a biopsy specimen obtained from the epidural tumor showed sheets of intermediate-size cells with vesicular chromatin, prominent nucleoli, and scant cytoplasm positive for CD34, CD33, lysozyme, CD43, and CD45 diagnostic of myeloid sarcoma. Taken together, this case is best classified as BP-CML. CML presents in BP in less than 5% of cases. Extramedullary BP is even less common in patients with CML and more frequently occurs in acute myeloid leukemia (AML) at the time of initial diagnosis or relapse. Central nervous system (CNS) relapse of CML and AML in a setting of hematopoietic stem cell transplantation (HST) has been reported. Case 327 was a 34-year-old woman diagnosed with CP-CML, treated with nilotinib, who developed lymphoblastic BP with leptomeningeal involvement. She achieved a complete remission upon induction chemotherapy, including dasatinib, and underwent allogeneic matched unrelated HST. Approximately 1 year later, while on maintenance dasatinib therapy, the patient developed right-side facial numbness and diplopia and was shown to have a left temporal enhancing mass with evidence of relapse in cerebrospinal fluid Image 3. Concurrent BM examination showed trilineage hematopoiesis, a normal male (donor) karyotype, and no evidence of BCR-ABL1. Dasatinib has been reported previously to show efficacy in preventing recurrent CNS disease due to its high blood-brain barrier penetrance. Cases of isolated CNS relapse in patients taking dasatinib have been related to the emergence of dasatinib-resistant BCR-ABL1 mutated clones, yet another mechanism of disease progression in CML treated by TKIs.
(Enlarge Image)
Image 2.
Accelerated phase chronic myelogenous leukemia as demonstrated by peripheral blood and bone marrow (BM) features with concurrent myeloid sarcoma presenting as a soft tissue mass. A, BM with significant fibrosis and decreased cellularity. B, Reticulin stain shows prominent fibrosis. C, Sheets of blasts seen on the biopsy specimen of soft tissue mass. (Case 201, courtesy of S. L. Ondrejka, DO, and colleagues.)
(Enlarge Image)
Image 3.
Isolated central nervous system relapse of lymphoblastic blast phase chronic myelogenous leukemia in a patient after a hematopoietic stem cell transplant. A, Magnetic resonance imaging scan showed a left temporal lobe intra-axial enhancing mass. B, Numerous blasts were seen on a cytospin of cerebrospinal fluid. (Case 327, courtesy of X. Zhang, MD, PhD, and colleagues.)
The emergence of additional cytogenetic abnormalities in a BCR-ABL1–positive clone is not uncommon upon CML progression and occurs in approximately 60% of patients. Chromosome gains or losses can occur in advance of morphologic evidence of progression. The most common unbalanced abnormalities are +8, additional Ph chromosome, i(17q), +19, +21, −Y, and −7. Less commonly, recurrent cytogenetic abnormalities typically seen in de novo AML, such as t(8;21), inv(16), t(15;17), and inv(3q)/t(3;3), have been reported. Case 127 is an example of BP with t(15;17)(q24.1;q21.2)/PML-RARA that has been reported elsewhere. The patient was initially diagnosed with CP-CML and achieved complete cytogenetic remission on imatinib. Imatinib was discontinued for unknown reasons, and 1 month later, the patient developed coagulopathy and leukocytosis with numerous promyelocytes with morphologic features similar to that of a microgranular variant of acute promyelocytic leukemia (APL) Image 4. Flow cytometry showed an immunophenotype seen frequently in the microgranular variant of APL with blasts/promyelocytes positive for CD2, CD13, CD15, CD33, CD34 (dim, subset), CD38, CD56, CD64, CD117, and myeloperoxidase. A complex karyotype with t(15;17) (q24.1;q21.2) and t(9;22)(q34;q11.2) was seen in 20 metaphases. The patient achieved complete remission on all-trans retinoic acid and arsenic therapy with FISH negative for BCR/ABL1 and PML/RARA. Reverse transcriptase–PCR showed low-level BCR/ABL1 transcripts and was negative for PML/RARA. The patient died 1 month later of multiorgan system failure, an outcome similar to other reported BP-CML cases with t(15;17). Another 43-year-old man (case 8) with BP-CML with a recurrent cytogenetic abnormality, inv(16) (p13.2q22)/CBFB-MYH11, was presented. One year after the diagnosis of CP-CML the patient developed BP with inv(16) (p13.2q22) and t(9;22)(q34;q11.2). Despite a typically favorable prognosis in de novo AML with t(15;17), inv(16), or t(8;21), the prognosis of patients with BP-CML with these recurrent abnormalities is usually poor.
