Immunophenotypic Profile Predictive of KIT Activating Mutations in AML1-ETO
Immunophenotypic Profile Predictive of KIT Activating Mutations in AML1-ETO
Translocation (8;21)/AML1-ETO is considered a favorable cytogenetic abnormality in acute myeloid leukemia (AML). However, associated KIT activating mutations confer poor outcome. The immunophenotype associated with KIT mutations in AML1-ETO has not previously been elucidated. We retrospectively reviewed the immunophenotype by flow cytometry of 56 cases of AML with t(8;21) and compared them with 100 cases of AML without t(8;21). In 21 t(8;21) cases, we sought KIT mutations by direct sequencing. Although CD19 and CD56 were aberrantly expressed in 42 (75%) of 56 and 46 (82%) of 56 cases, respectively, with t(8;21), these markers were only expressed in 4% and 25%, respectively, without t(8;21) (P < .001). However, the 5 KIT-mutated cases (D816H, 3; D816Y, 1; and N822K, 1) of t(8;21) AML had diminished CD19 expression (P = .04) with definite CD56 expression (P = .30) on myeloid blasts. Our study suggests that KIT activating mutations in AML with t(8;21) are associated with diminished CD19 and positive CD56 expression on leukemic blasts and, thus, can be phenotypically distinguished from AML1-ETO leukemias without KIT mutations.
Translocation (8;21)(q22;q22) results in the production of a detectable AML1-ETO fusion transcript by juxtaposition of the 5' portion of the acute myeloid leukemia (AML) gene on chromosome 21 to the 3' portion of the ETO gene on chromosome 8. AML with t(8;21) accounts for 8% of adult AML and 12% of childhood AML. The vast majority of cases occur de novo. AML1-ETO translocation confers a favorable prognosis with a complete remission rate of 96% to 98% and a 5-year overall survival approaching 70% and failure-free survival of more than 35 months. Despite this favorable outcome, a subset of patients has a precipitous course owing to high incidence of relapse. This subset is associated with the presence of an activating mutation in the KIT receptor tyrosine kinase domain.
c-KIT is a proto-oncogene that encodes a type III transmembrane tyrosine kinase, which is a receptor for the cytokine stem cell factor, also known as mast cell growth factor. Interaction of stem cell factor with KIT promotes dimerization and autophosphorylation of the receptor at specific tyrosine residues and transmits a series of biochemical signals that elicit a variety of cellular responses essential for the development of bone marrow stem cells. Expression of KIT in AML confers proliferative advantage, whereas KIT mutations confer resistance to apoptosis induced by chemotherapeutic agents.
KIT mutations are commonly found in core binding factor leukemias, seen in 12.7% to 48.1% of AML1-ETO leukemia. Mutations at the Asp816 and Arg822 in the tyrosine kinase domain (TKD) are the 2 most common KIT point mutations in t(8;21). The D816V mutation causing a substitution of valine for aspartic acid was the first c-KIT gene mutation to be identified in a mast cell leukemia cell line. The KIT D816 mutation is seen in 10.5% of t(8;21) AML and is associated with poor outcome (median overall survival of 304 days in KIT D816+ AML vs 1,836 days in KIT D816– AML). Cases of t(8;21) with the TKD D816 mutation are associated with a significantly higher incidence of relapse (80%-90%) than cases with unmutated KIT TKD (29%-35%).
Aberrant expression of lymphocyte antigens on blasts with obvious myeloid differentiation is a well-recognized phenomenon. In fact, CD19 expression is highly specific for AML, with t(8;21) occurring in 63% to 80% of adult and pediatric AML cases with t(8;21) and only rarely in the absence of AML1-ETO translocation. The positive predictive value of t(8;21) with the pattern of CD34+/CD19+/CD56+ was 100% in 1 series. In association with trisomy 4, lower expression of CD19 and higher expression of CD33 and CD56 have been reported. Specific immunophenotypic patterns have not been reported in association with activating mutations in the KIT TKD.
The aim of this study was to characterize the immunophenotype of AML1-ETO leukemia and concurrent cytogenetics of myeloid blasts with and without KIT activating mutations. A retrospective chart review of immunophenotype and cytogenetics was performed in 56 cases of AML with t(8;21) and compared with 100 consecutive cases of AML without t(8;21). KIT TKD mutation analysis was performed on 21 AML t(8:21) cases for which frozen cytopellets were available, by direct sequencing of the 212-base-pair (bp) polymerase chain reaction (PCR)-amplified region of exon 17 that encompasses nucleotide substitutions resulting in D816 and N822 mutations.
