Genetic and modelling of childhood leukaemia group
Paediatric cancers affect about 1/600 child and represent the second cause of death in western countries. The haematological malignancies account for approximately 45% of the paediatric cancers. Clinical features of these paediatric cancers suggest that they have different bases compared to the corresponding adult cancers. However, the precise molecular mechanisms of transformation and the bases for the exclusive association between several genetic alterations and paediatric cancers are unclear.
The overall goal of our studies is to identify the molecular bases of patient with haematological malignancies in order to understand the mechanisms of transformation and develop novel therapeutic strategies. For this purpose, we perform high-throughput sequencing analyses to identify the genetic landscape of leukaemia and perform functional analyses through the development of cellular and preclinical in vivo models (e.g. transgenic and patient-derived xenotransplantation). We have mainly focused our studies on acute megakaryoblastic leukaemia (AML-M7 or AMKL), a subtype of AML mainly diagnosed in children. AMKL are frequently diagnosed in patients with the constitutive trisomy 21 (Down’s syndrom) predisposition associated with a 80% survival rate. However, the majority of AMKL cases are diagnosed de novo and are associated with a 3-year survival of only 14% to 34% despite high-intensity chemotherapy.
In the past years, we have identified and characterised several genetic alterations found in de novo pediatric AMKL including:
- A chromosomal translocation t(1;22)(p13;q13) leading to the expression of the OTT-MAL fusion protein in about 20 % of patients (Mercher et al. PNAS 2001).
- A chromosomal inversion inv(16)(p13;q24) leading to the expression of a fusion oncogene between CBFA2T3 (a.k.a ETO2) and GLIS2 in about 30% of patients (Thiollier et al. JEM 2012).
- Chromosomal alterations leading to fusion oncogene involving NUP98 and MLL genes. Patients with these alterations are part of a more heterogeneous groups patients presenting in a common upregulation of several HOX genes.
About 10-15% of patients do not present any known mutations and remain to be characterised.
Our functional studies showed that the OTT-MAL fusion aberrantly activates the Notch signalling pathway (Mercher et al. JCI 2009), which plays a role in the normal development of the erythro-megakaryocytic lineage (Mercher et al. Cell Stem Cell 2008), but the expression of OTT-MAL alone does not efficiently induce AMKL in mice. Among the other most prevalent mutation in AMKL, we investigated the mutations in signalling proteins, such as MPL, the thrombopoietin receptor, or JAK kinases (~15-20% of AMKL)(Malinge et al, blood 2008). In mice, co-expression of OTT-MAL with an MPL mutant led to the first model of AMKL with a short latency (Mercher et al. JCI 2009).
The ETO2-GLIS2 fusion involves the GLIS2 transcription factor regulated by Hedgehog pathway activation and ETO2-GLIS2 patients exhibit an Heghegog pathway expression signature (Thiollier et al JEM 2012). While the Hedgehog pathway has been implicated in other malignancies, the ETO2-GLIS2 fusion represents the first example of direct alteration of this pathway in leukaemia. We are currently investigating the molecular bases of ETO2-GLIS2 in leukaemogenesis. To closely reproduce the chromosomal and genetic alterations found in these leukaemia, in the laboratory we are now using CRISPR/Cas9 genome engineering strategies in human and murine cells.
In parallel, we pursue the development of primary patient samples-derived xenotransplantation models to reproduce the features of the human AMKL disease (Thiollier et al, JEM 2012) and we are performing preclinical testing to assess the efficacy of novel therapeutic strategies. As part of a collaboration with John Crispino (Chicago, USA) we have established the efficacy of Aurora A kinase on human AMKL cells (Wen et al. Cell 2012). Cathy Ignacimouttou and Aurélie Siret are currently developping novel bioluminescent models using primary patient cells to improve the analysis of targetted treatments.
Finally, our work also focuses on the study of chromosomal numerical alterations in leukaemia (e.g. monosomy, trisomy, hyperdiploidy). Indeed, the functional impact of dosage imbalances of one or more genes remains unclear in leukaemogenesis. Trisomy 21 is frequently observed in pediatric acute leukaemia (AL) pediatric. Acquired trisomy 21 is found in 15% of the B-cell acute lymphoblastic leukaemia (B-ALL, 15%) and >25% of AMKL. In addition, patients with constitutive trisomy 21 (Down’s syndrome, DS) have an increased risk of developing leukaemia during childhood, compared to the general population (20 to 50 times higher for B-ALL and up to 500 times higher for AMKL). We showed that trisomy of 33 genes (Ts1Rhr mouse model of trisomy 21 for the minimal Down Syndrome Critical region in human), cooperates with GATA1 and MPL mutants for the development of AMKL in mice, thereby reproducing in vivo the acquisition of molecular abnormalities found in patients. Among the genes on chromosome 21, trisomy of DYRK1A, a protein kinase with a capacity of autophosphorylation and whose activity is solely regulated by its expression level, predisposes to AMKL development by inhibiting the calcineurin/NFAT signalling pathway (Malinge et al JCI 2012). We are currently investigating the impact of isolated trisomy of DYRK1A or other chromosome 21 genes on normal megakaryopoiesis, AMKL and B-ALL development.