AML1-ETO Needs a Partner: New Insights into the Pathogenesis of t(8;21) Leukemia

The detailed characterization of genetic and molecular aberrations in acute myeloid leukemia (AML) has substantially improved our understanding of the pathogenesis of this disease. With an incidence of up to 12% in all AML cases, the translocation t(8;21), forming the AML1-ETO fusion gene, is one of the most common genetic aberrations in AML. Experimental data have shown that AML1-ETO is not sufficient to induce leukemia by itself, but has to collaborate with other genetic alterations for leukemic transformation. These data are supported by observations in AML patients, who recurrently show activating mutations of the receptor tyrosine kinase FLT3 or c-KIT together with the AML1-ETO fusion gene. These findings might have clinical implications and provide a rationale to test RTK inhibitors in the treatment of patients with core binding factor AML and concurrent activating RTK mutations.     

Hematopoietic stem cell expansion and distinct myeloid developmental abnormalities in a murine model of the AML1-ETO translocation

The t(8;21)(q22;q22) translocation, which fuses the ETO gene on human chromosome 8 with the AML1 gene on chromosome 21 (AML1-ETO), is one of the most frequent cytogenetic abnormalities associated with acute myelogenous leukemia (AML). It is seen in approximately 12 to 15% of AML cases and is present in about 40% of AML cases with a French-American-British classified M2 phenotype. We have generated a murine model of the t(8;21) translocation by retroviral expression of AML1-ETO in purified hematopoietic stem cells (HSC). Animals reconstituted with AML1-ETO-expressing cells recapitulate the hematopoietic developmental abnormalities seen in the bone marrow of human patients with the t(8;21) translocation. Primitive myeloblasts were increased to approximately 10% of bone marrow by 10 months posttransplant. Consistent with this observation was a 50-fold increase in myeloid colony-forming cells in vitro. Accumulation of late-stage metamyelocytes was also observed in bone marrow along with an increase in immature eosinophilic myelocytes that showed abnormal basophilic granulation. HSC numbers in the bone marrow of 10-month-posttransplant animals were 29-fold greater than in transplant-matched control mice, suggesting that AML1-ETO expression overrides the normal genetic control of HSC pool size. In summary, AMLI-ETO-expressing animals recapitulate many (and perhaps all) of the developmental abnormalities seen in human patients with the t(8;21) translocation, although the animals do not develop leukemia or disseminated disease in peripheral tissues like the liver or spleen. This suggests that the principal contribution of AML1-ETO to acute myeloid leukemia is the inhibition of multiple developmental pathways.     

Ring chromosome 8 and translocation t(8;21) in a patient with acute myeloblastic leukemia

Recurrent translocation t(8;21)(q22;q22) acute myeloid leukemia (AML) is often associated with secondary chromosome changes of which the clinical significance is not clear since they do not seem to impair the prognosis. Uncommon chromosome changes may lead to the identification of leukemogenetic factors associated with t(8;21) since the AML1/RUNX1-ETO fusion gene resulting from the translocation is thought to be unable alone to induce leukemia. We here report a patient with AML, t(8;21) and ring chromosome 8 resulting in partial chromosome 8 deletion. Another patient with partial 8q deletion has been previously reported. It is suggested that more attention be paid to the genes located in distal 8q in relation to leukemogenesis.     

Linkage mapping of the AML1 gene on human chromosome 21 using a DNA polymorphism in the 3' untranslated region.

We have detected a polymorphism in the 3' untranslated region of the AML1 gene, which is located at the breakpoint on chromosome 21 in the t(8;21)(q22;q22.3) translocation often associated with patients with acute myeloid leukemia. Informative CEPH families were genotyped for this polymorphism and used to localize the gene on the linkage map of human chromosome 21. The AML1 gene is located between the markers D21S216 and D21S211, in chromosomal band 21q22.3.     

Contribution of an aged microenvironment to aging-associated myeloproliferative disease

The molecular and cellular mechanisms of the age-associated increase in the incidence of acute myeloid leukemia (AML) remain poorly understood. Multiple studies support that the bone marrow (BM) microenvironment has an important influence on leukemia progression. Given that the BM niche itself undergoes extensive functional changes during lifetime, we hypothesized that one mechanism for the age-associated increase in leukemia incidence might be that an aged niche promotes leukemia progression. The most frequent genetic alteration in AML is the t(8;21) translocation, resulting in the expression of the AML1-ETO fusion protein. Expression of the fusion protein in hematopoietic cells results in mice in a myeloproliferative disorder. Testing the role of the age of the niche on leukemia progression, we performed both transplantation and in vitro co-culture experiments. Aged animals transplanted with AML1-ETO positive HSCs presented with a significant increase in the frequency of AML-ETO positive early progenitor cells in BM as well as an increased immature myeloid cell load in blood compared to young recipients. These findings suggest that an aged BM microenvironment allows a relative better expansion of pre-leukemic stem and immature myeloid cells and thus imply that the aged microenvironment plays a role in the elevated incidence of age-associated leukemia.     

Translocation (3;21)(q26;q22) in secondary leukemia. Report of two cases and literature review.

The authors report two cases of secondary myelodysplasia and acute myeloid leukemia with t(3;21)(q26.3;q22) as the only cytogenetic abnormality in neoplastic bone marrow. This translocation was identified as a rare, recurring, non-random aberration in chronic myeloid leukemia less than five years ago and in secondary acute myeloid leukemia and myelodysplasia in 1990. The known and suspected cases in the literature are reviewed.     