(Enlarge Image)
Image 4.
Blast phase chronic myelogenous leukemia with t(15;17)(q24.1; q21.2);PML-RARA. A, Hypercellular bone marrow (BM) with sheets of blasts/promyelocytes. B, BM aspirate shows features of the microgranular variant of acute promyelocytic leukemia. C, Conventional cytogenetic analysis showing concurrent t(15;17)(q24.1;q21.2) and t(9;22)(q34;q11.2), among other abnormalities {46,XX,der(3)t(3;15)(q21;q15),t(15;17)(q24.1;q21.2),t(9;22)(q34;q11.2),der(15)t(3;15),del(17)(q21)[20]}. (Case 127, courtesy of C. C. Yin, MD, PhD, and colleagues.)
Although most cases of BP-CML have a myeloid immunophenotype, approximately 30% of patients with CML can develop B-lymphoblastic BP. Rare cases of the T-lymphoblastic immunophenotype also have been reported. Case 302 was a 22-year-old woman with CML who developed BP with an unusual B/myeloid mixed phenotype. Flow cytometry immunophenotypic analysis showed that the blasts were positive for B-cell (CD19, CD22) and myeloid antigens (CD13, CD33, CD117), along with CD34, TdT, CD10, and HLA-DR. Myeloperoxidase was detected in a subset of blasts by immunohistochemistry.
Progression of CML typically occurs in a clone carrying BCR-ABL1. However, additional chromosomal abnormalities, including recurrent translocations, can also occur in Ph-negative (Ph-neg) clones. The most common additional chromosomal abnormalities are trisomy 8 and loss of chromosomes 7 and Y. These abnormalities can be transient or related to a development of Ph-neg myelodysplastic syndrome (MDS) or AML and have been reported in the setting of interferon and TKI therapy. It has been hypothesized that targeting ABL1, which is involved in DNA repair, leads to genetic instability and additional chromosomal abnormalities in Ph-neg stem cells. Several patients with CML illustrating this scenario were presented in the workshop.
A Ph-neg clone developed in a 55-year-old man with CP-CML (case 368). The patient received intermittent imatinib therapy and was noted to have anemia and thrombocytopenia along with 4% blasts in the PB smear. The BM was hypercellular with dyserythropoiesis, dysmegakaryopoiesis, and 11% blasts Image 5. Conventional karyotyping showed a Ph-neg clone with inv(Y) and monosomy 7. The patient received azacitidine with no response and subsequent induction chemotherapy, upon which he returned to CP-CML. The Ph-positive clone was accompanied by a minor clone with chromosomal abnormalities seen in the myelodysplastic phase. Case 14 was a 56-year-old man with CP-CML who achieved complete molecular remission on second-line treatment with dasatinib. One year later, the patient developed pancytopenia; at this time, BM was hypercellular with dysplastic myeloid elements and 23% blasts Image 6A and Image 6B. Conventional cytogenetic analysis demonstrated Ph-neg clones with trisomy 8 and 10. A similar progression was observed in the patient presented in case 147. This 82-year-old woman with CP-CML was treated with modified doses of imatinib and subsequently dasatinib over a period of 5 years. The patient showed progressive cytopenias, and eventually blasts were seen in the PB smear. The BM showed variable cellularity with 32% blasts Image 6C and Image 6D, and no significant dyserythropoiesis or dysgranulopoiesis was seen. Cytogenetic analysis showed two leukemic clones: monosomy 7 and del(6) in 19 metaphases and one metaphase with the Ph chromosome and no other abnormalities. The above cases underscore the importance of BM examination and continued cytogenetic monitoring of patients with CML on TKI therapy.
(Enlarge Image)
Image 5.
