RESUMO
The SH2 domain-containing tyrosine phosphatase PTPN6 (SHP-1, PTP1C, HCP) is a 68 kDa cytoplasmic protein primarily expressed in hematopoietic cell development, proliferation and receptor-mediated mitogenic signaling pathways. By means of direct dephosphorylation, it down-regulates a broad spectrum of growth-promoting receptors, including the Kit tyrosine kinase, activated to elicit a prominent cascade of intracellular events by stem cell factor binding. The pivotal contribution of PTPN6 in modulating myeloid cell signaling has been revealed by the finding that shp-1 mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten (me/me) and motheaten viable (me(v)/me(v)) mice. Association of PTPN6 with c-Kit and negative modulation of the myeloid leukocyte signal transduction pathways prompted us to examine the expression of the protein tyrosine phosphatase PTPN6 gene in CD34(+)/CD117(+) blasts from acute myeloid leukemia patients. We identified and cloned cDNAs representing novel PTPN6 mRNA species, derived from aberrant splicing within the N-SH2 domain leading to retention of intron 3. Sequence analysis of cDNA clones revealed multiple A-->G editing conversions. The editing of PTPN6 mRNA mainly occurred as an A-->G conversion of A(7866), which represents the putative branch site in IVS3 of PTPN6 mRNA. Evidence that editing of A(7866) abrogates splicing has been obtained in vitro by using an edited clone and its backward clone generated by site-directed mutagenesis. The level of the aberrant intron-retaining splice variant, evaluated by semi-quantitative RT-PCR, was lower in CD117(+)-AML bone marrow mononuclear cells at remission than at diagnosis, suggesting the involvement of post-transcriptional PTPN6 processing in leukemogenesis.
Assuntos
Processamento Alternativo , Leucemia Mieloide/genética , Proteínas Tirosina Fosfatases/genética , Edição de RNA , Doença Aguda , Sequência de Bases , Células HL-60 , Humanos , Peptídeos e Proteínas de Sinalização Intracelular , Íntrons , Dados de Sequência Molecular , Proteína Tirosina Fosfatase não Receptora Tipo 11 , Proteína Tirosina Fosfatase não Receptora Tipo 6 , Proteínas Tirosina Fosfatases/metabolismo , RNA Mensageiro/análise , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Proteínas Tirosina Fosfatases Contendo o Domínio SH2 , Análise de Sequência de RNA , Células Tumorais CultivadasRESUMO
A G-->T transversion at nucleotide 2467 of the c-KIT gene leading to Asp816-->Tyr (D816Y) substitution in the phosphotransferase domain has been previously identified in a patient with rapidly progressing AML-M2 and mast cell involvement; the patient's blasts had a 47,XY, +4,t(8;21)(q22;q22) karyotype. Herein we confirm the simultaneous presence of both major chromosomal changes by multicolor fluorescence in situ hybridization (FISH) on interphase CD34+ mononuclear cells. By setting up culture leukemic blasts, spontaneous differentiation of adherent cells with mast-cell like features was proved by histochemical and immunoenzymatic analyses. Fluorescence in situ hybridization evidence of trisomy 4 confirmed the origin of differentiated cells from the leukemic blasts. Semiquantitative polymerase chain reaction (PCR) and phosphoimage densitometry of wild-type and mutated KIT alleles on bone marrow blasts made it possible to demonstrate that chromosome 4 trisomy led to a double dosage of the mutated KIT allele. This finding, and that of trisomy 7 and MET mutation in hereditary renal carcinoma represent the only cases of human tumors in which an increased number of chromosomes carrying an oncogene activated by point mutation have been detected.
