NF1 Microdeletions in Pediatric MLL-Rearranged AML and T-ALL: A Novel Mechanism for RAS Activation.

Oligo array-comparative genomic hybridization (CGH) is a technique to detect DNA copy number changes (amplifications and/or deletions), and can be used to identify novel genetic abnormalities in cancer. Searching for such abnormalities in specific selected subgroups of pediatric leukemia, we analyzed array-CGH data obtained on 212 samples (71 with MLL-rearranged acute myeloid leukemia (AML), 38 with normal karyotype (NK) AML, and 103 with T-cell- acute lymphoblastic leukemia (T-ALL). Cryptic deletions involving the neurofibromatosis type 1 (NF1) locus, del(17)(q11.2), were identified in 2/71 MLL-rearranged AML and 3/103 T-cell ALL samples, all cases without clinical evidence of NF1, but were not observed in the NK AML samples. The single copy loss of the NF1 locus was confirmed by NF1 specific MLPA assay. Subsequent mutation analysis revealed mutations in the remaining NF1 allele in 3 patients, confirming its role as a tumor-suppressor gene in cancer, since both were restricted to the leukemic cells. Furthermore, NF1 expression in the del(17)(q11.2)-positive leukemias was significantly lower in the patients tested compared to the other patient samples without this deletion. NF1 is an autosomal dominant genetic disorder caused by mutations in the NF1 gene. A small proportion of NF1 patients show germline NF1 gene deletions, in contrast to the confirmed somatic deletions described here in our leukemia patients. Other mutations in NF1, such as inactivation of both alleles by point mutations, or loss of one wild-type allele as an effect of uniparental disomy for the affected chromosome 17 has been observed in NF1 patients with JMML, may have been missed in our cohort. NF1 is a negative regulator of the RAS pathway (RAS-RAF-MEK-ERK pathway). Other mutations in this pathway have previously been associated with myeloid malignancies, and result in RAS activation, apoptotic resistance and proliferation of leukemic cells. We identified 22% N- and K-RAS and 1.4% PTPN11 mutations in our MLL-rearranged AML subgroup. RAS activation may therefore play an important biological role in at least 25% of MLL-rearranged AML samples. In conclusion, our data show that NF1 microdeletions are involved in leukemogenesis in pediatric leukemias, and that RAS-activation may be especially involved in the underlying biology of MLL-rearranged AML.