Patient developed myelodysplastic syndrome in a Philadelphia chromosome–negative clone after therapy with imatinib. A, Bone marrow (BM) was hypercellular with increase in blasts, dyserythropoiesis, and dysmegakaryopoiesis. B, Dysplastic megakaryocytes with separated nuclear lobes. C, Dyserythropoiesis. D, BM aspirate smear showed 11% blasts. (Case 368, courtesy of P. Kovarik, MD.)
(Enlarge Image)
Image 6.
BCR-ABL1–negative acute leukemias arising in patients with chronic myelogenous leukemia. A, Frequent blasts in bone marrow (BM) aspirate smear. B, Cytogenetic analysis showed trisomy 8 and 10 and no Philadelphia (Ph) chromosome. (A and B, Case 14, courtesy of J. Zhou, MD, PhD, and colleagues.) C, Numerous blasts seen in BM aspirate smear. D, Cytogenetic analysis showed two clones, one of which included the Ph chromosome. (C and D, Case 147, courtesy of T. Polsky, MD, PhD, and colleagues.)
Disease Progression in CML
CML is an MPN originating in a hematopoietic stem cell carrying the BCR-ABL1 fusion gene. BCR-ABL1 is a result of the t(9;22)(q34;q11.2) in which ABL1 and BCR genes on chromosomes 9 and 22, respectively, are split and form a fusion gene residing on the minute derivative chromosome 22, also known as the Philadelphia chromosome (Ph). The BCR-ABL1 fusion leads to a constitutively activated tyrosine kinase and cytokine-independent myeloid proliferation. The annual incidence of CML is one to two cases per 100,000 persons per year. The median age at diagnosis is 50 to 60 years; however, pediatric and adolescent patients have been reported. The natural history of the disease prior to the availability of tyrosine kinase inhibitors (TKIs) was bi- or triphasic with a chronic phase (CP) followed by an accelerated phase (AP) or blast phase (BP). Survival rates and prevention of progression to the advanced phase have dramatically improved in the era of TKIs. This therapy is not curative and requires long-term maintenance to suppress the Ph-positive clone since a CML stem cell persists despite targeted treatment. With the advent of TKIs, additional mechanisms of disease progression have emerged. These mechanisms include tyrosine kinase domain (TKD) mutations and the emergence of BCR-ABL1–negative clones. The discussion of the former is beyond the scope of this report. The latter and other typical and unusual presentations of CML progression have been illustrated by the workshop cases and are discussed in the following paragraphs.
Clinical Features
Approximately 20% to 50% of patients with CML are asymptomatic at the time of initial diagnosis, which is often made on routine physical examination with a CBC demonstrating an elevated WBC count. Approximately 90% of patients present in CP and, when symptomatic, show fatigue, malaise, weight loss, night sweats, and abdominal pain related to splenomegaly. Less common findings include bleeding or thrombosis related to abnormal platelet counts and/or function, gouty arthritis due to elevated levels of uric acid, gastric ulcers due to increased histamine levels, and leukostasis due to high WBC, leading to dyspnea, confusion, and priapism. In most untreated patients with CML, the CP is typically prolonged, is slowly progressive, and gradually evolves into the AP. Approximately 20% of patients in CP will progress directly into BP without a transition through AP. Both AP and BP are characterized by more prominent symptoms, and the latter may include extramedullary blast proliferation (myeloid sarcoma) most commonly in lymph nodes, skin, and soft tissues.
Morphologic, Immunophenotypic, and Genetic Features of CP
Patients with CP-CML typically have leukocytosis (12–500 × 10/L; median, 100 × 10/L) with neutrophilia and a left shift with overrepresentation of myelocytes ("myelocyte bulge") and segmented neutrophils. Rare blasts can be seen. Dysgranulopoiesis is absent. Absolute basophilia is a constant feature. Absolute eosinophilia and monocytosis also may be present. Monocytes typically constitute less than 3% of the peripheral blood (PB) differential count, and p190 fusion protein (minor breakpoint region of BCR gene, exons 1–2) is associated with significant monocytosis resembling that seen in chronic myelomonocytic leukemia. Mild to moderate normocytic normochromic anemia is typically present, and poikilocytosis, including teardrop forms and/or nucleated RBCs, is associated with bone marrow (BM) fibrosis. The platelet count can be normal or elevated. Thrombocytosis can be observed in patients with CML associated with p230 fusion protein, when the breakpoint occurs in the u-BCR region.
