RESUMO
MLL/AF4 and AML/MTG8 represent two leukemic fusion genes, which are most frequently found in infant acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML), respectively. We examined the influence of MLL/AF4 and AML1/MTG8 fusion genes on the expression of TERT coding for the telomerase protein subunit, and subsequently telomerase activity in t(4;11)-positive ALL and t(8;21)-positive cell lines, respectively. MLL/AF4 suppression diminished telomerase activity and expression of TERT. Blocking pro-apoptotic caspase activation in conjunction with MLL/AF4 knockdown enhanced the inhibition of TERT gene expression, which suggests that MLL/AF4 depletion does not reduce TERT expression levels by inducing apoptosis. Knockdown of HOXA7, a direct transcriptional target of MLL/AF4 fusion gene, caused a reduction of telomerase and TERT to an extent similar to that observed with MLL/AF4 suppression. Chromatin immunoprecipitation of SEM cells, using ectopically expressed FLAG-tagged Hoxa7, indicates HOXA7 binding site in the TERT promoter region. Furthermore, suppression of the AML1/MTG8 fusion gene was associated with severely reduced clonogenicity, induction of replicative senescence, impaired TERT expression and accelerated telomere shortening. We thus present findings that show a mechanistic link between leukemic fusion proteins, essential for development and maintenance of leukemia, and telomerase, a key element of both normal and malignant self-renewal.
Assuntos
Subunidade alfa 2 de Fator de Ligação ao Core/genética , Proteína de Leucina Linfoide-Mieloide/genética , Proteínas de Fusão Oncogênica/genética , Leucemia-Linfoma Linfoblástico de Células Precursoras/genética , Leucemia-Linfoma Linfoblástico de Células Precursoras/patologia , Telomerase/genética , Apoptose , Western Blotting , Senescência Celular , Imunoprecipitação da Cromatina , Cromossomos Humanos Par 11/genética , Cromossomos Humanos Par 21/genética , Cromossomos Humanos Par 4/genética , Cromossomos Humanos Par 8/genética , Subunidade alfa 2 de Fator de Ligação ao Core/antagonistas & inibidores , Subunidade alfa 2 de Fator de Ligação ao Core/metabolismo , Proteínas de Homeodomínio/genética , Proteínas de Homeodomínio/metabolismo , Humanos , Hibridização in Situ Fluorescente , Proteína de Leucina Linfoide-Mieloide/antagonistas & inibidores , Proteína de Leucina Linfoide-Mieloide/metabolismo , Proteínas de Fusão Oncogênica/antagonistas & inibidores , Proteínas de Fusão Oncogênica/metabolismo , RNA Mensageiro/genética , RNA Interferente Pequeno/farmacologia , Proteína 1 Parceira de Translocação de RUNX1 , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Telomerase/metabolismo , Telômero/genética , Translocação Genética , Células Tumorais CultivadasRESUMO
The chromosomal translocation t(8;21) is associated with 10-15% of all cases of acute myeloid leukaemia (AML). The resultant fusion protein AML1/MTG8 interferes with haematopoietic gene expression and is an important regulator of leukaemogenesis. We studied the effects of small interfering RNA (siRNA)-mediated AML1/MTG8 depletion on global gene expression in t(8;21)-positive leukaemic cell lines and in primary AML blasts using cDNA arrays, oligonucleotide arrays and real-time reverse transcription-polymerase chain reaction (RT-PCR). Suppression of AML1/MTG8 results in the increased expression of genes associated with myeloid differentiation, such as AZU1, BPI, CTSG, LYZ and RNASE2 as well as of antiproliferative genes such as IGFBP7, MS4A3 and SLA both in blasts and in cell lines. Furthermore, expression levels of several genes affiliated with drug resistance or indicative of poor prognosis AML (BAALC, CD34, PRG2, TSPAN7) are affected by AML1/MTG8 depletion. In conclusion, siRNA-mediated suppression of AML1/MTG8 cause very similar changes in gene expression pattern in t(8;21)-positive cell lines and in primary AML blasts. Furthermore, the results suggest that the specific targeting of AML1/MTG8 function may be a promising approach for complementing existing treatment strategies.
