Roland Berger

A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera

Myeloproliferative disorders are clonal haematopoietic stem cell malignancies characterized by independency or hypersensitivity of haematopoietic progenitors to numerous cytokines. The molecular basis of most myeloproliferative disorders is unknown. On the basis of the model of chronic myeloid leukaemia, it is expected that a constitutive tyrosine kinase activity could be at the origin of these diseases. Polycythaemia vera is an acquired myeloproliferative disorder, characterized by the presence of polycythaemia diversely associated with thrombocytosis, leukocytosis and splenomegaly. Polycythaemia vera progenitors are hypersensitive to erythropoietin and other cytokines. Here, we describe a clonal and recurrent mutation in the JH2 pseudo-kinase domain of the Janus kinase 2 (JAK2) gene in most (> 80%) polycythaemia vera patients. The mutation, a valine-to-phenylalanine substitution at amino acid position 617, leads to constitutive tyrosine phosphorylation activity that promotes cytokine hypersensitivity and induces erythrocytosis in a mouse model. As this mutation is also found in other myeloproliferative disorders, this unique mutation will permit a new molecular classification of these disorders and novel therapeutical approaches.

Various types of rearrangements target TLX3 locus in T-cell acute lymphoblastic leukemia

Most chromosomal translocations observed in T‐cell acute lymphoblastic leukemia (T‐ALL) often produce transcriptional activation of transcription factor oncogenes. Ectopic expression of the TLX3 (also known as HOX11L2) gene has been shown to be associated with a cryptic t(5;14)(q35;q32) translocation specific for a subtype of T‐ALL. Here we report several examples of variant and alternative translocations resulting in expression of TLX3 in T‐ALL, and we describe three of these translocations in detail. In particular, the CDK6 gene was rearranged in two t(5;7)(q35;q21) translocations. In two additional instances, fusion of the BCL11B (also known as CTIP2) and RANBP17/TLX3 loci were shown to result from subtle genomic insertion/deletion within these loci. This study further underscores that TLX3 expression in T‐ALL is strongly associated with the presence of genomic rearrangements.

Transforming potential of the T-cell acute lymphoblastic leukemia-associated homeobox genes HOXA13, TLX1, and TLX3

The importance of HOXA genes in T‐cell acute lymphoblastic leukemia (T‐ALL) has recently been recognized. We report a novel chromosomal translocation in a T‐ALL patient that maps upstream of the HOXA13 gene and downstream of the BCL11B/CTIP2 locus. Analysis of HOXA gene transcription demonstrated massive expression of HOXA13, whereas the other HOXA genes were unaffected. A genomic rearrangement of the HOXA locus associated with exclusive expression of HOXA13 was observed in a second patient. This situation resembles chromosomal translocations activating genes of the TLX/HOX11 family in T‐ALLs. To compare the leukemogenic properties of HOXA13 to that of TLX proteins, cohorts of lethally irradiated mice were transplanted with bone marrow transduced with a retroviral vector expressing TLX3 or HOXA13. Cells transduced with TLX3 or HOXA13 could not be detected in the peripheral blood of mice post‐transplantation and none of the mice developed malignancies. Cotransduction of the HOX cofactor MEIS1 with TLX3 or HOXA13 did not alter this outcome. However, in a myeloid clonogenic assay HOXA13 and TLX3 extended the proliferation of progenitors similarly to what was observed for TLX1. Altogether, our results strongly suggest the absolute requirement for cooperative events in association with homeobox gene up‐regulation to induce T‐cell leukemogenesis.

Deregulation of cyclin D2 by juxtaposition with T-cell receptor alpha/delta locus in t(12;14)(p13;q11)-positive childhood T-cell acute lymphoblastic leukemia.

Abstract:  Objectives: The t(12;14)(p13;q11) – a recurrent translocation in childhood T‐cell acute lymphoblastic leukemia (T‐ALL) – has very recently been molecularly characterized in one case, which displayed overexpression of the cyclin D2 gene (CCND2). Patients and methods: We have characterized two pediatric t(12;14)‐positive T‐ALLs using fluorescence in situ hybridization (FISH), cDNA microarray, and real‐time polymerase chain reaction (PCR). Results: FISH revealed breakpoints (BPs) in the T‐cell receptor alpha/delta locus (14q11) and in the vicinity of the CCND2 gene at 12p13. To investigate the expression of genes in 12p13, cDNA microarray analysis was performed. Expression data for eight genes, including CCND2, surrounding the 12p BP were compared with those in other T‐ALLs. The t(12;14)‐positive T‐ALL displayed an increased expression of CCND2 compared to the controls, whereas the expression of the other genes was similar in all T‐ALLs. Expression of CCND2 and two additional genes (PARP11 and FGF23), close to the 12p BP, was investigated with real‐time PCR of the two t(12;14)‐positive cases and four controls. Neither PARP11 nor FGF23 displayed expression differences among the T‐ALLs, whereas CCND2 was clearly overexpressed in both t(12;14)‐positive cases as compared to the mean expression level in the controls. Conclusion: We have confirmed, in two additional cases, that the recurrent T‐ALL‐associated t(12;14) results in overexpression of cyclin D2. The t(12;14) is the first neoplasia‐associated translocation shown to result in overexpression of cyclin D2. Furthermore, it is the first example of a T‐cell neoplasm with a targeted deregulation of a member of a cyclin‐encoding gene family.