BM is hypercellular due to markedly increased granulopoiesis with a prominent left shift. The left shift is easily appreciated in the BM biopsy samples due to increased immature precursors along bony trabeculae. Eosinophilia may be present. Blasts constitute less than 5% of a BM differential count in CP; however, the exact cutoff signifying progression to AP varies in different classification schemes. Erythropoiesis is decreased. The characteristic "dwarf" megakaryocytes are small with hypolobated nuclei. The number of megakaryocytes ranges from decreased to markedly increased. In a proportion of patients, aggregates and sheets of megakaryocytes are seen that are often accompanied by reticulin fibrosis and may herald disease progression. Pseudo-Gaucher cells and sea-blue histiocytes may be present.
Immunophenotypic abnormalities reported in CP-CML include decreased expression of CD16 and CD32 on mature neutrophils and aberrant expression of CD56 by the myeloid series. Myeloid blasts can show decreased expression of CD62L and aberrant expression of both CD56 and CD7. The major diagnostic utility of flow cytometric or immunohistochemistry-based immunophenotyping is limited to AP or BP, when the determination of lineage of the blast population is necessary. In a limited number of cases with prominent reticulin fibrosis and hemodilute aspirate smears, a CD34 immunohistochemical stain can assist in quantifying blasts in sections of BM biopsy and aspirate clot specimens.
The defining t(9;22)(q34;q11.2) can be seen in BM and PB cultures of more than 90% of patients with CP-CML. A small subset of patients shows variant translocations involving additional chromosome(s), such as 1p36.1, 3p21, 11q13, 12p13, and 17q25. Finally, in rare cases, patients with CML have cryptic abnormalities that cannot be appreciated by conventional cytogenetics. In these cases, BCR-ABL1 genes can only be identified by fluorescence in situ hybridization (FISH) or polymerase chain reaction (PCR). The most common additional chromosomal abnormalities observed at the time of diagnosis of CP-CML are loss of chromosome Y, gain of chromosome 8, a second Ph chromosome, and isochromosome 17q. Genomic microarray studies have provided additional insights into the spectrum of abnormalities in CP-CML. Single-nucleotide polymorphism array analysis showed clonal alterations in approximately 20% of patients. Most genomic losses are observed at or around the BCR and ABL1 genes. Interestingly, select gene polymorphisms, such as the T allele of BIM exon 5, were associated with delayed major molecular response to imatinib therapy, potentially leading to more frequent TKD mutations and TKI resistance.
In summary, diagnosis of CP-CML is typically straightforward, with most patients showing characteristic morphologic and laboratory features. The potential caveats include rare atypical presentations resembling other myeloproliferative or myelodysplastic/myeloproliferative neoplasms such as (1) CNL, when leukocytosis includes predominantly segmented neutrophils with no or minimal left shift; (2) ET, in cases with marked thrombocytosis and minimal leukocytosis; and (3) chronic myelomonocytic leukemia, in CML cases with significant monocytosis. Less often, a diagnosis of CML is made when a patient initially presents in the AP or BP.
How do we Define Disease Progression in CML?
Staging of CML at the time of initial diagnosis and classification of disease progression have been topics of ongoing discussions. Most published classification schemes rely on a combination of pathologic, laboratory, and clinical parameters. However, the cutoff criteria for advanced stage disease vary, and clinical validation of various staging systems is complex due to the ongoing changes in therapeutic regimens that have become available in the past 15 years. The 2008 World Health Organization (WHO) classification, most commonly used by pathologists, adopted staging criteria based on a literature review and the consensus of the Clinical Advisory Committee Table 1. Other staging systems have been proposed and clinically validated and continue to be used in TKI clinical trials. The most common are the MD Anderson and European LeukemiaNet staging systems.
Disease progression in CML can also be defined by cytogenetic or molecular events such as additional cytogenetic abnormalities in a BCR-ABL1–positive clone, TKD mutations, BCR-ABL1 amplification, and emergence of BCR-ABL1–negative clones. Many of these parameters are not currently included in a formal staging system. In the following paragraphs, we discuss manifestations of CML progression using WHO recommended criteria for CML staging. The CML cases submitted to the workshop are summarized in Table 2.