Abstract and Introduction
Abstract
Translocation (8;21)/AML1-ETO is considered a favorable cytogenetic abnormality in acute myeloid leukemia (AML). However, associated KIT activating mutations confer poor outcome. The immunophenotype associated with KIT mutations in AML1-ETO has not previously been elucidated. We retrospectively reviewed the immunophenotype by flow cytometry of 56 cases of AML with t(8;21) and compared them with 100 cases of AML without t(8;21). In 21 t(8;21) cases, we sought KIT mutations by direct sequencing. Although CD19 and CD56 were aberrantly expressed in 42 (75%) of 56 and 46 (82%) of 56 cases, respectively, with t(8;21), these markers were only expressed in 4% and 25%, respectively, without t(8;21) (P < .001). However, the 5 KIT-mutated cases (D816H, 3; D816Y, 1; and N822K, 1) of t(8;21) AML had diminished CD19 expression (P = .04) with definite CD56 expression (P = .30) on myeloid blasts. Our study suggests that KIT activating mutations in AML with t(8;21) are associated with diminished CD19 and positive CD56 expression on leukemic blasts and, thus, can be phenotypically distinguished from AML1-ETO leukemias without KIT mutations.
Introduction
Translocation (8;21)(q22;q22) results in the production of a detectable AML1-ETO fusion transcript by juxtaposition of the 5' portion of the acute myeloid leukemia (AML) gene on chromosome 21 to the 3' portion of the ETO gene on chromosome 8. AML with t(8;21) accounts for 8% of adult AML and 12% of childhood AML. The vast majority of cases occur de novo. AML1-ETO translocation confers a favorable prognosis with a complete remission rate of 96% to 98% and a 5-year overall survival approaching 70% and failure-free survival of more than 35 months. Despite this favorable outcome, a subset of patients has a precipitous course owing to high incidence of relapse. This subset is associated with the presence of an activating mutation in the KIT receptor tyrosine kinase domain.
c-KIT is a proto-oncogene that encodes a type III transmembrane tyrosine kinase, which is a receptor for the cytokine stem cell factor, also known as mast cell growth factor. Interaction of stem cell factor with KIT promotes dimerization and autophosphorylation of the receptor at specific tyrosine residues and transmits a series of biochemical signals that elicit a variety of cellular responses essential for the development of bone marrow stem cells. Expression of KIT in AML confers proliferative advantage, whereas KIT mutations confer resistance to apoptosis induced by chemotherapeutic agents.
KIT mutations are commonly found in core binding factor leukemias, seen in 12.7% to 48.1% of AML1-ETO leukemia. Mutations at the Asp816 and Arg822 in the tyrosine kinase domain (TKD) are the 2 most common KIT point mutations in t(8;21). The D816V mutation causing a substitution of valine for aspartic acid was the first c-KIT gene mutation to be identified in a mast cell leukemia cell line. The KIT D816 mutation is seen in 10.5% of t(8;21) AML and is associated with poor outcome (median overall survival of 304 days in KIT D816+ AML vs 1,836 days in KIT D816– AML). Cases of t(8;21) with the TKD D816 mutation are associated with a significantly higher incidence of relapse (80%-90%) than cases with unmutated KIT TKD (29%-35%).
Aberrant expression of lymphocyte antigens on blasts with obvious myeloid differentiation is a well-recognized phenomenon. In fact, CD19 expression is highly specific for AML, with t(8;21) occurring in 63% to 80% of adult and pediatric AML cases with t(8;21) and only rarely in the absence of AML1-ETO translocation. The positive predictive value of t(8;21) with the pattern of CD34+/CD19+/CD56+ was 100% in 1 series. In association with trisomy 4, lower expression of CD19 and higher expression of CD33 and CD56 have been reported. Specific immunophenotypic patterns have not been reported in association with activating mutations in the KIT TKD.
The aim of this study was to characterize the immunophenotype of AML1-ETO leukemia and concurrent cytogenetics of myeloid blasts with and without KIT activating mutations. A retrospective chart review of immunophenotype and cytogenetics was performed in 56 cases of AML with t(8;21) and compared with 100 consecutive cases of AML without t(8;21). KIT TKD mutation analysis was performed on 21 AML t(8:21) cases for which frozen cytopellets were available, by direct sequencing of the 212-base-pair (bp) polymerase chain reaction (PCR)-amplified region of exon 17 that encompasses nucleotide substitutions resulting in D816 and N822 mutations.