The t(10;11)(p13;q14) is a recurring translocation resulting in the fusion of the CALM and AF10 genes. The leukemogenic CALM/AF10 fusion genes codes for a 1595 amino acids protein. This translocation was first identified in a patient with hystiocytic lymphoma and was subsequently found in patients with AML, T-ALL and malignant lymphoma. This translocation is found in younger patients and is associated with a poor prognosis.
The CALM/AF10-associated leukemias can exhibit myeloid, lymphoid or mixed lymphoid-meyloid features, indicating a stem cell or an early commited progenitor as the target cell of leukemic transformation. At the present time the target cells in CALM/AF10-associated leukemogenesis are unknown. It is also not known which target genes are up or downregulated by the presence of the CALM/AF10 fusion protein.
To answer these questions, the following experiments were performed:
1) Five transgenic mouse lines, two expressing CALM/AF10 under the control of the immunoglobulin heavy chain enhancer promoter and three under the control of the murine proximal Lck promoter were generated. Although the CALM/AF10 expression was confirmed to be present and specific to the cells targeted by the promoters used (B- and T-cell progenitors for IgH and Lck promoters, respectively), the transgenic animals did not show a phenotype that could be detected after meticulous clinical, haematological, immunological, flow cytometrical and immunohistopatological analysis .
2) We performed molecular characterization of several CALM/AF10 patient samples: A group of 13 patients with different types of leukemia: case 1 (AML M2), case 2 (Acute Biphetnotypic leukemia), case 3 (Pre T-ALL), case 4 (Acute Undifferentiated Leukemia), case 5 (PreT-ALL), cases 6 and 7 (ProT-ALL), case 8 (T-ALL), case 9 (AML), case 14 (T-ALL), case 15, 16 and 17 (AML) with a t(10;11) translocation detected by cytogenetic analysis suggesting a CALM/AF10-rearrangement. The samples were analyzed for the presence of the CALM/AF10 and AF10/CALM fusion transcripts by RT-PCR and sequence analysis. All these patients were found to be positive for the CALM/AF10 fusion. In addition, we analyzed a series of twenty-nine patients with T-ALL with T-cell receptor ≥¥ rearrangement. Among these patients, four (case 10 to 13) were positive for the CALM/AF10 fusion transcript, indicating a high incidence of CALM/AF10 fusions in this group of leukemia.
Three different breakpoints in CALM at nucleotide 1926, 2091 and a new exon, with 106 bases inserted after nt 2064 of CALM in patient 4 were found. In AF10 four breakpoints were identified: at nucleotide position 424, 589, 883 and 979. In patient 16 we found an extra exon before nt 424 of AF10. In seven patients it was also possible to amplify the reciprocal AF10/CALM fusion transcript (case 1, 3, 4, 8, 9, 10 and 14). There was no correlation between disease phenotype and breakpoint location. Ten CALM/AF10 positive patients were analyzed using oligonucleotide microarrays representing 33,000 different genes (U133 set, Affymetrix). Analysis of microarray gene expression signatures of these patients revealed high expression levels of the polycomb group gene BMI1, the homeobox gene MEIS1 and the HOXA cluster genes HOXA1, HOXA4, HOXA5, HOXA7, HOXA9, and HOXA10. The overexpression of HOX genes seen in these CALM/AF10 positive leukemias is reminiscent to the pattern seen in leukemias with rearrangements of the MLL gene, normal karyotypes and complex aberrant karyotypes suggesting a common effector pathway (i.e. HOX gene deregulation) for these diverse leukemias. In addition, the general pattern of gene expression of CALM/AF10 patients when compared to other leukemia subtypes and to normal bone marrow was dominated by a global downregulation of genes some of them with function identified as related to important molecular mechanisms, such as membrane trafficking, cell growth regulation, proliferation, differentiation and tumor suppression.
3) We cloned CALM/AF10 fusion gene into a vector that allowed us to induce the expression of CALM/AF10 using doxycycline in transiently and stably-transfected NIH3T3 and HEK293 cells. This system will be an important tool to identify direct CALM/AF10 target genes and to answer the question whether continued CALM/AF10 expression is necessary to maintain the CALM/AF10-associated expression pattern