What are the Manifestations of Disease Progression in Patients With CML? How are They Modified by TKI Therapy?
According to the WHO classification, the objective morphologic criteria of AP include the presence of 10% to 19% blasts in PB or BM and basophilia of more than 20% (Table 1). However, there is a concern that these parameters also may indicate more progressive disease. Other features such as megakaryocytic proliferation and fibrosis are more subjective and may require comparison with an original diagnostic BM specimen since approximately 30% of patients with CML have marked reticulin fibrosis at the time of original diagnosis. The morphologic features of AP frequently coincide with other staging criteria, such as progressive therapy-resistant leukocytosis, cytopenias, and/or splenomegaly. Case 140 illustrated the common coexistence of multiple features of AP and an interesting evolution of disease on imatinib therapy. This 82-year-old man originally presented with AP-CML with a WBC count of 200 × 10/L, 11% blasts in BM, and significant BM fibrosis. After 5 months of low-dose imatinib, the patient developed a prominent BM and PB basophilia of more than 20% Image 1. The BM blast count at that time decreased to 7%, which is below the threshold of 10% defining AP-CML. Flow cytometry immunophenotyping confirmed the presence of CD34- and/or CD117-positive precursors and a substantial population of atypical basophils positive for CD117; low-density CD45 and low side scatter; decreased-density CD123, CD13, and CD33; and negative for HLA-DR (Image 1). Of note, normal basophils are positive for dim CD45, CD123, CD9, CD22, bright CD38, dim CD33, dim CD25, and CD36 and negative for HLA-DR, CD34, CD117, CD64, and CD19. Abnormal basophils frequently show decreased-density CD38 and remain positive for CD34 or CD117, as seen in this case. The recognition of basophils is also important due to an overlap with a blast gate and a potential to overestimate blast percentage. In summary, this case illustrates that the features of AP-CML may change over time and that several characteristics of disease progression may be present simultaneously in a single case.
(Enlarge Image)
Image 1.
Accelerated phase chronic myelogenous leukemia defined by more than 10% blasts in bone marrow (BM) develops marked basophilia during therapy with imatinib. A, Initial BM showed 11% blasts and 4.5% basophils. B, Flow cytometry immunophenotyping demonstrated a prominent population of blasts (in red) and immunophenotypically aberrant basophils (in green). C, Subsequent BM showed mild decrease in the number of blasts and a significant increase in basophils (30%) and eosinophils (21%). D, Similar features were seen by flow cytometry. (Case 140, courtesy of B. Sander, MD, PhD.)
Similarly, case 201 showed several features of AP-CML in BM and PB. The patient had progressive fatigue, weight loss, low back pain, and "saddle" anesthesia. The CBC showed leukocytosis with 16% blasts and 25% basophils, anemia, and thrombocytopenia. The computed tomography scan showed a prominent soft tissue mass invading the right acetabulum and iliac muscle, diffuse "osseus metastases" with an epidural mass, and splenomegaly. BM was inaspirable. The BM biopsy specimen showed hypocellular BM with osteomyelosclerosis and collagen deposition (MF-3) Image 2. No increase in blasts was seen on CD34 immunostain. The karyotype and FISH confirmed the presence of t(9;22)(q34;q11.2)/BCR-ABL1. Based on BM and PB parameters alone, this case would be classified as AP-CML. However, a biopsy specimen obtained from the epidural tumor showed sheets of intermediate-size cells with vesicular chromatin, prominent nucleoli, and scant cytoplasm positive for CD34, CD33, lysozyme, CD43, and CD45 diagnostic of myeloid sarcoma. Taken together, this case is best classified as BP-CML. CML presents in BP in less than 5% of cases. Extramedullary BP is even less common in patients with CML and more frequently occurs in acute myeloid leukemia (AML) at the time of initial diagnosis or relapse. Central nervous system (CNS) relapse of CML and AML in a setting of hematopoietic stem cell transplantation (HST) has been reported. Case 327 was a 34-year-old woman diagnosed with CP-CML, treated with nilotinib, who developed lymphoblastic BP with leptomeningeal involvement. She achieved a complete remission upon induction chemotherapy, including dasatinib, and underwent allogeneic matched unrelated HST. Approximately 1 year later, while on maintenance dasatinib therapy, the patient developed right-side facial numbness and diplopia and was shown to have a left temporal enhancing mass with evidence of relapse in cerebrospinal fluid Image 3. Concurrent BM examination showed trilineage hematopoiesis, a normal male (donor) karyotype, and no evidence of BCR-ABL1. Dasatinib has been reported previously to show efficacy in preventing recurrent CNS disease due to its high blood-brain barrier penetrance. Cases of isolated CNS relapse in patients taking dasatinib have been related to the emergence of dasatinib-resistant BCR-ABL1 mutated clones, yet another mechanism of disease progression in CML treated by TKIs.
