Frequent Mutation of the Polycomb-Associated Gene ASXL1 in the Myelodysplastic Syndromes and in Acute Myeloid Leukaemia.

Abstract 419

The identification of genes which are frequently mutated in the myelodysplastic syndromes (MDS) is critical to a full understanding of the molecular pathogenesis of this disorder. Recently, a newly identified gene mutation in MDS has been described; mutations of the ASXL1 (additional sex combs 1) gene have been reported in 4 out of 35 MDS patients (11%) and also in chronic myelomonocytic leukaemia (CMML) by Gelsi-Boyer et al (Br J Haematol 2009, 145:788-800). ASXL1 belongs to a family of three proteins regulating chromatin remodelling. The mutations were all found in exon 12 of the gene and are predicted to lead to the truncation of the C-terminus of the protein, which contains a PHD finger. We have screened ASXL1 for mutations in a large group of patients with MDS and AML in order to determine their frequency and any association with disease progression. Moreover, we have determined the effect of these mutations on global gene expression patterns and gene pathways. Mutation analysis of ASXL1 using direct sequencing was performed on DNA extracted from the bone marrow or peripheral blood samples of a total of 210 patients with MDS, AML or MDS/MPD and mutations were observed in 45 out of the 210 patients (21.4%). All mutations were observed in exon 12 of ASXL1. Mutations were found in 21 out of 121 MDS patients (17.4%); 0 out of 23 RA patients (0%), 0 out of 30 5q- syndrome patients (0%), 5 out of 22 RARS (22.7%), 1 out of 7 RCMD (14.3%), 2 out of 9 RCMD-RS (22.2%), 6 out of 15 RAEB-I (40.0%) and 7 out of 15 RAEB-II (46.7%). Thus mutation of ASXL1 represents one of the most common gene mutations in MDS and is strongly associated with more advanced forms of this disorder. Mutations were also found in 16 out of 44 CMML patients (36.4%) and in 8 out of 45 AML patients (17.8%), 21 of which were MDS-related AML. The majority of the mutations identified in this study were frameshift mutations caused by deletion or duplication of a nucleotide. The most common mutation was c.1934dupG p.Gly646TrpfsX12, found in 28 patients (13.3%). To investigate whether the mutations were acquired, we analyzed germline DNA from T cells of 7 cases. In all cases examined, the mutations were restricted to the non-lymphoid cells, indicating that they were acquired. Three patients with mutation of ASXL1 were shown by SNP array analysis to have a deletion mapping to 20q, encompassing the ASXL1 gene. Gene expression profiling, using the Affymetrix U133 Plus 2.0 arrays, was performed on CD34+ cells obtained from a subset of the MDS patients (n=29) and CMML patients (n=7) with and without mutation of ASXL1. Using hierarchal clustering, we obtained a good separation between cases with and without a mutation. Ingenuity pathway analysis showed that the most significantly deregulated pathway in MDS patients with a mutation of ASXL1 was the Retinoic Acid Receptor (RAR) activation. ASXL1 has been postulated to act as a ligand-dependent coactivator for RAR (Cho et al, J Biol Chem 2006, 281:17588-98). The most significantly deregulated ontology (functional) groups according to DAVID analysis included chromosome rearrangement. We conclude that ASXL1 is the most frequently mutated gene in advanced MDS known so far and we suggest that it may play a role in disease progression. These data suggest that the inactivation of ASXL1 may play an important the role in the molecular pathogenesis of MDS, AML and MDS/MPD.