Acquired pericentric inversion of chromosome 9 in acute myeloid leukemia

Pericentric inversion of chromosome 9 involving the qh region is relatively common as a constitutional genetic aberration without any apparent phenotypic consequences. However, it has not been established as an acquired abnormality in cancer. Among the three patients reported so far in the literature with acquired inv(9), only one had acute myeloid leukemia (AML). Here we describe an unique case where both chromosomes 9 presented with an acquired pericentric inversion with breakpoints at 9p13 and 9q12 respectively, in a AML patient with aberrant CD7 and CD9 positivity. Additionally, one der(9) also showed short arm deletion at 9p21 to the centromeric region and including the p16 gene. The constitutional karyotype was normal. This is probably the first report describing an acquired inv(9) involving both chromosomes 9 in AML. The possible significance of this inversion is discussed.     

Acute promyelocytic leukemia with unusual karyotype

Acute myeloid leukemia (AML-M3) is associated with the translocation t(15;17)(q22;q12-21) which disrupts the retinoic acid receptor alpha (RARA) gene on chromosome 17 and the PML gene on chromosome 15. We report a two-year-old patient with AML-M3 without the usual translocation t(15;17). Cytogenetic studies demonstrated normal appearance of chromosome 15 while the abnormal 17 homologue was apparently a derivative 17, der(17)(17qter-cen-q21:), the rearrangement distinctly shows deletion at 17q21 band and the morphology corresponding to an iso chromosome i(17q-). This case report is a rare cytogenetic presentation of acute promyelocytic leukemia (APML).     

Leukemogenic AML1-ETO fusion protein upregulates expression of connexin 43: the role in AML 1-ETO-induced growth arrest in leukemic cells

AML1-ETO, a fusion protein generated by the chromosomal translocation t(8;21), is frequently associated with acute myeloid leukemia (AML). In addition to blocking differentiation, AML1-ETO is also shown to induce growth arrest in AML cells, which is unfavorable for leukemogenesis harboring the t(8;21) translocation. However, its precise mechanism is still unclear. Here we provide the first demonstration that the conditional expression of AML1-ETO by the ecdysone-inducible system dramatically increases the expression of connexin 43 (CX43), together with growth arrest at G1 phase in leukemic U937 cells. We also show that the CX43 induction inhibits the proliferation of U937 cells at G1 phase, while the suppression of CX43 expression by small interfering RNA (siRNA) effectively overcomes the growth-inhibitory effect of AML1 -ETO in leukemic cells. Furthermore, either AML1-ETO or CX43 induction elevates cell-cycle negative regulator P27(kip1) protein by inhibiting its degradation, which is antagonized by siRNA against CX43. Taken together, our data indicate that CX43 plays a role in AML1-ETO-induced growth arrest possibly through the accumulation of P27(kip1) protein. The potential mutation or/and epigenetic alterations of CX43 and its related gene(s) deserve to be explored in AML1-ETO-positive AML patients.     

Transforming human blood stem and progenitor cells: A new way forward in leukemia modeling

MLL-AF9 (MA9) is a leukemia fusion gene formed upon translocation of the AF9 gene on chromosome 9 and the MLL gene on chromosome 11. MA9 is commonly found in acute myeloid leukemia (AML) and occasionally in acute lymphoid leukemia and is associated with intermediate to poor outcome. The specific signaling pathways downstream of MA9 are still poorly understood. We have recently described a model system whereby we expressed the MA9 fusion gene in human CD34+ Umbilical Cord Blood (UCB) cells and showed that these cells transformed to acute myeloid or lymphoid leukemia when injected into immunodeficient mice. The Mixed Lineage Leukemia oncogenes are unique in this model system in that they promote full transformation of primary human blood cells, while all other leukemia-associated oncogenes tested thus far have induced only partial phenotypes. Here we provide an update on the use of this system for modeling human leukemia and its potential application for therapeutic testing of novel compounds to treat the disease. We focus specifically on the Rho family of small guanosine triphosphatases (GTPases) as potential therapeutic targets, which we have implicated in the pathogenesis of AML associated with MA9 expression.     

Complex chromosome abnormality mimicking t(8;21)in an acute myeloblastic leukemia

A patient with acute myeloblastic leukemia (AML) and karyotypic abnormality mimicking t(8;21) is reported. Because of the uncommon morphology of blasts for AML with t(8;21), and absence of AML1-ETO fusion in the RT-PCR analysis, fluorescence in situ hybridization (FISH) was applied to precise the abnormality. FISH revealed a complex rearrangement involving chromosomes 7, 21 and 8, and involvement of the AML1 gene without evidence of AML1-ETO fusion. This case of unusual AML illustrates the necessity of collecting all hematologic, cytogenetic, and eventual FISH and RT-PCR data to analyse genetic rearrangements in leukemic patients.     

Targeted therapy: The new lease on life for acute promyelocytic leukemia, and beyond

Leukemia, a group of hematological malignancies characterized by abnormal proliferation, decreased apoptosis, and blocked differentiation of hematopoietic stem/progenitor cells, is a disease involving dynamic change in the genome. Chromosomal translocation and point mutation are the major mechanisms in leukemia, which lead to production of oncogenes with dominant gain of function and tumor suppressor genes with recessive loss of function. Targeted therapy refers to treatment strategies perturbing the molecules critical for leukemia pathogenesis. The t(15;17) which generates PML-RARα, t(8;21) that produces AML1-ETO, and t(9;22) which generates BCR-ABL are the three most frequently seen chromosomal translocations in myeloid leukemia. The past two to three decades have witnessed tremendous success in development of targeted therapies for acute and chronic myeloid leukemia caused by the three fusion proteins. Here, we review the therapeutic efficacies and the mechanisms of action of targeted therapies for myeloid leukemia and show how this strategy significantly improve the clinical outcome of patients and even turn acute promyelocytic leukemia from highly fatal to highly curable.