(Enlarge Image)
Image 2.
Accelerated phase chronic myelogenous leukemia as demonstrated by peripheral blood and bone marrow (BM) features with concurrent myeloid sarcoma presenting as a soft tissue mass. A, BM with significant fibrosis and decreased cellularity. B, Reticulin stain shows prominent fibrosis. C, Sheets of blasts seen on the biopsy specimen of soft tissue mass. (Case 201, courtesy of S. L. Ondrejka, DO, and colleagues.)
(Enlarge Image)
Image 3.
Isolated central nervous system relapse of lymphoblastic blast phase chronic myelogenous leukemia in a patient after a hematopoietic stem cell transplant. A, Magnetic resonance imaging scan showed a left temporal lobe intra-axial enhancing mass. B, Numerous blasts were seen on a cytospin of cerebrospinal fluid. (Case 327, courtesy of X. Zhang, MD, PhD, and colleagues.)
The emergence of additional cytogenetic abnormalities in a BCR-ABL1–positive clone is not uncommon upon CML progression and occurs in approximately 60% of patients. Chromosome gains or losses can occur in advance of morphologic evidence of progression. The most common unbalanced abnormalities are +8, additional Ph chromosome, i(17q), +19, +21, −Y, and −7. Less commonly, recurrent cytogenetic abnormalities typically seen in de novo AML, such as t(8;21), inv(16), t(15;17), and inv(3q)/t(3;3), have been reported. Case 127 is an example of BP with t(15;17)(q24.1;q21.2)/PML-RARA that has been reported elsewhere. The patient was initially diagnosed with CP-CML and achieved complete cytogenetic remission on imatinib. Imatinib was discontinued for unknown reasons, and 1 month later, the patient developed coagulopathy and leukocytosis with numerous promyelocytes with morphologic features similar to that of a microgranular variant of acute promyelocytic leukemia (APL) Image 4. Flow cytometry showed an immunophenotype seen frequently in the microgranular variant of APL with blasts/promyelocytes positive for CD2, CD13, CD15, CD33, CD34 (dim, subset), CD38, CD56, CD64, CD117, and myeloperoxidase. A complex karyotype with t(15;17) (q24.1;q21.2) and t(9;22)(q34;q11.2) was seen in 20 metaphases. The patient achieved complete remission on all-trans retinoic acid and arsenic therapy with FISH negative for BCR/ABL1 and PML/RARA. Reverse transcriptase–PCR showed low-level BCR/ABL1 transcripts and was negative for PML/RARA. The patient died 1 month later of multiorgan system failure, an outcome similar to other reported BP-CML cases with t(15;17). Another 43-year-old man (case 8) with BP-CML with a recurrent cytogenetic abnormality, inv(16) (p13.2q22)/CBFB-MYH11, was presented. One year after the diagnosis of CP-CML the patient developed BP with inv(16) (p13.2q22) and t(9;22)(q34;q11.2). Despite a typically favorable prognosis in de novo AML with t(15;17), inv(16), or t(8;21), the prognosis of patients with BP-CML with these recurrent abnormalities is usually poor.
(Enlarge Image)
Image 4.
Blast phase chronic myelogenous leukemia with t(15;17)(q24.1; q21.2);PML-RARA. A, Hypercellular bone marrow (BM) with sheets of blasts/promyelocytes. B, BM aspirate shows features of the microgranular variant of acute promyelocytic leukemia. C, Conventional cytogenetic analysis showing concurrent t(15;17)(q24.1;q21.2) and t(9;22)(q34;q11.2), among other abnormalities {46,XX,der(3)t(3;15)(q21;q15),t(15;17)(q24.1;q21.2),t(9;22)(q34;q11.2),der(15)t(3;15),del(17)(q21)[20]}. (Case 127, courtesy of C. C. Yin, MD, PhD, and colleagues.)
Although most cases of BP-CML have a myeloid immunophenotype, approximately 30% of patients with CML can develop B-lymphoblastic BP. Rare cases of the T-lymphoblastic immunophenotype also have been reported. Case 302 was a 22-year-old woman with CML who developed BP with an unusual B/myeloid mixed phenotype. Flow cytometry immunophenotypic analysis showed that the blasts were positive for B-cell (CD19, CD22) and myeloid antigens (CD13, CD33, CD117), along with CD34, TdT, CD10, and HLA-DR. Myeloperoxidase was detected in a subset of blasts by immunohistochemistry.
BCR-ABL1–Negative Myelodysplastic Syndrome and Acute Leukemia in Patients With CML
Progression of CML typically occurs in a clone carrying BCR-ABL1. However, additional chromosomal abnormalities, including recurrent translocations, can also occur in Ph-negative (Ph-neg) clones. The most common additional chromosomal abnormalities are trisomy 8 and loss of chromosomes 7 and Y. These abnormalities can be transient or related to a development of Ph-neg myelodysplastic syndrome (MDS) or AML and have been reported in the setting of interferon and TKI therapy. It has been hypothesized that targeting ABL1, which is involved in DNA repair, leads to genetic instability and additional chromosomal abnormalities in Ph-neg stem cells. Several patients with CML illustrating this scenario were presented in the workshop.
A Ph-neg clone developed in a 55-year-old man with CP-CML (case 368). The patient received intermittent imatinib therapy and was noted to have anemia and thrombocytopenia along with 4% blasts in the PB smear. The BM was hypercellular with dyserythropoiesis, dysmegakaryopoiesis, and 11% blasts Image 5. Conventional karyotyping showed a Ph-neg clone with inv(Y) and monosomy 7. The patient received azacitidine with no response and subsequent induction chemotherapy, upon which he returned to CP-CML. The Ph-positive clone was accompanied by a minor clone with chromosomal abnormalities seen in the myelodysplastic phase. Case 14 was a 56-year-old man with CP-CML who achieved complete molecular remission on second-line treatment with dasatinib. One year later, the patient developed pancytopenia; at this time, BM was hypercellular with dysplastic myeloid elements and 23% blasts Image 6A and Image 6B. Conventional cytogenetic analysis demonstrated Ph-neg clones with trisomy 8 and 10. A similar progression was observed in the patient presented in case 147. This 82-year-old woman with CP-CML was treated with modified doses of imatinib and subsequently dasatinib over a period of 5 years. The patient showed progressive cytopenias, and eventually blasts were seen in the PB smear. The BM showed variable cellularity with 32% blasts Image 6C and Image 6D, and no significant dyserythropoiesis or dysgranulopoiesis was seen. Cytogenetic analysis showed two leukemic clones: monosomy 7 and del(6) in 19 metaphases and one metaphase with the Ph chromosome and no other abnormalities. The above cases underscore the importance of BM examination and continued cytogenetic monitoring of patients with CML on TKI therapy.
(Enlarge Image)
Image 5.
Patient developed myelodysplastic syndrome in a Philadelphia chromosome–negative clone after therapy with imatinib. A, Bone marrow (BM) was hypercellular with increase in blasts, dyserythropoiesis, and dysmegakaryopoiesis. B, Dysplastic megakaryocytes with separated nuclear lobes. C, Dyserythropoiesis. D, BM aspirate smear showed 11% blasts. (Case 368, courtesy of P. Kovarik, MD.)
(Enlarge Image)
Image 6.
BCR-ABL1–negative acute leukemias arising in patients with chronic myelogenous leukemia. A, Frequent blasts in bone marrow (BM) aspirate smear. B, Cytogenetic analysis showed trisomy 8 and 10 and no Philadelphia (Ph) chromosome. (A and B, Case 14, courtesy of J. Zhou, MD, PhD, and colleagues.) C, Numerous blasts seen in BM aspirate smear. D, Cytogenetic analysis showed two clones, one of which included the Ph chromosome. (C and D, Case 147, courtesy of T. Polsky, MD, PhD, and colleagues.)
